Setup Theory
1:Tires
Tires are the most important element in the quest to get a car
to handle well. They're the only link between the car and the earth. That link
depends solely on the friction between the surface and the tire's contact patch,
so let's have a look at how friction works.
1.1 On-road tires
1.1.1 Friction
The formula for friction between two surfaces is side load = µ *
weight. µ Is the coefficient of friction.
For a rubber tire, µ is definitely not constant; it varies with temperature,
pressure and more importantly, amount of slip. This is represented in the next
graph.
On the horizontal axis is the amount of slip, from 0%(no slip, the tire just
rolls along) to 100% (Either the tire is standing still and the vehicle is
moving, or the vehicle is standing still, but the tire is moving). On the
vertical axis is the coefficient of friction. In the left part of the graph,
slip within the tire is dominant, also known as tire squirm. This happens when
the tire deforms under load, and the contact patch moves relative to the axle.
This also causes slip angles to exist. In the right part, slip between the two
surfaces is dominant; the tire starts to slide sideways a little. It is
remarkable that µ reaches its maximum when there is a little slip, usually it's
between 5% and 15%. That's because rubber interacts with the surface in a very
special way.
In fact, the reason why the graph has such an odd shape is because it's a
combination of things, there are two separate mechanisms involved: hysteresis
and adhesion.
The first component, adhesion, is the phenomenon that the outermost atoms of the
rubber molecules are in direct contact with the outer molecules of the surface.
Rubber is a polymer, and its molecular structure resembles spaghetti of strings
of atoms, the surface is most of the time crystalline, in which the atoms are
more closely together. So when there is a speed difference between the two, the
'atom strings' in the rubber will be stretched. Some molecular bonds will break,
and new ones will be formed. This process repeats itself as the one surface is
dragged over the other. Obviously, breaking and stretching molecular bonds, and
moving atoms around takes energy, and hence also a force. That is the adhesion
force. It reaches its maximum when the speed difference is somewhere between
0,03 and 0,06 meters per second.
The second component, hysteresis, exists because rubber is being deformed. As
the tire carcass is being distorted, in some areas the rubber gets compressed,
and in other areas it gets stretched. For stretching to be possible, the atoms
must move alongside each other, and as always, it's an irreversible process
because of friction. The friction will make the tire heat up. Again, all this
takes energy, and thus a force. That force is the hysteresis force, which is
very similar to the adhesion force, only its size is determined by the internal
friction in the rubber.
As the weight on the tire and the amount of slip vary, the proportion of the two
components changes. For example, if there is more slip, the hysteresis component
will be dominant over adhesion. If the rubber compound is very soft, and the
temperature is high and the surface smooth, adhesion will be the dominant force.
Note that all the above is valid for very hard racing surfaces, like asphalt or
really hard clay. If the surface is soft, it's the deformation of the surface
that causes the friction force to exist: the spikes on the tires dig into the
surface, and make grooves into it. In that case, the graph doesn't have a
section that's curved down; µ always increases as the weight on the tire and the
amount of slip increases. It's a totally different mechanism. That's also the
reason why when an on-road car takes a turn, and transfers weight onto the
outside tires, its cornering power decreases, while when an off-road car does
the same thing, its cornering power increases. So it makes sense for on-road
cars to have a high roll stiffness (think anti-roll bars), and for off-road cars
to have a very low one.
1.1.2 The traction circle
Now that we know how friction works, and how it is usually
maximal when there is a little slip, let's find out how it influences the car's
handling.
Unless the tire's thread isn't symmetrical, friction is the same in all
directions, and it also has a maximum value, which is also the same in all
directions. This can be represented by the traction circle.
The vertical component of the graph represents acceleration and deceleration,
and the horizontal component represents turning left and right. The maximum
amount of grip is represented by the edge of the circle, and the area of the
circle represents the amount of grip of the tire on the road.
Naturally, the fastest way around a track is to use your tires to their very
limit. So, to brake as fast as possible, you will need to take the tires to
point C on the graph. If you brake too hard, and you exceed point c on the
graph, you will skid, and your braking distance will increase. You might even
lose control. The same thing goes for acceleration: if you exceed point a, you
will experience a lot of wheelspin, and you'll accelerate slower. It's also
possible to exceed the grip limits when cornering (points D(black) and B), and
spin out.
But the hardest parts to judge aren't the axis lines, it's the parts in between.
Point D for example (the green one) represents a situation where the car is
turning right and accelerating. Note that D (green) is on the edge of the
circle, yet the car isn't accelerating or turning at its maximum speed, it's
somewhere in between. Let's say you are accelerating as fast as possible (point
A), and you steer a little towards the left. On the graph, this means you're at
a point left of a, which is outside the circle, so the tires will break loose,
and the car won't turn (front wheel drive) or spin out (Rwd). Another
interesting fact is that in order to get the most cornering power, there
shouldn't be any power applied to the wheels. (Points B and D (black)) And
conversely, in order to get the fastest possible acceleration or braking, no
steering should be applied.
Keep in mind that the radius of the traction circle represents the maximum
adhesion force, and this is proportional (well, kind of, as explained in the
previous paragraph) to the vertical load on the tire. So, in brief: the size of
the circle increases as more vertical pressure is exerted on the tire, and it
decreases if there's less vertical pressure on it. The circle doesn't even exist
when there's no pressure on the tire. It makes sense, because a tire
that's hanging in the air can't resist any lateral force.
1.1.3 Slip angles
You might have wondered what exactly happens when you go beyond
the traction circle, and how your car will react. Slip angles provide a clear
way of describing this.
A slip angle is the angle between where the tire is pointing and where it
actually going. Each tire has its own slip angle.
A tire that's not slipping has a slip angle of zero degrees. But 'slip' can be
internal as well as external; the contact patch doesn't need to be slipping
relative to the road, twisting of the tire's carcass is also a form of slipping.
This next drawing represents a car taking a turn at low speed. All four slip
angles are zero.
Assuming the car has the correct Ackermann effect and no rear toe-in, the car
can turn with none of the tires slipping. Note that the imaginary (well they're
not so imaginary when I draw them out for you) lines through the four axles
intersect at one point. That's the point the car is turning around. Sort
of like the apex of the corner the car is taking.
This is a typical situation when cornering speed is low, and all four tires have
more or less the same weight on them.
But...unfortunately, things aren't always like you want them to be. One common
condition is understeer. This happens when the front tires don't have
enought weight on them, and they start to slip, hence creating a slip angle.
The slip angle of the front tires is the angle between the blue lines and the
green lines.
The car is not turning around the point you'd expect, or want it to turn. (where
the blue lines intersect, point N) Instead, it's turning around the intersection
point of the green lines(point U), which makes for a larger turning radius than
expected. This is understeer: when the turning radius is bigger than you'd like
it to be.
The opposite can also happen: the rear tires can have insufficient weight on
them, and start to slip. this usually leads to a condition called oversteer,
where the turning radius is smaller than you'd expect it to be.
Here, the rear tires have started to slip, creating slip angles at the rear of
the car. The inside front tire has also started to slip. This is because the car
can't be turning around two separate points at the same time. in this case, the
car is turning around point O, (whereas the driver would have expected it to be
turning around point N.) When a car is cornering, the lines representing the
slip angles always intersect at the point the car is rotating around. If they
don't, the tire with the least amount of weight on it (in this case the inside
front) will develop a slip angle.
Notice that the point which the car is rotating around (O) is now much closer to
the center of the car, and more towards the front. The car will turn very
sharply, much sharper and earlier than expected.
Plain over- and understeer are very common conditions, but in reality, all sorts
of wacky things can happen.
For example: you can power slide around the corner.
Although the front wheels are steered to the left, the car is turning to the
right. (countersteering) The rear tires are sliding at an extreme angle.
No need to say this requires some serious driving skill.
1.2 Off-road tires
Off-road tires operate quite differently from on-road tires.
They usually have some sort of tread pattern with pins that dig into the soil,
or a series of small pins that scrape the top surface. This is entirely
different from the friction model described above, where you had a smooth, hard
surface and a uniform rubber contact patch.
Off-road traction is generally more complicated, the curves have more complex
shapes, there are more types of soil, more transients, and there are more
variables.
1.2.1 Tread Pattern
Size of the pins/blocks
There's a basic rule of thumb that says:"The softer the dirt,
the bigger the pins need to be.". Long pins work by penetrating the (soft) soil,
and short pins usually work by scraping off the upper layer of the soil.
Bear in mind that long pins and very soft compounds don't mix very well, then
the spikes just bend over instead of penetrating the dirt.
Density of the pins/blocks
For a given tire width, the density of the pins is inversely
proportional to the weight supported by each pin. There's usually an optimum,
where the tire works best. For example: if a certain tire works very well when
it's heavily loaded, but doesn't feel right when it isn't loaded, the tread
pattern is probably too dense. This can happen in very dusty or soft conditions.
Tires for sticky mud usually have a very low spike density, because too much
spikes cause too much stiction for the amount of traction they create, slowing
the car down.
Pins or blocks
Round pins provide grip that feels the same in all directions,
it feels consistent and it's somewhat easier to slide. Very useful on difficult,
low-traction, bumpy tracks.
Square blocks feel more 'edgy', they can give the impression of generating
slightly more grip, especially on smooth, hard surfaces. The forward traction
they generate also feels nicer.
Center tread
Some tires have a larger tread pattern in the center, often
shaped like an X, an I, or a T. In all cases, it increases forward traction
dramatically.
Note that on 4WD cars, more forward traction from the front tires can also mean
more steering.
1.2.2 Rubber compound
"The softer the surface, the harder the compound, and vice
versa."
Some synthetic surfaces, like carpet or polished floors require specially
formulated compounds, such as Schumacher's Yellow compound, and Medial Pro's
Indoor compound.
E.g. Losi Gold, ProLine XTR, Schumacher Blue & Green.
These work well on very soft surfaces, such as mud, damp, loose dirt and fresh
grass. The idea is that the tire doesn't move, but the soil does.
Hard compound tires aren't sensitive to changes in foam insert.
Medium compounds
E.g. Losi Silver, ProLine M2, Schumacher Silver.
These compounds work well on most dirt surfaces. They're the best choice if the
track is very dusty, or is starting to break up.
Soft compounds
E.g. Losi Red, ProLine M3, Schumacher Pink.
Soft compound tires work very well in blue-groove conditions, when there's
enough rubber deposit on the track to make it darkish gray or black. They also
work well on very hard surfaces, where the rubber compound is more important
than the tread design.
1.2.3 Carcass shape
Round carcass
These tires have a rounded contact area, so they're not at all
sensitive to camber changes. They're excellent for bumpy, rutted conditions;
they'll provide consistent traction and won't hook into the ruts and make the
car flip over. The downside is that they don't generate as much forward
traction.
Flat carcass
These tires have a flat, or almost flat contact area. They
provide excellent forward traction, and if camber is correct, also excellent
cornering traction, but only on smooth surfaces. In bumpy sections, they feel
inconsistent and can make the car flip over easily.
1.2.4 Rim size
Taller rims
If you use a slightly taller rim, for example if you use an old
2.0 inch tire on 2.2 inch rims, you'll stretch the tire's sidewall a little,
making it stiffer and flatter. If you overdo this, the tire's carcass is bent
out of shape, and traction is very poor. But done correctly, it can make the
tire feel just a little more responsive and sure-footed, but maybe not as good
in bumps.
Wider rims
Using slightly wider rims seems to be in fashion now, probably
because they combine very well with very soft compound tires. Using a slightly
wider rim stretches the carcass, making it lower, wider and more firm. This
makes the tire feel more direct, and a little better for smooth tracks.
1.2.5 Foam inserts
All tires, except maybe hard coumpound tires, need foam inserts
in order for the carcass to keep its shape. The softer the carcass, the denser
the foam needs to be. It's best that the shape of the foam is matched to the
shape of the carcass, so often the foam will need some trimming around the
edges. Also, when you're using wide rims, you might need a wider foam.
The foam that comes with the tires is usually the best choice, you could use a
slightly softer one for bumpy tracks and a slightly harder one for smooth
tracks, but you'll get a very similar result by just trimming the stock foam
differently. If the variations are relatively small, using a bigger or a more
dense foam has the same effect.
2: Suspension Components
2.1 Springs
The most common variety of springs are coil springs (see pic), these are usually
placed around the damper housing to form a spring-damper unit. A spring is an
elastic device that resists movement in its direction of work. The force it
exerts is proportional to the movement of one of its ends. Or to put this into a
mathematical equation: Force = movement * spring constant. A high value for the
spring constant makes for a stiff spring, and a low value makes for a soft
spring.
For progressive springs the spring constant will increase as the spring goes
deeper into its travel, and for regressive springs it will decrease with travel.
Most coil springs are slightly progressive, because as they compress, some of
the coils start touching each other, especially near the top and the bottom, and
hence the number of active coils decreases.
So math wise, springs aren't very complicated, but handling wise, they are. The
problem is that they work in two dimensions: left to right and front to rear.
For example: a car with soft springs will experience a lot of body roll in fast
turns, but it will also dive very hard under heavy breaking and squat a lot
while accelerating. This is because the springs have to absorb the torques that
are generated (see roll center and anti-squat), and soft springs need to be
compressed over a larger distance to be able to absorb a certain force. (If this
doesn't make sense, I suggest you take another good look at the graph) Note that
both observations have the same effect: more load on the front tires. So you
might think: "Why make a big deal out of this, the effect is the same." It's a
big deal because by the time you have read all of the chapters, you'll be able
to adjust a car's lateral balance independent from its longitudinal balance, but
for now, just remember that spring stiffness affect s just about anything: bump
handling, roll stiffness, pitch stiffness,… .
In general, you could say that stiffer springs yield less grip on that end of
the car, and conversely, softer springs yield more grip. This is because springs
inhibit weight transfer, both front-to-rear and left-to-right: for the same
cornering, acceleration or braking force a stiffer spring will compress less,
resulting in less chassis movement and thus also less weight transfer, and a
soft spring will compress a lot, resulting in a lot of weight transfer.
But, you won't always be able to use the spring you want: on small,
high-frequency bumps, stiff springs will make the car bounce, resulting in a
loss of grip. So you need softer springs, because they allow the tires to stay
in contact with the ground. On smooth tracks however, stiff springs are the way
to go, they will also help the car's jumping ability and responsiveness.
2.2 Damping
Damping is needed to absorb the energy associated with suspension travel. That
suspension travel can be induced by bumps, or lateral or longitudinal
acceleration. Without damping, the magnitude of the suspension movement would
never stop increasing, leading to a very humorous situation. In terms of energy,
damping absorbs most of the energy the car receives as it moves, unlike
springs, who store the energy, and release it again. Imagine a car with
no damping driving on a bumpy road. The subsequent impacts from the bumps on the
tires would make the suspension bounce very intensely, which is not a good
thing. Dampers absorb all the excess energy, and allow the tires to stay in
contact with the ground as much as possible. This also indicates that the
damping should always be matched to the spring ratio: never run a very stiff
spring with very soft damping or a very soft spring with very stiff damping.
Small changes however can give interesting results. Damping that's a bit on the
heavy side will make the car more stable; it will slow down both the vehicle's
pitch and roll motions, making it feel less twitchy. Note that damping only
alters the speed at which the rolling and pitching motions occur, it does
not alter their extent. So if you want your vehicle to roll less, adjust the
anti-roll bars, or the springs, but not the dampers.
Something you can adjust with the damping rate is the speed at which the
suspension rebounds: if a car with soft springs but hard dampers is pushed down,
it will rebound very slowly, and a car with stiff springs and light damping will
rebound very quickly. The same situation occurs when exiting corners: in the
corner, the weight is transferred, and the chassis has rolled and/or dived, but
when the steering is straightened out, and the cornering force disappears, the
chassis comes back to its original position. The speed at which this happens is
controlled by the damping rate. So the car with the soft springs and hard
damping will tend to want to continue turning when the steering is straightened.
It will also tend to continue running straight when steering is first applied;
it will feel generally unresponsive, yet very smooth. The car with firm springs
and soft damping will be very responsive: it will follow the driver's commands
very quickly and aggressively.
You may not always be able to use the spring and damping rates you'd like,
because of bumps. Small, high-frequency bumps require soft settings for both
damping and springs. You can't use such soft settings for big, harsh bumps,
because the car would bottom out a lot, so you'll need to set your car a little
stiffer. On very smooth tracks you can use very stiff settings for both springs
and damping.
But it's not quite as simple as that: even in the simple dampers used in R/C
cars, there is a difference between high-speed and low-speed damping. Maybe I
should point out that the speed which is being referred to is the speed of the
shaft in relation to the housing, not the speed of the car. In most full-scale
cars, the difference is implicated by means of an array of spring-operated
valves in the piston. In less sophisticated damper units, as used in R/C, the
difference is an effect of the inherent properties of the fluid being used.
If there's anything a racing enthusiast needs to know about fluid dynamics, it's
that there are two basic ways for a fluid to flow; laminar and
turbulent. A flow is said to be laminar if the particles move parallel to
each other, creating flow lines that never intersect. Laminar flow occurs when
the velocity is low, the fluid has a high viscosity, and the surface is smooth
and well-rounded. A flow is said to be turbulent if the particles move randomly,
creating eddies. Situations where the velocity is high, the fluid is thin and
the surface is rough favor turbulence. In case of turbulence, a lot more energy
is required(or wasted, depends how you look at it) because there is a lot more
friction between the particles. Also, for a laminar flow the pressure
(resistance, in case of a damper) is proportional to the velocity of the fluid
whereas in case of turbulence, it's proportional to the velocity squared. There
is no strict distinction between the two types; there's a big gray area in
between . To predict whether or not a flow is turbulent, the Reynolds number is
used. It's defined as Re = D * V /n .
D is the diameter, V is the velocity of the fluid, and
n is its viscosity. If Re is smaller
than 2000, the flow is most likely to be laminar, if it's in between 2000 and
4000 it's something in between, and if it's greater than 4000, the flow is most
likely turbulent.
Now consider a typical R/C damper unit: you have oil of a certain viscosity
passing through orifices of a certain diameter at a certain speed. Some oil
flows around the outside of the piston, this is almost always laminar, since the
gap between the piston and the housing is so narrow, so it creates a lot of
drag. For the oil flowing through the holes in the piston however, it's hard to
predict. When the shaft speed is very low it will be laminar, and when it's high
it will be turbulent. Exactly when the transition will happen is hard to
predict, but easy to feel: because the resistance of the shock is proportional
to the shaft speed when the flow is still laminar, and proportional to the shaft
speed squared the very next moment, when the flow has turned turbulent, it feels
like a kind of hydraulic lock has occurred because the difference in resistance
is usually quite substantial. The transition is sometimes also described as
'pack'; it feels as if the shock 'packs up'.
This effect can both be useful and unwanted: it can prevent your car from
slapping the ground when landing from a jump, but it can also make your car
bounce very badly over sharp ruts or bumps taken at high speed. So it's pretty
important to get this adjustment right.
The way to achieve this is to select the right piston and shock oil: both the
combination of a piston with small holes and a low viscosity oil and the
combination of a piston with large holes and a high viscosity oil will yield the
same static damping; it will feel the same when you bump your car by hand. It
will also make the car handle the same in low-speed transitions, such as smooth
cornering and low-frequency bumps. But the real difference is in the high-speed
damping: the first combination will pack up very rapidly because of the low
viscosity fluid and the increased fluid velocity. (the same amount of oil has to
pass through smaller holes in the same amount of time, so its speed must be
higher) The second combination will have a relatively high resistance to
turbulence, because of the very thick fluid which flows at a much lower speed.
Hence, turbulence will occur at much higher shaft speeds, or it may not occur at
all.
So selecting the right piston and oil depends largely on the track layout.
Killer jumps or chassis-wrecking bumps require pistons with small holes to
prevent the chassis from slapping the ground and usually making the car very
unstable. On the other hand, if the track has lots of bumps or is very rutted,
any packing up of the shocks would make the car bounce and thus very unstable.
In that case you should try pistons with large holes.
Note that judging if the holes in the pistons are too small or too large isn't
as straightforward as you'd like it to be; because the shock absorbers aren't in
direct contact with the ground, there is some elasticity to the whole suspension
system. Suspension arms aren't infinitely rigid and neither are rims so expect a
little flex, and hence also a little bounce from them. Then there there's some
more elasticity in the tires, although this is a far less 'bouncy' form of
elasticity. These effects are most apparent when your car lands off a big jump,
and it bounces up a little, without the chassis having touched the ground. It
means the pistons are way too small, which makes the shocks lock up too fast, so
the impact has to be taken up by the elasticity in the suspension arms and the
rims.
2.3 Roll center
Predicting how a car will react when forces are applied at the tires is not
easy. The force can be absorbed, split, converted into a torque... by all sorts
of suspension components. To avoid all of this you can try to find the roll
center of your car and try to predict the reaction of the car from there. A roll
center is an imaginary point in space, look at it as the virtual hinge your car
hinges around when its chassis rolls in a corner. It's as if the suspension
components force the chassis to pivot around this point in space.
Let's look at the theory behind it first. The theorem of Kennedy tells us that
if three objects are hinged together, there are at most three poles of movement,
and they are always collinear, i.e. they are always on one line. To understand
what a pole really is, consider the analogy with the poles of the earth: as
earth rotates, the poles stay where they are. In other words, the earth rotates
around the imaginary axis that connects the two poles. Now this is a
3-dimensional analogy, in the case of the roll center we only need two
dimensions at first. So a pole of an object (or a group of objects) is like the
center point of a circle it describes.
If we look at the suspension of a typical R/C car, with a lower A-arm and an
upper link, we see a bunch of objects that are all hinged together. These
objects include the chassis, the upper link, the A-arm, and the hub. For now we
consider the hub, the axle and the wheel as one unit. First, let's look at the
chassis, the upper link and the hub. They are hinged together, so the theorem of
Kennedy applies. The pole of the upper link and the hub is the ball joint that
connects them, because they both hinge around it. The pole of the upper link and
the chassis is also the ball joint that connects them. So if we now look at the
chassis, the upper link and the hub, we have already found two of the three
poles, so if there is a third one, it should be on the imaginary line that
connects the other two. That line is drawn in red on the next drawing.
The same applies to the bottom half of the suspension system, the pole of the
lower A-arm and the hub is the outer hinge pin, the pole of the A-arm and the
chassis is the inner hinge pin, so if there is a third pole it should be on the
line that connects the other two. That line is also drawn in red . If your car
uses ball links instead of hinge pins, the axis through the centers of the two
balls makes up a virtual hinge pin.
If the two red lines intersect, the pole of the hub/wheel and the chassis is the
intersection point I . Point I is sometimes referred to as 'virtual pivot', or
as 'instantaneous center'. This pole can give us information about how the
suspension moves.
The distance from point I to the centerline of the tire is sometimes referred to
as 'swing axle length' , it's as if the hub/wheel is attached to an imaginary
swing axle which hinges around point I. Having that long swing axle would be
equivalent to having the double wishbone-type suspension, but the actual
construction would be very impractical. Nevertheless it serves as a good
simplification. The swing axle length, together with the angle, determine the
amount of camber change the wheel will experience during the compression of the
suspension. A long swing axle length will cause very little camber change as the
suspension is compressed, and a very short one will cause a lot.
If the upper link and the A-arm are perfectly parallel to each other, the two
red lines won't intersect, or, in other words, the intersection point I is
infinitely far removed from the car. This isn't a problem though: just draw the
green line (in the next drawing) parallel to the two red ones.
The two red lines should always intersect on the side of the center of the car,
if they intersect on the outside, camber change will be bizarre: it will go from
negative to positive back to negative, which is not a good thing for the
consistency of the traction.
The wheel and the ground can also move relative to each other; let's assume the
wheel can pivot around the point where it touches the ground, which is usually
in the middle of the tire carcass. That point is the pole of the tire and the
ground. As it is drawn, a problem might arise when the chassis rolls: the tires
might also roll, and hence the contact point between the earth and the tire
might shift, especially with square-carcass tires that don't flex much.
Now we can apply the theorem of Kennedy again: the ground, the wheel and the
chassis are hinged together, we have already found the pole of the wheel and the
ground, and the pole of the wheel and the chassis. If the pole of the ground and
the chassis exists, it should be somewhere on the line that connects the other
two poles, drawn in green in the next drawing.
The same procedure can be followed for the other half of the suspension, as in
the picture below. Again a green line will be found the pole of the ground and
the chassis should be on. The intersection point of the two green lines is the
pole of the ground and the chassis. (Circled in purple)
That point(purple), the pole of the chassis and the ground is also called the
roll center of the chassis. It gives us information about how the chassis
moves in relation to the ground. Theoretically, the ground could rotate around
it while the chassis would sit still, but usually it's the other way around; the
chassis rotates around it while the ground sits still.
The roll center is also the only point in space where a force could be applied
to the chassis that wouldn't make it roll.
The roll center will move when the suspension is compressed or lifted, that's
why it's actually an instantaneous roll center. It moves because the
suspension components don't move in perfect circles relative to each other, most
of the paths of motion are more random. Luckily every path can be described as
an infinite series of infinitely small circle segments. So it doesn't really
matter the chassis doesn't roll in a perfect circular motion, just look at it as
rolling in a circle around a center point that moves around all the time.
If you want to determine the location of the roll center of your car, you can
either 'eyeball' it by imagining the lines and intersection points, or you can
get a really big sheet of paper and make a scale drawing of your car's
suspension system.
Now that we know where the roll center (RC) is located, let's look at how it
influences the handling of the car. Imagine a car, driving in a circle with a
constant radius, at a constant speed. An inertial force is pulling the car away
from the center point, but because the car is dynamically balanced, there should
be a force equal but opposite, pulling the car towards the center point. This
force is provided by the adhesion of the tires.
In principle, the inertia force works on all the different masses of the car, in
every point, but by determining the center of gravity (CG) it's possible to
replace all of the inertia forces by one big force working in the CG. It's as if
the total mass of the car is packed into one point in space, the CG. If the CG
is determined correctly, both conditions should be perfectly equivalent.
The forces generated by the tires can be combined to one force, working in the
car's roll center.
Viewed from the back of the car, it looks like this:
Two equal, but opposite forces, not working in the same point generate a torque
equal to the size of the two forces multiplied by the distance between them. So
the bigger that distance, the more efficiently a given pair of forces can
generate a torque onto the chassis. That distance is called the roll moment.
Note that it is always the vertical distance between the CG and the RC,
since the forces always work horizontally.
The torque generated by the two forces will make the chassis roll, around the
roll center. This rolling motion will continue until the torque generated by the
springs is equally big, only opposite. The dampers determine the speed at which
this happens. Note that the roll torque is constant, well at least in this
example where the turning radius is constant, but the torque supplied by the
springs increases as the suspension is compressed. (See chapter 'springs') The
difference between the two torque's, the resultant, is what makes the chassis
lean. This resultant decreases because the torque supplied by the springs
increases. So the speed at which chassis roll takes place always decreases, and
it reaches zero when both torque's are equal. So for a given spring stiffness a
big roll moment will make the chassis roll very far in the corners, and a small
roll moment will make the chassis lean over less.
So at any given time, the size of the roll moment is an indication of the size
of the torque that causes the chassis to lean over while cornering.
Now; a different problem arises; the location of the roll center changes when
the suspension is compressed or extended, most of the time it moves in the same
direction as the chassis, so if the suspension is compressed, the RC drops.
This little animation shows how the height of the RC changes as the suspension
is compressed. The height of the CG also changes a little, because the position
of all of the unsprung mass changes relative to the chassis changes. So it's
really hard to tell if the roll moment actually increases or decreases.
Also, when the car corners, and the chassis leans over, the RC usually moves
away from the chassis' centerline.
Most R/C cars allow for the length and position of the upper link to be changed,
and thus change the roll characteristics of the car. The following
generalizations apply in most cases. An upper link that is parallel to the lower
A-arm will make the RC sit very low when the car is at normal ride height, hence
the initial body roll when entering a corner will be big. An upper link that is
angled down will make the RC sit up higher, making the initial roll moment
smaller, which makes that particular end of the car feel very aggressive
entering the corner. A very long upper link will make that the roll moment stays
more or less the same size when the chassis leans over; that end of the chassis
will roll very deeply into the suspension travel. If not a lot of camber is
used, this can make the tires slide because of excessive positive camber. A
short upper link will make that the roll moment becomes a lot smaller when the
chassis leans; the chassis won't roll very far.
Until now, we've ignored the fact that there are two independent suspension
systems in a car; there's one in the front and one in the rear. They both have
their own roll center. Because the 'chassis' parts of both systems are connected
by a rigid structure, the chassis, they will influence each other. Some people
tend to forget this when they're making adjustments to their cars; they start
adjusting one end without even considering what the other end is doing. Needless
to say this can lead to anomalies in the car's handling. Having a very flexible
chassis can hide those anomalies somewhat, but it's a far cry from a real
solution.
Anyway, the front part of the chassis is forced to hinge on the front RC, and
the rear part is forced to hinge on the rear RC. If the chassis is rigid, it
will be forced to hinge on the axis that connects both RCs (purple), that axis
is called the roll axis. (red)
The position of the roll axis relative to the cars CG tells a lot about the
cornering power of the car; it predicts how the car will react when taking a
turn. If the roll axis is angled down towards the front, the front will roll
deeper into its suspension travel than the rear, giving the car a 'nose down'
attitude in the corner. Because the rear roll moment is small relative to the
front, the rear won't roll very far; hence the chassis will stay close to ride
height. Note that with a car with very little negative suspension travel (droop)
the chassis will drop more efficiently when the car leans over. With the nose of
the car low and the back up high, a bigger percentage of the cars weight will be
supported by the front tires, more tire pressure means more grip, so the car
will have a lot of grip in the front, making it oversteer. A roll axis that is
angled down towards the rear will promote understeer. Remember that the position
of the roll centers is a dynamic condition , so the roll axis can actually tilt
when the car goes through bumps or takes a corner, so it's possible for a car to
understeer when entering the corner, when chassis roll is less pronounced, and
oversteer in the middle of the corner because the front RC has dropped down a
lot. This example illustrates how roll center characteristics can be used to
tune a car to meet specific handling requests, from either the driver or the
track.
In general, you could say that the angle of the upper link relative to the A-arm
determines where the roll center is with the chassis in its neutral position,
and that the length of the upper link determines how much the height of the RC
changes as the chassis rolls. A long, parallel link will locate the RC very low,
and it will stay very low as the car corners. Hence, the car (well at least that
end of the car) will roll a lot. An upper link that's angled down, and very
short will locate the RC very high, and it will stay high as the chassis rolls.
So the chassis will roll very little. Alternatively, a short, parallel link will
make the car roll a lot at first, but as it rolls, the tendency will diminish.
So it will roll very fast at first, but it will stop quickly. And a long link
that's angled down will reduce the car's tendency to roll initially, but as the
chassis rolls it won't make much of a difference anymore.
In terms of car handling, this means that the end where the link is angled down
the most (highest RC) has the most grip initially, when turning in, or exiting
the corner, and that the end with the lowest RC when the chassis is rolled will
have the most grip in the middle of the corner. So if you need a little more
steering in the middle of the corners, lengthen the front upper link a little.
(Be sure to adjust camber afterwards) If you'd like more aggressive turn-in, and
more low-speed steering, either set the rear upper link at less of an angle, or
increase the front link's angle a little.
Now you might ask yourself: what's the best, a high RC or a low one? It all
depends on the rest of the car and the track. One thing is for sure: on a bumpy
track, the RC is better placed a little higher; it will prevent the car from
rolling from side to side a lot as it takes the bumps, and it will also make it
possible to use softer springs which allow the tires to stay in contact with the
bumpy soil. On smooth tracks, you can use a very low RC, combined with stiff
springs, to increase the car's responsiveness and jumping ability. More about
this in chapter 6.
2.4 Anti-squat
Anti-squat describes the angle of the rear hinge-pins relative to the horizontal
plane. Its purpose is to make the car squat less when accelerating. (Squatting
is when the rear of the car drops down when the car accelerates)
More anti-squat will give more 'driving traction': there will be more pressure
on the rear tires as you accelerate, especially the first few meters. At the
same time, it will give more on-power steering, because the car isn't squatting
much. The disadvantage is that the car has an increased tendency to become
unstable entering corners, especially in the rear. Reducing the anti-squat angle
has the opposite effect: a lot less on power steering, and more rear traction
when the car isn't accelerating as much anymore. The car will also be a lot more
stable entering corners. It also affects the car's ability to handle bumps: more
anti-squat will cause the car to bounce more when accelerating through bumps,
but it will increase the car's ability to absorb the bumps when coasting.
Reducing the anti-squat does the opposite: it improves the car's ability to soak
up the bumps under power, but reduces it while coasting.
2.5 Ride height
Proper ride height is very important, too low and the vehicle will bottom out a
lot, too high and the risk of traction rolling will be unnecessarily big. Equal
ride height front and rear is a good starting point. Raising or lowering ride
height on one end of the car changes the steering characteristics of the car,
the lowest end will have a slightly bigger percentage of the cars static weight.
But, more importantly, the roll center will also be lowered, making that
particular end of the car roll deeper when the car corners, making it sit even
lower and thus having more grip.
You should also be aware that changes in ride height usually influence the
amount of down-travel too, which, as explained in the next section, can have
serious consequences.
2.6 Suspension travel
The amount of negative suspension travel (downtravel) a car has can have a huge
effect on its handling; it influences both the mount of roll and the amount of
pitch the chassis will experience.
In this animation we see a car with a lot of downtravel as the chassis rolls
into a turn. The chassis is free to roll, and the height of the CG doesn't
change very much.
In this animation we see a car with almost no downtravel as it rolls into a
turn. The chassis is pulled down as it rolls, effectively lowering the CG.
So, if one end of the car has less downtravel than the other, that end will be
forced down more in a turn, which makes for more grip at that end, especially in
the middle part of the turn, where weight transfer is more pronounced. Very
little downtravel at the front will give a lot of steering, especially when
entering a corner at high speed, or very violently. Very little downtravel at
the rear will give a lot, and consistent traction throughout the turn.
But that isn't all there is to it: the amount of suspension travel also
influences the car's longitudinal balance, i.e. when braking and accelerating.
An end with a lot of downtravel will be able to rise a lot, so chassis pitch
will be more pronounced, which in turn will provide more weight transfer. For
example: if the front end has a lot of downtravel, it will rise a lot during
hard acceleration, transferring a lot of weight onto the rear axle. So the car
will have very little on-power steering, but a lot of rear traction. A lot of
downtravel at both ends, combined with soft springs, can lead to excessive
weight transfer: on-power understeer, and off-power oversteer. The cure is
simple: either reduce downtravel, or use stiffer springs.
There are also some disadvantages of having very little suspension travel: the
bump handling and the car's jumping ability may suffer, it will bottom out very
easily.
Limiting suspension droop has another interesting effect: you can use it to
reduce traction rolling. (When the car flips over because it has too much
traction.) As you can see from the two animations above, a car with less droop
will have a lower CG as it turns, which is exactly what you need in an eternal
struggle against traction rolls. Often it's a better solution than using stiffer
springs and harder tires, it's even beter than reducing ride height or adding
anti-roll bars.
2.7 Anti-roll bars
Anti-roll bars are like 'sideways springs', they only work laterally. Here's how
they work: if one side of the suspension is compressed, one end of the bar is
lifted. The other end will also go up, pulling the other side of the suspension
up also, basically giving more resistance to chassis roll. How far and how
strongly the other side will be pulled up depends on the stiffness and the
thickness of the bar used: a thin bar will flex a lot, so it won't pull the
other side up very far, letting the chassis roll deeply into its suspension
travel. Note that the bar only works when one side of the suspension is extended
further than the other, like when the car is cornering. When both sides are
equally far compressed, like when the car is braking, the bar has no effect. So
anti-roll bars only affect the lateral balance of the car, not the longitudinal
balance.
Unfortunately, anti-roll bars aren't the only things affecting the car's roll
stiffness; they work in conjunction with the springs and dampers. Suppose you
add an anti-roll bar at the rear of your car without changing any of the other
settings. When the car enters a turn, the chassis starts to roll. Normally, the
suspension on the outside of the turn would compress, and the one on the inside
would extend, making for a lot more pressure on the outside tire. With the
anti-roll bar however, the suspension on the inside will be compressed, so the
chassis will roll less, and the rear of the car will sit lower than normal. So
the rear has more weight on it, and it's distributed more evenly over the two
tires. This makes for a little more, and more consistent traction. Remember that
this is in the beginning of the turn, the situation is different in the middle
of the turn. Normally, without the anti-roll bar, the chassis would stop rolling
when the roll torque is fully absorbed by the outsid e spring. But with the
anti-roll bar, some of that torque is absorbed by the anti-roll bar, and used to
compress the inside suspension. So the outside suspension won't be compressed as
much as it normally would, making the rear of the chassis sit up higher than
normal, so less weight is on the rear of the car, and more at on the front. It's
as if suddenly the rear has become stiffer, making for more steering and a
little less rear traction. Rear traction is more consistent however, because the
weight is distributed more evenly over the rear tires, unless the track is
really bumpy, that is; anti-roll bars can really mess up a car's rough track
handling, so they're rarely used on bumpy tracks. Adding an anti-roll bar at the
front of the car has a similar, but opposite effect: it decreases steering, but
makes it much smoother and more consistent. It can stop the front from 'biting
into' the surface too much, making the turning radius big and smooth. This can
come in handy on large, wide tracks.
Math-wise, the torsion stiffness of the middle part of the bar goes up with the
fourth power of the bar's diameter, and for the two side parts, torsion
stiffness goes up with the square of the diameter. Keep this in mind while
changing anti-roll bars.
2.8 Shock mounting locations
Most R/C vehicles have several possible mounting points for the shock absorbers,
both at the upper mount (area 1)and at the A-arm.(area 3) By mounting the shocks
in a different position, spring action can be altered. The question is: how will
this affect the handling, or the 'feel' of the car? To understand this, first
you need to know about wheel rates.
A wheel rate is an equivalent spring rate at the wheel; it's the spring rate of
a spring that would give the same stiffness as the current one, if it was to be
attached right at the centerline of the wheel. After all, that's where the
traction forces act: at the wheel.
A wheel rate is defined as motion ratio² * spring rate, and motion ratio is the
distance between the lower shock mounting position and the inner hinge pin
divided by the distance between the inner hingepin and the tire's centerline.
Or: wheel rate = spring rate * (D1/D2)²
Motion ratio is somethimes referred to as 'installation ratio'.
The formula tell us that the closer the bottoms of the shocks are mounted to the
middle of the chassis, the softer the wheel rate will be.
Note that if you change the lower shock mounting location, you change both the
shock angle and the motion ratio, but it's usually the change in motion ratio
that has the biggest effect. The amount of suspension travel also changes, which
can also affect the car's handling.
The angle of the shocks, a, has a more subtle effect than the lower mounting
position: it changes the way the motion ratio subtly changes as the suspension
is compressed.
The shock angle isn't constant either: it gets bigger as the suspension is
compressed. This effect is more pronounced as the shocks are more laid down, so
the more inclined the shocks are, the more progressive the wheel rate will be.
So think of the top mounting positions as a means of fine-tuning spring and
damper rates, and changing the progressiveness.
Keep in mind this isn't perfectly correct: if the centerline of the tire doesn't
intersect with the outer hinge pin, a considerable part of the forces acting on
the tire are transmitted to the chassis along the upper link. Nevertheless, it's
a very good approximation.
Since the shocks' angle changes their progressiveness, it also influences the
shaft speed: if the shock is laid down(progressive), shaft speed will increase
as the shock is compressed, if it is close to vertical(linear), shaft speed
won't vary a lot with suspension travel. Obviously, this affects high-speed
damping too; it affects when the transition from low-speed to high-speed damping
occurs. It will occur earlier when the shock is closer to vertical, because when
it is inclined, it takes some time (and some positive suspension travel) for the
shaft to 'speed up', and reach the same shaft speed. So inclining the shocks
more has more or less the same effect as using a piston with slightly bigger
holes, and mounting it more upright has the same effect as using a piston with
slightly smaller holes.
I find that changing the lower mounting location of the shocks comes in handy
sometimes when you want to change the amount of negative suspension travel, but
you don't feel like altering the length of the shock, or when you need the
springs to be just a little stiffer or softer. Changing the top mounting
location is a very subtle adjustment, I like to change it after all of the
other, more important adjustments have been made, and the car is handling more
or less the way I want it to. It's especially helpful to alter the 'feel' of the
steering entering corners. Now I don't know if this applies when the springs'
action is very progressive, but the more the shocks are stood up (less
inclined), the more direct their action will be entering the corner. For
instance: if the front shocks are close to vertical, and the rears are somewhat
laid down, the car will have a lot of turn-in steering; it will be very
responsive. If the rears are close to vertical ,and the fronts are more laid
down, t he car won't have a lot of turn-in, but it will have more steering in
the middle of the turn; it will 'square'. In some cases, the rear might actually
begin to slide. It works much in the same way as having stiff springs or heavy
damping: if you have stiff springs, or heavy damping up front, the initial
reaction when you enter a turn will be very strong. In the middle part of the
corner the car will probably understeer, but it's the initial reaction that
gives the car a 'responsive' character. Even roll center works this way: a very
high roll center in the front will make the car turn in very aggressively, but
understeer in the middle of the corner. It's nice if you like an aggressive car
you can 'throw' into the corners, but I doubt it's the fastest way round the
track. Conversely, if the rear roll center is set very high, the car will turn
in very gently, and possibly oversteer after that.
3: Alignments
3.1 Camber
Camber describes the angle between the tire's centerline and the vertical plane.
If the wheels of the car lean inwards, the camber angle is said to be negative,
if they lean outward, the angle is said to be positive. It is usually measured
at ride height, and angles of -0.5 to -3 are the most common.
First of all, positive camber is never used, only negative. Negative camber is
necessary because when a car turn into a corner, it experiences chassis roll,
which increases the tires' camber angle. Also, because most rubber tires are
quite flexible, they get a little deformed in the direction of the center of the
corner. If the car doesn't have any negative camber, only the tires' outer edge
and sidewall would touch the ground, which isn't beneficial for traction. A
tire's coefficient of traction (grip) increases as it's contact surface
increases, so the ideal situation would be that the tire would stay
perpendicular to the ground at all times, and that it wouldn't deform under
heavy side load. Unfortunately, this isn't the case; most of the time you have
to find the best compromise. The problem is that if you want maximum forward
traction, you have to set the camber to 0°, and if you want maximum cornering
action you have to set it to a few degrees negative, depending on the softness
of the suspension and tire carcass. So you can't have both, but you can try to
make the best possible compromise. The easiest way is to set camber so the tires
wear evenly across their surface, that way you can be sure every part of the
surface is used to the maximum of it's potential. Keep in mind that a car with
very soft suspension settings and very little camber change will need more
negative camber than a car with a very stiff suspension and In very bumpy
offroad conditions however, it can be beneficial to use more camber than would
be needed for uniform wear across the surface. The excess camber stabilizes the
car in large bumps and reduces the risk of catching a rut and flipping over.
Camber can also be used as an adjustment to attain a desired handling effect,
but I definitely don't recommend this: a non-optimal camber setting always
yields less traction, which inevitably makes the car slow.
3.2 Caster
Caster describes the angle c between the kingpin and the vertical plane. In case
of a double wishbone-type of suspension, the axis through the centers of the
ball links serves as a 'virtual hinge pin'. If the kingpin is leaning back, as
in the pic, the caster angle is said to be positive. Negative caster (kingpin
leaning towards the front) is never used. Note that the contact patch between
the tire and the ground is behind the intersection point of the extension of the
kingpin and the earth. (Dimension d) This will cause the wheels to 'trail'.
The caster angle will cause excessive camber in the front wheels as they are
steered, lifting the front of the car up. This lifting effect is what causes the
front wheels to have a tendency to straighten out when there's no steering force
applied: when the wheels are pointed straight ahead, the chassis sits at its
lowest position, steering the wheels requires some force, to lift the car up.
When the force is removed, gravity will return the wheel to their original
position. The bigger the caster angle, and the heavier the car, the stronger
this effect is. Also, the bigger he caster angle, the bigger the camber
difference induced when the wheels are steered. This camber difference is to
compensate for the chassis roll and tire squirm when the car is cornering.
Hence, a lot of caster will provide more steering in high-speed corners, where
chassis roll is more pronounced, and whilst turning in. It will also make the
car more stable in rough conditions, and the car's straight-line stabili ty will
also be improved. A small caster angle will provide more steering in low-speed
corners, and less turn-in.
Note that the caster angle isn't always constant. Cars with double wishbone
suspension, where both wishbones aren't parallel, will experience a change in
caster angle as the suspension is compressed or extended. If the lower wishbone
has less kickup than the upper wishbone, the caster angle will decrease as the
suspension compresses, like when the car is turning or braking. This is called
'reactive caster'.
3.3 Toe Angle
The wheels on a car seldomly point straight ahead. The drawing on the left shows
a car with toe-out at the front and toe-in at the rear. Toe-in
means the tires point inwards, and toe-out means the tires point outwards.
Both front wheels try to pull the car to the side. They won't be able to,
because on the other side there's an equally big, opposing force, but the can
try. These forces are indicated by the green arrows. So the wheels aren't
pointing in the direction in which they are going (white arrow). This creates a
slip angle, as explained in chapter 1.
So, in theory, the car isn't going anywhere. But, this in an instable situation:
suppose the car hits a slight bump on one side, or it is steered just a little
bit. This will induce more load on one of the two front tires. More load means
more grip, so the tire can also pull to the side a little stronger. In the case
of a small steering correction, the force on the other side will also get
smaller, because the weight is transferred, not induced because of a bump. So
you end up with one wheel pulling to one side very strongly, and the other wheel
pulling the other side, but not nearly as hard. As a result, the two forces
don't suspend each other any more; there is a resultant force to one side, which
will steer the car. This is bad news, because this will in turn cause weight
transfer, worsening the problem. The driver can try to correct this by
countersteering, but if it's not done perfectly, you end up with the same effect
in the other direction. This will make the car wave from side to side, or in t h
e worst case, fishtail.
Toe-out causes instability, so there's no point in using it at the rear of your
car, it will make it undrivable. But, in the front, there's the stabilizing
effect of caster. That's why a little toe-out is sometimes used in front, as
long as the car has sufficient caster, instability on the straights won't be a
problem. The 'instable effect' will be noticeable though: while turning into
corners. Turning in will feel more immediate and more aggressive.
At the rear of the car there's some toe-in. This will also create two opposing
forces, but this time it's a stable system. If for whatever reason one force
would become greater than the other, the car will be steered in the direction
that causes weight transfer onto the tire that was losing weight in the first
place. So the tire which was losing grip ends up with more weight, and hence
more grip. The system stabilises itself, this is also called negative feedback.
Toe-in causes stability: it will create an effect that steers the car straight
ahead. It's mostly used at the rear, where it will prevent the rear from
'stepping out' when the tires are constantly brought to the edges of the
traction circle, and any bump can make them lose grip. To the driver it will
feels as is the rear is really 'locked in', as if there's some invisible force
keeping the rear on track. There's a downside to this too: in corners,
especially low-speed ones, this can take away a lot of steering. The effect can
be so big the gripping power of the front tires is hardly sufficient to turn the
car. In other words, excessive rear toe-in makes a car understeer.
Toe-in at the front does basically the same thing: it stabilises the front. This
can be a nice effect if you want to calm down the front of your car while
accellerating. It will also make you lose a little steering; turn-in will be far
less aggressive.
There's something else all sorts of toe do: they take away lag. The opposing
forces, small as they usually are, take away all the slop in the suspension, and
pre-load the tire in a lateral way. They slightly deform the tire carcasses,
which makes the car react quicker, without any lag.
The downside to having lots of toe is primarily the loss of energy, or speed:
more speed is being scrubbed off as the toe angle increases, since the tires'
slip angle will be equally big. The grippier the track ,the bigger the energy
loss will be, so avoid lots of toe when there's a lot of grip. Also, lots of toe
will create big slip angles even when the car is running straight ahead. This is
not a good starting point, since lots of slip makes a tire lose grip.
The toe angle is measured in degrees, it's the angle between the centerlines of
the two tires.
Normal values for toe at the front range from -1.5 to +1.5, any more would cause
strange handling effects. At the rear, 0 to 3.5 degrees of toe-in are commonly
used, a little less for on-road.
3.4 The Ackermann Effect
As you might have guessed, the Ackermann effect was 'discovered' by someone
named Rudolf Ackermann, back in the days of horse drawn carts. It's all about
finding the right angles for the front wheels for the car (or cart) to steer
correctly.
This is the situation when the car isn't turning: the lines extending the front
and rear axles do not intersect. None of the tires is slipping and all of the
axles are angled the same. (0°)
This is the same car taking a relatively wide turn. The point where the axle
lines intersect is the point about which the car is turning; it's the center of
the circular path the car is following.
Notice that if none of the tires is slipping, the inside front wheel is angled
just a little more than the outside front wheel. For wide turns, it isn't much,
but it's a start.
This is the car negotiating a relatively tight turn. The radius of the circle
the car is describing is very small.
Notice that there is quite a difference in angle between the two front wheels.
The difference gets even bigger when the turn is even tighter. That's the
Ackermann effect.
As it turns out, making a steering rack that produces the perfect angles to
satisfy Ackermann's theory is very hard, if not impossible. However, several
elementary types of steering mechanisms produce angles that approximate the
ideal very well. After all, there's always some free play in the steering
components, and the tires can take up the rest of the rest by means of sidewall
deformation.
That theory is nice, and for every day street cars it's nice to have the correct
ackermann setting at full lock because it will make your car turn better and
smoother in tight corners, like when you're parking or manouvering. But when
you're racing the tires tend to slip anyway, so you can fool around with the
angles a little.
The angle between the two front tires is referred to as 'the Ackermann angle',
and it can be varied by adjusting the steering linkages.
A large Ackermann angle gives you smooth, predictable steering. You'll be able
to round the corners nicely without all four tires trying to force the car in a
different direction. A smaller Ackermann angle on the other hand can give you
more aggressive steering, especially entering the corners. However, it isn't
guaranteed that the front won't wash out now and then. Neither is a smooth
cornering radius. It can be useful an high-traction tracks, if your car tends to
oversteer in the middle of the corners, and you'd like a little more turn-in.
Not to mention the psychological impact on the person driving right in front of
you when you're entering corners.
4: Weight Transfer
Any vehicle has a certain weight, in static conditions (no movement), that
weight is distributed as explained in chapter 6. Newton's third law, force =
mass * acceleration, implies that whenever the vehicle accelerates in any
direction, additional forces occur. For example, when your car lands after
having taken a jump, its downward velocity decreases rapidly. Basically, it
stops falling down quite suddenly. The extra force associated with this equals
the mass of the car times its acceleration.
To put this into a numerical example: suppose the car has been falling down for
1 second. Right before it hits the crust, its downward velocity equals G * 1
second = 10 m/s. Suppose the car weighs 1 kilogram = 10 Newton, and its
suspension needs 0.1 second to absorb the impact of the landing. The force
exerted on the tires is 1kg * 10m/s / 0.1s = 100N = 10kg. So the split second
the car lands, it weighs 100 N instead of 10 N.
The point I’m trying to make here is the following: the weight of a
vehicle, and hence also its tire loads, varies all the time. Its mass
stays constant though. Well actually it doesn't; it varies with speed, but for
now let's pretend Mr Einstein has never been born, it doesn't make any
significant difference anyway. Unless of course you're so fast you approach the
speed of light, in which case there's no valid information on this site :-)
Additional forces exist whenever there's acceleration, or in other words,
whenever the value or the direction of the velocity changes.
Why is this important, you may ask. Simple: the amount of weight on each tire
determines its gripping capabilities, and hence, the handling of the car. In
fact, it's safe to say that controlling the weight transfer is the single most
important thing in racing, and also what distinguishes good drivers from
mediocre ones. Knowing where your car's weight is and keeping your tires right
on their limits without suddenly crossing it is what makes you fast.
Because we race in a 3D world, weight transfer can occur in 3 directions. (in a
4D world it'd be too easy to cheat I guess) If there's movement up or down,
weight can be added or subtracted. The vertical dimension is a little different
from the two horizontal ones, because of gravity. Your car can go from being its
normal weight (on a flat road) to being several times as heavy (hitting an
uphill section or the face of a bump) to being weightless (flying through the
air) to weighing less than normal (on the downside of a bump or elevated piece
of track). And don't forget, there's also downforce, which also increases the
car's weight. Luckily, in the other two dimensions, the total weight is
constant, it can only shift from one side to the other. Like when you're
accelerating, weight is shifted from the front wheels onto the rear wheels, but
the total remains the same. When you're negotiating a corner it's the same deal,
just in another dimension: weight shift from the inside tires onto the outside
tires, a nd again, the total remains the same. For simplification, let's ignore
the first case, and pretend the track is completely flat from now on.
Consider this car: its CG (purple) is located right in the center. It's rolling
along at a constant speed (if it's standing still its speed is constantly zero),
on a perfectly flat surface, straight ahead, so all tires are loaded equally. If
the car's weight is W, the weight on each wheel is W/4.
Note: there's a small inconsistency in the drawings: the cyan colored arrows are
to be interpreted as the weight that's on the tires. The inconsistency is the
fact that all other forces are drawn as being exerted on the car, and
these are drawn as being exerted by the car.
4.1 Lateral weight transfer
This is the same car, negotiating a turn. It's still travelling at the same,
constant speed on a flat surface, only the direction of its speed vector is
changing. The radius of the turn is also constant.
First of all, not all forces are drawn, it would have overloaded the picture. On
each tire there's a horizontal force, the four of them counteract the yellow
one. They're basically the gripping forces of the tires on the road. The weight
vector isn't drawn either, it should be acting downwards at the Cg. (purple) the
other ones are drawn.
The yellow arrow indicates the centripetal force. It's a result of the inertia
of the entire car. It acts in the CG, and is directed away from the center point
of the circular path the car is following. This force results in a torque
exerted on the car, which has to be counteracted as described in Newton's second
law. The counteraction is in the fact that the outside tires are loaded heavier
than the inside ones. Because the total weight of the car stays constant, all of
the weight that is removed from the inside tires is added to the outside tires.
In other words, the weight has shifted, towards the outside of the turn.
The results are important as well as numerous. Unequal loading of the tires
usually means that the total amount of grip is reduced, resulting in less
cornering power. Also, the torque mentioned before can make the body roll. In
this example the car's body won't roll, because it hasn't got any suspension.
The body won't roll either if the CG is on the roll axis. The fact if the body
rolls or not doesn't influence the amount of transferred weight. (supposing the
CG doesn't move around that much as the car rolls) It does however determine
where most of the transferred weight is going to go. Sure, in this example it's
easy: whatever comes off the left front is going on the right front and whatever
comes off the left rear is going onto the right rear. In other words, the car is
symmetrical, and it stays that way because it has no suspension. But in a real
car it's different: suppose the rear of a real car rolls way more than the
front, because the rear RC is very low or the rear springs are very soft. More
wei ght will be transferred onto the right rear tire. Sure, there will also be
some weight transferred onto the right front, but it will be less. This makes
for an understeering car. And on top of that, there's braking and accelerating,
they also transfer weight from front to rear or the other way around. That's why
you can usually loosen up an understeering car by braking a little; it transfers
weight to the front, where you can use it to provide a little more front
traction to steer your car. All things considered, determining how much of the
weight is going where isn't easy. Lots of things come into play, like spring
rates, anti-roll bar stiffnesses, roll center heights, suspension travel,...
First, let's focus on how much weight is transferred. It's easy to derive that
the amount of weight transferred, in essence the difference in weight on the two
tires, is equal to the centripetal force (yellow) multiplied by the height of
the CG divided by the track of the car. The centripetal force is equal to the
car's lateral acceleration, expressed in Gs, times its total weight. The lateral
acceleration is equal to the speed of the car squared divided by the radius of
the turn.
From this we can conclude that the amount of transferred weight is proportional
to the height of the CG, and inversely proportional to the track of the car.
This is why most race cars are built as low as possible and as wide as the
regulations allow; it minimizes weight transfer, which in turn prevents a
decrease in overall traction. It is also proportional to the car's static
weight, another reason why race cars have to be as light as possible, again to
minimize weight transfer. The amount of weight transferred is also dependant on
factors that don't refer to the car itself, like its speed, and the radius of
the circle which approximates the path it is following. The fact that the amount
of weight transferred is proportional to the radius of the turn is one of the
reasons why a large, smooth radius is the fastest line when cornering: it
minimizes weight transfer, so it maximises overall grip and cornering power.
4.2 Longitudinal Weight Transfer
It's basically the same deal as lateral weight transfer, but it works in another
dimension. This time the car is running straight ahead, (white arrow) on a flat
surface, but it's accelerating. In other words, its speed is increasing, or in
the words previously used, its speed vector is increasing in size but its
direction remains the same.
Again some forces aren't shown: the forces that propel the car for instance.
They act in the contact patches of the driven wheels and are in the same
direction as the white arrow. The car's weight also isn't drawn.
As always when an object is subjected to acceleration, there's an inertia force,
acting in the CG (drawn in yellow) This results in a torque, which is
compensated by a shifting of the weight from the front tires onto the rear
tires. Whatever weight comes off the front tires is added at the rear tires, and
as long as the acceleration happens in a straight line, it happens
symmetrically.
Having a little more weight on the back tires can be a good thing for a rear
wheel drive car that's accelerating in a straight line, but because of the
reduced weight at the front, it will understeer horribly. The same principles
apply as with lateral weight transfer: because the tires are unequally loaded,
the total amount of grip available is reduced. That's why the absolute quickest
way around a corner is without braking or accelerating. Needless to say you need
a well balanced car for that, one that doesn't under - or oversteer. Another
thing that applies is that the weight transfer is independent of the vehicle's
pitch angle. The car will probably dive when braking and squat while
accelerating, but the amount of weight is unaffected by this. Unless of course
the height of the CG changes dramatically. This can happen if the car has a lot
of negative suspension travel. Imagine that the front lifts off quite high, and
the rear doesn't squat very much. The CG will rise a little, which in turn
promotes more weight transfer. This is how wheelies begin, the CG rises, which
makes for more weight transfer, which makes for more rear traction which can
cause the front to rise even more... it's kind of like an avalanche effect.
The formula for the amount of weight transferred is very similar to the previous
one. Now it's the inertia force times the CG height, divided by the wheelbase.
(instead of track) the inertia force is equal to forward or rearward
acceleration times the car's mass. So a low CG and a longer wheelbase produce
less longitudinal weight transfer. And as always, the transferred weight is
proportional to the acceleration.
As explained earlier, race cars have to be low and wide, so from the previous
you'd expect that they'd also have to be as long as possible. This is not always
the case. Rear-wheel drive cars, for example, can use some more weight on their
rear tires as they accelerate, to keep them from spinning out. A little
understeer under acceleration allows you to really put the power down without
having to worry about wheelspin.
But that's not the only reason; cars with long wheelbases tend to handle a
little sluggish in tight, low-speed corners, they're not nimble enough. So, in
general, a long wheelbase is better for big, smooth, high-speed tracks, and a
short wheelbase is better for tight, twisty tracks.
5: gearing
This is probably the most straightforward issue of all. In most cases, the best
gear ratio is the one that allows your car to accelerate as fast as possible,
without over-revving your motor on the straight. So finding the right gear ratio
is quite simple in most cases: just make sure the motor reaches maximum revs
towards the end of the longest straight.
However, in some cases it can be beneficial for your lap times to over- or
undergear your car: a slightly undergeared car will accelerate faster, so the
track is full of short straights where acceleration is very important, you might
want to consider a smaller ratio. Overgearing is frequently used to prevent
wheelspin under acceleration, mostly on low-traction tracks. It can make the car
somewhat easier to drive. It's only advisable on tracks where wheelspin is a
problem.
A gear ratio indicates the number of revolutions of the motor has to do for the
wheel to complete one full revolution. For example: a ratio of 9.0 means that
when the motor has done 9 complete rotations, the wheel will have done just one,
if the motor has done 18, the wheel will have done 2,...
Here's the tricky part: a higher number means a smaller ratio, and conversely a
smaller number means a higher ratio. So 9.0 is a taller or higher ratio than
10.0. Think about it: if the motor has to do less rotations for the wheel to do
one (smaller number), the car will go faster for the same rpm, which means a
taller ratio.
6: Weight and power distribution
6.1 Weight distribution
Weight distribution is very important; not only does it affect the static weight
on the different tires, it also affects how the weight shifts in dynamic
conditions.
The easiest way to judge weight distribution is to determine the car's Center of
Gravity (CG). This is a point in space where the mass of the entire car is
accounted for. Because of its location, it can be used to simplify the effects
of inertia forces. In reality, every little bit of mass is subjected to inertia,
but it's much easier to make use of an equivalent condition: assume all the mass
of the object is concentrated in its center point, i.e. it's CG. So instead of
having to figure out how every part of a 1.5kg car reacts to a certain force, we
only have to figure out how a weightless car with a 1.5kg dot in it's center(the
CG) reacts to it. The latter is much easier: the force only works in the CG, and
not in the rest of the car.
Of course, this only works when the CG is determined correctly. I think that's a
lot of work, and it might not be accurate, so I propose a different method. It's
based on the fact that when an object is statically balanced, its CG is right
above the point where it's supported. By applying this in three different
planes, you can determine an object's CG. Here's an example.
Here we have an object with a heavy part (dark) and a lighter part (bright) we'd
like to determine the CG of. Since the right part is heavier the CG will
probably be located somewhere at the right.
We try to balance it on a sharp edge, and this is the position in which the
object stays put. So we know the CG is somewhere right above the point where the
object is supported.
The red line contains all the points above the point where the object was being
supported, so the CG has to be located somewhere on the red line.
We can follow the same procedure, but in a different dimension. Again, we can
draw a red line on which the CG is located.
Because this is a 2D example, trying to balance the object 2 times is sufficient
to determine it's CG(circled in purple). For a car, which has 3 dimensions,
you'll need to do it 3 times. It might impose some practical problems, but this
is where you'll have to use your imagination.
Now that we know where the car's CG is located, we can easily calculate the
amount of weight on the tires, and the weight distribution.
First, let's have a look at the front-to-rear weight distribution:
The wheelbase is the distance between the front and rear axle, F is the distance
between the CG (green) and the front axle, R is the distance between the CG and
the rear axle.
Weight on the front axle = weight of the car*(R/WB)
Weight on the rear axle = weight of the car*(F/WB)
Or, in percentages:
Front weight percentage = (R/WB)*100%
Rear weight percentage = (F/WB)*100%
Obviously, this will have its effects on handling: more weight on a tire means
more grip. So if the CG is located further towards the rear, the car will have a
lot of rear traction, which is nice to have if acceleration is important. If the
CG is located further towards the front, the car will have a lot of steering,
but it might lack rear traction, which increases the risk of spinning out.
In some cases, lateral weight distribution is a major concern, especially in
so-called LTO(left turn only) cars, who race on oval tracks. It's basically the
same deal:
TW is the treadwith, the distance between the centers of the tires at the axle,
E is the distance between the CG(green) and the centerline of the left side
tires, I us the distance between the CG and the centerline of the right side
tires. If the front and rear axles aren't equally wide, E and I have to be
measured at the CG.
Weight on left side = (I/TW)*weight of the car
Weight on right side = (E/TW)*weight of the car
Or, in percentages: left side weight percentage = (I/TW)*100%
Right side weight percentage = (E/TW)*100%
Note that if you need to know the amount of weight on one tire, you need to
multiply the weight of the car by 2 factors, one of the lateral balance, and one
of the longitudinal balance, for example:
Weight on left front tire = Weight of the car*(I/TW)*(R/WB)
Weight on right front tire = Weight of the car*(E/TW)*(R/WB)
Weight on left rear tire = Weight of the car*(I/TW)*(F/WB)
Weight on right rear tire = Weight of the car*(E/TW)*(F/WB)
Note that this is only true when the car isn't tweaked; spring preload should be
the same on the left and right hand side.
Again, having the CG away from the center of the car has consequences for the
car's handling: having it toward the left improves the car's ability to turn
left, but it might make it very difficult to drive the car in a straight line,
especially under acceleration.
The height of the CG is also very important: it determines the car's roll
characteristics and weight transfer. More about this in
chapter 2.
Sadly enough, that isn't all there is to it; inertia has been left out,
rotational inertia to be more precise. Here's an example:
These drawings represent two cars, the first one on the left has all the heavy
stuff (blue) located at its ends, far removed from the CG (purple). The second
one on the right has all the heavy stuff lined up right in the middle, very
close to the CG. Both cars weigh just as heavy, and their CGs are in exactly the
same place.
So both cars will transfer the same amount of weight while braking or cornering,
and their roll angles will also be identical. Yet they won't handle the same,
because their rotational moment of inertia is different. The first car will
react slowly, turn in a little sluggishly and it will generally be more
reluctant to change direction. Some might say it is slow, others might find it
very stable, it's the same thing. The second car will feel like the opposite: it
will change direction very quickly, and it will feel very nimble, and thus also
unstable.
So, rotational moment of inertia doesn't change how far the car's chassis
moves, it changes how fast it does so. It's kind of like swinging a
baseball bat with a big, heavy tip: you'll need a lot of effort to get it going,
and once you get it going, there's not much you can do to alter its course.
The rotational moment of inertia can be calculated too: the rotational moment of
inertia of a body around an axis is the sum of all the elementary masses of the
body multiplied by their distance to that axis squared. For
simple-looking bodies like cylinders, cubes and cones and such, you can do this
by hand, but for real-life applications you'll need a sophisticated CAD program.
Note that it's also important around which axis you're calculating the
rotational moment of inertia. Consider the following example:
These drawings represent identical cars, except for the fact that they have
their weight distributed differently: the first one has its heavy components
(blue) lined up along its lateral axis (purple), and the second one has its
heavy stuff lined up along its longitudinal axis.
Consider the first car. If we calculate the rotational moment of inertia around
its lateral axis, we have to multiply all of the masses with their distance to
the axis squared. In this case, we have to multiply most of the mass with a very
small distance squared, resulting in a very small value. On the other hand, if
we calculate its rotational moment of inertia around its longitudinal axis (not
drawn), we have to multiply most of the mass with a very large distance squared,
resulting in a large value. So, the first car has a very large moment of inertia
around its longitudinal axis, and a very small one around its lateral axis. In
other words, this car will react very slowly while cornering; it will move from
side to side (roll) very slowly. But, it will move from front to rear (pitch)
very easily, this might be beneficial for quick braking, but it will make the
car bounce back and forth in bumps, making it very unstable.
For the second car, the opposite is true: it has a large value for its
rotational moment of inertia around its lateral axis (not drawn) and a very
small one around the longitudinal axis. This means that the car will roll
quickly, and be very responsive in turns, but it will be very stable front to
rear. This helps stabilize the car in bumps while maintaining good cornering
abilities.
Maybe now you can understand the hype about mid-mounted motors in full-scale
cars: the motor is by far the heaviest item, so by positioning it centrally, the
car's rotational moment of inertia is reduced, making for a more nimble handling
car.
6.2 Power distribution & Differentials
If you should remember anything from Chapter 1, tires, than it's the fact that
whenever you apply more power to a tire (braking or accellerating), you lose
lateral grip. So naturally, power distribution is very important in setting up a
car.
A number of devices alter a car's power distribution: differentials, be it the
gear type or ball type, torsen diffs, solid axles, one-way pulleys, one-way
front differentials, and viscous couplings of all sorts.
6.2.1 Ball differentials
Ball differentials are the most commonly used. They can be adjusted for
tightness (and slippage), which makes them very versatile.
It's not a bad idea to have your manual with you at all times, mainly for the
section about the diffs, in case they get gritty and you need to clean and
rebuild them. Having your manual with you is not a bad idea PERIOD. If
something's not on this site, it's in your manual. Basically all you have to do
is keep your ball diffs clean and smooth, and make sure they don't slip.
You *can* use them to alter your car's handling. If you adjust your front diff a
little tighter, you'll experience more understeer entering a corner, but more
steering and a little more front traction exiting it. The car will also feel
more stable. If you tighten the rear diff, it will become easier for the rear
end to break loose. But I don't recommend using diff tightness to alter under/oversteer
balance. The reason is that a tight diff scrubs off speed in the corners. A car
with loose diffs will coast through the corners more easily, without scrubbing
off much speed.
6.2.2 One-way pulleys
A one-way pulley, or one-way bearing mounted onto the central driveshaft if your
car is shaft-driven, allows the front axle to rotate faster than the rear axle.
In other words, the front wheels are allowed to freewheel. But there's still a
(ball) diff on the front axle.
The main disadvantage is that you lose braking on the front wheels. On a
high-traction track where you don't really need to brake hard, this isn't a
problem. However, it is a disadvantage on low and medium-traction tracks, it can
make the car spin out when you do as much as touch the brakes. The main
advantage of a one-way pulley is a higher cornering speed. The fact that the
front wheels can freewheel lets the car flow through the corner with a lot less
tire scrub, resulting in higher cornering speeds.
Some cars have an adjustable one-way pulley, where you can adjust the amount of
friction imposed on the one-way bearing. This a very useful adjustment: it
allows you to alter the percentage of front brake bias, from 0 to 100%. If you
crank down the adjustment nut, and lock up the one-way, you can slam the brakes,
and your car will brake very hard, with the front wheels sliding out, making
your car understeer somewhat. If you loosen the adjustment nut, most, if not all
of the braking will be done by the rear wheels. You'll gain a lot of steering,
and cornering speeds will be higher.
6.2.3 One-way front diff
A one-way front diff contains two one-way bearings; one for each wheel. So it
acts like a diff, but only in the forward direction: the front wheels can only
rotate faster than the rears, but not slower. So off-power it acts much in the
same way as a completely loose one-way pulley, but under power (accellerating
out of corners), it makes the car a lot more stable, and provides more and more
consistent front traction. this is because the inside front wheel can't
'unload'. Front traction stays extremely consistent, no matter how much power
you apply to the front axle. Another difference is that there's no more front
differential action off power, just freewheeling. So difference between a fully
loosened center one-way and a one-way front diff is that with the one-way
pulley, you'll still experience a little friction from the front diff when
turning in. This can make chicanes and difficult tight turns a little more
managable. But on the downside, you lose the sweet on-power feeling of a one-way
diff. No pain, no gain.
So, with a one-way front diff, you get no front braking, no front differential
action off power, high cornering speed, and excellent accelleration out of the
corner. On really high-grip, open tracks with smooth, flowing high-speed
corners, you need to use a one-way in order to be competitive.
6.2.4 TORSEN diffs
TORSEN stands for TORque SENsing. A torsen diff 'senses' where the torque is
going, and locks up if one wheel is a lot faster than the other one. It's based
on the fact that a worm gear can drive a regular gear, but a regular gear can't
drive a worm gear. So if one wheel (usually the inside one) loses traction and
wants to 'unload', the diff senses it, and sends most of the torque to the other
wheel. This allows you to really lay the power down without wheelspin. Torsen
diffs are often found as center and/or front diffs in 1/8 scale IC off-road
buggies. Apparently, these have s***loads of power, and don't care about the
extra rotating mass.
6.2.5 Gear diffs
Gear diffs are a little less sophisticated as ball diffs, but they can handle
more power. They're a lot heavier, and you need to use silicone oil or grease
from different viscosities to adjust them. As far as adjustment is concerned,
the principles are the same as for ball diffs.
6.2.5 Solid axles
A solid axle, or spool is like a fully locked diff with no moving parts. Which
means it's super-solid, light and easy to adjust. But there's no differential
action at all, so a lot of speed is scrubbed off in the corners. A solid front
axle makes a car very, very hard to turn in, but stable in accelleration and
decelleration. A solid rear axle gives a lot of steering.
Solid rear axles are common in 1/8 scale IC on-road cars. These use very wide
foam tires and have powerful engines, so they don't care about a little tire
scrub in the corners.
6.2.6 Viscous couplings
Viscous couplings, such as Losi's Hydra drive and Schumacher's Visco drive are
an addition to a slipper clutch. They reduce stiction and give the clutch much
more of a smooth, flubber-like feel. The advantage is simple: more and more
consistent traction in bumpy sections. The downside is a considerable weight
penalty.
7: Downforce
In essence, downforce is a force that's pushing down on the car, and therefore
also on the tires. In a way, this is like 'free grip': it doesn't require any
weight shifting, that's also why most of the time, the distinction is made
between 'mechanical grip' and 'aero grip'. It isn't totally free though: more
downforce also means more drag, i.e. more wind resistance, so you'll always have
to find the best possible compromise between grip and aerodynamical drag.
Another thing that's very important is the fact that downforce is proportional
to speed, more precisely the speed of the air relative to the car. So at low and
very low speeds, downforce is negligible. Remember this when you have to set up
your car for slow corners: downforce won't have a lot of influence. At high
speeds however, downforce is in most cases dominant over the mechanical
settings: in high-speed corners, adjusting downforce is the only way to go.
8: Balance
Balance is without a doubt one of the most important requirements for a well
handling car, you might say it is the very key to having a fast,
confidence-inspiring car.
If I were to define 'a well-handling car', I'd say something like "A
well-handling car is a car that even my grandmother could drive competitively.".
What I'm trying to say is that a car should be as foolproof as possible.
Granted, one of the key features of a good driver is his/her ability to
compensate all of the car's ill handling effects, but driving, a balanced,
predictable car will come in handy when it really counts, when the pressure and
the adrenaline levels are up and the finish is near. Plus, a smooth car is
always faster.
It's mainly because there are two suspension systems in a car: one in the front
and one in the rear, and they are forced to work together. As you might expect,
if they're not tuned to do more or less the same thing, 'working together' can
become very difficult.
Consider the following example: a car with a perfectly symmetrical weight
distribution, i.e. the same amount of weight on the four tires, and all the
springs and dampers are of equal stiffness. But the rear roll center is much
lower than the front. If this car were to enter a corner, it would turn in
nicely, but after that, it would understeer horribly. Let's have a look at what
actually happens. The front has a high roll center, so the roll moment will be
very small; the front end of the chassis will not want to roll very far. In the
rear, it's the opposite: the RC is very low, so the roll moment is very big; the
rear end of the chassis will want to roll very far. Since the spring rates, or
for that matter, wheel rates are equal at both ends, the force that tries to
prevent the chassis from rolling will be the same, so the rear will want to 'outroll'
the front. For instance, the front will want to roll 2 degrees, and the rear
will want to roll 10 degrees. Obviously, if the chassis is a rigid structure, t
h is will not happen, it will roll to the average of the two roll angles: 6
degrees. So neither the front, nor the rear is where it should be. The front
would have transferred the right amount of weight from the inside front to the
outside front tire if the chassis would have rolled 2 degrees, but it rolled 6
degrees, so a lot more weight has been transferred onto the outside front tire
than expected. In the rear, the opposite has happened: if the chassis would have
rolled 10 degrees, the right amount of weight would have been transferred onto
the rear outside tire, but since the chassis only rolled 6 degrees, a lot more
weight has remained on the inside rear tire. This is not a very healthy
situation, and it doesn't allow the car to take a corner in a smooth,
predictable, and fast manner. The excess weight on the inside rear tire, and the
high amount of grip associated with it will push the car towards the outside of
the corner; it will cause an understeer condition, especially under power. The
excess weight, and grip, on the outside front tire will cause an unpredictable
oversteer situation. So you see, the front and the rear are fighting each other,
and this will only become worse when throttle or brake is applied. If more
throttle is applied, more weight will be transferred to the rear, so there will
be even more weight on the rear inside tire, causing an even bigger push. More
brake means more weight on the outside front tire, which in normal circumstances
is a good thing, but in this case there already was too much weight on it, so it
will result in more oversteer. Needless to say, this is not the fastest way
around the corner, and the car will require an incredible amount of driving
skill. Having springs in the rear that are too soft would have caused a similar
problem.
So, what does this example teach us? It teaches us that the front and the rear
of the car should always be able to work together, but that's not all; we can
also use the insight it provided us to tune a car to our specific handling
needs, by purposely changing the balance.
To be able to judge if your car is balanced or not, you'll need some kind of
baseline comparison. I use the following, imaginary set-up: a car with the CG
right in the middle, and identical springs, dampers and roll centers front and
rear. It's pretty obvious this car is balanced, but usually, front wheel drive
cars will have more weight on the front axle, and rear wheel drive cars will
have more weight on the rear axle. This is easy to compensate: just increase
spring stiffness and damping by the same ratio as the CG moved. For example: if
the rear has twice as much weight on it than the front, use springs that are
twice as stiff in the front, and use a damper rate that's also twice as thick.
If you try this, you'll find that it's very easy to drive, predictable and
stable, and generally well suited to extreme track conditions. But you might not
find it aggressive enough, or think that it's got a little too much steering for
our taste,….
But there's a remedy for that; you can make the changes you like, as long as you
don't disturb the balance too much. For instance, you can move the weight
forward a little, but use a stiffer spring in front. This will give you more
weight on the front tires, statically. So you'll get more turn-in, and probably
a little more on-power steering, but you'll lose some rear traction. Or you can
use an anti-roll bar in the rear, but use slightly softer rear springs. This
will give you more steering in the middle part of the corner, and it will give
you more forward traction. Another thing that's being used a lot is to use a
higher RC in the rear than in the front, combined with stiffer springs (and
damping) up front, and softer ones in the rear. This makes for a very stable
car: it will turn in sharply at first, because of the stiff springs up front,
but then, it will understeer a little, because with the stiff springs and heavy
damping up front, it takes some time to transfer the weight onto the outside
front t i re. This happens a lot faster in the rear. But eventually, when the
weight is fully transferred, the car will steer very well. This setup can be
very fast: the car can be 'thrown' into the corner, without losing a lot of
speed because of the mild understeer. Then, at the apex of the turn, some
braking will probably be needed, but after that the car will be very stable
again, like in the entrance of the turn, which makes a high exit speed possible.
9: Making adjustments
9.1 Before Making Any Adjustments
You can’t expect to be able to make your car handle better by making setup
adjustments if the car’s not in perfect working order to begin with. Any small
malfunction can mess up a car’s handling to the extent that it’s impossible to
correct using setup adjustments. The following is a list of such malfunctions.
So if your car is handling very badly, you might want to check this list before
making any setup adjustments. Keep in mind that in oval racing some of these are
purposely introduced to obtain an asymetrical handling pattern.
9.1.1. Unequal tire size
Different diameter tires on one end of the car, like for example when the left
front tire is bigger than the right front, will make it pull to the right, both
under acceleration and deceleration. The biggest tire has a little more weight
on it, because the spring at that side is compressed a little further. So it
will have a little more traction, and on top of that, it will have a greater
‘rollout’. Those are two reasons why the biggest tire will want to travel a
larger distance, pulling the car to the opposite side. So if your car pulls to
one side while accelerating and to the other while braking, you might want to
check the tires’ size first.
If the tires up front differ in diameter from the ones in the rear, other kinds
of handling defects can occur. If your car is front or rear wheel drive only,
and you have compensated the difference in ride height, the only difference will
be a difference in grip. A larger diameter tire will have a slightly bigger
contact patch than a smaller one, and hence it will provide just a little more
grip.
If your car is full-time 4WD, tire diameters can influence the amount of
steering your car has very dramatically. Suppose the front tires are larger than
the ones in the rear. The front end of the car will tend to travel a greater
distance than the rear. Around a corner, the paths covering the largest distance
are located towards the outside, and the short ones are located on the inside.
So, the front end will want to go around the outside, and the rear will want to
go right on the inside, making the car point outwards. In other words, the car
will understeer horribly. It will feel like a ‘sticky’ kind of understeer too,
slowing down won’t help very much, and braking will only help for a very short
time because the car will very quickly try to return to its understeering state.
The opposite can also happen: bigger tires in the rear will make the car
oversteer very badly.
And also, the difference in 'rollout' will cause extra friction, which can
significantly reduce top speed. Equal diameter tires are definitely the way to
go for 4WD cars.
9.1.2. Tweak
Tweak describes the difference in weight on two tires on the same end of the
car. For example, if there's more weight on the left front than on the right
front tire, the car is tweaked. The unequal tire load will make the car pull to
the side when accelerating or braking, or even make it spin out for no apparent
reason. The causes can be numerous: unequal tire size, difference in shock
length or preload, tweaked (bent) chassis, ...
There are several telltale signs that your car is tweaked. For example: if it
turns more sharply to one side, even though the left/right weight distribution
is 50/50, if it hooks (oversteers) to one side, and understeers to the other
side,...
9.1.3. Excessive Friction
Things like A-arms that aren't moving freely, faulty ball joints, bent or
scratched shock shafts, tight steering mechanism, loads of crud in bearings or
bushings,... all cause uneven tire loads and thus also erratic handling. The
only remedy is regular maintenance; I suggest you have a good look at all the
parts of your car after every race.
9.2 Making Adjustments
The purpose of making adjustments is to make the car go faster around the track,
or to make it more controllable, or often both. A car that's easier to drive
often causes lower, more consistent lap times, and more importantly, it will
inspire the driver more confidence, which comes in handy when the going gets
rough.
Before you even start thinking about changing your car's setup, consider these
two things: firstly, is the car in perfect working order? Be sure that all of
the suspension components operate freely, without excessive play, and that the
car isn't tweaked. Things like that can really mess up a car's handling.
Secondly: the fisrst things to consider are always the tires. Any time spent
trying to get a car that's on the wrong tires to handle is wasted time; it won't
be fast enough anyway.
The first, and most important step towards making an adjustment is determining
the cause of the handling deficiency you wish to cure. Experienced
drivers/mechanics or people with a lot of insight in vehicle dynamics will just
know or feel this. In order to know what to change, you need to know what each
element does, and does not do. For instance: changing the front toe angle does
not change the balance of the car, it just changes the way the car reacts going
into corners. From all the previous chapters, it should be clear, but in
reality, it can be difficult to judge the difference between what the car is
doing and what you'd like it to do.
Here are a few examples.
When you're racing on a big, flowing track that has a lot of shallow, rhythmic
bumps in it, don't be tempted to use both a soft setting for springs and damping
and a very low roll center: the chassis will roll from side to side in every
small bump, resulting in a very unstable, unpredictable car with very little
traction. In this case, stiffening both the damping and the springs will
increase traction, but it won't be the best solution, that would be
raising the RC, and making sure it stays high when the chassis rolls. Note that
when the bumps are sort of rhythmic, soft settings for dampers and springs will
make the chassis 'resonate' to the bumps, which causes a weird form of
instability. In that case, damping and maybe springs that are a little harder
would be better; it will make the car skip over the bumps instead of plunging
into every one of them very deeply. And it's not just side-to-side movement that
can be excessive on a bumpy track: if your car has a lot of negative suspension
travel, its chassis can suffer from excessive pitch: it will kind of rock back
and forth. The answer is simple: reduce the downtravel. Even though this will
make the car bottom out on large jumps and bumps a little more, it will be a lot
more stable in bumpy sections, especially sections where the car is accelerating
or braking.
11: Driving
A fast car isn't all it takes to win races, you need good driving skills too.
11.1. The basics
There are two ways to be fast: you can either travel a smaller distance, or do
it at higher speed. Or you can combine both. This means that the path your car
travels should be short as well as without any sharp turns. Let's look at an
example:
This drawing shows the ideal line through a simple, 180 degree corner. The ideal
line is drawn in red, and the track edges in black. This type of driving line is
called Out In Out: approaching the corner, you take the outside, as far
as possible, you take the inside line in the middle of the corner, and you take
the outside again on the exit. The middle part of the corner, drawn in blue, is
called the apex of the turn, or the clipping point. The wider you can
make the radius of the line, the faster you can take the corner. Or in other
words, the less speed you'll lose.
Note that all the braking should be done in the straight line, before the
circular part begins. The acceleration too begins after the circular part, when
the car is tracking straight. During the circular part, the car's velocity is
constant. It has to be, assuming that the radius of the path is constant and the
tires are delivering maximum grip.
The same goes for any type of corner: the line with the largest radius is the
fastest one. A 90° turn is pictured: brake, turn in, keep the radius of the turn
constant, so don't accelerate or brake, and then straighten up and accelerate.
You begin and end the corner on the very outside, and almost clip the apex in
the middle part.
The most important thing isn't to know where the ideal line is and try to stay
on it, it's being able to stay on it the whole time, without any wild moments,
going sideways or braking too soon or too late,... Consistency is the key: never
lose the momentum. You lose more time if you mess up once than you can gain
rounding 10 other corners perfectly.
11.2. Advanced technique
In the previous examples we only considered one single corner, preceded and
followed by a short straight. But this isn't always the case, like in chicanes,
or turns preceding a long straight. If there is another sharp turn or a long
straight following the corner, the Out In Out line might not be the fastest
line. For example: exiting one corner very wide might mess up the entrance to
the next one because you're still on the wrong side of the road. in that case,
the Slow In Fast Out type of cornering is probably faster.
The Slow In Fast Out type of cornering is based on the fact that usually, a car
can brake harder than it can accelerate. As shown in the drawing, you brake a
little later and harder, and turn in more sharply. But from that point on you
can take the corner at a higher speed than you would using the Out In Out line.
Note that the clipping point has shifted towards the exit of the turn. As a
result, you enter the straight at a much higher speed. For corners that are
followed by a long straight, the Slow In Fast Out line is always the fastest
one, because the you can enjoy the increase in speed the whole length of the
straight, gaining precious time.
Exactly how asymmetric the ideal line is depends a little on the length of the
straight, and the difference between braking and accelerating, the line is more
asymmetric if the straight is longer and if the difference is bigger.
There are at least two downfalls to this type of cornering. First, it's very
important that nothing goes wrong exiting the corner: if for example you exit a
little too wide and you have to make a little steering correction to avoid
hitting the outside, you'll lose all of the extra speed you're carrying, or
possibly more. Secondly, because you brake later and more towards the outside,
you leave the door wide open for anyone waiting for a chance to pass you. So
it's not wise to be using this type of cornering when there's somebody right
behind you, waiting to make a pass.
Finding the ideal line through a series of turns and straights is just a matter
of putting it all together: try to find the largest possible radius so you lose
as little speed as possible, try to find a path that's as short as possible, and
try to enter the straights at as high a speed as possible. Easier said than
done, I know, but that's why practice is so important.
Regardless of which technique you use, by far the most important phase is the
phase where you brake, and turn in. You'll find that if you've go the braking
and turning in bit down, there isn't much left that can go wrong, all you do
next is steer and accelerate. Conversely, if you mess up the braking part,
you'll have your work cut out for you trying to round the corner without losing
too much time.
Finding the fastest way around a series of corners has another interesting
quirk: at the very limit, the car's trajectory becomes predefined. Suppose a car
is rounding a corner at or very near the limit. Traction is at its maximum
(somewhere on the edge of the traction circle), and so is its speed. This means
there's no more room for adjustment. No corrections can be made, simply because
that would require a little extra traction, and that just isn't there. Maybe if
the car's speed was reduced, some extra traction would become availble, but this
decrease in speed would also necessitate some extra traction. So there's really
no way out. So, once a car has been 'thrown', or 'put' into a corner, and it's
taking it at the limit, the car is like a thrown dart: its trajectory is
predestined. This has two consequences. First, it means that braking into a turn
and turning in are by far the most important factors in choosing a good driving
line. Like I mentioned before, but if you brake right, and you turn in at the
right time, you're 98% there; the rest of the corner isn't important, or hard to
take. The second consequence is the fact that going around a track the fastest
possible way means making no corrections at all. If you're able to make
corrections, it means you're not already on the limit. This is why smooth
drivers are always faster: they are on the edge 100% of the time.
11.3. Jumping
(This section only applies to Off-road driving.)
Jumping a car isn't that hard, you just need to hit the face of the jump
squarely, and land shiny side up.
You need to line up your car before it hits the face of the jump; it needs to
hit the jump absolutely straight, with none of the wheels slipping. Keep this in
mind.
You can alter your car's attitude when it's in the air by pushing or releasing
the throttle. Because of the inertia of the tires, more throttle makes its nose
go up, less throttle makes the nose go down. If it needs be, you can even jab
the brakes, which will bring the nose down rapidly. Note that in this phase, the
effect of the size and position of the rear wing can be felt, a large, angled
rear wing can prevent a car from landing nose-first. Also, there's a subtle
difference between 4WD and RWD cars, they just react differently to throttle
movements. A RWD car will rotate around its rear tires, a 4WD car will sort of
'float'.
Try to land on the down slope of the jump, if there is one, you'll be able to
hit the throttle much sooner. A well set-up car will settle directly after
landing, while a poorly set-up car will bounce around a few times before
becoming stable enough to hit the throttle.
11.4. Passing
When it comes to making a pass, you need 3 things: you need to be faster, or
carry more momentum, at least, and you need a gap, large anough to be able to
squeeze through. You also need balls ;-) There's one principal rule about
passing: never, ever go around the outside, unless there's no other way. The
outside line is a dangerous place: it's usually full of dust and crud, it's
longer, and there's always the possibility your opponent sends you off the
track. Then, you need to be faster, somehow. There are two ways to do this. The
first is to gain momentum coming out of a corner, and pass your opponent down
the straight. This is the easiest and least dangerous technique. The other
possibilty is to dive down the inside when entering a corner. You brake as late
and as hard as you can, put the car on the inside, and hope your opponent gives
way. The tricky part is rounding and exiting the corner properly; since you
probably didn't enter the corner ideally, you need to concentrate on the middle
and exit part, and make sure your opponent doesn't pass you again. But since
you're in the lead now, you can always 'keep the door firmly shut'.
This setup guide assumes you have some sort of 'standard setup' to
begin with. One should come with your kit. You can also find standard setups on
manufacturers' websites.
It also assumes your car is in perfect working order. (Bearings spinning freely,
nothing dragging the ground, no binding in the suspension,...)
Tires
Tires are always the first element in setting up a
car. If you've got the right tires and inserts, you're 99% there.
|
Caster
Caster is a very sensitive adjustment!
Adding or removing a few degrees of caster can transform the steering
balance of a car. |
| More |
More caster aids stability, especially at high speeds.
More caster generally suits large, open, high-speed tracks. |
| Less |
Less caster increases steering drastically.
Steering feels much more direct, the car turns tighter and faster.
Small amounts of caster are suitable for tight tracks. |
Toe
| Rear Toe-in |
This is one of the most sensitive adjustments! One degree goes a
long way.
Stabilizes the car greatly. It makes the rear end 'stick'. The more
toe-in you use, the more the rear of the car sticks. This is especially
apparent going into and coming out of turns.
But more toe-in makes the difference between sticking and breaking loose
bigger.
Large amounts of toe-in (2.5 ... 3 degrees) scrub off a little speed in
the straights. |
| Rear Toe-out |
Rear toe-out is never used. It makes the rear of the car very, very
unstable. |
| Front Toe-in |
Stabilizes the car in the straights, and coming out of turns.
It smoothes out the steering response, making the car easy to drive.
It can make the car turn a little more in the middle and exit parts of a
turn. |
| Front Toe-out |
Increases turn-in steering a lot.
But can make the car wandery on the straights.
Never use more than 2 degrees of front toe-out! |
Camber
Camber is best set so the tires' contact patch is as big as possible
at all times. So with a stiff suspension and firm tires you'll need less
camber than with a soft suspension or tires with big, flexible
sidewalls.
If the tires wear evenly across their contact patches, camber is about
right. |
Pinion/Spur
Smaller Gear Ratio
(bigger number means smaller ratio) |
More punch and accelleration.
More runtime.
Lower top speed. |
Bigger Gear Ratio
(smaller number means bigger ratio) |
Less punch, but more top speed.
Less runtime. |
| Smaller Pinion Gear |
Smaller gear ratio |
| Bigger pinion Gear |
Bigger gear ratio |
| Smaller Spur Gear |
Bigger gear ratio |
| Bigger Spur Gear |
Smaller gear ratio |
| Overall Ratio |
Overall Ratio = (Spur/Pinion)*Internal Gearbox Ratio |
Rollout
(mm/rev) |
Rollout = (Pi*Tire Diameter)/Overall Ratio |
Motors
More Turns
(e.g. 13x2 or 14x3) |
More runtime.
Less power, and smoother response.
Easy to drive. |
Less Turns
(e.g. 9x2 or 8x3) |
Less runtime.
More power.
Harder to drive. |
More Winds
(e.g. 11x4 or 12x5) |
Slightly more runtime.
Feels very smooth, has a nice powerband. Very useful on slippery tracks.
More top-end. |
Less Winds
(e.g. 12x1 or 11x2) |
Slightly less runtime.
Feels very punchy, but has less top-end. |
More Timing Advance
(e.g. 6 to 8mm) |
Less runtime.
More punch, and more top speed.
More wear on the comm and brushes.
Motor gets hotter. |
Less Timing Advance
(e.g. 4 to 6mm) |
More runtime.
Easy on the comm and the brushes.
Less punch and top speed. |
| Stiffer Brush Springs |
More power at low revs.
Slightly lower top speed because of increased friction.
Better for high currents and bumpy tracks. |
| Softer Brush Springs |
More power at hight revs, but less punchy.
Higher top speed.
Good for low current draw. |
TIP: You get slightly more punch and a slightly more efficient motor
if you use a slightly stiffer brush spring on the + side.
The easiest way to do this is to hold one leg of the spring with pliers
and gently bend the leg 5 to 10 degrees more. |
Springs
| Stiffer |
Stiffer springs make the car feel more responsive, more direct.The
car reacts faster to driver input
Stiff springs are suited for tight, high-traction tracks, which aren't
too bumpy.
Usually, when you stiffen the whole car, you lose a small amount of
steering. |
| Softer |
Softer springs are better for bumpy and very large and open tracks.
They can also make the car feel as if it has a little more traction in
low-grip conditions.
Springs that are too soft make the car feel sluggish and slow. |
| Stiffer Front |
The car has less front traction, and less steering. It's harder to
get the car to turn, the turn radius is bigger and the car has a lot
less steering exiting corners.
On very high-grip tracks, if the track itself feels tacky or sticky,
very stiff springs are the way to go. |
| Softer Front |
The car has more steering, especially in the middle part and the
exit of the corner.
Front springs that are too soft can make the car hook and spin. |
| Stiffer Rear |
The car has more steering, in the middle and exit of the turn. This
is especially apparent in long, high-speed corners.
But rear traction is reduced. |
| Softer Rear |
The car has generally more rear traction, in turns as well as
through bumpy sections and while accellerating. |
Damping
| Heavier |
Thicker oil (heavier damping) makes the car more stable, and makes
it handle moore smoothly.
If damping is too heavy, traction could be lost in bumpy sections. The
car will also change direction slower. |
| Softer |
Soft damping makes the car react quicker. |
| Damping should always be adapted to the spring ratio; the suspension
should never feel too 'springy' or too slow. |
| Heavier Front or Softer Rear |
The turn radius is wider, but smoother. The car doesn't 'hook'
suddenly.
The car is easier to drive, and high-speed steering feels very nice.
Easy to drive. |
| Softer Front or Heavier Rear |
The steering reacts quicker.
More and better low-speed steering. |
Weight Distribution / Battery Location
| More towards the front |
More front-end grip, all the time. But the front also feels more
inert.
If you overdo it, it feels like you're riding on the front tires, and
the rear doesn't do anything but follow the front.
Rear traction is reduced. |
| More towards the rear |
More rear-end grip, so the rear feels more planted. But if the rear
does swing out, it's usually very sudden and more unpredictable. |
One-way
| Without one-way, or tighter setting |
You can brake much later and harder.
If you brake hard enough to lock the front wheels up, or almost lock
them up, the front end slides. (You lose steering.)
The car could be slower in tight turns if traction is decent.
Much easier to drive on slippery tracks. |
| With One-way, or looser setting |
With a one-way, the car can take corners faster. It rolls through
them more easily and freely.
Braking can be tricky, and it can result in the rear end sliding out.
A one-way *diff* feels very nice coming out of turns: it's very stable
while accellerating, and it gives more on-power steering. |
Weight Distribution / Battery Location
| More towards the front |
More front-end grip, all the time. But the front also feels more
inert.
If you overdo it, it feels like you're riding on the front tires, and
the rear doesn't do anything but follow the front.
Rear traction is reduced. |
| More towards the rear |
More rear-end grip, so the rear feels more planted. But if the rear
does swing out, it's usually very sudden and more unpredictable. |
Upper Shock Mounting Location
| More Inclined |
Has a more progressive, smoother feel.
More lateral grip.
Having all shocks inclined makes the car very easy to drive, and it
feels like the car has more grip, but it's not always fast... |
Less Inclined
(More Vertical) |
More direct feel.
Less lateral grip. (side-bite) |
| Front more inclined than rear |
Steering feels very smooth.
A little more mid-corner steering.
Mounting the rear shocks very much upright can result in the rear end
feeling unpredictable. It can also makt the rear end jitter in turns.
|
| Rear more inclined than front |
Feels agressive turning in, but for most of the time the car has a
little less steering.
The car has a lot of side traction in the rear, and the turn radius
isn't very tight. |
Anti-Roll bar
| Anti-roll bars are best used on tracks where traction is consistent.
|
Adding an anti-roll bar, or stiffening it, reduces traction at that
end of the car. So it feels like the opposite end has more grip.
If the track is smooth enough, it also makes the grip level feel a
little more consistent.
Anti-roll bars reduce body roll in turns, so they make the car feel more
direct, and make it change direction quicker. |
| Stiffer Front |
An anti-roll bar at the front of the car reduces low-speed steering.
The turning radius will be larger, but smoother and very consistent.
It reduces 'hooking' by preventing front end roll.
The car will have more rear traction in turns. |
| Stiffer Rear |
Adding an anti-roll bar to the rear of the car gives more steering.
the car steers tighter, also at low speeds.
On a very smooth track, it can make sliding easier. |
Anti-Squat
| More |
More anti-squat generally makes the rear of the car more sensitive
to throttle input.
The car has more steering while braking, and also a little more powering
out of corners. |
| Less |
Less anti-squat gives more side-bite, on-power and while braking.
It feels easier to drive in low-grip situations. |
Note that anti-squat only works when you're accellerating or
braking, it does absolutely nothing when you're coasting through turns.
The harder you brake or accellerate, the bigger the effect of anti-squat
is.
|
Roll Center / Camber links
| Long Link |
A long link gives a lot of body roll in turns.
It feels as is the body is willing to keep on rolling, until in the end,
the springs prevent it from rolling any further.
The car has more grip in corners, especially the middle part. But: if
there already is a lot of traction, long camber links can slow the car
down in turns. |
| Short Link |
A short link makes that the body doesn't roll as far, its tendency
to roll drops off as it rolls.
It feels as is the car generates a little less grip. |
More Parallel Link
(More Parallel to lower arm) |
A parallel link gives a little more roll than an angled one.
It feels very smooth, and consistent as the body rolls in turns. |
Angled Link
(Distance between arm and link is smaller on the inside) |
An angled link makes it feel as if the car has a tendency to center
itself (level, no roll), other than through the springs or anti-roll
bar.
It gives a little more initial grip, steering into corners. It makes it
very easy to 'throw' the car.
The body rolls a little less than with parallel links.
It's possible to use softer settings for damping and spring rate than
with parallel links, without destabilising the car. |
| Beware that you should always keep an eye on the balance of your
car; large differences in roll center front vs. rear will make the car
feel less consistent and less confidence-inspiring. |
| Longer Front |
The front rolls and dives more in turns.
Lots of steering in mid-corner.
Could make the car hook. |
| Shorter Front |
The front feels very stable.
A little more turn-in, but less steering in mid-corner. |
| Longer Rear |
More rear traction in turns, and coming out of them.
Rear end slide is very progressive, not unpredictable at all.
Make sure that there's enough rear camber though, or you could lose rear
traction in turns. |
| Shorter Rear |
The rear feels very stable. It breaks out later and more suddenly,
but if it does, the slide is more controllable.
It makes the front dive a little more, which results in more steering,
especially when braking. |
| More Angled Front |
Turn-in is very agressive.
The front feels as if it wants to roll less than the rear. |
| More Angled Rear |
The rear end is rock-solid while turning in. It feels very
confident. |
Internal Travel Limiters / Droop / Downtravel
Less Droop
(more internal limiters) |
The car changes direction faster, and corners flatter. It feels
generally more responsive and more direct.
Adding a lot of travel limiters is only advisable on smooth,
high-traction tracks.
Reducing droop is the most effective way to stop your car from traction
rolling on high grip tracks, you can use as little as 2 or 3 mm. |
More Droop
(less internal limiters) |
Less internal shock spacers give better handling on bumpy tracks,
and more and more consistent traction on difficult tracks. |
Less droop in front,
more at the rear |
The car changes direction faster.
It turns in very well, but it could lose front traction halfway throught
the turn. |
More droop in front,
less at the rear |
Makes the car brake better.
Rear traction feels consistent. |
Ride Height
| Higher |
The car feels better in bumpy sections.
It can feel tippy, or even flip over in high-grip conditions. |
| Lower |
The car feels more direct, and it can potentially corner a bit
faster.
It's also harder to flip the car over. |
| Lowering one end of the car, or putting the other end higher up,
gives a little more grip at the lowest end, but try to avoid big
differences in ride height between the front and the rear. |
Kickup / Anti-dive
| The assumption is made that if kickup is changed, caster stays the
same. (This usually requires different caster blocks.) |
| More Kickup - Less anti-dive |
Much better through bumps.
More forgiving to drive. |
| Less Kickup - More anti-dive |
More turn-in steering.
The car dives less while braking, and the front lifts less while
accellerating.
Maybe a little more braking traction, and a little more on-power
steering too. |
Wheelbase
| Shorter |
A short wheelbase makes the car feel very nimble, and good in tight
turns.
This is a good idea for very small and tight tracks. |
| Longer |
The car becomes a lot more stable, and better in wide, high-speed
turns.
This is good on wide-open tracks. |
Shock Pistons
| The assumption is made that if pistons are changed, the viscosity of
the oil is also adapted, to give the same static feel. (Same low-speed
damping)
|
| Smaller Holes |
Smaller holes mean more 'pack'. Pack means the damping gets very
stiff, or almost locks up, over sharp bumps.
Small holes are good for smooth tracks. |
| Bigger Holes |
Bigger holes mean less pack. The point at which the damping gets
stiff (where the shock 'packs up') occurs a lot later, at higher shock
shaft speeds.
Big holes are very good for bumpy tracks. The car is more stable and has
more traction in the bumpy sections. It won't be thrown up over sharp
bumps, the suspension will soak them up a lot better. |
Bump Steer
Bump steer in generally undesirable. If the angle of the wheels does
change as the suspension is compressed, the wheels should move outward.
(steering less)
More bump steer can make a car have less steering, and be a little bit
more stable in bumps. |
© Copyright 2001 by Bruno
'Elvo' Heremans.