M-Racing (RC-setup.com) Car Setup
Guide
A setup system is an invaluable tool in helping you properly setup your RC race car to win races.
However, being able to measure your RC car or RC truck's settings is just
part of RC car handling tuning.
This guide is intended to introduce some concepts and explain how
different RC car settings affect your RC car's handling. We are continually
adding to this guide and hope that it will help beginners and advanced
RC drivers alike.
Camber angle
A wheel with a negative camber angle
Camber angle is the angle between the vertical axis of the
wheel and the vertical axis of the vehicle when viewed from the front
or rear. If the top of the wheel is further out than the bottom (that
is, away from the axle), it is called positive camber; if the
bottom of the wheel is further out than the top, it is called
negative camber.
Camber angle alters the handling characteristics of a car. As a
general rule, increasing negative camber improves grip on that wheel
when cornering (within limits). This is because it gives the tire that
is taking the greatest proportion of the cornering forces, a more
optimal angle to the road, increasing its contact patch and
transmitting the forces through the vertical plane of the tire, rather
than through a shear force across it. Another reason to have negative
camber is that a rubber tire tends to roll on itself while cornering.
If the tire had zero camber, the inside edge of the contact patch
would begin to lift off of the ground, thereby reducing the contact
patch. By applying negative camber, this effect is reduced, thereby
maximizing the contact patch.
On the other hand, for maximum straight-line acceleration,
obviously the greatest traction will be attained when the camber angle
is zero and the tread is flat on the road. Proper management of camber
angle is a major factor in suspension design, and must incorporate not
only idealized geometric models, but also real-life behavior of the
components: flex, distortion, elasticity, etc.
Most RC race cars have some form of double wishbone suspension
which allow you to adjust camber angle (as well as camber intake).
Camber Intake
Camber intake is the measure of how much the
camber angle changes as the suspension is compressed. This is
determined by the length and angle between the top and bottom
suspension arms (or turnbuckles). If the top and bottom suspension
arms are parallel, camber will not change as the suspension is
compressed. If the angle between the arms is considerable, the camber
will increase as the suspension is compressed.
A certain amount of camber intake is desirable to
maintain the face of the wheel parallel to the ground as the car rolls
into a corner.
Note: the suspension arms should be either
parallel or closer to each other on the inside (car side) than on the
wheel side. Having suspension arms that are closer to each other at
the wheel side than at the car, will result in camber angles that vary
radically (and a car that behaves erratically).
Camber intake will define how the roll-center of
your race car behaves. The roll center of your car will in turn
determine how weight will be transferred when cornering and this will
have an important effect on handling (more on this later).
Caster Angle
Caster (or castor) angle is the angular
displacement from the vertical axis of the suspension of a wheel in a
car, measured in the longitudinal direction (angle of the kingpin when
looked at from the side of the car). It is the angle between the pivot
line (in a car - an imaginary line that runs through the center of the
upper ball joint to the center of the lower ball joint) and vertical.
Caster angle can be adjusted to optimize a car's handling
characteristics for particular driving situations.
The pivot points of the steering are angled such that a line drawn
through them intersects the road surface slightly ahead of the contact
point of the wheel. The purpose of this is to provide a degree of
self-centering for the steering - the wheel casters around so as to
trail behind the axis of steering. This makes a car easier to drive
and improves its straight line stability (reducing its tendency to
wander). Excessive caster angle will make the steering heavier and
less responsive, although, in off-road racing, large caster angles are
used to improve camber gain in cornering.
Toe-In and Toe-Out
Toe is the symmetric angle that each wheel makes with the
longitudinal axis of the vehicle, as a function of static geometry,
and kinematical and compliant effects. This can be contrasted with
steer, which is the symmetric angle, i.e. both wheels point to the
left or right, in parallel (roughly). Positive toe, or toe in
is when the front of the wheel points in towards the centerline of the
vehicle.
Front toe angle
In general, increased front toe in (i.e. the fronts of the
front wheels are closer together than the backs of the front wheels)
provides greater straight-line stability at the cost of some
sluggishness of turning response, as well as a little more drag as the
wheels are now driving a bit sideways.
Toe-out in the front wheels, will result in more responsive
steering and quicker turn-in. However, front toe-out usually means a
less stable car (i.e. more twitchy).
Rear toe angle
The rear wheels of your race car should always be adjusted with
some degree of toe in (although 0 degrees of toe is acceptable under
some conditions). In general, the more rear toe-in, the more stable
your car will be. Keep in mind, however, that increasing toe angle
(front or rear) will result in decreased straight line speed
(particularly when racing stock electric motors).
One related concept is that the proper toe for straight line travel
of a vehicle will not be correct while turning, since the inside wheel
must travel around a smaller radius than the outside wheel; to
compensate for this, the steering linkage typically conforms more or
less to Ackermann steering geometry, modified to suit the
characteristics of the individual vehicle.
Ackermann steering geometry
Ackermann steering geometry is a geometric arrangement of
linkages in the steering of a car designed to solve the problem of
wheels on the inside and outside of a turn needing to trace out
circles of different radii.
When a vehicle is steered, it follows a path which is part of the
circumference of its turning circle, which will have a centre
point somewhere along a line extending from the axis of the rear axle.
The steered wheels must be angled so that they are both at 90 degrees
to a line drawn from the circle centre through the centre of the
wheel. Since the wheel on the outside of the turn will trace a larger
circle than the wheel on the inside, the wheels need to be set at
different angles.
The Ackermann steering geometry arranges this automatically by
moving the steering pivot points inward so as to lie on a line drawn
between the steering kingpins and the centre of the rear axle. The
steering pivot points are joined by a rigid bar, the tie rod, which
can also be part of the steering mechanism. This arrangement ensures
that at any angle of steering, the centre point of all of the circles
traced by all wheels will lie at a common point.
Slip angle
Slip angle is the angle between a rolling
wheel's actual direction of travel and the direction towards which it
is pointing. This slip angle results in a force perpendicular to the
wheel's direction of travel -- the cornering force. This cornering
force increases approximately linearly for the first few degrees of
slip angle, and then increases non-linearly to a maximum before
beginning to decrease (as the wheel slips).
A non-zero slip angle arises because of deformation in the tire. As
the tire rotates, the friction between the contact patch and the road
result in individual tread 'elements' (infinitely small sections of
tread) remaining stationary with respect to the road.
This tire deflection gives rise to the slip angle, and to the
cornering force.
Because the forces exerted on the wheels by the weight of the
vehicle are not distributed equally, the slip angles of each tire will
be different. The ratios between the slip angles will determine the
vehicle's behavior in a given turn. If the ratio of front to rear slip
angles is greater than 1:1, the vehicle will tend to understeer, while
a ratio of less than 1:1 will produce oversteer. Actual instantaneous
slip angles depend on many factors, including the condition of the
road surface, but a vehicle's suspension can be designed to promote
specific dynamic characteristics.
A principal means of adjusting developed slip angles is to alter
the relative roll front to rear by adjusting the amount of front and
rear lateral weight transfer. This can be achieved by modifying the
height of the Roll centers, or by adjusting roll stiffness, either
through suspension changes or the addition of an anti-roll bar.
Weight Transfer
Weight transfer refers to the
redistribution of weight supported by each tire during acceleration
(both longitudinal and lateral). This includes accelerating, braking,
or turning. Understanding weight transfer is crucial for understanding
vehicle dynamics.
Weight transfer occurs as the vehicle's center of gravity (CoG)
shifts during automotive maneuvers. Acceleration causes the car’s mass
to rotate about a geometric axis resulting in relocation of the CoG.
Front-back weight transfer is proportional to the ratio of the center
of gravity height to the vehicle's wheelbase, and side-to-side weight
transfer (summed over front and rear) is proportional to the ratio of
the center of gravity height to the vehicle's track as well as it’s
roll center (explained later).
For example, when a car accelerates, its weight is transferred
towards the rear wheels. You can witness this as the car visibly leans
to the back, or "squats". Conversely, under braking, weight transfer
toward the front of the car will occur (the nose "dives" toward the
ground). Similarly, during changes in direction (lateral
acceleration), weight transfer to the outside of the direction of the
turn occurs.
Weight transfer causes the available traction at all four wheels to
vary as the car brakes, accelerates, or turns. For example, because of
the forward weight transfer under braking, the front wheels do most of
the braking. This bias to one pair of tires doing more `work' than the
other pair results in a net loss of total available traction.
If lateral weight transfer reaches the tire
loading on one end of a vehicle, the inside wheel on that end will
lift, causing a change in the handling characteristics. If it reaches
half the weight of the vehicle it will start to roll over. Some large
trucks will roll over before skidding, while on-road cars usually roll
over only when they leave the road.
Roll center
The roll center of a vehicle is the
imaginary point marking the center of where the car will roll (when
cornering) when looked at from the front (or behind).
The location of the geometric roll center is solely dictated by the
suspension geometry. The official definition of roll center is: "The
point in the transverse vertical plane through any pair of wheel
centers at which lateral forces may be applied to the sprung mass
without producing suspension roll".
The significance of the roll center can only be appreciated when
the vehicles center of mass is also considered. If there is a
difference between the position of the center of mass and the roll
center a “moment arm” is created. When the vehicle experiences lateral
acceleration due to cornering, the roll center moves up or down and
the size of the moment arm, combined with the stiffness of the springs
and roll bars (sway bars in some parts of the world) dictate how much
the vehicle will roll while cornering.
The geometric roll center of the vehicle can be found by following
basic geometrical procedures when the vehicle is static:
Draw imaginary lines parallel to the suspension arms (in red). Then
draw imaginary lines between the intersection points of the red lines
and the bottom center of the wheels as shown in the picture (in
green). The intersection point for these green lines is the roll
center.
You should note that the roll center will move when the suspension
is compressed or lifted, that's why it's actually an instantaneous
roll center. How much this roll center moves as the suspension is
compressed is determined by the suspension arm length and the angle
between the top and bottom suspension arms (or turnbuckles).
As the suspension is compressed, the roll center
will become higher and the moment arm (distance between roll
center and the car’s center of gravity (CoG in the picture)) will
decrease. This will mean that as the suspension is compressed (when
taking a corner, for example), the car will have less tendency to keep
rolling (which is good, you do not want to roll over).
When using higher grip tires (foam), you should
set the suspension arms so that the roll center is raised
significantly as the suspension is compressed. On-road nitro cars have
very aggressive suspension arm angles to raise the roll center as the
car corners and prevent roll-overs when running with foam tires.
Running parallel, equal-length suspension arms
will result in a fixed roll center. This means that as the car leans
over, the moment arm will be forcing the car to roll more and more. As
a general rule of thumb, the higher the center of gravity of your car,
the higher the roll center should be to avoid a roll-over.
Bump Steer
Bump Steer is the term for the tendency of
a wheel to steer as it moves upwards through the suspension travel.
On most cars, the front wheels usually toe-out, that is, the front of
the tire moves outwards, as the suspension is compressed. This gives
roll under steer (when you hit a bump when turning, the car tends to
straighten). Excessive bump steer increases tire wear and makes the
vehicle twitchy on rough roads.
Bump Steer and Roll Steer
In a bump, both wheels rise together. In roll one wheel rises as
the other falls. Typically this produces more toe in on one wheel, and
more toe out on the other, thus giving a steering effect. In a simple
analysis you can just assume that the roll steer is the same as bump
steer, but in practice things like the anti-roll bar geometry have an
effect that modifies it.
Bump steer can be made more toe-out in jounce by lifting the outer
ball joint or dropping the inner ball joint. Usually only small
adjustments are required.
Understeer
Understeer is a term for a car handling
condition during cornering in which the circular path of the vehicle's
motion is of a markedly greater diameter than the circle indicated by
the direction its wheels are pointed. The effect is opposite to that
of the oversteer and in simpler words understeer is the condition in
which the front tires don't follow the trajectory the driver is trying
to impose while taking the corner, instead following a more straight
line trajectory.
This is also often referred to as pushing, plowing, or refusing to
turn in. The car is referred to as being 'tight' because it is stable
and far from wanting to spin.
As with oversteer, understeer has a variety of sources such as
mechanical traction, aerodynamics and suspension.
Classically, understeer happens when the front tires have a loss of
traction during a cornering situation, thus causing the front-end of
the vehicle to have less mechanical grip and become unable to follow
the trajectory in the corner.
Camber angles, ride height, tire pressure and centre of gravity are
important factors that determine the understeer/oversteer handling
condition.
It is common that manufacturers configure cars deliberately to have
a slight understeer by default. If a car understeers slightly, it
tends to be more stable (within the realms of a driver of average
ability) if a violent change of direction occurs.
How to adjust your car to reduce understeer
You should begin by increasing the negative camber of the front
wheels (never above -3 degrees in an on-road sedan or 5-6 degrees on
an off-road car).
Another way to reduce understeer is to decrease the rear wheel
negative camber (which should always be <= 0 degrees).
Yet another method to reduce understeer is to reduce the size or
remove the front anti-roll bar (or to increase the size of the rear
anti-roll bar).
It is important to notice that any adjustment made will result in a
trade-off. Cars have a limited amount of grip that can be distributed
between the front and rear wheels.
Oversteer
A car is said to be oversteering when the
rear wheels do not track behind the front wheels but instead slide out
toward the outside of the turn. Oversteer can throw the car into a
spin.
The tendency of a car to oversteer is affected by several factors
such as mechanical traction, aerodynamics and suspension, and driver
control.
Limit oversteer happens when the rear tires exceed the limits of
their lateral traction during a cornering situation before the front
tires do, thus causing the rear of the vehicle to head towards the
outside of the corner. More generally oversteer is the condition when
the slip angle of the rear tires exceeds that of the front tires.
Rear wheel drive cars are generally more prone to oversteer, in
particular when applying power in a tight corner. This occurs because
the rear tires must handle both the lateral cornering force and engine
torque.
The car's tendency toward oversteer is generally increased by
softening the front suspension or stiffening the rear suspension (or
adding a rear roll bar). Camber angles, ride height, and tire
temperature ratings can also be used to tune the balance of the car.
An oversteering car is alternatively referred to as 'loose' or
'tail happy'.
How do you differentiate Oversteer and Understeer?
When you turn into a corner, oversteer is when the car turns more than
you expected and understeer is when it turns less than you expect.
To Oversteer or to Understeer,
that is the question
As mentioned before, any adjustment made will result in a
trade-off. Cars have a limited amount of grip that can be
distributed between the front and rear wheels (this can be enhanced
through aerodynamics, but that’s another story).
All race cars develop a greater lateral (i.e. sideslip) velocity
than is indicated by the direction in which the wheels are pointed.
The difference between the circle the wheels are currently tracing and
the direction in which they are pointed is the slip angle. If
the slip angles of the front and rear wheels are equal, the car is in
a neutral steering state. If the slip angle of the front wheels
exceeds that of the rear, the vehicle is said to be understeering.
If the slip angle of the rear wheels exceeds that of the front, the
vehicle is said to be oversteering.
Just remember that an understeering car goes into the boards nose
first, an oversteering car goes into them tail first, and with a
neutral-steering car, both ends hit the boards at the same time. J
Other Important factors to consider
Any vehicle may understeer or oversteer at different times based on
road conditions, speed, available traction, and driver input. The
design of a vehicle, however, will tend to produce a particular
"terminal" condition when the vehicle is pushed to and past its limits
of adhesion. "Terminal understeer" refers to a vehicle which,
as a function of its design, tends to understeer when cornering loads
exceed tire traction.
Terminal handling balance is a function of front/rear relative roll
resistance (suspension stiffness), front/rear weight distribution, and
front/rear tire traction. A front-heavy vehicle with low rear roll
stiffness (from soft springing and/or undersized or nonexistent rear
anti-roll bars) will have a tendency to terminal understeer: its front
tires, being more heavily loaded even in the static condition, will
reach the limits of their adhesion before the rear tires, and thus
will develop larger slip angles. Front wheel-drive cars are also prone
to understeer because not only are they usually front-heavy,
transmitting power through the front wheels also reduces their grip
available for cornering. This often leads to a "shuddering" action in
the front wheels which can be felt in the car as grip is suddenly
being changed from planting the engines power on the road and
steering.
Although understeer and oversteer can each cause a loss of control,
many automakers design their vehicles for terminal understeer in the
belief that it is easier for the average driver to control than
terminal oversteer. Unlike terminal oversteer, which often requires
several steering corrections, understeer can often be reduced simply
by reducing speed.
Understeer is not just present during acceleration through a
corner, it can also be found during heavy braking. If the brake
balance (the strength of the brakes in terms of the front and rear
wheels) is too heavy at the front this can cause understeer. This is
caused by the front wheels locking and losing any effective steering.
The opposite is true if the brake balance is too strong towards the
rear wheels causing the rear end to spin out (like a child skidding on
a bicycle). In ordinary road cars a safe brake balance (tending
towards slight understeer) must be found.
Racing drivers, on asphalt surfaces, generally prefer a neutral
condition (with a slight tendency toward understeer or oversteer,
depending on the track and driver preference) because both understeer
and oversteer conditions will scrub off speed while cornering. In rear
wheel drive cars understeer is generally faster on a circuit because
the rear wheels need to have some grip available to accelerate the
vehicle out of the turn.
Spring rate
Spring rate is a component in setting the vehicles ride height and
its location in the suspension stroke. Spring rate is a ratio used to
measure how resistant a spring is to being compressed.
Springs that are too hard or too soft will both effectively cause
the vehicle to have no suspension at all.
Wheel rate
Wheel rate is the effective spring rate when measured at the wheel.
This is as opposed to simply measuring the spring rate alone.
Wheel rate is usually equal to or considerably less than the spring
rate. Commonly, springs are mounted on control arms, swing arms or
some other pivoting suspension member. Lets assume the spring moved
0.75 inches, the lever arm ratio would be 0.75 to 1. The wheel rate is
calculated by taking the square of the ratio (0.5625) times the spring
rate. Squaring the ratio is due to two effects. The ratio applies to
both the force and distance traveled.
Suspension Travel
Travel is the measure of distance from the bottom of the suspension
stroke (when the vehicle is on a stand and the wheel hangs freely), to
the top of the suspension stroke (when the vehicles wheel can no
longer travel in an upward direction toward the vehicle). Bottoming or
lifting a wheel can cause serious control problems. "Bottoming" can be
done by either the suspension, tires, chassis, etc. running out of
space to move or the body or other components of the car hitting the
road.
Damping
Damping is the control of motion or oscillation, as seen with the
use of hydraulic shock absorbers. Damping controls the travel speed
and resistance of the vehicles suspension. An undamped car will
oscillate up and down. With proper damping levels, the car will settle
back to a normal state in a minimal amount of time. Most damping in
modern vehicles can be controlled by increasing or decreasing the
thickness of the fluid (or the size of the shock valve holes) in the
shock absorber.
Anti-dive and Anti-squat
Anti-dive and Anti-squat are expressed in terms of percentage and
refer to the front diving under braking and the rear squatting under
acceleration. They can be thought of as the counterparts for braking
and acceleration as Roll Center Height is to cornering. The main
reason for the difference is due to the different design goals between
front and rear suspension, whereas suspension is usually symmetrical
between the left and right of the vehicle.
Anti-dive and Anti-squat percentage are always calculated with
respect to a vertical plane that intersects the vehicle's Center of
Gravity. Consider Anti-dive first. Locate the front Instant Centers of
the suspension from the vehicle's side view. Draw a line from the tire
contact patch through the Instant Center: this is the tire force
vector. Now draw a line straight down from the vehicle's center of
gravity. The Anti-dive is the ratio between the height of where the
tire force vector crosses the center of gravity plane expressed as a
percentage. An Anti-dive ratio of 50% would mean the force vector
under braking crosses half way between the ground and the center of
gravity.
Anti-squat is the counterpart to Anti-dive and is for the rear
suspension under acceleration.
Circle of forces
The Circle of Forces is a useful way to think about the
dynamic interaction between a vehicle's tire and the road surface. In
the diagram below we are looking at the tire from above, so that the
road surface lies in the x-y plane. The vehicle that the tire is
attached to is moving in the positive y direction.
In this example, the vehicle would be cornering
to the right (i.e. the positive x direction points to the center of
the corner). Note that the plane of rotation of the tire is at an
angle to the actual direction that the tire is moving (the positive y
direction). That angle is the slip angle.
The magnitude limit of F is limited by the dashed circle,
but it can be any combination of the components Fx (turning)
and Fy (accelerating or braking) that does not exceed the
dashed circle. If the combined Fx and Fy forces exceeds the boundaries
of the circle, the tire looses grip (you slide or spin out).
In the example, the tire is generating a component of force in the
x direction (Fx) which, when transferred to the vehicle's
chassis via the suspension system in combination with similar forces
from the other tires, will cause the vehicle to turn to the right. The
diameter of the circle of forces, and therefore the maximum horizontal
force that the tire can generate, is affected by many things,
including the design of the tire and its condition (age and
temperature range), the qualities of the road surface, and the
vertical load on the tire.
Critical Speed
Oversteering cars have an associated instability mode, called the
critical speed. As this speed is approached the steering becomes
progressively more sensitive. At the critical speed the yaw velocity
gain becomes infinite, that is, the car will continue to turn with the
wheel held straight ahead. Above the critical speed a simple analysis
shows that the steer angle must be reversed (counter steering).
Understeering cars do not suffer from this, which is one of the
reasons why high speed cars tend to be set up to understeer.
Finding the Zen (or at least a balanced car)
A car that tends neither to oversteer nor understeer when pushed to
the limit is said to have neutral handling. It seems intuitive that
race drivers would prefer a slight oversteer condition to rotate the
car around a corner, but this isn't usually the case for two reasons.
Accelerating early as the car passes the apex of a corner allows it to
gain extra speed down the following straight. The driver who
accelerates sooner and/or harder has a large advantage. The rear tires
need some excess traction to accelerate the car in this critical phase
of the corner, while the front tires can devote all their traction to
turning. So the car must be set up with a slight understeer or "tight"
tendency. Also, an oversteering car tends to be twitchy and ill
tempered, making a race car driver more likely to lose control during
a long race or when reacting to sudden situations in traffic.
Note that this applies only to pavement racing. Dirt racing is a
different matter.
Some successful
drivers do prefer a bit of oversteer in their cars, preferring a car
which is less sedate and more willing to turn into corners (or inside
their opponents). It should be noted that the judgment of a car's
handling balance is not an objective one. Driving style is a major
factor in the apparent balance of a car. This is why two drivers with
identical cars often run with rather different balance settings from
each other. And both may call the balance of their cars 'neutral'.
Copyright Notice:
Some of the
definitions found here were based on text found on wikipedia (www.wikipedia.com).
Feel free to print these out and / or distribute this document as long
as you give proper credit.
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