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The M-Racing (RC-Setup.com - Website no longer in service) guide is intended to help beginners and advanced racers alike. Please let us know what you think of it!
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.
Table of content:
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 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 (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 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 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 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 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.
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 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.
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 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.
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 - Table of content
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.
Other Important factors to consider - Table of content
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 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 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.
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 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 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.
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.
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) - Table of content
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. |
