Suspension - Camber, Caster and Toe

Owned by Adam Marchand
Last updated: Mar 31, 2019 by Robin Pearce

https://www.carbibles.com/guide-to-car-suspension/


Suspension design, regardless of application, is driven by three main parameters: camber, caster and toe. These parameters control the location of the suspension, and hence the wheel, within 3D space relative to the car. These three parameters allow for the engineer to precisely control the behaviour of the car through turns and on straightaways, and can in turn "tune" a car to a certain style of driving. It is important to understand what each of these parameters does to the performance MSXII, such that appropriate values can be chosen to give the desired performance of the car.

Camber

Camber is defined as the angle between the "true" vertical and the centreline of the tire. Positive camber is when the wheel points "away" from the centre of the car, and negative camber is when the wheel points "towards" the centre of the car. Camber primarily affects the performance of a car through a corner, by adjusting the contact area between the tire and the ground, hence controlling the amount of understeer and oversteer the car delivers. Under and over steer is simply the sensitivity of the car to steering inputs: a car oversteers if it turns harder than expected via a turn of the wheel, and understeers if it does not turn as sharply as expected. The larger the area of contact between the wheel and the ground, the more friction generated throughout the turn, which in turn increases the sticking power of the car throughout a corner and makes turning extremely sharply at speed difficult. In general, zero and negative cambers are considered "useful" as they provide the largest area of contact between the tire and the ground, and generate the largest sticking force.


Camber is obviously a dynamic value: the angle can change greatly from the initial "neutral" position when the car is at rest. This poses some complexity to the design of MSXII's suspension: the implementation of front double wishbone suspension coupled with rear trailing arm suspension means each half of the car must have drastically different camber profiles to optimize driver safety and car performance. Typically, dual wishbone front suspensions have very small positive camber to force the car to understeer through a turn, as this is simultaneously the safest and most stable option. A positive camber will result in an inner tire (i.e. the tire closest to the inside of the turn) camber of around zero, and an outer tire camber of around -1° throughout a turn. As both of these values are <0°, this is considered "useful" camber and will provide the greatest sticking power and control throughout turning of the car. Camber values in the range of +1 to +1.5° are common in the car industry - MSXII used +1° of camber which provided good control in dynamic tests.


The rear suspension is much more straight forward, as the suspension can (theoretically) only pivot on a singular plane, with no 3D rotation. Additionally, the trailing arm suspension will not encounter a drastic shift in angle throughout turns and banks, and will more or less be parallel with the side of the car. MSXII used 0 camber in the rear suspension. 


Caster

Caster is the relative angle between the steering axis of the wheel (The centreline of the car coilover or upright) and the true vertical axis. Positive caster is when the steering axis points away from the front of the car, and negative caster is when the steering axis points towards the front of the car. Caster controls the tendency of a car's wheels to naturally return to the "straight" position while in motion, with positive caster tending to straighten wheels in motion. Using the example of a shopping cart, which has a high positive caster on the front wheels, we can see how the forward motion of the cart "pulls" the rear wheels into the straight position. As the cart moves forwards, the steering axis drags the rear wheels, which in turn induces a force pulling the rear wheels into line with the front wheels. Caster also generates another car parameter known as "trail", which is the distance between the point of contact between the ground and the steering axis, and the point directly below the wheel axel. Larger amounts of trail make the suspension more "returnable": the rear wheels will have a much higher tendency to realign themselves with the front wheels. Again, the shopping cart example demonstrates this, as the wheels are very quick to snap back to the aligned position. 


While most cars do not respond as aggressively to caster angles as they do changes in camber, it is still something to consider when optimizing and designing suspension, especially in the case of a car as light as MSXII. The camber change generated by a caster angle is typically desirable (<0°), but care should be taken to avoid over-cambering the wheels while turning. Primarily, the caster angle must be the same on both sides of the car. If these angles are not identical, the car will have a tendency to pull in one direction, forcing the driver to constantly readjust even on straight track. Additionally, too great of a caster angle will increase straightaway stability but drastically increase the understeer of the car: the large angle will make the wheels tend to align themselves aggressively, making turning much more difficult. Typical values of caster are cited at +3° to +5°, decreasing as vehicle weight increases. Due to the relatively low mass of MSXII, a high caster angle of +5° was used.


Toe

Toe is the alignment of the tires relative to each other. Toe-in (rarely called positive toe) is when a pair of tires face towards each other, where as toe-out (or negative toe) is when the tires face away from each other. On the vast majority of cars for all applications, from in-town driving to formula, only the leading tires (i.e. closest to the front of the vehicle) have any toe, where as the back tires are typically parallel to each other. Toe is measured as the angle by which the leading tires are "out of parallel" with each other. Toe is the component of suspension design that impacts the most aspects of a car's performance, affecting the straight line stability, corner entrance/exit and tire wear rate. It should be noted that aggressive amount of toe-in/out can lead to accelerated tire wear and early failure of tires. Toe-in tends to a greater straight line stability, where as toe-out allows for shaper entrances and exits out of turns by generating more oversteer. 


Toe is typically left at zero for most consumer cars, as these cars are driven for often hours every day without being subjected to extreme performance tests like aggressive turning and straightaway sprints. MSXII, however, must negotiate several tests of both control and agility that an average consumer car typically does not experience. Toe-in will allow for MSXII to perform exceptionally well in straight line trials, where as toe-out will allow for sharp cornering in the figure-8 and slalom tests, despite requiring more steering correction in straight line trials. This will require more discussion, as the trade-off of choosing one form of toe over the other may simply be too great to validate a toe angle with our suspension. MSXII had no toe.