How to Measure
The first stage of adjusting and tuning your caster angle is to first measure and understand what your vehicle currently has. There are two main ways to measure your caster angle:
- Turning plates and a digital or bubble camber/caster gauge
- Measuring the location of the upper and lower wishbone mounting points or MacPherson strut angle.
The first technique is the quickest and fastest way to measure caster and can also be done trackside which is especially useful if you are tuning your caster on a track day or whilst racing. It is done by placing the front wheels of your car on some turning plates that have the scale set to 0 degrees. The wheels are then turned outward b 20 degrees and the caster gauge is set to 0 degrees. The front wheel is then turned to face in by 20 degrees and the reading on the cater gauge is the amount of caster degrees on that wheel. This can then be repeated for the other wheel.
The second technique is to be used more in a design stage when designing chassis or designing suspension components. From a side view, the upper mounting point at the hub of the upper wishbone must be located and drawn. Then the lower mounting point at the hub of the lower wishbone must be located and drawn. Finally the vertical centre line of the wheel must be drawn. All of these points only need to be drawn in 2D from the side view. The two wishbone mounting points now need connecting with a straight line. The angle between the two lines is the caster angle of that wheel.
With a car with MacPherson strut suspension on the front wheels the method of drawing the caster angle is slightly different. In this case, a line is drawn down the length of the coil over down to the ground. A second line is then drawn vertically down the centre line of the wheel. Again, this is done in 2D from the side view of the car. The angle between these two lines is the caster angle of that wheel.
How to Adjust
There a few ways that caster can be adjusted on car. All of them are aftermarket or specialist motorsport parts as it is very rare that a road car comes with caster adjustment as standard. Some of the most popular ways of adjusting caster are:
- Adjustable Arms
One way to adjust caster is with an adjustable arm. In the case of a MacPherson strut system this can be in the form of an adjustable tension rod that holds the lower arm forwards or backwards in the wheel arch. The screw can be adjusted to move the wheel forwards or backwards adjusting the caster angle.
- Offset bushes
One common way to adjust caster is to install an offset caster bush. This bush mounts the back or front of the lower wishbone to the car and has the centre bolt hole offset in a forward direction. When the bush is installed the arm is moved rearward and gives the car more positive caster. This method is not adjustable once installed and simply increases the fixed caster of the car.
- Offset top mounts
Another way to adjust the caster of a car with a MacPherson strut set up is to install offset top mounts on the coil over. These are often sold with the main purpose to be camber adjustable. Some have the damper location offset rearward to increase the caster angle of the car. Once again this position is fixed as is not adjustable once installed.
- Adjustable Hubs
In high end motorsport where the suspension components are custom made and are designed to be adjustable the use of shim packs is often used on the hubs. Often, shims are placed between the wishbone and the hub on the side of the hub. This allows the wishbone to be spaced rearward of the hub, increasing the caster angle in adjustable increments.
Effects of Adjustment
The effects of increasing the positive caster of your car have multiple benefits that can reduce issues that are being fed back from drivers about handling. The improvements that can be made are:
- Increasing the steering feel of the car giving the driver more feedback through the steering wheel. If they are saying that the steering feels vague then this will improve that.
- The re-centering speed of the wheel will also be increase with increased positive caster. Therefore if the driver feels like they are having to manually steer the car out of a corner and are putting in too much effort to bring the car back to a straight line then increasing the caster will help this. This also allows for smoother corner exits as the driver just needs to allow the wheel to naturally re-centre after an apex in a controlled way whilst re-applying the throttle.
- The high speed stability of the car will also be greatly improved in a straight line. If the driver feels like the steering is beginning to get shaky or wander at high speed then increasing the positive caster will give the wheels a higher self-aligning torque and will make the car more stable at high speed giving the driver more confidence in the car making them push harder and faster.
- Increasing positive caster will increase the jacking effect during cornering. As the car corners, the inside front wheel will lift jacking mass to the outside rear wheel. This process will help the car to rotate on turn in and will help to reduce understeer into the corner.
Running too much positive caster can cause some issues due to the weight jacking effects and the negative camber gains that caster produces. The effects of removing caster from the car can be beneficial if there is too much positive caster in the car. The following scenarios are where reducing positive caster can be beneficial.
- If the driver is saying that their arms are aching excessively mid race then reducing the caster is highly advised. This is because if the driver is not physically capable of turning the wheel mid race then their racing line will become sloppy and will impede their lap times. Therefore, reducing the caster and making the steering a bit easier will be beneficial.
- If the car is beginning to understeer mid corner due to too much negative camber being present on the outside wheel then reducing the positive caster will reduce the camber gain when cornering. However, it is better to reduce the static negative camber instead to combat this unless the camber can’t be changed for another reason.
- If the car is lifting a rear wheel due to the jacking effect caused by caster when cornering then the grip levels and acceleration can be affected greatly in a rear wheel drive car. It can also cause oversteer. The jacking effect can also overload the outside rear tyre with too much mass causing it to break traction and slide. Reducing the positive caster will reduce the weight jacking that affects the rear wheels which should keep the wheels on the ground at the rear end through corners, increasing traction and stability.
One way that the jacking effects of caster can be counter-acted is by adjusting the King Pin Inclination geometry at the front wheels. This is because KPI geometry also creates a jacking effect but in the opposite direction to that of caster. Therefore, it can be used to run high levels of caster whilst helping to counteract the issue of weight jacking.
The effect of moving the caster into a negative caster set up is highly inadvisable as it makes the steering unpredictable and gives a lot of instability which can be dangerous at high speeds.