Although the dynamics of modern vehicles are now strongly influenced by electronic control systems, there are still purely mechanical parameters that determine driving behavior in a very impactful way. One of these is the caster angle.
Definition of caster angle
For a vehicle with MacPherson type suspension geometry, from a side view, a line can be drawn down to the ground along the largest dimension of the spring-shock assembly. The angle this line makes with the vertical is the caster angle.
In a double wishbone suspension (see figure below), on the other hand, from a side view it is possible to draw a line passing through the attachments of the upper and lower triangles to the wheel hub. Again, the angle this line makes with the vertical is the caster angle.
Caster angle and driving feeling
The first purpose of the caster angle is to give stability to the steering wheels, which would otherwise be unmanageable. Since the center of the contact area is some distance away from the steering axis extension, a realignment moment (torque) is generated every time an attempt is made to deflect the wheel from straight travel. This self-aligning moment is felt every time the steering wheel is turned, noting a certain opposition of the same, which tends to return to the “center”, that is to the zero angle. If the angle is small, the realignment will be slow and bland, vice versa with higher angles there is a more marked return of the steering and a greater hardness of the same.
In common road cars the caster angle value is usually between +3 and +5 degrees and is usually more pronounced on rear-wheel drive cars. In fact, on those with front-wheel drive and all-wheel drive it tends to be more contained, because there is already a self-aligning effect caused by the traction, which produces a forward displacement of the center of the contact area of the tire, which generates a moment which tends to bring the wheels parallel to the direction of travel. These cars have a low positive caster angle as the steering needs to be light and manageable and to reduce vibration. In sports and competition cars, the caster angle can assume much greater values (for example between +12 and +13 degrees).
Why can a caster angle be good for one car but may not be ideal for another? On Assetto Corsa Competizione we can in fact find cars like the Porsche 911 GT3-R that require a low caster angle, equal to +6.7 degrees and other cars that require a higher angle, such as the Ferrari 488 GT3 Evo and the Mercedes AMG GT3 Evo, greater than +12 degrees, and then there are cars like BMW with a caster angle of about +9.4 degrees.
To explain this, we introduce another parameter: the mechanical trail.
The caster angle in turn determines another important parameter, namely mechanical trail. Still looking at the vehicle from the side, the mechanical trail is the distance between the center of the contact area and the point of junction between the ground and the steering axis.
This parameter is influenced not only by the caster angle, but also by the geometry of the suspension and with it by the position of the steering axis. Therefore, with the same caster angle, it is possible to have different values of the mechanical trail, as shown in the examples “a”, “b” and “c” of the image below where, with a caster angle of approximately +18 degrees, we have three different values of the mechanical trail (X 1 ≠ X 2 ≠ X 3).
Remember: other factors that affect the mechanical trail are tire and rim size, inflation pressure and tire wear.
The mechanical trail therefore assumes a significant importance since its length actually determines the magnitude of the self-aligning moment: a shorter length of this parameter will correspond to a slower and weaker realignment, vice versa with a more extended arm there will be a more energetic return.
Dynamic cornering behavior
When cornering, the caster takes on even more importance, because there is a variation in the magnitude of the camber angle of the steering wheels. More precisely, the outer wheel will tend towards a more negative camber (the minus sign is when the top of the wheel tilts towards the center of the car), while the inner wheel will tend towards a positive camber. This happens in a proportional way, i.e. the greater the caster angle, the greater the effect of the variation of the camber in the curve (the camber obtained in the curve is called dynamic camber).
The effect is called “camber recovery”, that is, the suspension’s ability to increase camber when cornering, preventing the outer tire from moving its contact area towards the outside of the tread, thus decreasing its contact footprint and hence the grip.
Summary of the effects of caster adjustment
Increasing the caster angle has multiple advantages that can mitigate the problems encountered on the track. The improvements that can occur are:
- increased steering feel of the car, with more feedback via the steering wheel.
- the speed of re-centering of the steering wheel will also increase with the increment of the positive caster. So, if we feel like we need to manually realign the steering wheel when exiting a bend and that we are trying too hard to get the car back in a straight line, the increased caster will help.
- the car’s high-speed stability on straight lines will also benefit. If we feel that the steering is starting to wobble at high speed, the increase in positive caster will give the wheels higher self-aligning torque and make the car more stable at high speed.
- finally, a positive caster angle allows immediate and precise insertion as soon as you steer. This is due to the fact that there is a greater difference in height between the front wheels, ie lifting of the internal wheel and lowering of the external one; therefore, due to the load transfer, the external side of the vehicle is more loaded than the internal one (the internal rear wheel tends to lift). This provides a contribution of self-steering torque which aids in cornering.
However, a setting with too much positive caster angle can also cause problems:
- especially in long duration races, if our arms begin to get tired, it is highly recommended to reduce the positive impact thus making the steering lighter.
- if the car starts to understeer mid-corner due to excessive negative camber on the outer wheel, lowering the positive caster will result in a reduction in cornering camber recovery.
Resolving handling issues associated with caster modifications
Below are summarized the main adjustments of caster angle for solving the most common handling problems of the car.
Understeer
- curve input: increases the caster.
- curved travel: increases the caster.
- curve exit: increases the caster.
Instability when cornering
- curve input: increases the caster.
- curved travel: increases the caster.
- curve exit: increases the caster.
Tire overheating
- reduce caster.
Overly sensitive steering
- increases the caster.
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