Even though the name uses the word “slip”, this characteristic has nothing to do with slipping or skidding.
Slip angle is the angular difference between the direction of the tyres’ contact patch with the road and the direction of the wheel rim. A non-zero slip angle arises because of deformation in the tyres’ carcass and tread. A slip angle results from a combination of tyre sidewall flex and tyre traction.
When a wheel travels in a direction other than the one its contact patch is pointing, then you have a SLIDE angle rather than a slip angle. A car in fact travels in the direction of the contact patch and not the direction of the wheel rim.
To see the affect whilst the car is stationary is simple. Small movements of the steering can be made before traction is broken and the wheel actually turns. This is due to the tyres flexing. As the slip angle increases, more of the contact patch is slipping. Eventually, so little of the contact patch is slipping, that traction is lost and the tyre begins to slide. Slide traction increases linearly virtually until traction is lost, then increases more gently allowing the driver some degree of control before final loss of traction.
Unlike the rear axle, if a large amount of slip you can adjust the steering to keep the car traveling in the desired direction. This situation is known a ‘Understeer‘.
It can be seen in the diagram left that both rear wheels have a large amount of slip and inside front tyre is also generating slip.
Increasing the slip angles at the rear tends make the car looser (oversteer). Likewise, reducing the slip angles at the rear can be interpreted by the driver as making the car feel ‘tighter’.
Increasing the weight transfer towards the front, increases the slip angles on the fornt wheels. This may require a larger steering wheel angle (Understeer). Reducing weight transfer will have the opposite effect.
Items that effect weight transfer and slip are:
- Spring rates
- Spring locations
- Sway bar rate
- Weight distribution
- Brake Balance
- Height of the roll centers
- roll stiffness
- Addition of an anti-roll bar
What else effects slip angles?
- Acceleration and braking (primarily because of weight transfer and toe-angle changes)
- Tyre profile size
- Camber – More camber equals greater camber thrust. Camber thrust attempts to turn your wheel into a turn in the direction of lean. Therefore more camber thrust gives smaller slip angles.
As passenger cars are often only exposed to low lateral accelerations, Ackermann steering is set-up to ensure that all wheels roll freely with no slip angles because the wheels are steered to track a common turn centre.
Race cars can operate at high lateral accelerations and therefore all the tyres operate at significant slip angles and the loads on the inside wheels are much less than the outside wheels due to the lateral load transfer.
Race cars use Parallel or even Reverse Ackermann, as low speed steering geometry would cause the inside tyre to be dragged along at much higher slip angles than needed. This would result in raised tyre temperatures, slowing the car down due to slip induced drag.