Caster Angle

In the side view the inclination of the Steering Axis (also known as kingpin axis) is called ‘Caster Angle’.

If the ‘Steering Axis’ doesn’t pass through the centre of the wheel then there is a side view ‘Steering Axis Offset’ present. The distance from the steering axis to the centre of the tyre footprint is called ‘Trail’ or ‘Caster offset’.

Caster angle and trail are of importance when designing the suspension geometry.

The effects to consider are :

  • As trail is increased more steering torque or effort is needed.
  • Caster angle will cause the wheel to rise and fall with steer. This effect is opposite from side to side and causes roll and weight transfer. Leading to an over-steering effect.
  • Caster angle has a positive effect on steer-camber. With positive caster angle the outside wheel will camber in a negative direction and the inner wheel in a positive direction, causing both wheels to lean into the turn.
  • The size of the mechanical trail due to caster may not be too large compared to the Pneumatic trail from the tyre. The pneumatic trail will approach zero as the tyres reaches the slip limit. This will result in lowering the self-centering torque that is present due to the lever arm between the tyres rotation point at the ground and the point of attack for the lateral force. This will be a signal to the driver that the tyre is near breakaway. This “breakaway signal” may be lost if the mechanical trail is large compared to the pneumatic trail.

image: Caster Angle

Caster is measured in degrees by using a caster camber gauge. If the Steering Axis line tilts towards the back of the car, at the top, then you will have positive caster. If the line tilts forward then you would have negative caster.

  • Positive caster provides the directional stability.
  • Too much positive caster will make the steering effort difficult, however power steering will allow you to run more.
  • Negative caster requires less steering effort but can cause the car to wander when driving in straight lines.
  • The caster angle has positive effects during cornering but too much caster causes weight transfer that will have an over-steering effect.
  • The possibility to adjust the camber angle from 0° to about -4° will be very helpful during the testing of the car.
  • Setting the camber to 0° minimizing the rolling resistance during acceleration.
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