Glossary
What is Caster? Definition, Typical Angles & Handling Effects
Caster is the angle of the steering axis viewed from the side of the car, positive when the top of the axis leans towards the rear. Positive caster adds negative camber to the outside wheel as the steering is turned, improves straight-line stability and gives heavier, more communicative steering. Race cars typically run +4° to +8°, with speedway cars using a caster split to help the car turn left.
| Setting | Steering feel | Camber gain in corners | When to use |
|---|---|---|---|
| +2° to +4° | Light | Low | Cars without power steering, tight street circuits |
| +4° to +6° | Medium | Moderate | Most touring and GT cars |
| +6° to +8°+ | Heavy | High | Open-wheelers and cars with limited static camber adjustment |
| Split (LF +2°, RF +5°) | Pulls left | Asymmetric | Speedway and oval racing |
How caster works
Because the steering axis is tilted, turning the wheel tips the outside tyre into negative camber and the inside tyre into positive camber. That camber gain only appears when steering lock is applied, so caster lets you run less static negative camber and keep a bigger contact patch for braking, while still having the camber you need mid-corner.
The trade-off is steering effort and a tendency for the car to jack weight diagonally as lock is applied, which on some cars upsets corner entry.
Adjusting caster
Caster is normally set by moving the upper ball joint forward or back with shims, offset bushes or adjustable radius rods. On strut cars it is set with caster plates at the strut top. Always re-check camber and toe afterwards.
Frequently asked questions
Does caster affect tyre wear?
Only indirectly. Caster itself does not scrub the tyre on the straights the way toe does, but the camber gain it produces changes how the tyre loads mid-corner, which shows in temperature spread.
Can caster cause understeer?
Too little caster can, because the outside front tyre runs out of camber in long corners. Too much can also feel like understeer if the steering becomes so heavy that the driver under-steers the car.
What is caster split?
Running different caster angles left and right. The car pulls towards the side with less caster, which is used on ovals so the car naturally wants to turn left.
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