The rearward or forward tilt of the steering axis when viewed from the side of the vehicle, affecting straight-line stability, steering return-to-center, and high-speed handling.
Caster is one of the three primary wheel alignment angles, alongside camber and toe. It describes the tilt of the steering axis (the line through the upper and lower ball joints or strut axis) when viewed from the side. Positive caster means the steering axis tilts backward at the top (the contact patch trails behind the steering pivot point), which is the standard setup for virtually all modern vehicles. Negative caster tilts forward at the top and is rarely desirable.
Positive caster creates a self-centering effect because the tire contact patch trails the steering pivot, creating a mechanical tendency to return to straight-ahead. More positive caster increases straight-line stability and return-to-center feel, which is why performance and track cars often run more caster than the factory baseline. The trade-off is increased steering effort (more caster makes steering heavier).
Caster is not adjustable on most stock vehicles. Lowering a car can alter effective caster depending on suspension geometry. Aftermarket upper control arms or strut mounts on some platforms allow caster adjustment. Track-day and autocross competitors often maximize caster within the available range for stability and feel benefits. Caster imbalance between left and right wheels causes the car to pull to one side.