Caster

Caster is an alignment angle that measures the forward or backward tilt of the steering axis (the pivot line of the wheel assembly) when viewed from the side of the vehicle. It is measured in degrees and plays a critical role in steering stability, self-centering, and tracking.

Positive caster occurs when the steering axis tilts toward the rear of the vehicle. This is standard on most passenger vehicles because it creates a caster trail that pulls the wheels straight when moving, providing stable high-speed tracking and returning the steering wheel to center after a turn. Negative caster occurs when the steering axis tilts toward the front of the vehicle, which makes steering light but can cause steering wander or instability at speed.

Symptoms of incorrect caster include the vehicle pulling to one side (usually toward the side with less positive caster), heavy steering, or a steering wheel that doesn't return to the center position. Caster does not typically cause tire wear on its own, but it heavily impacts steering feel and stability.

Caster angles are adjusted during a wheel alignment by adding or removing shims, adjusting strut rod bushings, or using adjustable upper control arms. Bent struts or damaged spindles are common mechanical causes of caster misalignment.