Caster is an angular displacement of the vertical axis of the suspension of a steered wheel in a vehicle, measured in the longitudinal direction. It is usually expressed in degrees.

Positive the caster increases stability, as well as steering ease and comfort while cornering. Negative caster reduces high-speed stability and cornering camber recovery and increases tire wear, as well as steering effort.

Changing a vehicle’s caster angle involves adjusting the upper and lower pivot points of each front and rear wheel so that the line created by those two points lines up with the steering axis from the center of each wheel. This is done by a wheel alignment technician with specialized equipment and skills.

The caster angle is one of the primary wheel alignment angles. Other important angles include camber and toe, but caster is the most common.

The Science of Smooth: Understanding the Mechanics of Coaster Wheels

To determine a caster angle, an experienced alignment technician will need to turn the wheels of the car about 15 degrees in each direction to measure a horizontal line that extends from the center of the wheel to the center of the front and back turning wheels. The caster angle is then compared to the vehicle’s specifications for proper directional steering.

Most common road cars have a positive caster angle value between +3 and +5 degrees, which helps with high-speed stability and steering ease while cornering. Increasing a positive caster angle can also improve steering wheel returnability and decrease tire wear, as well as steering effort and road shocks.