caster

Caster is the forward or backward tilt of the steering axis relative to vertical, measured in degrees. When you turn the steering wheel, the front wheels pivot around this tilted axis rather than a truly vertical line. A positive caster angle tilts the steering axis backward, so the contact patch of the tire trails behind the pivot point. This geometry is what makes your steering wheel return to center after a turn and what keeps the car stable at highway speed.

The steering axis is an imaginary line that runs through the upper and lower ball joints (or equivalent suspension pivot points). In most cars, this axis is tilted 5 to 10 degrees toward the rear of the vehicle. This tilt is built into the suspension geometry during design; it is not the same as camber (which tilts the wheel itself) or toe (which angles the wheels left and right). The caster angle is typically greatest on the outside wheel during a turn, which is why outer wheels feel more responsive to steering input.

Excessive caster causes heavy steering effort and can make the car feel nervous at speed, while insufficient caster reduces directional stability and self-centering. After a collision, front-end work, or suspension replacement, caster angles often drift and must be reset during alignment. Caster is measured with the same computerized alignment rig used for camber and toe, but it requires the wheel to be turned through its full range to calculate properly. Most shops cannot adjust caster without exchanging shims in the suspension or bending suspension arms.

The term derives from the caster wheels found on furniture and shopping carts: a wheel mounted on a swiveling pivot. A furniture caster with a long trailing offset (from pivot to ground contact) naturally swings into line with direction of motion and resists side-to-side wandering. The automotive steering axis caster angle produces the same stabilizing effect, which is why engineers adopted the name over a century ago.