Caster: tilt angle vs. swivel wheel

In everyday English, a caster is a small, freely rotating wheel attached to the leg of a piece of furniture or equipment. Shopping carts, office chairs, and hand trucks all ride on casters. The word comes from the verb "cast," meaning to throw or rotate, because the wheel can swivel in any direction.

In automotive engineering, caster means something entirely different: it is the angle between the steering axis and a true vertical line, measured in the side view of the vehicle. When the steering axis tilts forward (toward the front of the car), the caster angle is positive. When it tilts backward, the caster is negative. This angle is measured in degrees and is a critical part of wheel alignment geometry, separate from camber and toe.

Positive caster improves directional stability and causes the wheels to self-center after a turn, much like the casters on a shopping cart naturally straighten when you stop pushing. A car with zero caster (steering axis perfectly vertical) would feel twitchy and require constant correction. Too much positive caster can make the steering feel heavy and can cause vibration. Most road cars are set between 2 and 4 degrees of positive caster.

The small furniture wheel and the automotive geometry share a common root in their behavior: both involve how an axis or pivot point is angled relative to vertical. On a shopping cart caster, the swivel pivot sits slightly forward of the contact patch, creating a stabilizing effect. On a car wheel, the steering axis itself leans forward of the wheel's contact patch, producing the same self-centering behavior. The terminology crossed over because the mechanical principle is identical, even though the scale and application are vastly different.