Caster
Caster is the fore-and-aft inclination of the steering axis when viewed from the side of the vehicle.
What the angle influences
With positive caster, the upper steering-axis point lies behind the lower point in the common convention. Together with mechanical trail, tire forces, steering-axis geometry, and steering assistance, caster contributes to steering self-alignment, effort, feedback, and camber change as the wheels turn.
Caster angle is not the same as trail, and a larger angle does not by itself guarantee better stability. The steering axis may be defined by physical joints or a virtual intersection of suspension links. Ride height, wheel travel, compliance, and tolerances can alter the measured value.
Specifications must state side, sign convention, steering position, ride height, and load condition. Cross-caster—the left-to-right difference—can affect directional behavior even when each individual value is within a broad range.
For steering-axis motion and its interaction with wheel travel, see Suspension Kinematics and Vehicle Dynamics.