Caster angle is the angular measure from the vertical of the suspension of a steered wheel in a car or other vehicle, measured in the longitudinal direction. It is to the angle between the pivot line (an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to take optimize car performance in particular driving situations.

The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact point of the wheel. The purpose of this is to provide a degree of self-centering for the steering - the wheel castors around so as to trail behind the axis of steering. This makes a car easier to drive and improves its straight line stability (tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although, in racing, large caster angles are used to improve camber gain in cornering. Caster angles over 10 degrees with radial tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.

Note that the steering axis (the dotted line in the diagram below) does not have to pass through the center of the wheel, this allows the Caster to be set independently of the mechanical trail, which is the distance between where the steering axis hits the ground, in side view, and the point directly below the axle. The interaction between castor angle and trail is complex, roughly speaking they both aid steering, caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case of the shopping wheel trolley the system is undamped, but stable, as the wheel oscillates around the 'correct' path. The shopping trolley setup has a great deal of trail, but no caster. Complicating this still further is that the lateral forces at the tire do not act at the center of the contact patch, but at a distance behind the nominal contact patch. This distance is called the pneumatic trail and varies with speed, load, steer angle, surface, tire type, tire pressure and time. A good starting point for this is 30 mm behind the nominal contact patch.

Compare to: Camber angle, toe (automotive)