The caster angle or castor angle is the angular displacement of the steering axis from the vertical axis of a steered wheel in a car, motorcycle, bicycle or other vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car an imaginary line that runs through the centre of the upper ball joint to the centre of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimise their car's handling characteristics in particular driving situations.
Arthur Krebs proposed placing the front axle of a car at a positive caster angle in his UK patent of 1896, entitled Improvements in mechanically propelled vehicles.. In it he stated it was intended "To ensure stability of direction by means of a special arrangement of fore-carriage, that is to say, to re-establish automatically the parallelism of the two axles of the vehicle when there is no tendency to keep them in any other direction, or after a temporary effort has caused them to diverge from said parallelism. [...] The axle of the fore-carriage is situated a suitable distance behind the projection of the axis of the pivot-pin in order to ensure the stability of direction above referred to."
The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the center of the contact patch of the tyre on the pavement by a distance called trail. The purpose of this is to provide a degree of self-centring for the steering -- the wheel casters around in order to trail behind the axis of steering. This makes a vehicle easier to control and improves its directional stability (reducing its tendency to wander). Excessive caster angle will make the steering heavier and less responsive, although in racing large caster angles are used for improving camber gain in cornering. Caster angles over 7 degrees with radial tyres are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.
The steering axis (the red dotted line in the diagram above) does not have to pass through the centre of the wheel, so the caster can be set independently of the 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 caster angle and trail is complex, but roughly speaking they both aid steering: caster tends to add damping, while trail adds 'feel', and returnability. In the extreme case, such as the caster wheel on a shopping trolley, the system is undamped but stable, as the wheel oscillates around the 'correct' path. The shopping trolley/cart setup has a great deal of trail, but (somewhat confusingly) no caster. Complicating this still further is that the lateral forces at the tyre do not act at the centre 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, tyre type, tyre pressure and time. A good starting point for this is 30 mm behind the nominal contact patch.
When the front suspension of a vehicle is aligned, caster is adjusted to achieve a self-centring action in the steering, which affects the vehicle's straight-line stability. Improper caster settings will require the driver to move the steering wheel both into and out of each turn, making it difficult to maintain a straight line.
In the context of bicycles and motorcycles, caster is more commonly referred to as "head angle", "rake angle" or "rake and trail", especially in American English. The terms caster or castor angle are still predominantly used in British English.
Some bicycle constructors refer to the angle subtended by the mechanical trail at the wheel centre as the caster .
caster n : the slight usually backward tilt from vertical of the axis of the steering mechanism of an automobile for giving directional stability to the front wheels
castor angle, the angle at which the steering-head of the front wheels of a motor vehicle is set