The caster 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 center of the upper ball joint to the center of the lower ball joint) and vertical. Car racers sometimes adjust caster angle to optimize their car's handling characteristics in particular driving situations.
Off-road vehicles such as agricultural tractors generally use positive camber. In such vehicles, the positive camber angle helps to achieve a lower steering effort. Also, some single-engined general aviation aircraft that are primarily meant to operate from unimproved surfaces, such as bush planes and cropdusters, have their taildragger gear's main wheels equipped with positive-cambered main wheels to better handle the deflection of the landing gear, as the aircraft settles on rough, unpaved airstrips.
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. British English still predominantly uses the term castor or castor angle.
Some bicycle constructors refer to the angle subtended by the mechanical trail at the wheel centre as the caster .
Front end alignment
When a vehicle's front suspension is aligned, caster is adjusted to achieve a self-centering 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.
Positive caster angle
The pivot points of the steering are angled such that a line drawn through them intersects the road surface slightly ahead of the contact patch of the tire on the pavement. The purpose of this is to provide a degree of self-centering for the steering — the wheel casters around in order to trail behind the axis of steering. This makes a car easier to drive 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 tires are common. Power steering is usually necessary to overcome the jacking effect from the high caster angle.
Trail or trailing
The steering axis (the red dotted line in the diagram above) does not have to pass through the center of the wheel, so the caster can 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 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 of the shopping trolley wheel, 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 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 thepneumatic 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.
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, stating 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."