steering is usually necessary to overcome the jacking effect from the high caster angle. Some front-end alignment calls for different right-side and left-side caster. This is called cross caster, and the difference is called the spread. Cross camber may also be specified, but not usually both. 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 trail, which is the distance between where the steering axis intersects the ground, in side view, and the point directly below the axle. Caster angle and trail both influence the steering, albeit in different ways: caster tends to add damping, while trail adds "feel" and returnability. The caster wheel on shopping carts are an extreme case the system is undamped but stable, as the wheel oscillates around the "correct" path. The construction has relatively high trail, but no caster, which allows changing of direction with minimal force. In this case the lateral forces at the tire do not act at the center of the contact patch, but at a point behind the center. 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 center of the contact patch. Front-end alignment When the front suspension of a vehicle 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. Two-wheeled vehicles 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 center as caster. See also Bicycle and motorcycle dynamics Camber angle Toe (automotive) Trail Vehicle dynamics Caster References Automotive steering technologies