Editing Caster angle (section)

==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 cart]]s are an extreme case{{snd}} 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 [[wikt:lateral|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&nbsp;mm behind the center of the contact patch.{{Citation needed|date=July 2020}}