Caster angle

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.

[...] 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.

"[4] The steering axis is angled such that a line drawn through it intersects the road surface slightly ahead of the center of the contact patch of the tire on the pavement by a distance called trail.

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 vehicle easier to control and improves its directional stability (reducing its tendency to wander).

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.

θ is the caster angle, the red line is the pivot line, and the grey area is the tire.
Front suspension of a race car ‍ — the caster angle is formed by the line between upper and lower ball joint
An example of a chopper with a raked fork at an extreme caster angle