The caster is the tilting of the front steering axis, either forward or backward from the vertical (1). A backward tilt is positive, and a forward tilt is negative. If one wheel has more positive caster than the other, it will cause the wheel to pull toward the center of the vehicle. This will cause the vehicle to move or lead toward the side with least positive caster.

The caster influences the directional control of the steering, but does not affect the tire wear. The weak springs or overload will affect the caster.