Steering Axis Inclination

Toe-Out On Turns

Tracking

Caster

Caster (fig. 8-48) is the steering angle that uses the weight and momentum of the vehicles chassis to lead the front wheels in a straight path. Caster is the backward or forward tilt of the steering axis that tends to stabilize steering in a straight direction by placing the weight of the vehicle either ahead or behind the area of tire-to-road contact.

Caster controls where the tire touches the road in relation to an imaginary center line drawn through the spindle support. It is NOT a tire wear angle. The basic purposes for caster are as follows:

To aid directional control of the vehicle

To cause the wheels to return to the straight-ahead position

To offset road crown pull (steering wheel pull caused by the slope of the road surface)

Caster is measured in DEGREES starting at the true vertical (plumb line). Manufacturers give specifications for caster as a specific number of degrees positive or negative. Typically, specifications list more positive caster for vehicles with power steering and more negative caster for vehicles with manual steering (to ease steering effort). Depending upon the vehicle manufacturer and type of suspension,

Figure 8-48. - Caster angle.