Wheel Geometry: Caster
Caster is the angle at which the swivel pin axis (C) deviates from the vertical (A) when viewed from the side, It is given in degrees.
When the swivel axis is inclined rearwards, as shown, caster is said to be positive (+) and when inclined forwards it is said to be negative (-). The Saab 9-5 has positive caster.
Caster tends to keep the wheels pointing in the straight-ahead position and so helps the driver to steer a straight path. In addition, more powerful steering wheel return action is obtained with a greater caster angle.
Caster is not adjustable.