Wheel Alignment Specifications & Procedures: Adjustment: Caster

Measure frame angle directly behind cab using a bubble protractor or inclinometer. Determine existing caster. Determine difference between frame angle and caster angle. A down-in-rear, frame angle must be added to a positive caster angle.
An up-in-rear frame angle must be subtracted from a positive caster specification. A down-in-rear frame angle must be subtracted from a negative caster specification.
An up-in-rear frame angle must be added to a negative caster specification. Measure distance from top of jounce bumper bracket on lower control arm to bottom of frame crossmember.
To increase caster, add shims between forward upper control arm retaining bolt and frame, or subtract shims from rear retaining bolt. To decrease caster, subtract shims from forward bolt, or add shims to rear bolt.