Caster is the angle from the top suspension pivot point to the lower ball joint, i.e.; the line around which the suspension pivots. Caster is a functional angle in that the higher it is, the more stable the steering while turning. High caster also make the steering “want” to return to center after turning. Vans have a caster angle of 7.25 degrees, most cars are 1 1/2 to 3 degrees. The higher angle is needed to stability on a top heavy car. The further the steering is turned, the more the tires lean into a curve. High caster is the main reason Vanagons handle so much better than the earlier Type 2 Buses. High caster leads to inner and outer edge wear on the front tires, so tires need to be rotated at proper intervals.
Camber is the angle of the tire in and out at the top versus the bottom. Camber functions for side to side stability. High camber corners better, but wears the inside edge of the tire. Toe, in alignment terms, is the in and out angle of the front of a tire in relation to the rear of the tire. On most cars, the tires toe in at the front. This assists the vehicle to stay straight on the road. If the toe gets too high, the tires scuff as you drive, and rapid tire wear will result.
When an alignment is done, the caster must be checked first. Caster, camber, toe: that is the order of things. Adjusting caster changes the other two settings. Adjusting camber changes toe but not caster. Adjusting toe changes only toe, caster and camber are unaffected. Camber and toe are termed Live Adjustments, meaning that as one changes the alignment adjustments on the car, the corresponding number changes on the alignment machine’s screen. Caster is done mathematically by the machine. It requires a 5 minute check cycle, turning the tires in and out to different positions and waiting for the machine to signal for the next position. Caster is calculated by measuring the amount of camber when a tire is turned and comparing that to the camber measurement with the tire straight ahead. At the end of a cycle, a caster figure is generated. Each time the adjusters get moved, the cycle must be repeated to check the caster. Takes a lot of time. Most cars don’t have adjustable caster. Some mechanics don’t feel inclined to adjust caster.
Some cars don’t have alignment adjustments in the rear. However many of the modern cars and all of the vans that we work on have said adjustments in the rear, for camber and toe. Since the rear wheels don’t steer, there is no caster adjustment. If you want your Mark VI or later Volkswagen, your Eurovan or Vanagon, or any Audi to track correctly, the rear adjustments need to be checked and reset with every other set of tires, or after any large pothole, road hazard or accident.