See Figure 1
Caster is the tilt of the axis that the strut rotates about. The axis, called the king pin inclination, allow the tires to be pushed as opposed to being dragged by the car. Caster allows the wheels to self-center and return to the straight ahead position after the wheels have been turned. Excessive caster will numb the steering response of the vehicle.
Caster is not adjustable. If the caster measurement is not correct, a suspension part is bent or broken, or the body has been damaged.
KING PIN INCLINATION
See Figure 2
The king pin is an imaginary line drawn from the top of the strut bearing to the ball joint at the base of the strut. This is the axis that the strut rotates about. The angle at which it makes with true vertical is the inclination.
The king pin inclination allows for road shock transmission to the steering wheel to be reduced, lower the amount of effort required to turn the wheels and also provides self returning force to straighten the vehicle after a turn. King pin inclination in addition to camber determines the contact patch of the tire on the ground.
King pin inclination is not adjustable and usually is not measured.
See Figure 3
Camber is the inclination of the wheel to true vertical. Camber allows the tire to place a larger contact patch on the ground as the vehicle rolls. Camber can be negative with the top of the wheel tilted in towards the vehicle or positive with the top of the wheel tilted out. Excessive negative camber will wear the tire on the inside edge and on the outside edge if the camber is too far positive.
High performance and racing tires tend to need more negative camber then a standard street tire. A race prepared vehicle may have up to 3 degrees negative camber to compensate for roll. In a high performance application, the camber should be adjusted as many degrees negative as there are degrees body roll. This will cause the tire to be straight up and down at maximum roll and have a tire contact patch that is flat to the ground. If these levels of camber where used on a street car, the tires would wear the inside edges of the front tires in a very short period of time, plus exhibit twitchy steering response.
Camber is adjusted by moving the top of the strut mount in or out as needed. There are offset strut mounts available to change the camber.
See Figure 4
Toe is measured with the wheels in the straight ahead position and is the difference between distances between the front of the wheels and at the rear of the wheels. Toe is considered toe-in if the tires are closer at the front of the wheels than at the back of the wheels. Toe-out is when the wheels are closer at the front than at the rear. Toe-in allow stability and toe-out causes a very sharp and unpredictable turn-in response if it is excessive.
Toe is adjusted by changing the length of the tie rods. The tie rods are adjusted evenly with the steering box centered.