WHAT CASTER DOES Caster primarily affects steering stability, effort and return. Tilting the steering axis fore or aft causes the wheel spindle to travel in an arc causing the suspension to lift as the wheels are steered. Negative caster is undesirable because it creates an unstable condition that can result in steering instability and wander. Positive caster is beneficial because it aids steering return and helps the wheels self-center and run straight. But is also increases steering effort slightly. The higher the caster angle, the greater the steering effort. For these reasons, most vehicles specify only about 1 to 4 degrees of positive caster. Some European luxury cars specify up to 8 or 9 degrees of caster to provide extra steering stability at high speeds.
HOW CASTER CHANGES Like camber, the caster angle can change as the vehicle is loaded. Raising or lowering the rear of a vehicle can change the caster angle. A one inch change in rear ride height can change caster up to a full degree, depending on the length of the vehicle. Raising the rear of a vehicle decreases caster, which reduces steering effort but may increase steering instability. Lowering the rear of the vehicle (which occurs when the vehicle's trunk or cargo area is loaded), increases caster, which improves steering stability and return but may also increase steering effort. To eliminate such changes, some luxury cars have automatic load leveling systems with air shocks in the rear.
HOW CASTER AFFECTS CAMBER & TIRE WEAR Though caster is not considered a tire wear angle, it does affect camber and can contribute to shoulder wear under certain circumstances. Caster affects camber because of the backward tilt of the steering axis. As long as a vehicle is traveling straight ahead, caster has no effect on camber. But when the wheels are turned, caster forces the spindles to follow a slightly curved path rather than a flat trajectory. This causes the wheels and tires to lean slightly as they are steered, a condition known as "camber roll."
CAMBER ROLL Camber roll actually has a beneficial effect on cornering because it causes the tires to lean into a turn. Turning to the left, for example, causes the left spindle to angle down as it pivots and the right spindle to angle up. This adds positive camber to the left wheel and negative camber to the right. This helps the wheel roll towards the direction it is turning for improved handling and traction. Page