What is Caster in wheel alignment?
In a wheel alignment process, the caster is the tilting of the uppermost point of the steering axis either forward or backward (when viewed from the side of the vehicle). Caster determines how to control the direction of the vehicle from the steering but does not affect the tire wear. Also, excessive load on a vehicle or a weak rear spring will affect the caster.
Difference between positive (+) caster & negative (-) caster
(Case A) When the rear of the vehicle is lower than its designated trim height, the front suspension moves to a more positive caster.
(Case B) If the rear of the vehicle is higher than its designated trim height, the front suspension moves to a less positive caster.
How does it matter
(Case A) Withmore positive caster, the front of the vehicle will be higher, which will make the wheels pull toward the center of the vehicle. This condition will affect the stability of the vehicle, causing it to pull or lead to the side with the amount of positive caster.
(Case B) With too little positive caster, the front of the vehicle will be lower, which will make the steering difficult at high speed and wheel return ability will be diminished when coming out of a turn. You will also notice vibrations in your steering wheel.
In such circumstance, you can either adjust the caster by undergoing a wheel alignment or your mechanic will have to check the axle of your car to understand why the rear of the car is getting low or high