Caster Definition
Caster can be defined as the forward or rearward tilt of the projected steering axis from true vertical, as viewed from the side. This line is formed by extending a line through the upper and lower steering knuckle pivot points. For vehicles with front control arms, visualize the line extending through the upper and lower ball joints. On strut equipped vehicles, the line extends through the lower ball joint to the center of the upper strut mount. Caster is always viewed from the side of the vehicle.When the upper pivot point is rearward of the lower pivot point, caster is positive. If the upper pivot is forward of the lower pivot point, caster is negative. When the two points are straight up and down from each other, the caster is zero. A maximum side to side variation of ±.5° is recommended on most vehicles. Caster is NOT a normal tire wearing angle and is used as a directional control for stability and steering returnability.


Front of Vehicle Caster Effect

Caster effect is necessary so that the load of the vehicle is "carried" through the steering axis line formed on the upper and lower pivot points. Positive caster gives a vehicle directional stability because the tire is being pulled along by the load which is projected in front of the center of the tire contact area. This causes a vehicle with positive caster (point of load ahead of the point of contact) to be harder to steer away from the straight ahead position. With Positive caster, road surface variations have a minimal effect on the tire, the tire will continue to go straight. When a tire has a Negative caster condition, where the projected steering axis point of load is behind the tire point of contact, a vehicle will have a tendency to be easier to steer but will lack directional stability. A vehicle with negative caster is affected by any road surface variation such as small road irregularities or bumps. With the point of load pushing the tire along (negative caster), any bumps or road irregularities which are encountered have a tendency to immediately affect directional stability and vehicle handling.


Front Caster Effects
Effects of Positive Caster
Vehicles usually have some positive caster specified since this promotes directional stability, however, excessive positive caster can cause two problems. The first is that excessive caster will cause a high level of road shock to be transmitted to the driver when the vehicle hits a bump, etc. The second problem is that a tire/wheel assembly with positive caster has a tendency to toe inward when the vehicle is being driven. If one side has more positive caster than the other, this causes it to toe inward with more force than the other side. This will cause a lead or pull to the side with least amount of positive caster.

Effects of Caster on Tire Wear
When set with a substantial amount of caster, the spindle travels in a vertical arc, causing it to move up and down and raise and lower the wheels as the steering wheel is turned. Because of this, camber changes occur. With a high amount of positive caster, the camber changes that occur, especially at low speeds in tight turns, cause the tires to show wear on their shoulders. In high speed cornering, the vehicle tends to continue straight ahead when the steering is initially turned. Due to this, and the amount of camber change that takes place when a spindle travels through its arc of travel, the shoulders of the tires on a vehicle may scrub and wear. When a left turn is made at a fairly high rate of speed with a vehicle which has positive caster, the caster of the left front wheel changes toward positive but the momentum of the vehicle is in a straight ahead direction. This causes the inside of the left front tire to scrub as it is turned. Just the opposite effect takes place on the right wheel as the vehicle is turned left at high speed. The right front wheel's camber will go negative but the outside edge of the tire is scrubbed because of the vehicle's momentum to go straight. On some vehicles setting caster more than +2.5° will cause scrub problems.

Caster Definition
Caster can be defined as the forward or rearward tilt of the projected steering axis from true vertical, as viewed from the side. This line is formed by extending a line through the upper and lower steering knuckle pivot points.