Caster, the slant of the steering axis from true vertical, reduces steering effort and improves tracking or directional control. Basically, positive caster gives the vehicle its tendency to track straight ahead on its own. Although caster doesn't directly affect tire wear on the vast majority of vehicles, it may indirectly influence it on a few of them.
To understand caster, look at a front wheel from the side of the vehicle. Picture a line running through the steering pivots down to the roadway. The steering pivots on a traditional short-arm/long-arm (SLA) suspension are the upper and lower ball joints.
On a typical MacPherson strut front end, the upper strut mount and lower ball joint are the steering pivots.
The line running through the steering pivots defines the caster angle. Caster is zero when the line is perfectly vertical, positive when the line leans toward the rear of the vehicle and negative when the line tilts toward the front of the vehicle.
Most vehicle manufacturers specify a slightly positive caster value (about 1 to 3 degrees).
The accompanying illustration shows that with the steering axis tilted back for positive caster, the axis line projects itself to a point on the roadway ahead of the tire. To grossly simplify, projecting the axis line ahead of the tire ``pulls'' the tire forward.
Observing a bicycle helps explain caster and its effect on handling. Note that all bicycle forks are designed with lots of positive caster. Projecting the steering axis line far ahead of the bike's front tire enables an agile person to ride the bike ``no hands!''
On the other hand, the negative caster on a shopping cart's front wheels causes the cart to track poorly on its own.
Net result: Positive caster helps the tires return to the straight-ahead position and track straight ahead on their own, thus improving the vehicle's overall feeling of stability and steerability.
Strictly speaking, caster is not a tire-wearing angle. However, the more positive caster a vehicle has, the more its tires lean over during turns.
In other words, increasing positive caster increases camber change during turns. Camber change on turns is sometimes called camber roll.
Some vehicles are designed with extraordinary positive caster-sometimes twice the normal amount. When such a vehicle is driven mostly in the city, it experiences frequent camber roll because a city vehicle turns lots of corners. Consequently, its tires may develop a shoulder-scuffing pattern associated with camber wear.
Abnormal caster conditions
Generally speaking, right and left caster should be within one-half degree of each other. Assuming right and left camber angles are equal or nearly equal, a vehicle will pull toward the side with the least amount of positive caster.
Whereas caster describes the front or rear tilt of the steering axis, SAI (steering axis inclination) is the inward or outward tilt. Like caster, SAI helps the tires return to center and track forward on their own. (For a detailed discussion of SAI theory and diagnosis, refer to the May 17, 1993, Parts & Labor service supplement.)
MacPherson strut vehicles, which dominate the marketplace, usually operate with two times or more SAI as a traditional SLA suspension. Due to the effect of greater SAI, a strut suspension may tolerate more side-to-side caster variation without pulling.
Remember that excessive caster variation during braking causes the vehicle to pull only when you apply the brakes. This condition is often misdiagnosed as brake trouble.
But if the brakes are working smoothly, the root cause is probably cracked or fatigued strut rod or radius rod bushings. The bad bushings allow excessive lower control arm movement during braking, which causes a sudden caster change and the resulting pull during braking.
Excessive positive caster increases low-speed steering effort and ride harshness or road shock.
On the other hand, excessive negative caster reduces steering stability at higher speeds and may cause wander. It also reduces the ease with which the steering returns to center after turns.
Any problem that allows the steering pivots to move fore or aft changes caster. That includes worn ball joints, bad strut rod bushings, worn upper strut mounts or worn control arm bushings on SLA suspensions.
Finally, getting an accurate caster setting on trucks may require an alignment technician to compensate for the truck's frame angle. (For more information on this, procedure, see Parts & Labor, July 13, 1992.)