Another most important wheel alignment angle is caster, which is the forward (negative) or rearward (positive) tilt of the steering axis as observed from the side. Caster is often measured in degrees, and only applies to the front wheels because they are the only ones that steer (apart from the few oddball Japanese cars that had four-wheel steering).
Caster is a weird angle because it doesn’t affect tire wear directly. It’s biggest effect is on steering stability, steering effort and steering return. So it is mostly the most ignored angle.
Most cars have a small amount of positive caster to give quick steering return and high speed stability. This occurs because caster forces the spindle to angle down slightly as the wheels turn. This carries the chassis and brings more weight to bear on the wheels as they turns. The net effect is that caster helps keep the wheels kept straight ahead for improved steering stability, and helps the wheels return to the straight ahead position after turning. Many European luxury sedans have a lot of caster for this very purpose because it gives a more stable feel at highway speeds. The badside is that it increases steering effort and steering feedback to the driver.
What happens if caster is out of specs? If there is too much difference in caster side-to-side, it can cause a vehicle to drift or pull to one side. Some alignment specs require a slight difference in caster to compensate for road crown. But as a standard, caster should usually be within half a degree side-to-side.
The same kind of issues that can cause camber misalignment can cause caster misalignment: a bent spindle, mislocated strut tower, bent strut, worn or collapsed control arm bushing, bent control arm or a weak or broken spring. So if any of these parts are replaced, caster should be checked and readjusted as necessary after the parts have been installed.
Ride height can equally affect caster. Spring sag or overloading a vehicle can alter ride height up to several inches, which can change caster measurements by up to a degree or more. This may contribute to steering instability or change steering effort. So checking ride height is also an important part of aligning the wheels. If ride height is below specs, weak springs should be replaced. Upgrade opportunities here include installing variable rate springs, air springs, overload shocks or air-assist shocks on a vehicle that is used for towing or hauling heavier than normal loads.