What is caster?

The wheel is never located directly under the shock/strut mounting point on the car.  The amount by which the wheel leads/trails the mounting point is caster.  Positive is wheel ahead, negative is wheel behind.

In more detail, from a post by Kris Rozon:

"Think of caster as the amount the actual wheel is either ahead, or behind the top strut mount. If the wheel is ahead of the top mount, as in one of those Harley's with the big Chopper-Bar front fork things, then that is positive caster.

If you had negative caster, that is the same as a shopping cart's rear wheels. They trail the mounting point.

The more positive caster you have, the tighter the steering and the more the steering wheel will want to center itself. I am not aware of any side effects of too much caster, but there must be some. Obviously too much negative caster would be bad (kinda like the wobbly wheel on the shopping cart).

So, if one wheel is a great amount more negative or positive than the other, then you will notice a pull in the steering wheel. This is what some on the digest have experienced regardless of numerous alignments and tire balances. The steering wheel will also seem to turn one way better than the other. The front of the DSM is not alignment-friendly."

Some have reported that too much positive caster promotes road wander. Ernie Coursolle (Dakota) pointed out, based on his experience with pre-86 Corvettes, that not enough positive caster may also cause wander. The '84 'Vette reportedly tended to wander around a lot with only 3 degrees of positive caster. Once the specs were changed in 1986 to 6 degrees, the 'Vette lost the wander, and realigning the '84 to '86 specifications eliminated the wander on the older car. Still, most agree that extreme positive caster is a bad thing, which simply means that too much of a good thing is just as bad as not enough - just like everything else.