Most of you have felt this at some point, especially if you've ever lowered or raised a vehicle, or changed the suspension significantly without compensating for the resultant change in steering geometry. Typically, bumpsteer manifests itself as a tendency for the front end to dart or wander without driver input, especially on a less-than-ideal surface, forcing a concentrated effort to keep the vehicle in a straight line. What's actually happening is the wheel is steering as the suspension moves over the road irregularities because the length between the spindle and the rack or gearbox lengthens, but the tie rod does not. The result is the spindle rotating, or toeing, outward slightly to compensate. Depending on the severity, on the street it can be a minor hassle all the way up to dangerous; on the track excessive bumpsteer is always a liability since it will limit control and traction if a bump or dip is encountered while cornering.
Most modified cars have some small degree of bumpsteer; the real goal is simply to make it small enough that it doesn't interfere. To achieve that, the tie rod has to travel on an arc parallel to the one the spindle follows. That's why bumpsteer often increases noticeably when lowering or raising a vehicle-it changes the angle of the tie rods and consequently the arc on which they travel. The only way to combat it is to try and get everything back in line; in the case of lowered cars, by either raising the rack or steering box (usually not an option), or lowering or raising the outer tie-rod attachment point at the spindle accordingly.
This is a term that gets thrown around mostly in racing circles, but having a grasp on what it means will still help with understanding the rest of the concepts. To locate an object in space, we need three things: the X-, Y-, and Z-axis coordinates. When relating to cars, the X-axis is lengthwise, and the Y-axis is side-to-side. However, that doesn't tell us anything about how the object is oriented. Yaw is the concept that describes angular motion and how a car rotates around the Z, or vertical, axis. By definition, a car is always in yaw through a turn, simply because it is headed in a different direction than the nose is pointing-which, of course, means that yaw is related to the slip angle.
Why is that important to know? Because there is only so much yaw that can be exerted on a vehicle before control is lost, and knowing how to make adjustments to keep it in check dictates how much speed can be carried through a corner.