The short story here is that excessive bump steer increases tire wear and makes the vehicle more difficult to handle on rough roads.
What is bump-steer?Bump steer is when the front wheels move up and down, we want the front wheels to maintain a particular direction. It's most important for the wheels to have minimal bump when negotiating turns. There are certain elements of the construction of the front end components that will make this happen.The angles of the upper and lower control arms, meaning a line extending through the center of rotation of the ball joints and inner mounts of each arm, intersect at a point we call the instant center (IC). This is one of the components used to determine the moment center location. In order to have near zero bumpsteer, the intended goal, we need to have the tie rods on each side point toward the IC for its side. This is one of two criteria for near zero B/S.The other thing we need is for the tie rod to be a specific length. That length must be equal to the distance formed by 1) a line extending through the centers of rotation of the tie-rod ends, and 2) the tie-rod line intersection with a) lines extending through both the upper and lower ball joints, and b) the plane that passes through the inner chassis mounts. This can get a little complicated because although the ball joints do form a single line, the chassis mounts form a plane because of the front and rear mounts.So, the inner tie-rod intersection point is where the tie-rod line intersects the plane of the inner mounts and the outer line intersection point is where it intersects the ball joint line. A three dimensional geometry program can simulate this very well, but most of us don't have the luxury of owning and knowing how to operate one of those. If so, we must go through the process of physically measuring the B/S in our cars.What Creates Bumpsteer When the tie rod is not aligned with the IC and/or the length is wrong for the system, we have B/S. As the wheel moves vertically, the wheel will either steer left or right. We will refer to the direction from a driver's perspective only, in this discussion.If the tie rod was pointed so the tie-rod line passes below the IC, then the wheel will bump-in (toward the centerline of the car) as the wheel travels up, and bump-out when the wheel travels down. If the tie-rod line passes over the IC, then we will have bump-out as the wheel travels up, and bump-in when the wheel travels down.If the tie rod were too short, we would have bumpsteer in when the wheel travels in both directions from the static ride height position. If it were too long, then the wheel would bump-out as the wheel traveled in both directions from ride height.These indicators can tell us if we have either a tie rod alignment problem or a tie rod length problem. In some cases, both may be present and that causes a very erratic motion of the wheel. To determine which, record each inch for several inches of travel in both directions from static ride height and note the tendencies. You might have perfect alignment and a tie rod that is wrong for length. This could be due to a poorly designed drag link or the wrong width rack-and-pinion steering unit.
Source: Hot Rod Network