Bump Steer is when your wheels steer themselves without input from the steering wheel.
This undesirable steering is caused by bumps in the track interacting with improper length or angle of your suspension and steering linkages. Most car builders design their cars so that the effects of bump steer are minimal.
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 wich is called 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, you need to have the tie rods on each side point toward the Instant Center (IC) for its side. This is one of two criteria for near zero Bump Steer (B/S).
The other thing you 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,
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 eccass to that, so we must go through the process of physically measuring the Bump Steer (B/S) in our cars.
When the tie rod is not aligned with the instant Center (IC) and/or the length is wrong for the system, we have Bump Steer (B/S). As the wheel moves vertically, the wheel will either steer left or right.
Want to read more about Bump Steer... Hotrod.com did a splendid article about it, wich you can find HERE