Bump Steer Explained - Chassis

In the past we have provided information about all of the alignment aspects including toe-front and rear, rearend alignment and rear steer, Ackermann, and front-to-rear alignment. Every team I have ever worked with either did the bumpsteer itself or had someone come in and do it at least once in a particular car's life. We know now that is not enough.

Certain setups have evolved to where your bumpsteer might not be what you think it is. When you make changes to your antidive or moment center, you may be changing your bumpsteer characteristics. So, when trying a new setup such as the BBSS asphalt setups, you might be introducing B/S and not know it. The greater amount of travel associated with the BBSS setups may be out of the range you last checked your B/S for. As a result, the car may begin to behave erratically and cause the driver to be uncomfortable with the new setups and the culprit might be a problem with the steering.

We will use the term, near zero bumpsteer simply because most cars will never be able to have absolute zero bumpsteer. If you can get your bump to less than 0.010-inch of bump per inch of vertical travel, you will have a good design where the driver will not feel the movement. That is a little less than 1/64-inch of toe movement. If you are inclined to want a small amount of bump-in some direction for one or both front wheels, go for it, but we don't recommend any amount of bump.

Basics of Bumpsteer As 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 we are negotiating the 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.