Definition of bumpsteer: Bump-steer is a change in toe* angle caused by the suspension moving up or down. Bump-steer is built into the geometry of the suspension and steering system, and has nothing to do with turning the steering wheel. The effect of bump-steer is for the wheel to toe-in or toe-out when the suspension moves up or down. This toe change or "steering" occurs any time the suspension moves, whether it is from body roll, brake-dive, or hitting a bump in the road. Bump steer is undesirable because the suspension is steering the car instead of the driver.
Cause of bumpsteer: The front wheels do not move directly straight up or down when the car hits a bump. Instead, the wheel follows an arc, or curving path, that pushes the wheel slightly inward (towards the centerline of the car) or outward (away from the car) in response to vertical wheel movement. The outer tie-rod (which connects the steering rack to the wheel) also moves in-and-out in an arc as it moves up and down. If the rate which the outer tie-rod arcs in or out does not match the rate the wheel moves in or out, the wheel will be turned by the tie-rod. This is bumpsteer.
The center point of the arc traveled by the wheel (known as the instant-center) is controlled by the location and angle of the moving suspension links. This point moves as the ride height changes. In contrast, the arc of the outer tie-rod is controlled by the position of the steering rack, which is fixed. In order to eliminate bumpsteer, both the length and the center point of the two arcs must be the same. However, since the instant center moves with ride height, bumpsteer cannot be eliminated throughout the entire range of suspension travel. Therefore suspension designers concentrate on minimizing bumpsteer within the range of movement closest to factory ride height. Changing the ride height or other suspension components may move the suspension outside this narrow "optimized" window.
NOW, basically in our trucks if you are lowered, what happens is the tie rod is on top of the pivot of the spindle, when you lower your truck it "shortens" the path it is able to take, putting it at a angle and bind. So that when your truck hits a bump, as the tire moves upward the shortened tire rod steering linkage will cause abrupt changes in toe,(distance between the two front tires) causing the truck to dart left or right abruptly
with the bump steer kit, this is corrected by setting the tie rod under the spindle mount, letting the tie rod and steering linkage geometry be corrected, thus in the event of a bump so forth in the road, as the tire moves upward, the toe setting stays within its parameters, thus less darting left to right. If you are lowered, its a great addition to handling.
Paul at Razors Edge did an awesome job with the bumpsteer kit that is now available
