The trends in setting up our stock cars have influenced bumpsteer issues so much that we need to rethink how today's setups might have an adverse effect on the bumpsteer characteristics of our cars.
"Bumpsteer" is traditionally viewed as a product of movement of each wheel vertically caused by the car diving, rolling, and going over bumps. Over the past few years, changes in the setup of our race cars have caused us to take a different view of bumpsteer design.
For asphalt racing with associated high amounts of downforce combined with softer spring setups, the cars no longer run the straightaways at ride height. With some cars, aero downforce may reduce the "at speed" ride height by an inch or more. Having zero bumpsteer at ride height is no longer valid for those cars.
Conversely, many dirt Late Model cars produce high amounts of lift when accelerating down the straights and a lot of jacking effect in the turns. Traditional ranges of wheel travel related to the measurement of bumpsteer are no longer valid in some cases.
Bumpsteer is affected as we discover the need for better roll center designs and actually make changes to our cars. Many chassis have vertically adjustable mounting points designed to facilitate quick changes to control-arm angles. Teams can change control-arm lengths to achieve better camber change characteristics, and this causes changes to the inner mounting-point planes. Since bumpsteer is affected by the control-arm angles versus tie-rod angle, as well as tie-rod length associated with the control-arm pivot planes, the tie rod must be adjusted for its length and angle in order to keep bumpsteer under control.
The soft spring setups used by many teams create another element to consider: Chassis dive on entry under heavy braking must be controlled to prevent the car from making contact with the racing surface. Anti-dive is incorporated to help control excess dive. The changes to control-arm mounting angles from a side view cause a rotation of the spindle in that same view and, therefore, a vertical movement of the outer end of the tie rod. As the tie-rod angle changes, so does the amount of bumpsteer.
Starting with the basics, bumpsteer is affected by four things:1) Tie-rod angle versus control-arm angle-the tie rod must be pointed at the instant center formed by extensions of the upper and lower control arms through the pivot points.
2) Tie-rod length-the tie rod must be a specific length in order to avoid having excessive changes in bumpsteer.
3) Steering arm movement-if the spindle rotates from a side view as the wheel travels vertically, the outer end of the tie rod will move vertically more or less than the ball joints. If not considered in adjustment, this changes the angle of the tie rod and produces bumpsteer.
4) Turning the steering wheel-with the large amounts of camber used in the left-front (LF) and right-front (RF) wheels, the simple task of turning the steering wheel causes the outer tie-rod ends to move vertically with results similar to those associated with No. 3 above.
Bumpsteer is measured by using two dial indicators placed against a plate bolted to the wheel hub. As we jack the wheel up or down, the indicators resting on the plate move. Record the amount of movement in increments of inches of wheel travel.
We used to start at a wheel height that represented the spindle location at ride height, then bump the wheel up or down and record how far each dial indicator moved at the front and rear of the plate. The difference in movement was thought to be how much bumpsteer we had per inch of wheel travel at each inch from zero height. We need to reconsider this approach somewhat for several reasons.