First, if our aero package is refined so our "ride" height is minus 1 inch at speed, then that is where zero should be for the bumpsteer procedure. Second, if we are steering the car at the most critical phase of the lap related to bumpsteer, we need to steer the wheels before we check the bumpsteer in the car. With cambers greater than zero, the outer ends of the tie rods will change height as the wheels are turned. In turn, this changes the tie-rod angles. Remember, tie-rod angle is one of the influences on bumpsteer.
Third, on most bumpsteer devices I have seen, the dial indicators are much more narrow than the diameter of the tire. One particular brand of fixture measured only 15.5 inches between the points of the indicators. An 86-inch-circumference tire has a diameter of 27.37 inches. The amount we measure as a difference in movement of the dials must be multiplied (in this case) by 1.77 (27.37 divided by 15.5 equals 1.77) to get the real bumpsteer numbers at the outer edge of the tire. We usually equate the bumpsteer differences to toe measurements, so we really need to know the difference in movement out at the tread surface front to rear by applying the correct multiplier.
What do we want to see for bumpsteer numbers? Zero may not be ideal, and there are a lot of different opinions among racers. We never want bumpsteer that increases the angle of attack or steering angle of either wheel. The outside (right-side) wheel should not bump in, and the inside (left-side) wheel should not bump out. Should the wheel continue to steer left as the driver negotiates the turn, he or she might need to back steer and that is very unsettling. If the wheels bumped out toward the outside of the track, the driver only needs to increase the steering effort to adjust to the movement.
The LF wheel on most stock cars that race on most average banked tracks moves up and down an inch or so. Again, aero downforce at speed may move this range lower from actual ride height.
The RF wheel is mostly in bump from 1 to 4 inches. Aero might account for the first inch, and dive and roll at mid-turn adds an additional 2 to 3 inches of movement in most cases.
High-banked tracks of 16 or more degrees will push the wheel movements associated with mechanical (as opposed to aero) downforce up to 4 or more inches. Take into account the type of track when deciding how much range you will use to measure the bumpsteer amounts.
In your minds, stand back from your car and think about all of the front suspension systems and how they might affect bumpsteer. Take careful measurements so you will know exactly what amount of bumpsteer is present in each wheel, and adjust accordingly. Remember, we want correct bumpsteer movement mostly in the turns where the car is at the very limits of lateral adhesion and any slight change in steering might cause one or both front tires to lose grip.
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