Steering Affects BumpSteer
With this system, the wheel...
With this system, the wheel can remain on the ground and the chassis can be moved to measure bumpsteer. This may be difficult to do with low ride heights and it's very difficult to simulate the mid-turn configuration for setups that travel extreme amounts.
When we steer our front wheels, we change the angles of our tie rods due to caster, camber, and degree of spindle on both sides. The tie-rod ends travel in an arc that is not parallel to the ground. This changes the outer tie-rod height and therefore the B/S. It's for this reason that we recommend doing your B/S with the wheels both straight ahead and then again with the wheels turned equal to mid-turn steering at the track you will run.
I have had car builders tell me proudly that their cars had near zero bumpsteer. I asked them what it was in the turns and I got a puzzled look. In past years, I used the Mitchell program for studying the frontend geometry, including steering. Bill Mitchell originally wrote that for Ford and it's considered one of the better three-dimensional programs. Performance Trends has one too.
With a three-dimensional program, I could do a B/S analysis with the car at ride height, or any other attitude including with varying degrees of dive and roll. I could roll the car and lower it to simulate the mid-turn attitude and then bump the wheels to see if the steering angle changed. I did a few late '90s Cup cars and Craftsman trucks and was amazed at how perfect the systems were. I got near zero B/S no matter what attitude I put the car in. You can simulate this in the shop too by moving the chassis and steering the wheels before you check the bump.
Measuring BumpSteer, Some Tips
The most common way to measure...
The most common way to measure bumpsteer is to support the car on jackstands and using a jack, move the spindle up and down in the range of motion it will have during a racing lap. If your wheel travel is 3-4 inches, moving the wheel 1 or 2 inches up and down will not simulate the conditions. For most asphalt racing, both front wheels travel up in relation to the chassis. For dirt cars, the left front may well travel down considerably.
We can measure our B/S using several different types of equipment. There is the double-dial caliper system, the single-dial system, and the laser system. Each one will tell us if the wheels steer when they are in bump or rebound.
BumpSteer Gauge The most common tool is the bumpsteer gauge. It consists of a plate bolted to the hub and a stand that holds either one or two dial indicators. It comes in two configurations, the double-dial indicator type with a stationary stand and the single-dial type with a swinging stand. With the latter, when the wheel moves vertically, the stand follows the plate. With the double-dial type, the two dial pins are always moving one way or the other. If the system has zero B/S, then both dials will move together the same amount.
If the front dial moves farther as the wheel moves, then we have bump-in at that wheel. If it goes less, then we have bump-out. Be sure to count the number of turns each dial makes when moving the spindle vertically. Subtract the readings to find the B/S amount related to the distance the wheel has moved. We usually refer to B/S as decimal inches of bump per whole inches of travel.