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Thread: Bump Steer

  1. #1
    What is the max amount of bump steer that you want to have in bump/droop?

    I am shooting for toe in on both bump and droop. Right now I have at full bump (1.00 inch travel) .033 in. Does that seem reasonable or to much? Like I said in my ackerman thread I am using front steer.

    MSU FSAE '07

  2. #2
    .033" where?
    College dropout extraordinaire
    (formerly WWU Rev-Hone Racing)

  3. #3
    You probably don't want any bump steer, as it can cause the car to be unpredictable especially midturn. I don't know how much .033 is, and where you are measuring, but .033 on your car may be ok, on someone elses it may put it into a ditch. Other things like wheelbase and trackwidth will have effects on how noticeable this is.

  4. #4
    My question is: how much bump steer is too much? It takes time to build up traction force, so is it possible to get toe in or toe out in bump that doesn't cause instability because the tire doesn't have enough time to react and build up traction force?
    If this was the case, it would be possible to come up with roll steer that was very beneficial without any consequences in bump.

  5. #5
    The real issue in this case is that it's hard to separate roll and bump in the real world. It's also the case where you can't assume steady state vehicle motion throughout a turn, and you also can't assume that vehicle motion will follow a nice smooth line as it would in the perfectly ideal corner maneuver (especially with non-professional drivers).

    In decoupling the steering geometry from the suspension motion, the driver is able to control vehicle heading much better since the steering output is not deviating from the input due to bump and roll motion. If you're constantly having to react to changes in tire steered angle throughout a corner, it's going to affect smoothness through the corner and make the car increasingly harder to drive.

    You also have to realize that your maximum theoretical induced roll comes mid-corner at the apex if the car is driven on the "perfect line" which is not necessarily the place where you would like your maximum steering change (unless you're planning on having negative bump-steer that would decrease in roll, but that sounds even worse to drive). Granted, you can always induce a larger roll moment on corner entry by initiating a Scandinavian Flick maneuver, but good luck trying to control the car afterwards. Plus I'm sure the safety officials would really enjoy that...

    That being said, I've also seen a passive rear-steer system used to some degree of success that initiates a steer angle based on induced roll from a combination of built-in bump-steer and Ackerman geometry. However, the best rear-steer systems use an electronic control system to properly decouple steering motion from bump and roll inputs.

    So I suppose if I had to answer the initial question, I would say the proper amount of bump-steer is as little as possible in the front, and probably the same in the rear as long as you're not going for a passive steering system of sorts.
    Engineers aren't boring people, we just get excited about boring things.

  6. #6
    I assume your measurement of .033" is deviation of the front of the tire. While this is fairly small amount of bump-steer, it can certainly be improved based on the steering geometries I've worked on. It may be different in your case, but I would try to make that number a little smaller.

    Also, you need to consider the camber change in bump and the toe at those increments. Although toe in would cause stability in normal cases, at greater camber angles toe out bump-steer will actually cause the tires to move in more straight, unoppossing path. After you find the correct amount of bump-steer, make sure to check for roll steer, too.
    _ _
    Joel Harshbarger
    USF Motorsports

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