Your opinion on "bump steer", what's desired?

Brakes, Steering & Suspension

  1. bherman

    bherman Well-Known Member

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    Hi Guys', this may seem to be a silly question that only has one answer, you don't want any, but is that true? First let me say that I'm not an expert on this subject but I do have some practical experience with it. My definition of "bump steer" would be toe change during suspension travel. Common thinking would be this toe change creates a steering input without moving the steering wheel. If this toe change occurs evenly on both tires the only negative effect is passing through zero toe, which is unstable all by itself. If the suspension travel is different from side to side (much more typical) the result is divergent toe patterns, that creates steering inputs not desired by the driver. This condition is even more apparent during body roll (cornering), causing "roll steer".

    Many years back when we were working on road race cars we always attempted to tune all of the bump steer out of the cars. The cars were GTP cars, with carbon fiber "tubs", extremely stiff, very light, limited suspension travel and very fast. At the speeds those cars traveled (220+) any bump steer was not appreciated by the drivers, even small undulations in the track surface would make the vehicle twitchy, if they had any more than .010 bump steer (per side) through out the total suspension travel. This is very speed dependent and I'm not suggesting that our street cars need this kind of attention to bump steer geometry. If you're one of the racers on this site and your car is running 150 MPH you might want it as close as you can get it!

    This gets me to the question that I really wanted to ask. More recently we work closely with people that are focused on production suspension design. In most modern front suspensions bump steer is used to create an under-steering condition. A small amount of bump-out under load reduces the steering angle and keeps us non-race car drivers from getting ourselves in trouble. This typically varies from .050 to .080 toe-out during roll steer. I'm currently tuning the bump steer on my car, do you think I should make it under-steer a little?

    IMG_0996.JPG

    Checking bump steer, thanks for looking, Brian
     
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    • 493 Mike

      493 Mike FBBO Gold Member FBBO Gold Member

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      Mopar Action had a bump steer article in the last issue. Simple home built tool to check toe with also.
      Mike
       
    • multimopes

      multimopes FBBO Gold Member FBBO Gold Member

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      I don't think many alignment shops today are very good at understanding and adjusting our older cars. I have heard too many complaints of questionable results from well meaning attempts to find a happy medium. I knew only a couple people who really knew what they were doing and how doing a little this or tweaking that can make a huge difference. Unfortunately one died and the other moved far away and retired. The last guy who died owned a body and alignment shop and he had several sons who never wanted to learn the old school ways. When the old man croaked, they just had the alignment rack ripped out!
      In my case, I would be happy if I got the time to install my power setup in my car. I don't like dislocating my shoulders trying to park, LOL!
       
    • bherman

      bherman Well-Known Member

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      multimopes, thanks for your reply. I agree that a lot of knowledge is being lost as we lose the experienced people. On the other side the tools that we have available today (CAD and other software) enable us to examine things that weren't possible 20 years ago. Today knowing what the suspension is doing is easy. Where we need the "old guy" is knowing what we want it to do! Thanks again, Brian.

      493 Mike, I appreciate your reply and info. I haven't found that article yet bur I'm looking. Thanks, Brian.
       
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      • ckessel

        ckessel Well-Known Member

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        Checking the bump steer takes time. Between the customer not wanting to pony up because of the time required, $$$, and the shop not wanting to spend the time, again $$$, you get left mostly with "set the toe and let it go". If you find a shop willing to do it, don't be stingy.
         
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        • mopar 3 B

          mopar 3 B Well-Known Member

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          Its surprising what was accomplished with bubble gauges and a toe set bar. I was always more concerned with front end lift at speed than bump steer. This could more or less really have been a bump steer problem and just did not know it.
           
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          • ckessel

            ckessel Well-Known Member

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            On a $50-$80 alignment, you will only get camber/caster/toe set as thats about an hour or less to do. Something else to have them do is have them hang targets on all 4 wheels so you can see what the "thrust angle" is. You need to have the front aligned to the rear so the car isn't going sideways down the road. Having the thrust angle off can kill tires and make it twitchy. Hotchkis makes shims to put between the rear leaf spring front hanger and the unibody mount to help correct unless its too far off then you need to visit a frame shop or you have a bent diff. Maybe both.
             
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            • Scott Engelhardt

              Scott Engelhardt FBBO Gold Member FBBO Gold Member

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              I don't want to go off on a rage but a proper wheel alignment takes time. Time = money and people do not want to pay to get it done right. To properly align a vintage car could require several road tests and putting it on the rack more then once. When I aligned my Satellite it took 4 hours of tweaking and driving, part of that was seeing how much caster I could run with out making it hard to steer at low speed (no power steering) As far as bump steer my car does not have any checked with my alignment machine
               
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