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Thread: suspension geometry problems

  1. #1
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    suspension geometry problems

    Hi,
    today i tested my tube chassis on street. But that drive not very well.

    I see/think, that the steering rack/tie rod is the problem. My tie rod is parallel with the lower a-arm.
    I tested the suspension without springs and upper arm. when the spindle move up or down, the spindle go something left and right.

    Anybody know what's the correct place or have pictures.



  2. #2
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    Re: suspension geometry problems

    Your tie rod should be close to parallel to the lower arm but also needs to be the correct length. If the inner tie rod end is not in the right place you will never get rid of the bump steer. Due to the ackerman principal built into your suspension the inner tie rod end will need to be slightly outside of a line drawn between the upper and lower inner pivots for the a-arms. Remove your springs and shocks and leave the upper and lower a-arms in place and then raise and lower the spindle and see if the wheel toes in and out with the movement. If the toe movement is not too much you can sometimes reduce it by moving the rack up or down. There is a very good book by Steve Smith titled Advanced Race Car Suspension Development that has a good section in it about measuring and correcting bump steer. It shows graphs that will tell you what adjustments need to be made. Hope this helps.

  3. #3
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    Re: suspension geometry problems

    I can also recommend Herb Adams' book "Chassis Engineering". It has an excellent chapter on suspension geometry and how to minimize bump steer.

    As txbuilder said, placement of the inner tie rod is very critical. If you are looking at your chassis (or a drawing of your chassis) from the front of the car, you should draw a line from your upper control arm joints to the lower. Preferrably, you would want the inner tie rod joint to intersect as close to that line as possible.

  4. #4
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    Re: suspension geometry problems

    The toe movement is not much. But i think not good enough.

    I think some pics/graphs would be help. To see the line and the inner tie rod.

  5. #5

    Re: suspension geometry problems

    On the naerc chassis the rack can be adjusted upward by way of adding shims.
    Please post some pics.

  6. #6
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    Re: suspension geometry problems

    Quote Originally Posted by mlochala
    I can also recommend Herb Adams' book "Chassis Engineering". It has an excellent chapter on suspension geometry and how to minimize bump steer.

    As txbuilder said, placement of the inner tie rod is very critical. If you are looking at your chassis (or a drawing of your chassis) from the front of the car, you should draw a line from your upper control arm joints to the lower. Preferrably, you would want the inner tie rod joint to intersect as close to that line as possible.
    Since the outer tie rod end is outside of the line through the ball joints the inner tie rod end needs to be outside of the line through the inner pivots. This is due to ackerman. It is a balancing act to get everything to come together and it is usually not possible to completely eliminate all bump steer but it can be reduced to where it is not a problem.

  7. #7
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    Re: suspension geometry problems

    i might be sticking my foot in my mouth but here it goes. it is real hard to tell what the problem is with out seeing some numbers so if you really want to discuss what you have going on we will need some measurements and maybe a few pics. the can of worms you opened is allot deeper then most think and can really drive a person nutz trying to figure it out. for every action their is an equal opposite reaction. and that just can make a man go bald quick lol.

    anyways i have a couple questions for you. what chassis are you using? and if you are using the fiero front suspension are you using any spacers in the front?

    if you are using a custom set up. (custom made arms and spindles) what is your lengths? are you running swaybars? how much anti dive did you build into it? do you have the rack in a straight line with the tie rods to spindles? (as viewed from the top) if not how far forward did you place the rack? or how far to the rear did you place it?

    sorry for all the questions but even though you may think its your bump steer giving you problems. it could very well be a combination of things that is starting your whole problem. i all most have to say that you problem may not just be your bump steer if you have a bad roll center it will compound your issues and in some cases as i have seen in my days that could actually give you a similar feeling.

    also you may want to consider moving the rack back towards the rear of the car some a half inch is fine. what that can do for you is give the car a more nimble feeling. kind of like a quickness in the steering. if you move it forward it will give you a slower more stable feeling. and if your rack is not square to the chassis it will give you a weird feeling like the car is trying to dance around. you need to check everything you have done very carefully. but the thing to remember is that you are driving a street car not a race car 150-200 mph. so like txbuilder mentioned. if you can get your bump within say .040 you will be fine specially with the tread of the tires these days. we all ways had to get the bump within .010 but at the same time the cars where doing 150-200.

  8. #8
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    Re: suspension geometry problems

    I agree with everything that cliffscustomfab says. This is a rather complex subject with many variables that can have an affect on the handling.

  9. #9
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    Re: suspension geometry problems

    At the risk of souning dumb, what is it doing?

    Dave
    If you're not confused, you're not paying attention.

  10. #10

    Re: suspension geometry problems

    Get the Book, "Tune to Win" by Caroll Smith $20.00, Excellent Suspension Geometry Tech in very Understandable format. Its Like Chemistry 101 for Suspensions. The other books should also be in every car guys library

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