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Bump Steer Spacers

Old 10-05-2004, 06:02 AM
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Bump Steer Spacers

Bear with me, just doing a little brainstorming. Is the sole purpose of these spacers for bump steer, or are they there for suspension geometry as well? When not taking the tie rods into account, what is the optimum angle for the control arms to be at?

EDIT: My bad, I should have searched a little more before posting, found this link which helps, but my last question still stands.

http://www.reactionresearch.com/suspension.htm

Last edited by preith; 10-05-2004 at 06:20 AM.
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Old 10-05-2004, 09:01 AM
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Originally Posted by preith
Bear with me, just doing a little brainstorming. Is the sole purpose of these spacers for bump steer, or are they there for suspension geometry as well? When not taking the tie rods into account, what is the optimum angle for the control arms to be at?

EDIT: My bad, I should have searched a little more before posting, found this link which helps, but my last question still stands.

http://www.reactionresearch.com/suspension.htm
The info about bump steer spacers on the site isn't really correct. You really want the arms close to flat in an operating condition. Bump steer spacers can be used to achieve this on a lowered car. A lot of things are related in the front suspension. Bump steer spacers change RC height, position on the bumpsteer curve, and to some extent camber gain.

I hear all the time how camber gain is so important. In a strut based car you will seriously screw things up trying to fix that. As soon as the car rolls a little you will lose a lot of camber (think about the top if the strut moving). How little does it need to move to equal a degree. But what about laying it way over (heard that too). Then you have a huge KPI but somewhat lower scrub. You'll soon find that steering effects are much larger than any benefit you may get from camber gain. Most tires these days are very tolerant of a lot of negative camber, slicks not so much so.

If you make the JTR conversion make sure the arms are near level. If not you may find that while you have reduced bumpsteer you may increase the push at the front. You're rasing the roll center.

Sorry to sound like a pompous *** on this.

Cary
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Old 10-05-2004, 12:00 PM
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Thanks again for the insight. What I really should asked was, if bumpsteer was completly out of the equation, and suspension geometry was my only concern, what purpose would the spacers have? Would I still want the control arms paralel, or would it be advantageous to have a little more angle? Excuse my ignorance, but isn't having a higher roll center what you want?
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Old 10-05-2004, 03:05 PM
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Originally Posted by preith
Thanks again for the insight. What I really should asked was, if bumpsteer was completly out of the equation, and suspension geometry was my only concern, what purpose would the spacers have? Would I still want the control arms paralel, or would it be advantageous to have a little more angle? Excuse my ignorance, but isn't having a higher roll center what you want?
Ignorance is not asking the question. So you shouldn't worry about it. It's not like I really know all this stuff either. I'm just a little farther down the road. :-) And you should keep in mind that I only autox and hillclimb. So my advice pertains more to that than road racing. I don't pretend to know what you need to do there but most of this should carry over.

The spacers are useful to get the control arms back towards level. In the front of a Z this is the sweet spot for RC movement, lateral track change, etc. If you angle the arms downward (from a front view towards the outer balljoint) you're supposed to get this mythical camber gain that keeps the tire flat to the road and raise your RC. Everyone tells you this is good.

In reality what happens is that the tire contact patch is forced to laterally migrate as suspension compresses. It doesn't really slide according to current theory but adds to steering. This coupled with the higher RC loading the tire quicker will give you more understeer when you probably don't want it. In road racing it may be okay as the steering input is generally less and a lot slower.

Weight transfers is fixed with CG height and track width. You can't change it no matter what you do with shocks, springs, bars, RCs, etc. What you can change is the rate of change and the split front to back. And when you get a balanced car what you really have is one that creates slip angles at the front and back at the same time (also called being neutral).

Hope this helps,

Cary
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