posted January 14, 2005 06:26 AM
1)When running hoosier tires as opposed to Americn Racer, how would that affect where to posistion the roll center? Hoosiers having a taller, more flexiable sidewall than the AR tire will make a difference, correct?
Tire height differences won't effect the static roll center, but it will factor into body roll changes which would have an effect on the dynamic roll center.
2)I run a flat tight track and also a high bank wide track, do I need to adjust roll center from one to the other? I have no adjustments on my upper conrtrol arm mounts, are there ways to adjust without cutting and welding?
Adjusting the roll center from one track to another could be a good tuning tool. For that flat track, you might move the static rc further left in the car so you create a larger lever arm which will make more dynamic roll.
In stead of cutting your top brackets, can you drill another hole in there to alter the a-arm angle?
Coleman sells a threaded ball joint that allows you to adjust your front roll center.
Slick little unit. I highly recommend these. The ones I bought I could alter my RC by about 2" if memory serves me right.
3)If I understand correctly the lower the angle in the upper control arm, the lower the roll center, and conversly, higher angle, higher roll center.
To answer that properly, what do you mean by lower the angle? If you raise the upper a-arm balljt height and or lower the inboard mounting points, then you have lowered your roll center height.
4)Can roll center be too low? What is desired height for 3/8mile tacky high bank and flat quarter mile kind of sandy/dusty?
Yes, it can be too high and too low. Every thing I've read said to put it 3-5" above ground on the dirt. Some people will tell you 4-6". My modified was at 3" below ground before I first started tinkering with it. I'll tell you that it didn't work!
5)Does it matter which side of center roll center falls on? If so, what is recomended.
Yep, the further left of centerline the longer the lever arm you generate which loads the RF more on entry. It will definitely pinch the RF down under the car the further left you go.
In my opinion the only way to do this roll center redesign is with some CAD program or with a scaled model. In the past I've made models out of PVC pipe and cardboard. That is the only way to really learn it. The RC software would be good after you have learned the basics.
Also, just so you understand that when you go making RC changes you also change everything else like toe, caster, camber. And as WFO mentioned these suspensions are all one big compromise. You have to pick and choose what number is important.
I don't know how experienced you are with front ends but make sure you have an advanced understanding of caster,camber, bumpsteer, and toe curves. You might even plot some graphs of that data.
Some other good sources are the Carroll Smith books. Engineer to Win, Tune to Win, and I can't remember the 3rd one.
Good luck, its a satisfying avenue that most people haven't explored on our level.