haulinkraut

I have been watching the other bumpsteer measurement thread and had some questions that I thought would be better off not highjacking his thread. I have a lightened 996 turbo with a full cup suspension conversion with spindles and have been chewing through the inner tread on my rear tires. Uniform inner wear side to side on the tires mind you. My ride height is pretty close to 996cup specs. I do not have any bumpsteer adjustability with the stock 996 cup toe links. My toe was set at 15 per side. What are you cup guys seeing in the way of rear bumpsteer issues? Do you guys see that over or under a certain ride height requiring bumpsteer adjustments on the control arms and toe links? And does camber effect bumpsteer?

rbahr

Hi,

Actually, I would not mind - I learn a lot. To answer your question: I 'suspect' it depends on how you adjust your camber. If you use the top adjusters - as in the front of the car, then no, if you install shims, then yes.

My explanation is that bump steer can be explained as follows :from: http://www.motoiq.com/MagazineArticles/ID/3608



My suspicion is that when you add shims, you are changing the length of the LCA which will change the relationship of the arcs...

Ray

tcsracing1

I bumped steered my 997GT3.

It is something that requires experience. If you mess it up, the car will be worse.
It can take awhile to get the correct setting desired.

I had a rolex grand-am cup team bump steer my car using tarrett bits.

haulinkraut

Thanks for the input. That link didn't work for me. Would love to read it for sure. I would almost think that lengthening the control arms with really low ride height would help. That being said I guess what I am curious about then is if the 996cup is experiencing this too? Or is it because the are running under -3degrees camber at the ride height they run and the geometry works out? Or maybe with 1400lb springs they rear doesn't move enough to have bump steer issues?

Along the lines of control arm and toe arm arcs, whether you run -1.5 or -3.5 rear camber if the toe remains constant the only difference between to rotational arcs of the control arm and toe arm should be the ones induced by the ride height and the different inner amount locations at the subframe. No? Because in theory if you have to add say 5mm(hypothetical) of shims to the control arm to increase your camber then the length added to the toe link should also be 5mm to maintain toe. Or is this not the can based on the upward or downward incline of the control arm and toe links?

rbahr

I tried to fix the link...

I would need to instrument this, but I would thing that 1400lb springs don't move much.

WRT your question on LCA length - I don't know - suspect that in reality it is not linear.

Ray

lordpantsington

working link: http://www.motoiq.com/MagazineArticl...Toe-Steer.aspx

Leigh2

996's have inherent bump steer in the rear regardless of the camber setting. You can realistically only adjust the rear camber using shims in the rear it's really not recommended to monkey with the length of the dog bones.
Take the spring out, set up your strings and measure the toe change throughout the suspension travel. Adjust using a bump steer adjustable toe link and washers. It's not that complicated once your setup and doesn't need to repeated every alignment.