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      01-24-2013, 11:22 AM   #16
Pete_vB's Avatar

Drives: '69 GT3, GT4, 1M, 912
Join Date: Nov 2010
Location: SF Bay Area, Shenzhen, Oman

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Originally Posted by redux View Post
Originally Posted by Pete_vB View Post
I'm a bit surprised with your decision to pull the rear bar. Where are you getting this advice from?

I run one of my Porsches without the rear bar, but I wouldn't expect it to help stabilize the rear in your situation.

Running no rear bar means running stiffer rear springs to maintain overall roll compliance. Stiffer rear springs mean you can't put power down as well, causing the back end to step under power.

The other thing I could imagine contributing to instability is mismatched shocks. How have you tuned the shocks so far? Have you used principles like these to guide your shock tuning?
I have no problem with the back end stepping out on me, it's the rate of the transition that worries me. This was just an initial test of the baseline setup to get some numbers with the data logger. The dampers will go back on the shock dyno, valving may be changed, spring will be adjusted to suit, and the package will be track tested again. I have a long way to go with the street tire setup (at least 9 more test sessions) before I call it good enough to move on to things like R-comps and more horse power. By the time I done, every angle will have been investigated, and also tested by people that are much better behind the wheel than I am.
Not sure if you've missed the point. Stiffening the back springs as you've done as well as presumably stiffening the shocks has been the cause of the increased twitchiness you've experienced. Going stiffer with the mains is unlikely to help.

Find a good setup guy to test the car, not just a good driver.
1M, GT4, 1969 Porsche 911 w/ 997 GT3 Cup Motor (435hp & 2,100 lbs)