Bumpsteer Toe Arms (question)
 

Does anyone have any direct knowledge of just how much a bumpsteer toe arm will help in relation to a similar one without the bumpsteer adjustment.

For example the 996 street cars aside from the GT3 and GT2 suffer from bumpsteer at low ride heights. Will one of these special toe arms alleviate this?

Long story as I've mentioned before in another thread is that between 3 996 TT's all of which running pretty low, we have had about 12 driver rear tire failures in the last year and a half, sometimes after a day, sometime after a few laps. But the passenger rear is never affected. The separations have come on clockwise tracks with no banks but high compression areas. All three cars had 63" wings on the back and are 996 Turbo's running pretty low ride heights. I've concluded that the only reasonable explanation is bumpsteer toe out under load and with heavy cars with lots of DF it makes them go fast. So I'm wondering if the bumpsteer control arms will fix this or if I have to suck it up and raise the ride height.


TIA

Getting toe-out from bumpsteer in the rear will certainly make the car feel twitchy under compression, but I do not see where it could cause tire failure. If you have to run excessive toe-in to compensate for this, then maybe that is a factor. What part of the tires is failing, inner sidewall, outer sidewall or tread separation?

Inner tread (it's almost the corner of the tread and the sidewall). Some have separated on the sidewall though.

I should also mention that the cars don't really feel twitchy under cornering. But all of the tires are separating on the same side of the car on clockwise tracks. There is no other explanation that I can think of. The passenger side tires will run down to the cords while the drivers side fail over and over again.

We have made it to the cords on R6 vs full slicks (A005, R100, Michelin Blue) though which is even more puzzling. I had to go all the way down to -2.0 negative camber to get pretty even wear on the rears. Oddly enough prior to the wing I ran the Yoko A005 (which have failed on me since) for weekend after weekend after weekend with no problem on the same set. Springs were softer then, but that doesn't explain problems on one side only.

Well in theory the bumpsteer will help as you are reducing the slip angle of the tire

What hot pressures are you running?

What size tires are you running? If you are running a 650 diameter rear slick that is most likely the issue. These tires are meant to run on BMWs and not suitable for the loads on the rear of a 911. All of the manufacturers have a little asterisk next to the 650 diameter tires that say *not suitable as a rear tire for rear engined applications. I would bet this is the problem since you said you don't have it with R6's. You can get away with it on a light car but on a heavy 996 turbo with lots of power you're asking for trouble.

I don't think changing the bumpsteer is going to effect your rear tire problems at all...if the car had so much bump steer that it caused enough toe change to make tire failures, the car would be pretty unstable to drive.

All good points, however also consider that this has happened to hoosier R100's and Michelin blues and yellows as well and in bigger sizes, like 680 and up. I have run the 650 without consequence prior to adding stiffer springs and a rear wing. Besides, if that were the case wouldnt it wear on boths side equally? Only the loaded side is failing with perfect wear on the other side.

your problem is not a direct result of bumpsteer... It is possible your spring rates are wrong for the set-up & DF combination.

I am getting some softer springs ASAP because this was one of my thoughts as well. Front springs are 899 and rears are 1124. Ride heights are from Porsche measure points 105 front and 127 rear.


I would agree on the 650 slicks being the culprit except that before I put on stiffer springs and the wing, I bought some used 280/650/18 slicks from Bob Chick. I ran them at Barber two weekends, CMP one weekend and Road Atlanta one weekend and never had single problem. Still had some depth left on them too, and the wear was pretty good. I ate through a newer tire in one day on the new setup. I think the 650 clause for rear engine is for banked tracks.



BTW thanks for all the help.

I run a 645 rear slick (hooter r80/r100) with no issues per Hoosier and experience.

[QUOTE=wanna911;7855852]I am getting some softer springs ASAP because this was one of my thoughts as well. Front springs are 899 and rears are 1124. Ride heights are from Porsche measure points 105 front and 127 rear.


Your spring rate is appropriate for slicks.