Qwickrick

The limited slip differential in my GT3 is fried, and I'm getting severe "wandering" under heavy braking. I understand how a LSD works to prevent wheelspin under acceleration, but don't understand how it acts during deceleration to keep the car straight. I thought it was the ABS that sensed the decelleration rate of each wheel and made adjustments to line pressure accordingly to control this.

This is a common enough problem with 996 GT-3's, but I would like to understand the mechanics behind it.

I have a full RS suspension underneath, no rubber bushings, and new pads and rotors.

Thanks

dan212

An interesting article from Guards on LSD LINK

Qwickrick

Good article

ECS

Wow, great article. This is something I would love Excellence to do. Maybe under a technical racing section.

dbryant61

Qwickrick,

I'll read the article also when I have more time, but I'll offer some personal experience. I used to flat spot a rear tire way too often when I had an open diff, and spun twice under hard braking. I installed a LSD in April and have done two D.E.s since. Now I occasionally will lock up a front if trail braking very hard, but not a rear (so far). I can feel the rear move out just slightly and then stop. I'm guessing that is when it hits the LSD lock-up percentage. It feels very stable and I am able to begin braking deeper than before.

tkerrmd

the Guard LSD I have has 50/80 lock up. 80 on acceleration but dont forget the contribution of the deceleration which helps stabilze the car under hard braking and trail braking. Your stock GT3 unit probably is more like 40/60?
Get the Guard as a replacement.

Rassel

Do you trail brake and heal-toe?
What pads are you running?

Geoffrey

Tom, if you installed the Guard pressure rings in the 50/80 configuration, then you have 50 on acceleration and 80 on deceleration. The issue with the stock GT3 unit is the number of plates and the preload on them, not necessarily the locking ratio.

Qwickrick

Yes to both, and double clutch my heel/toe downshifts/I'm running cup rotors (new) and pagid 29 front/14 rears as I have for the past two seasons.
We already tested my stock LSD, and it's left the building.

tkerrmd

you are right, sorry my bad, thanks for picking that up

Rassel

Of course you know this already; trail braking and a little uneven rev match can unsettle the rear. This combined with the brake balance that you moved to the rear can make the rear twitch in braking. I presume if you press the clutch, the rear goes straight?

C.J. Ichiban

I think the stock LSD is a lot lower ratio, like 25/40

Bill Verburg

996 GT3 oem would have a GKN 40/65, acc./dec.

PorscheII

Rick

The GT3 LSD has brass friction discs ... which, while a lot better than carbon discs, still aren't really a track disc. Change those out for Porsche Motorsports or Guard clutch pack, and the difference will be astounding, regardless of the ramp angles.

Qwickrick

My problem came up at the end of long straights, like T1 at WGI, where I try to brake very hard, and very late...nothing to do with trail braking or unsettling downshifts. I first thought it was uneven braking surface, but then I had the same problem at T5A (bus stop) at the Glen. Same problem came up at T1 at LRP, and T1 at Lightning....and then decided it wasn't all the track surfaces going bad at the same time......then we tested the LSD and it was fried.