997.1 GT3 - rear unsettled on heavy braking - Rennlist Discussion Forums



997.1 GT3 - rear unsettled on heavy braking

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Old 02-21-2017, 12:46 AM   #1
Astrix
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Default 997.1 GT3 - rear unsettled on heavy braking

Hi,

I did search for this but came out empty handed (might have been my search terms)

Anyway, I've got a few track days in with my 997.1 GT3 and with the last track day (last october) I noticed that under heavy braking the back end starts to feel loose, almost as if its pushing from the back? It feels rather unsettled..

I'd put myself at currently driving the car 90% of its capability and I'm still quite conservative when it comes to pushing the car, I tend to do all my braking in straight lines and limit trail braking for obvious reasons.

Its not something I've noticed in previous outings, I am just wondering if there is anything mechanical or adjustment wise I should look into prior to the next outing.

FWIW running an aggressive road alignment and Dunlop SP Race tires in this instance

Cheers
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Old 02-21-2017, 12:55 AM   #2
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Two things contribute to this. Try additional rear wing to settle the rear and my understanding is to also check the status of your limited slip differential.
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Old 02-21-2017, 01:14 AM   #3
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Pretty much guarantee if your diff is stock it's toast and contributing to this.

I also went with a more bitey rear rotor to help give a tick more rear bias.

But you need to deal with the diff. That's job #1 if it's stock. Rebuild with Guard Diff parts or get a GTpro diff in full.
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Old 02-21-2017, 03:59 AM   #4
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As said before, if your LSD is stock it probably failed like all do.

Also standard rubber bushings on the suspension also causes some softness and movement to the car. And the rear toe links with solid ends help with minimizing the movement.
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Old 02-21-2017, 03:50 PM   #5
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Yep. Totally agree with spyerx and sin911. I'd get the preload on the diff checked first.
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Old 02-21-2017, 04:27 PM   #6
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Jack up one wheel in the rear with ebrake off (make sure you have plank of wood in front tire so car doesn't start moving). Turn lifted wheel. If turns with no resistance it's toast. Had guards lsd installed wheel would not spin. Mike
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Old 02-21-2017, 08:31 PM   #7
Astrix
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Thanks guys, I had it in the back of my mind that it could be limited slip related.

I do know the first owner did a lot of track work with the car, so this makes sense.

Typically when lsd fails it will go open, am I correct in thinking when these gt3 LSD fails its actually binding instead, causing it to lock/unlock?
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Old 02-21-2017, 11:45 PM   #8
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I have a 997.1 GT3 also and have experienced the rear end wiggle under hard breaking.
Turn 3 at BIR. Turn 5 at Road America. Both hard breaking zones after high speed straights.

I increased the angle of the rear wing and found significant improvement with no real impact on top speed.
This is an easy adjustment. There is a thread on the forum here with instructions. Any bozo can do it.

The other thing is to adjust your braking technique. Try a moment of easy braking pressure before you stand on it. This tends to settle the car.

Good luck!
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Old 02-22-2017, 01:36 AM   #9
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No,it's an open diff thet just spins and doesn't allow rear to set.
Also make sure your rear toe is right and in spec and not changing.
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Old 02-23-2017, 12:02 AM   #10
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All of the above
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Old 02-23-2017, 01:33 AM   #11
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If the *** wiggles under straight line threshold braking, your lsd needs rebuilding
You can play with the wing to mask the issue but rebuilding your lsd will be worth every penny and cure the issue
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Old 02-23-2017, 09:03 PM   #12
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From memory my wing is set to a more aggressive angle already, but I'll double check it when I get to the car next week.

I have also been talking to Matt at Guards and I'm in the process of getting the new LSD kit sent to us and hopefully installed for the next club day in 2 weekend time.

I may have a chat with my alignment guy too in the interim.

Will report back!

Cheers
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Old 02-23-2017, 10:03 PM   #13
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If you install a new diff just ensure your shop knows how to properly measure and set the lash on the ring/pinion. If you rebuild the existing diff and the stacks are identical you can leave your stock lash.

(At least I think this is how it works based on memory)
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Old 02-24-2017, 12:48 AM   #14
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Quote:
Originally Posted by Spyerx View Post
If you install a new diff just ensure your shop knows how to properly measure and set the lash on the ring/pinion. If you rebuild the existing diff and the stacks are identical you can leave your stock lash.

(At least I think this is how it works based on memory)
I ended up going with the upgraded internals for the OEM carrier over an entirely new unit, so I don't foresee any issues in the preloads and lash settings (unless we find something thats news to us), I am using a good friends diff shop to do it, those guys eat diffs and transmissions for breakfast so I don't expect any issues on the install.

I also ordered for my old 6.2 GT3 the same kit, turns out its way cheaper cos you don't need the upgrade ramps... lol oh well!
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Old 02-28-2017, 03:03 AM   #15
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Check the rear toe!...

Diff possibly but toe far more likely...
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