DanH

I've been investigating a handling trait on my car whereby it feels like there is a slight weight transfer at the rear when getting onto the power through & out a corner. Initial thinking ,from a recent thread about toe-links where I asked about this, was that my problem was probably bushing/eccentric related and monoballs should solve it.

Having spoken to a lot of people, I'm now coming around to the idea that it's more likely diff related and wanted to run it past a few of you knowledgeable chaps.

The symptom is as described above and happens on both sides equally. Once the guy suggested it was a diff issue, it all seemed to fall into place mentally as actually the car started to feel like it had quite an agressive/abrupt lockup right after I had it rebuilt with cup diff plates. Not knowing better I assumed this is how it was meant to be, but started to wonder again when a supercup driver took my car out and said something felt a bit odd at the rear although he couldn't put his finger on what. The car is still quick and it still grips after the 'transition' but it feels like it won't and its a case of having the confidence to drive through it which is what I'd been working on and getting much better at. I think it would be quite intimidating to a beginner to work the car like that as the transition is very close to the edge of the grip envelope and once its done you are working both ends of the car right through the corner (although in a nice way as setup+4 way dampers are excellent).

Talking to the guy who made the suggestion, who races cup cars etc and has done a fair bit of GT3 setup work, he says that the diff locking should be almost imperceptable if setup correctly. Is that the general consensus?

The rebuilt diff has very little preload as per the normal road going cars, so it almost feels to me like it locks up quite late on and does it too quickly. I'm guessing this is to do with how the ramp angles were set or it was shimmed. Is anyone able to explain what aspect of the diffs setup is causing the symptoms I describe?

Thanks for any help. Trying to get my head around the problem before going back to the guys who did the work so that I present my case in an accurate and correct fashion.

Dan

LVDell

The two differences I noticed between toe arms and diff was that toe arms shifting seemed to be more twitching while diff (under accel) seemed more progressive.

Have you had the car bump steered by a race shop?

DanH

No - wasn't even aware it was possible with standard rear toe links? It was resetup and corner weighted prior to my last track outing though and apart from this feels very good. The guy who made me think it was the diff also pointed out that bump steer is mostly engineered out on the RS anyway?

The feeling I'm getting isn't a twitching one, its almost like a weight transfer under throttle out of corners except the car doesn't feel like it rolling. It does this as I'm getting onto the power and loading the car up for the apex/exit but once the car has taken a set I don't feel it again that corner. Its just this confidence undermining feeling of something shifting at the back, which is why I was initially thinking bushes.

The reason I've changed my mind after talking to this guy :

- Him mentioning it made me think back to when I first noticed it and it pretty much was right after I had the diff done. I'm a little bit hazy on this because not long after I got the 4 ways put on, and also I wasn't sure if maybe my driving had improved hence the diff was meant to feel like that when worked properly.

- They have inspected the bushings and didn't appear to be any give.

The feeling isn't totally unprogressive, but its certainly not seemless, theres a very definite feel of something happening for a fraction of a second. Is your diff almost undetectable in terms of feeling something from the rear (as opposed to how it affects the car steering on the throttle etc)?

Is there much scope for adjusting the LSDs ramp angles etc? It almost feels like its engaging quite late and too agressively. Trying to get a handle on how it might have been rebuild that it could feel like this.

KOAN

What type of motor mounts do you have?

LVDell

Seems like the diff then. I didn't realize you have stock toe arms so that could very well be the culprit since there is no bump steer adjustment. This is a tough one.

It's not a motor mount issue. I'm pretty sure of that.

roberga

the dif is a very easy test that any good shop can test quickly. There are many that would say that if you have tracked you car for any time the dif is most likely gone. The clutch pack material is not very robust. Get cups or have a Paul Guard Transmission dif put in.

DanH

It was after I got the diff rebuilt with the cup friction pack that I started noticing this!!! (although only really realised this in hindsight as its only recently I've started to develop some confidence in my ability to feel setup etc)

I'm wondering what he could have done during assembly that might cause this issue. Different shimming or something? I have very weak knowledge of diff configuration unfortunately but would be great if I could go back to the shop with enough knowledge of what they may have done to not get confused into submission!

As to testing it, doesn't that only show preload? As per GT3 road cars I have very little. If its the diff, its the lack of progressiveness in the lockup thats the problem.

The other thing that strikes me is that my car apparently feels unlike any other gt3 the supercup driver had driven when it does this, so I think it must be something specific to my car and thats suggesting diff rather than bushes/mounts/toe arms etc. He's not a development driver though, hence not really able to help with diagnosis beyond reporting how it feels.

sechsgang

I think I may have the same issue...but I the shift actually feels interestingly helpful to me. That said, I am new to the car and its the first with a locking diff that I have...I dont feel it on EVERY corner, but corners like turn 10 at WGI I do a bit...

DanH

Not sure its my diff anymore. Supercup driver said its all the time, not just on exit of the corner. So maybe I was projecting what I thought was happening He's a busy guy though so hard to chat to him about it.

Holger B

Dan, sounds like you should replace the toe arms if you haven't already done so.

DanH

On the basis of replacing the bushes?

This is different to other GT3s that the cup driver has been in so they could possibly be worn although only 23k miles on the car, a lot from road driving.

So I don't think its down to not having solid eccentrics etc. Also it doesn't feel wobbly, more that it takes a set only after a slight transition.

Also need to mention that I have a 996 GT3 RS so it has modified uprights with a lower mount point for the rear toe links which eliminates the bump steer. Unless its knackered bushings its not bump steer I don't think.

996FLT6

Dan is your car unstable under hard braking? A fried LSD can make the car unstable. Regards mike

DanH

No its very good under braking.

The thing I'm wrestling with is that I'm the first 6 car with the intrax 4 way dampers from Oakley design. I can't completely eliminate them from the equation. I'm coming around to the idea that its eccentrics or camber arms being at an angle despite being the RS uprights. I need to get that checked.

I've been reading the threads about camber arms and that description of a 'wallow' doesn't seem a million miles from what I'm describing.

Does increasing the rear spring rate effect the likelihood of this effect? I'm on cup tyres and rear main spring is 1100 or 1200 I think (although there is a tender so effective rate is not the same I know).

Holger B

I don't know how the RS compares to the "comfort" version that we have here, but the stock toe links seem to move under load and that may be the cause of your problems. You've got a lot of moving parts (pun intended) to sort thru to locate the cause of your problem, so maybe this isn't it.

My biggest reason for replacing the toe arms was to eliminate the eccentrics. My car wouldn't hold the alignment very long with the stock arms and would quickly become undriveable. Problem solved thanks to Tarett.

996FLT6

Another thing to bear in mind if u felt this after the rebulit lsd with cup clutch packs maybe its the aggressive nature of the lsd thats causing the behavior. I know maybe the supercup driver think its a weird behavior but hey your car is still a street car versus a cupcar. With cup clutch packs it's probably more aggressive then our vanilla lsd and maybe that's why u feel more rear weight shift because lsd is acting up early. I have the guard 80/80 lsd and yes I feel lsd biting early. Is that what your feeling? Mike