spiller

Trying to get to the bottom of this annoying trait that my car seems to have had since I’ve owned it (two years). While driving on highways at speeds of 60 or greater, accelerating moderately and letting off (not 100% lift off, but with some intent) causes the rear to sway slightly to the right, then back to the left. My initial thought when I got the car was the rear toe was off, however I’ve had a number of wheel alignments, none of which have solved the issue.

The car has around 20,000 miles and for the most part suspension is stock. I have recently added Ohlins Road & Track.

My only theories are it’s either the rubber in the toe links, or more likely, the soft factory engine mounts allowing twisting of the drivetrain, causing the rear of the car to become unsettled. I feel I have a good grasp of 911 driving dynamics, but the exposure of this trait is annoying to me and I would like to fix it. Although this is my third iteration of the wonderful 996 GT3 platform, it is my first stock one and I’ve so far not really enjoyed the rubbery (for want of a better word) feeling from the rear of this car.

I would appreciate the inputs of the 996 GT brains trust!

Marv

Alignment issues will do that.

However, start with the basics. Insure the tires are correctly inflated. Any small mismatch in pressure will do the same. Also, check the wheels are straight and true.

Worn or damaged suspension parts should be checked and possible frame damage.

DHI

Probably just the TIRES !!!!!
Been there !
Sounds ridiculous but that is what it was in my case.

Check the fab dates on the tires.... likley unmatched.

If not this, pressures. But you too experienced for overlooking this.

Give the tire mismatch theory a go.

Report back !!!

ScottArizona

Could this be a worn diff? Probably not but just thought I’d throw it out there. When my diff was worn I didn’t notice this particular behavior but I did notice a very wobbly rear end upon hard breaking.

De Jeeper

My guess is weak motor mounts.

brontosaurus

I can feel my car shuffle a bit in the back when coming abruptly off throttle in a straight line & always chalked it up to the Guard diff. Motor mounts are OEM but replaced very recently, but I suppose I should check the alignment.

Marv

I have Guard diff plates and do not experience that.

Maybe a worn damper?

brontosaurus

Interesting. I wouldn't be surprised. Stock suspension &~55k miles.

Marv

That was about the mileage when I did my first refresh on my stock dampers. One began leaking, so I had all four rebuilt at Bilstein. The second time I got a little more mileage before those dampers started to fail and I bought Ohlins R&T.

FEKGT3

I experienced similar back and forth when coming off throttle. Thought the same about tires and even thought shock was failing. Ended up getting worse until diff failed completely. Had lsd rebuilt by guard and changed engine mounts as one was leaking. All better now.

spiller

Thanks everyone for the input. The tyres are new MPSC2 Connect and have done less than 500 miles. The diff is stock, but this car has done only two 1 hour track days in it’s life (in my hands) and the car exhibited this trait before it had ever been on a race track. The diff still feels tight when making tight turns so I’m led to believe my LSD is still in good condition. Admittedly I have not checked the break away torque but I feel I can rule tyres and diff out here.

Suspension is all stock low mile and more or less non tracked so I’m confident all the bushes are in top shape. Nothing has been flagged in the two annual services I’ve completed.

The odd thing is I don’t feel this sensation when driving the car quickly on twisty roads, nor did I feel it on track. It’s only there in the freeway setting as described above.

Which motor mounts are people recommending in 2022?

Marv

The diff is most likely shot. It will barely get you off the dealer's lot. There is a test for it that you could do. Basically you raise one rear wheel and turn it with a torque wrench. Someone here probably can explain it in greater detail.

Also, are you sure the damping adjustments are set equally left to right?

ScottArizona

It may not be your issue, but I highly recommend the OEM 993 RS motor mounts. They still have some rubber so you won’t get crazy vibrations or knocking but they are much much firmer than stock. You can piece together all the parts from pelican if you want to save some money or just order directly from Suncoast or the like.

black04

Well then it's definitely the differential. You will thank me for this info
It was worn out before you got the car.

More info

• Most of us don't understand or feel the benefits of a properly operating LSD because we don't take it to the track every weekend, thus all the suspension and tire red herrings that you are getting here
• I'd bet that half of us don't know our LSDs are worn out because it is not engaging at all i.e. It is the equivalent of an open diff. "How can a GT3 handle right if the diff is open, Joel?", you ask. Answer; There are a million 911's with open differentials and a billion Boxsters too. And they all corner like stink (that means "Great")
• As far as the instability you feel, that is because your LSD is not 100% open. The plates are gripping-releasing-gripping-releasing every time you touch the throttle OR brakes, depending on conditions.
• The stock plates are poor. For your rebuild, Go with Guards plates, for several reasons.

Joel

spiller

Well, I’m surprised, but I guess I shouldn’t be! When I ran this car on the track I had no traction issues, but it was a high speed circuit with minimal slow corner, hard acceleration zones. In certain heavy braking situations I remember hearing a groaning/clunking from the rear which would correspond to the releasing-gripping-releasing you explained. I wouldn’t say the car was unstable in the braking zone as per the many worn LSD descriptions I have come across. I was planning on doing a LWFW next year so I guess I’ll add some guard diff plates to the list!

thanks for the input everyone. I have saved some money on engine mounts at least (but maybe I’ll still do them).