cid042

Hi all !

I have a strange behavior with my - quite new to me - 996 cup. It is basically a 2005 996 Cup, with a RSR bodykit, and mating rims (so 30/68 on the rear). The shocks and springs are stock, and have just been rebuilt.

When reaching quite high lateral G, I have a somewhat brutal loss of grip, leading to snap oversteer, with a weird sound (see video). Having had a look at the rear tyres, I have a 2 to 3 cm strip on the outside, where the tyre seems to have skidded.

I have tried to add more rear ARB, and it tends to push the phenomenom at a higher G level and I get a little bit of "normal" sliding before the snap oversteer. My educated guess would be that I am reaching the bumpstops, but my mech argues that it is not possible and that the stops are like new...

Here is a video of the phenomenom : see from 2:16.


Any idea or advice are welcome !

MarcD147

not sure but could you have still to much brake on to deal with the steering input? (rear too light and comes around)

38D

Have you run the car more than 2-3 laps at a time? The video only shows a few laps, not enough enough time to get fully up to temp. With the big tires, it will take even longer to get them up to temp. It just sounded like cold/hard tires scrubbing on the ground. Take some tire temps to ensure you are getting them hot enough. Assume these are new tires and not old?

Q&A

Bonsoir Cedric. Looks indeed more like a very sharp turn, in combination with lift off oversteer and maximum weight transfer.
If dampers / springs / stabi's, alignment, ride height, are not very well sets up (also front vs. rear) you might need to work on trying various changes there too.
Maybe it's dialed in for too much oversteer now. Having a modified Cup car (RSR wide) could mean you need to work more on finetuning the set up.

The narrow body 996 Cup is quite sensitive to lift off oversteer as i experienced very often. Driving a 996 Cup fast, requires a sensitive touch.

Here some corners where you can see this lift-off oversteer happening too, on fairly new Michelin's.

cid042

Thanks all for you quick replies.

Yes, everything was up to temp. The vid is just a run of one of our tests of sway bars settings.

I tend to force the car to get the phenomenom. Listen to the grinding noise that goes with it. It feels line cold tyres, which is not the case... If I take a long corner and load smoothly the suspension, it does the same.

My last cup cars experiences (mainly 996 R, 997.1 and 997 Cup S) was a smooth beginning of slide on the edge. Never this kind of brutal loss of grip.

cid042

Forgot to add : The tyres in the pic have been used the last day for 2 30 min races.

The 2 daus before, I used new tyres and had the same behavior (even more, as it was easier to get hogh lateral G’s.

spiller

What are your alignment settings? I also find my 6 cup breaks away smoothly at the rear, although it’s difficult to catch a big slide. It is however very sensitive to lift off oversteer even just a small lift, as was my 996 GT3 road car.

coryf

Possibly the differential. It looked like the rear stepped out when you lightly went to power? Could be a broken diff housing or worn plates. How is the stability under braking?

ghhally

I was also thinking about the diff. Do you know the current setting of the LSD, pre-laod and ramps?

Steve113

yeh seems more mechanical issue . Nut and bold that car

cid042

Thanks again for all your inputs.

The car is stable under braking, but I have no idea of the diff state, as it has not been inspected(yet !).

I will try to match this with the datas I have, to see if there is some throttle application when this happens.

ghhally

Lift one side of the car in the rear, lock the other side. Use a torque wrench and put force on the lifted wheel. Bump the setting until the axle breaks loose.
This will tell you the breakaway force and will give you an indication on the state of the LSD.

cid042

I was wondering how to grt an idea if its state, thanks ! What would a good/bad value ?

ghhally

From the factory they had around 100Nm, 2005 cups even more.

cid042

Thanks !