996 GT3 floaty/unstable feeling

996 GT3 floaty/unstable feeling

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911p

Original Poster:

2,290 posts

127 months

Tuesday 11th March 2014
quotequote all
Was hoping some of you might have some advice on this subject and/or may have experienced this previously!

Yesterday at Brands Hatch the car felt extremely floaty over the straight which pitches and dips quite a lot. In addition, when travelling at speed it really felt like it wanted to follow all of the cambers in the road. Lastly, under braking the car also didn't feel very stable at all, where it is usually rock solid.

It didn't feel like this at Brands Hatch a couple of months ago where it felt completely planted all the way around the track. The only thing which has changed since then is the car has had new tyres (moved from MPS2 to Yoko AD08R).

They were driven on for 500 miles before the track day so fully bedded in. What could be causing this? Could one of the tyres be misshapen? Incorrectly fitted?

Is there also anything suspension related which might cause the car to feel this way?

Cheers all smile

arcamalpha

763 posts

111 months

Tuesday 11th March 2014
quotequote all
Mine's not a 996, but is very sensitive to tyres. I actually find worn out Cups best!

When you say unstable is it the front or rear?

Orangecurry

5,967 posts

153 months

Tuesday 11th March 2014
quotequote all
Tyre pressures?

911p

Original Poster:

2,290 posts

127 months

Tuesday 11th March 2014
quotequote all
Felt like it was the rear end.

Forgot to talk about pressures. When we were running 32F 35R the car felt really skittish at the rear end when exiting clearways onto the straight. When we moved the pressures up to 32F 39R the skittishness was mostly eliminated but the other symptoms remained. Very strange.

Trev450

5,786 posts

119 months

Tuesday 11th March 2014
quotequote all
I don't wish to patronise, but are they fitted the correct way round?

911p

Original Poster:

2,290 posts

127 months

Tuesday 11th March 2014
quotequote all
hehe Yes - we've checked all the obvious.

LaSource

2,384 posts

155 months

Tuesday 11th March 2014
quotequote all
I would suggest rear pressures are on the high side.
Other culprit for instability under braking is the rear diff.
Do you know if still original or replaced? Std or Motorsport plates?
If still original and the car has done over 25k miles of mixed driving then diff plates likely to be worn.

WinstonWolf

71,703 posts

186 months

Tuesday 11th March 2014
quotequote all
If you want it to be the best it can absolutely be book it in for a fettle with Chris at Centre Gravity.

911p

Original Poster:

2,290 posts

127 months

Tuesday 11th March 2014
quotequote all
With regards to pressures, I think the problem lies elsewhere. Felt even worse when they were lower.

Rear diff was rebuilt last year with Guards LSD plates. Car was also fully sorted at Center Gravity a few months ago!

gfreeman

1,047 posts

197 months

Tuesday 11th March 2014
quotequote all
Track day tyre pressures I assume were read when hot. Most tyres like MPSCs seem to go best around 30-32 all round hot on a 996 GT3.

Exceptions are Toyo for which I was reccomended 34F 38R hot by Steve Girdwood (Simply Tyres - those were the days). This transformed the handling when I was farting about at 32 all round and not feeling at all comfortable.

I have no experience of your tyre choice though but as GT3s are pretty sensitive you need to talk to someone who has that combination and knows what they are talking about.

Orangecurry

5,967 posts

153 months

Tuesday 11th March 2014
quotequote all
The key thing is what has changed since last time. If it's just the tyres, then the reason is.... tyres.

911p said:
Felt like it was the rear end.

Forgot to talk about pressures. When we were running 32F 35R the car felt really skittish at the rear end when exiting clearways onto the straight. When we moved the pressures up to 32F 39R the skittishness was mostly eliminated but the other symptoms remained. Very strange.
Is that 32/35 before you started a lap? During? What were the pressures immediately you came in?

911p

Original Poster:

2,290 posts

127 months

Tuesday 11th March 2014
quotequote all
I apologise in advance for these poor descriptions!

We started with 32F 35R hot, and the car had a strange skittish feeling, almost like the tyres were skipping across the track an inch at a time. Felt almost juddery - only when coming out of clearways though which is a little bumpy.

We then moved them to 32F 39R hot and the skittish feeling was gone, but all the other symptoms remained.

nxi20 who is an absolute track veteran in his 996 GT3 (and has used these tyres before) recommended 32F 35R hot.

Orangecurry

5,967 posts

153 months

Tuesday 11th March 2014
quotequote all
Did you try dropping the pressures down?

Otherwise the answer is that the PS2 plus whatever-pressure-you-ran-them-at was disguising a problem that has now been revealed.... but if CoG has set-up the car on PS2s and you liked it, then it's the new tyres that you don't like, because nothing else has changed.

gt3nor

113 posts

107 months

Tuesday 11th March 2014
quotequote all
Could be one or more of the dampers, how many miles on the car and has the dampers been rebuild anytime?

911p

Original Poster:

2,290 posts

127 months

Tuesday 11th March 2014
quotequote all
Good points. The sidewalls of the AD08Rs are way stiffer than PS2s, so if the dampers were worn they would be having to work much harder now over the bumpy stuff.

The dampers performed well on the damper testing machine at CG, but Chris did mention that the dampers were due a refurb which should be done some time this year. Maybe they are worn out.

ETA: car is on 52k and I'm unaware of it having a damper rebuild at any point.

Edited by 911p on Tuesday 11th March 16:35

slodge

510 posts

109 months

Tuesday 11th March 2014
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My 6.2 GT3 was on original dampers when I first got it at 50k+ miles. Felt sh*t on track initially, not very confidence inspiring and pretty skittish. The diff was also knackered. Cup diff plates, Ohlins, new bushes and a lovely geo have literally transformed it to a thing of communicative beauty. I run PS2s on the OEM rims on wet days and R888s on Allegerittas on dry days.

HTH

Cheers

Slodge

Zyp

11,104 posts

136 months

Tuesday 11th March 2014
quotequote all
If you haven't spent 10 grand on it recently, that'll be your problem.

smile

911p

Original Poster:

2,290 posts

127 months

Tuesday 11th March 2014
quotequote all
Slippydiff said:
They'll be toast, get 'em refurbed and start again smile
slodge said:
My 6.2 GT3 was on original dampers when I first got it at 50k+ miles. Felt sh*t on track initially, not very confidence inspiring and pretty skittish.
Cheers for the comments. We contacted Chris at CG and he reckons the rear off-side damper might be shot (it was weaker than the others on our visit last year). He said if both rear dampers weren't acting symmetrically it could cause a load of bump steer. Going to get it booked in for a damper refurb all round smile

Zyp said:
If you haven't spent 10 grand on it recently, that'll be your problem.
hehe

andy102

130 posts

116 months

Tuesday 11th March 2014
quotequote all
Zyp said:
If you haven't spent 10 grand on it recently, that'll be your problem.

smile
and the banter continues....

boxsey

3,414 posts

157 months

Tuesday 11th March 2014
quotequote all
Orangecurry said:
The key thing is what has changed since last time. If it's just the tyres, then the reason is.... tyres.
+1

AD08s are much more of a road tyre than a track tyre and I believe R version is very much the same. They start out with a lot of tread depth when new, 7 or 8 mm like most road tyres. Track tyres like MPSC and Toyo R888 start with much less tread depth (about 5 mm). With so much tread, the blocks will move around more. They might get better as they wear down but I think you would be better moving to a full track tyre like most other GT3 owners use for track work. The old saying of you only get what you pay for rings true when it comes to tyres for a GT3.