Originally Posted by cid042

Hi all,

At this stage, I need quite urgent help on this : This season, we enter a 996 GT3 (road car) in a championship where Michelin Sport Cup 2 are the mandatory tires. It is highly modded as a racecar : Central lock, stripped interior, and so on. This car was run with slicks till now, and set up with 996 cup springs (240 Nm/mm front & 260 Nm/mm rear). The 996 GT3 RS (Road car) was delivered with 45 Nm/mm front springs and 90 Nm/mm rears.

With the Sport Cup 2, the car bounces and everything proves that the springs are far too stiff.

We have ordered a study to a well known company to define the best spring rate, and thay conclude that it should be 160 Nn/mm front & 220 Nm/mm, and that seems still too high from my point of view. We have not changed anything yet, and do not want to do this 3 times to achieve a good setup. We have as much smooth track as bumpy ones in there (think Dijon, Magny Cours, Ledenon, Paul Ricard, Le Mans...).

Long story short : May anyone help us in choosing the good spring rates for this car ? It is not for an international championship, but as we all know : Do it once, do it well ;-)). The dampers will be revalved 996 cup (early ones, no adjustment).

Key facts :
* Weight : 1455 kg in total, with driver and half a tank
* Weight at the wheels :
* RL : 459 kg
* RR : 436 kg
* FL : 292 kg
* FR : 268 kg
I do not know the motion ratio, but it is the standard 996 GT3 one, if that may help. The car do not have rose joints in the suspension.

Many many thanks for any help !

Kindly yours,

Cedric

Originally Posted by haulinkraut

I am not a professional racer by any means, but here are my thoughts. I run 160nm front and 180nm rear on cup sachs 2-way dampers. I think this is quite soft even street/track driven. I am actually looking to up my rates. But, for my dampers I get no bounce. And what you will find is unless driven my a strict class regulation, most guys here will vary rates up and sometimes down depending on tracks and driving style. With the fixed dampening cup bilsteins you only have maybe 100lbs of window with spring rates to play with. I considered a fixed damper route when I did my cup conversion, but knowing that I would mess with spring rates I opted for the adjustable Sachs dampers. With fixed dampers you are kinda limited with how much you can play with spring rates unless you want to revalve every time. Bounce is likely caused by under-dampening. So I would strongly consider adjustable dampers seeing you are in somewhat uncharted territory. Just my .02 good luck.

Originally Posted by cid042

Hi,

Thank you for your feedback. Are you running on MPSC2 and still have the stock ABS ?

Bouncing is one thing, the other main issue is having no load transfer ini braking, leding to engaging the ABS on every single hard braking zone, with poor braking efficiency... No problem on this side with your setup ?

Originally Posted by cid042

I must admit that I am not sure of anything. That is my best guess.

When braking, I hit the ABS pretty quickly, and I do not feel so much load transfer. I tried to remove the mastervac, in order to get more feeling, as I thought the over amplification spoilt the feeling.

I previously tried this car with slicks, and it was quite ok. So my best guess is that the stock ABS is quite sensitive, and that I need more load on the front wheels for it not to come on. And that is only on the braking side of the things.

At turn in, the suspension do not seem to move a lot. The tires seems to be overwhelmed nearly instantly. It takes the middle of the turn to tame everything, then the exit lacks motricity. In fact, it is more about predictability and confidence.

At Ledenon, a few weeks ago in the turn called "virage du camion", there is a compression, and the track is quite bumpy. It felt like the rim was hitting the tyre/track, the pressures were ok (I even raised them a bit). In high braking zones, I have to brake very early and avoid curbs if I do not want to be thrown a few meters away.

The outside of my tyres are blueish. They really do not seem to work correctly, and I feel like overdriving them constantly.

Keep in mind that I am more used to slicks on a Cup S car that has lots of grip and brake feel and that I am not a professionnal driver, nor a suspension engineer. I am just feeling something I am not used to in other racecars.

Originally Posted by cid042

Originally Posted by cid042

Yes, right. And even with that, the stability of the rear in braking is far from perfect...

Originally Posted by AudiOn19s

What Diff is in the car? That's generally a bigger factor on stability under braking....well that and or bump steer while the car is diving under the brakes. Do you have adjustable bump steer toe arms at the front and back, I think you said no monoballs (rose joints) correct? What are the ride heights on the car.

Those camber values are great for slicks but in my opinion are too high for the PSC2 tire you are running.

Originally Posted by haulinkraut

Good thought on the diff. But I am curious if a **** diff would cause that drastic of ABS intervention?

And with still running rubber, I definitely agree. Especially in the rear. But just not sure that explains the ABS thing.