What damper settings to adjust for wiggling under braking?

Hi all,

I run Nitron R3's on a 996.2 GT3 using 255/35/18 295/30/18 RE71-RS tires on 110/140 f/r N/mm springs.

When the tires are new/fresh, when I brake hard in a straight line the car is rock solid and doesn't wiggle. It's when the tires are getting worn and have a decent amount of heat cycles does the car start to wiggle a lot under hard braking and the rear end doesn't feel that stable. Of course I adjust by being a lot more sensitive to brake pressure but ideally this doesn't happen.

Should I:

1) Add front compression. If yes, is it HS or LS? What damper velocity range is threshold braking?

2) Add rear rebound.

3) Do 1 and 2 at the same time?

Thanks!

increase Rear toe and move brake bias to the front.

Dampers should not have a much affect on braking.

As David suggested - these cars are VERY well sorted from the factory!

To clarify, during a run, you don't notice much, of any, difference, but over several runs you've noticed it getting a lot more 'twitchy'?

I was wondering if there is a change in hot pressures, and tyre tread temp' that is also a factor, as worn tyres have less tread depth to generate heat - have you been monitoring them?

Plus, of course, the chemical reactions to the compound during the cycles.

David - ok. I run 4mm total toe in in the rear right now. I may increase it but to be honest it really only happens when tires are worn, and it's not to the point where I feel I want to compromise how the car performs when tires are new.

Threshold braking should be LS compression, no?

Gord - when tires are new I feel the rear is rock solid during threshold braking. When tires are worn then there's some instability in the rear when braking hard. Minor steering corrections are needed but the car isn't to the point where it's tracking all over. All I was thinking about was maybe trying some minor damper adjustments, and adjust it back when tires are new or just leave it if the car isn't behaving too much different even after the damping adjustment.

I don't think it's pressures because it's not changing during a session. It's changing when tires are say, >60% worn.

Yes I monitor pressures closely. I keep it 31 front 32 rear hot.

OK, since this happens when the tires are worn. How are the tires wearing? Perhaps you have too much camber / toe.

Another possibility: If this is only happening at the end of a stint (not necessarily when the tires are worn, but more when they are hot). Have you checked your hot tire pressures? Perhaps you are getting too much temperature rise and "crowning" the tire. What happens to the braking balance if you start with 10kpa (1.5 psi) less rear tire pressure?

Lastly, rear engine Porsches have always required a bit of two-phase braking, first press the pedal smoothly, you will feel the car "sit down", then you apply maximum pressure. If you drive it like a downforce car (i.e. maximum brake force immediately) you can get that unstable rear feeling. I started driving street Porsches on the track in the mid-1980s, and this technique has always worked for me.

Thanks for the technique tip on braking. I do admit my braking skills aren't quite there yet... will give it another go.

Tires pressures - I'm definitely monitoring them closely to make sure it's always 31 front 32 rear hot, but sure let me try lowering the rear by a psi or so.

Tires are wearing relatively well but with a bit more inside tire wear given how much mileage I put on for the mountain roads, and driving to/from the mountains and track. I'm actually on a conservative alignment of -2.5/-2 camber +2mm +4mm total toe in front/rear.