atr911

Initial thoughts:

More toe in in the rear? Currently at 3 mm

Increase compression in the front?

Increase spring rate overall? Currently at 500 lbs

Putting an LSD in over the winter so that'll help alot, just wondering if there is a temporary solution to help.

Veloce Raptor

Add rear rebound.

And the LSD.

Or just give her rear end a nice slap.








Professional Racing and Driving Coach

Land Jet

I'm not nearly the driver VR is but my last instructor helped me with this problem in a 150mph braking zone. He had me braking hard initially and them slowly easing back on the brakes as I approached the turn in point. This easing back off the brakes took some of the weight transfer off the fronts and it helped settle the back end thus eliminating the wiggle.

RonCT

This is a question for the people that set up your car - too many variables. I spent about a month trying to figure out the rear end braking hop / wiggle when I went to Damptronics + H&R Sways on my 07 C2S. In the end, the senior engineer at Bilstein in CA suggested I was asking too much from a street oriented suspension and that I should go back to stock PASM / sways. Reverted and everything calmed down again. Often it's hard to mix and match and remain as balanced as the factory made the car in the first place. I'll say this about Gen 2 SPASM, they really did nail it this time.

2BWise

Or this

Or adjust brake bias toward the front.

Veloce Raptor

He does not have adjustable bias. And IMO adding front bump does not solve the rear end issue....but keeping the rear end better planted (ie more rebound) will.








Professional Racing and Driving Coach

SundayDriver

Don't you mean reducing (softening) the rebound in the rear? I know that some people say "more rebound" when they mean a more free rebound response. He needs the rear to drop quicker and make firm contact with the track. In my jargon, that is reducing rear rebound.

Larry Herman

+1. I think that there is a mis-impression that tightening the rebound "holds the car down better". Yes, it will limit initial body movement, but it does so at the expense of grip.

For those who do not agree, think about it this way - if you are tightening the rebound to hold the body down, what is holding the tires down?

beentherebaby

Less rear rebound damping force would allow the rear tires to stay on the pavement with more pressure than stiffer/increased rebound. More front bound compression force would be my choice unless the track is too rough for it. Either more front bump or less rear rebound could work but you'd need to test to see which is the best overall for the circuit.

SundayDriver

In general, I feel you are better off adjusting on the end that is causing the problem so I would start with adjustments at the rear. There are exceptions, but it needs far more than an internet post to understand if that is appropriate. That is my opinion, but it is backed by pretty much every race engineer that I have talked to or read.

atr911

Great info so far. The track is brand new and really smooth. I'm going to increase front compression for the next day and see how it feels.

I'm not 100% sure the rebound adjusters are working correctly as there is a problem with the detent on te adjusters. I'm going to have them rebuilt this winter.

atr911

Initially TPC recommended setting 5 all around but 7 for rebound in the rear. I'm going to drop rebound 1 all around and increase compression 1 all around for the track and see how it feels.

Carrera51

The LSD will help. My Carrera and a fixed tail and LSD, the 964 doesn't have either. The Carrera's rear is much more stable under hard braking. I am guessing that the tail reduces lift at the rear end more than the retractable tail element on the 964, and coupled together with the LSD, make the Carrera's rear end much more stable under hard braking at the end of long straightaways, like the back and front straights at VIR and the front straight at SP and WG. Does your retractable tail retract when you go below 80mph even though the car is still moving. The 964s stays deployed until the car comes to a stop.

I don't have adjustable shocks in either car so I am no help there.

beentherebaby

How do you know which end of the car is causing the problem? We know where the problem is sensed not where it is induced.

You could lower the rear rebound to improve tire contact pressure for stability but then you'd have more rear body lift under braking, meaning increased weight transfer to the front, defeating the intended goal.

The only way to know is to test, as I originally said. Increasing front bound force keeps the front from diving and the rear rebound keeps the rear from lifting so the overall effect is a more stable platform that doesn't pitch excessively under braking.

You can often achieve a desired objective via several methods but the secret is finding the best combination for the car/track/driver. The only means to know what works best is to test.

Larry Herman

Technically, that is not correct. You will still have the exact same amount of weight transfer, it will just happen slower if you add more rebound, at the expense of grip during that transition phase.

Maybe knowing exactly when (in the phase of braking) the back of the car "wiggles" would help to narrow in on possible solutions.