GT3 Steering Precision

[clord]

I've had my own GT3 for 1.5 weeks now and have driven a total 3 different GT3s on the track and on the street. They all seem to demonstrate a subtle steering behavior that I would like to describe and see if others have noticed.

It happens in a gentle, steady state turn to both the right and left and close to neutral throttle, that is, not under heavy acceleration nor under heavy overrun. It's all gentle enough that all tires are experiencing a very low-to-no slip angle at all. ... so, this is not about tires slipping/sliding at all. It happens in such gentle circumstances that this is also not about fore-aft weight transfer effects.

The behavior is that when transitioning from light, even steady speed throttle to lift-off/closed throttle the front end tucks subtly into the turn....enough to require a slight steering correction to maintain the turn's line. Open the throttle back just past neutral and the nose steers just a little wide of the line requiring another minor steering adjustment.

I've been thinking about the 60/40 asymmetric limited slip differential and how it may be creating this effect, but, my understanding of it's function would be that it would create the opposite effect. More lockup (60%) in overrun would create more understeer which would push the line wider on overrun, not tighter. I've also been speculating about "dynamic toe" geometry that Porsche has used in the rear suspensions of some cars since the 928's Weissach Axle and wondering if this an artifact of something like that. If so, it's happening at very low loadings.

What is your experience and what are your ideas?

[ldw]

Agreed and it is very noticable. It is a function of the somewaht stiffer spring rates compared to the standard C2 996 and more significnatly the shocks with higher compression valving and much higher rebound control. There is very little droop in the shocks. As a comparison, when the car is on a lift, the separation between the top of the tire and the fender is about the same as the standard stock ride height tire to fender distance of a C2 996 when it is resting on the ground! The described handling behavior is greatly magnified in the cup car race platform on slicks with its yet stiffer springs and differently valved shocks. It is the limited slip which in fact assists in this brief oversteer not being more than it is and when power put back on,...the car relatively understeers again.

[twisted]

I have felt the same thing in my GT3, the car feels a bit of oversteer at neutral power, mine will turn down into the sweeper corner/oversteer and sometimes if a bump is in the sweeper of a turn the car will move up the turn after it hit the bump.

What I have done to what appears to me to have reduceed this is to soften the rear sway bar. Mine came with both bars set at one hole from full stiff. So in the rear I moved the bar one hole further away from full stiff, so it's in the third hole not the second from full stiff. The other thing I did was set the tire presures to factory cold settings of 32 front and 39 rear.

[Speedfreak]

I wonder if this is what "EVO" magazine was talking about in their review of the GT3 and the Stradale. In their words,,,,,

"....911s can often feel reluctant to turn in, requiring a bit of a forearm shove to counter a natural tendency towards steady-state understeer, but it feels more pronounced in the GT3. You simply don't get the sense that the outside front tyre is fully hooked up, which never does much for your confidence, especially through high-speed corners. Without trying a Pirelli-shod GT3 it's hard to point the finger at the Michelins fitted to our test car, but experience in the current generation Carrera suggests the Italian rubber might sharpen things up."

[twisted]

As to the 'Michelins' I'm not sure if I like them. It's hard to feel them out, as where the limit is on them. I would say greasy when I first picked up the car. I'm not 100% sure on this but I think they are coming around some, still not sold on them. As to EVO saying understeer, I'm not 100% sure if I would agree to that, oversteer in sweepers yes and softening the rear bar on my car has helped this.

[Jean-Marc]

I do not believe that this is related to the GT3 only. I have the same on my 2000C2 (e-gas) when I run Pirelli Pzeros. It does not happen with Bridgestone SO3s. Did the cars you drove have Pirelli tires?

[clord]

It happens with the factory Michelin Pilot Sport II N2s. To my knowlege, all US GT3s delivered to date have come with that tire. I understand that PAG will supply GT3s with a Pirelli tire as well, sometime in the future. I doubt that the steering effect that I am noticing is a function of the tires. It feels like suspension or drivetrain to me.

[viken]

As you say, Craig. The tires have nothing to do with this. It is a typical function of a sports car and ldw gave you the best explanation as to what contributes to the feel.

BTW, I was under the impression that the Rosso was going to be supplied as of 9/03 production. But, now I hear the factory has installed many of these on early production GT3's. However, they are all outside North America. My car is still on the ship to dock today or tomorrow. I am very curious to see what the tires are.

Edited October 14, 2003 by viken

[skl]

Have only 800 miles on mine so far but have noticed the same thing- not objectionable just noticeable. As it gets colder here though I'm sure the tires aren't sticking as well when it's under 50 degrees F. I noticed last winter the few times I drove my E46 M3 on cold but clear roads it was a handfull. Those tires need some temp!

[jim911]

These cars have a lot of castor and mine had toe out. after doing an alignment and correcting the toe adj. can handles great. You will still get a little of that oversteer feeling because of the castor but much better.

[viken]

Hi Jim,

Where did you have the toe set at?

[jim911]

We used the factory specs. that GTSteve posted

[jim911]

1/16" toe in works fine. Car came with 1/8" toe out.

[viken]

1/16" toe in works fine. Car came with 1/8" toe out.

I am not sure I understand this, Jim. But, the stock toe out spec is + 8' which is roughly 1 mm which is a hair under 1/16". This is somewhat aggressive but gives you very precise and quick steering with a little dartiness in straight line. Going to 1/16" toe in will increase straight line stability but reduce steering response. Can you please describe the before and after behavior of the car?

Thanks!

[jim911]

With the toe out the car feels like there was a pivit point at the center of the dash, you had that oversteer feeling even though it did not realy come around. There is a lot of camber in these cars so by going to toe in car still turns in very good and that oversteer feeling is gone. Won't really know whats going to be final till I get on a track.