Where is Cary?

 

Yeah, where is Cary?

 

Last weekend, my friend Butthead was driving my 996TT with me in the car, and I am here to tell you that the car was rock solid at 185mph in the CA desert.

We even got the car airborne at 80mph, and I have the scrape mark on my lower front valence to prove it.

And this is on the bone stock setup with 41psi cold front and rear tire pressure on 18" tires with PSM off. Pyrometer (with frigging laser beam) reading indicated about 120 mid tread, and 114 corner tread on all four tires.

 

The ALMS season opening 12 hrs of Sebring is this weekend and teams have been in FL all week. Perhaps Cary is there.

Greg A

 

Actually, Cary has been working on a project in his shop that is due for testing tomorrow, so he has been slammed.

I'm sure he will answer your question in great detail as soon as he gets a chance!

 

Hey Cary,
Are you out there? I would appreciate your input.

 

Ken,

Sorry it’s taken me so long to chime in. I have about 10 different things going on of which most aren’t going the way I wanted. Anyway lots and lots of input here… I’ve only read part of it so, if I’m repeating anyone, I apologize up front.

These cars have a sweet spot for sure and when you go too low the geometry really goes to hell. The comment on the bump steer is the first thing that came to mind with me as well. It’s really easy to bump steer these cars in the rear with a multi-piece rear toe link. Even though you can zero bump this cars I'd still set it with a little toe gain in compression. One thing that is critical though, is that it’s exactly the same on both sides.

Another thing to consider is that on the rear of a GT2 is that they have very thin solid carriage mount spacer that raises the entire rear suspension about 15mm. So, in essence, the GT2 thinks it’s sitting 15mm higher in the rear than a stock car same chassis to ground height. This was done partially because the car was getting low enough that it was falling out of that sweet spot I was talking about.
For sure tires could be adding to this a great deal. Tires are getting complex enough in design that you finding people having to use different alignments for same sized tires by different manufactures. Never forget that tires are a 4 dimensional spring which have a tremendous effect on the cars suspension.

After all that, I still think the first thing I’d check would be the individual bump steer on each corner and the camber curves. Then, proper corner weights again, with mostly street use do 50/50 cross weights. Once you do that, let me know and we can go from there.

 

Cary,

Have you looked at the Goodyear F1 Fiorano tires. I am looking at the 335/30-18's and either 255 or 265/30-18's. The 335's have a 25.9 OD and the 255's are 26.0 and 26.3 OD respectively. The fronts will fit on my current set up. The rears might need just a tad more room maybe.

 

Cary,
First off, thanks for the response. It is great to hear from you! I would suggest we talk about this on the phone, but the last time we shot the sh_t, we talked for 2 hours!!!!
In any case, I value your opinion so I wanted to clarify/add a few things here.
From what you wrote, the most prominent point is the GT2's 15mm carriage spacer. Essentially, if I read this correctly, it has moved the pick up points 15mm upward, essentially preserving the linkage geometry, or at least bettering it from the lowering effect they have thrown in.
I must share something with you, however. When I was watching my car on the dyno several weeks ago (Mustang 4 wheel dyno), I was specifically looking at the relationship between the front tires and the front fenders. When you get a car going on a dyno like this, there are some resonances inherent with all of the moving parts so the car shakes and bounces slightly even though it is properly tied down. I am talking really small stuff, nothing that is indicative of an improper tie down.
Now here is where it gets really interesting. And this supports the theory in my first post in this thread.
Watching the front tire, as its turning on the roller, and its relationship to the fender, they are moving together!! The slightest up and down jouncing in the tire is also apparent in the fender. This means to me that the suspension damper valving is set very stiff, to the point that small, high frequency, oscillations will not be damped out!! That, coupled with the super small sidewall is analagous to no suspension at all!!!
Said another way.....great for a super smooth racetrack.....bad for roads with slight tire path ruts or the like.
Think about it. If we were to replace the shocks and springs with rigid bars, the only shock absorption would be through the tires. ANY, and I mean ANY road imperfections would upset the tracking of the car.....think go-kart here and how twitchy they feel.
I tend to be an extremist and I think I may have been overzealous setting the damper settings at 3 front and 2 rear. That would be 67% as stiff as is possible in the front (3 out of 9) and 78% as stiff as possible in the rear (2 out of 9). Probably not good for typical roads in NJ.
One last thing. With regard to your 15mm carriage spacer. I originally set the suspension to GT2 spec and then, subsequently, raised the rear slightly. This has created positive rake and, in effect simulated the same 15mm effect.
Cary.......your thoughts on my theory?

 

I spoke to Dave at Custom Alignment a few days ago about the issue of rear bump steer and lowered 996s. He says many shops don't check compression toe at all - just static toe- He has even seen other shops install lowering susp kits and use the wrong eccentric cams for setting the rear toe !
Question for Carey - is the diminished handling issue with excessive lowering with 996s due to the kinematics of the rear compression toe being out of the designed geometric control range ? Kinda scary if the rear wants to lead the front at high speed on a wavy or bumpy road.

 

KPV, the front end jumping up and down on the dyno is probably from the rear tires getting and loosing traction resulting in varying front lift. When the rear tires get good traction it tries to lift the front of the car off the ground.

 

ebaker,
Understandably so, and I emphasize...very minor...to avoid comments about incorrect tie downs, however, the fender and tire move together as if there are no shocks or springs in there. This observation, coupled with all of the other noted effects, means to me......too stiff.

 

Ken,

Stiff is when I angle down a driveway and one of the tires stays up.

 

I hear you on that one Chad. I often look on in amazement when I jack up the car from one of the jack points and the opposite side of the car lifts at the exact same time!!

 

If you want a high degree of response and control, get rid of the rubber bushings and go with metal monoballs and bushings for suspension and sway bars. Talk about a brick hard ride.