First off, the setup:

• 2002 996TT
• Bilstein PSS-9 (Euro-version)
• Ride height per GT2 spec (verified)
• Alignment spec per GT2 (verified)
• Tires Yokohama 235/35/19 front and 315/25/19 rear
• Stock TT sway bars
• PSS-9 settings 2 rear, 3 front
• Corner balanced by very reputable company

The question:

When I have taken the car in excess of about 140mph up to about 170mph, I find the handling to be very twitchy. I sense the car requires absolute, unbroken attention regarding steering (even in a straight line) otherwise it will go out of control. It seems like too much work compared to what I am used to. The best analogy I can think of is thinking of the wheel path ruts that trucks inevitably create on the highways. As you are driving in those same ruts, your car is sometimes channeled and upset very easily from following its normal path. I hope I have explained myself sufficiently.

I compare this to my recollection of my previous 993 C2 and a 944 turbo I had many years ago. I drove those cars up to 150mph and 165mph respectively and they felt very much in control.

My Theory:

Since my suspension geometry is not arbitrary and the GT2 geometry setup has been painstakingly followed, I believe the effect I am realizing is due to the short sidewalls of the 19's. My thoughts are that the smaller, more rigid, sidewalls transmit the lateral forces of wheel path ruts to the suspension without much sidewall flex.
Normally, with a taller sidewall (stock setup with 18's) these lateral forces, of road ruts at tire surface, are taken by parallel flexing of the two sidewalls together. The forces are sufficiently damped by the time they get to the suspension, thus not requiring steering correction. It becomes a relationship of the lateral force on the tread surface, the flexibility of the sidewalls and the inherent inertia of the car.

2 Analogies....
Place ball bearings on a flat surface, place a sheet of plywood on the ball bearings, connect a glass mount cell phone antena to plywood, push on tip of antenna to move plywood. Not gonna happen since the antenna is too flexible to overcome the inertia of the plywood. (Stock, flexible sidewall 18's)
Now, replace the antenna with a 2x4 rigidly attached to plywood. Pushing the tip of the 2x4 will undoubtedly move the plywood. (Stiffer sidewall 19's)

So, in conclusion, when the road imperfections (ruts) throw these lateral forces into the tire treads, the 19's transmit most of it to the chassis, as compared to the more flexible 18's, thus overcoming the inertia of the car and requiring steering input.

Does my theory hold water? Do you agree? I would really appreciate hearing from Cary on this one.

 

Ken...I am certainly not Cary (and therefore probably way out of my league in trying to contribute here) but have you thoroughly checked the bump steer at both ends of the car. If not, that could be it.

Good luck...

 

KPV,

I have 19's with GT-2 look and height. My car at 150+ or 200+ (I hit 212 in the autobahn) was amazing, straight as an arrow and very pleasant. I also drove a GT-2 at those speeds. How much negative camber to you have? What brand tires are you running?
Though I did had a problem last year and had to change the subframe chassis. The car was extremely loose and bad then. Though it was noticable at much lower speeds around 100-130mph.

 

John,
My knowledge of bump steer is wehn the toe changes due to suspension compression. This was typically combatted with steering rack spacers on my very old Datsun 280Z. Apparently, the variation of range of angle of steering tie rod at rest to full suspension compression would pull the wheels in (toe wise) towards each other. The shims preserved the unaltered geometry. I really don't think that is the issue here since others have not expressed this effect.

EL750,
I am running the Yokohama AVS Sports. I am running the following alignment specs:

Front camber -1 degree
Front total toe +.09 degrees
Rear camber -1.8 degrees
Rear total toe .25 degrees

These specs are precisely within the GT2 allowances as I have just double checked them.

 

Ken,

I am sure you are probably right, but I would have it checked out. It is a process of elimination after all. I have run into this issue before with cars that have been lowered...as have you apparently with your 280z. I gather you have checked all the suspension parts to ensure that nothing is loose, worn or broken.

 

Another variable to the suspesion geometry would be the lower control arms. The stock turbo arms are "shorter" than the GT2/3 arms, so when the car is lowered, you may be in GT2 spec regarding height/camber, but the geometry would still not be the same as on a GT2. Am I right here? If so, perhaps it is not the root cause but contributing to it along with the bump steer issue that John brought up?

 

Try a little more toe in. Try changing the air pessures.

 

Ken, I am not sure if you are aware of the differences, but there are a LOT of differences between a Turbo suspension and GT2. As Bob M mentioned, the geometries are quite different. Aligning your car to GT2 spec does not make it a GT2.

Also, a GT2 is quite twitchy at high speeds from the factory as well.

 

Ken,

tyson is right. why you would mimic a gt2 setup in a 4WD car? i don't understand.

a TT should be much more stable than a finely tuned GT2 at say 180 due to much higher dynamic stability provided by 4WD, especially on imperfect ORR road surfaces. This is why i switched to 4WD for ORR.

on my SY GT2 [alas no more she is], i found a very neutral setup [0 toe, little camber, etc] was best for very high speed, as recommended by CFR/Ruf Racer.

Most alignment guys and suspension tuners have no experience in sustained high speed handling, and thus lead you the wrong way. Cary will know i'm sure, and check with Roland, and ask Ruf.

 

I have corresponded with Mr. Lieb from Ruf and for my car he suggests:

-1.5 front camber
-2.0 rear camber
zero front toe (I think I am still going to go with a very small bit of toe-in so that I do not get toe-out under braking).
20' rear toe-in each side
1 cm rake

 

You took your C2 to 150?!!

 

So, I just got in and I am catching up on this thread and I read them all. I get to the bottom and there is Info's thread. I was rolling on the floor laughing my *** off!! For those that don't know, Info bought my old car. That is it in his avatar.

Now, to address your comments......

John S,
I actually park under the car. It is on a lift in the garage. I routinely look up and inspect things. Although nothing is glaring, I really don't think there is anything broken or loose.

Bob M,
I do recall reading that the turbo lower control arms were shorter. This in itself would reduce negative camber in the front assumiung all else is equal. Suspension 101 dictates more negative camber equates to more contact patch area of the outer tires in a corner. What is its impact on high speed stability though?

ebaker,
I know the frictional thrust imposed on the front tires (and rears for that matter) from driving tries to create more toe out. That is one of the purposes of toe in....so it balances to roughly 0 when in motion. I also know that toe out, when in motion, creates the effects I speak of......the unnerving jitteryness. Hmmmmm......bump steer....toe change.......

Tyson,
I know there are many differences between the GT2 and the turbo. I am not trying to make a GT2 from a TT. I was simply trying to "do the right thing" regarding the suspension alignment. I figured if I am going to drop the car (with the PSS-9's), why be simply arbitrary with the drop and setup? I figured that I have the specs for ride height and geometry for the GT2 and it is based off the same chassis. I figured that would be as purist of an approach as I could have. What were my other options....lower it some arbitrary amount and setup the suspension to what geometry spec? Should I have set up the geometry per the TT spec only with the GT2 ride height?
The specs for front total toe for the TT and GT2 are identical at +5 degrees.
The specs for front camber differ slightly. The TT is 0 and the GT2 is -1 degree.
The specs for rear toe per wheel for the TT and GT2 are identical at +10 degrees.
The specs for rear camber differ slightly. The TT is -1 degree and 25 minutes and the GT2 is -1 degree and 50 minutes.

Watt,
Why mimic? Well, if I am lowering the car, I looked at two choices.

1. Lower an arbitrary amount and set geometry to arbitrary setup.
2. Lower to a known and spec'd amount for another car that uses the same chassis and set geometry to that car (GT2)

Which option would you pick?

John S (again),
I am pretty damn close to what Mr. Lieb recommends. I have slightly less rear toe per side.

Mike,
Yes I did.

Cary,
Where the hell are you????????????

 

If you want the best job, my advice would be to have a racing professional set up your car according to how you want it to behave.

Like fly Cary or his friend Loren Beggs to set up your suspension from A-Z. Maybe even add some parts here and there....

 

Ty,
That is good advice. I am still waiting for Cary to chime in here.

 

Ken,

Are we looking at the wrong thing with suspension? I am wondering if it is possible that you have an inbalance between rear aero downforce and front with your changes there. Not enough front for proper high speed balance. Just a thought. You say it only happens at high speeds...