John,
I don't think it has anything to do with the aerodynamics. The reason I say this is because the symptom does exist at lower speeds as well. The big difference, however, is that at lower speeds it is not nearly as pronounced since things do not happen as fast. At close to 180mph, you are doing 3 miles a minute, and small deviations have large results. I really wish there was a better way to describe this.
Another thought I had was that the super short sidewalls plus the 2 and 3 (front and rear) settings on the PSS-9's have made the car perfect for a super smooth, perfectly paved road......stiff, stiff, stiff.
Maybe, especially considering I am in NJ, I should substantially back off the settings to something around 7 front and 6 rear. I again refer to my cell phone antenna vs 2x4 on plywood on ball bearing analogy. If the sidewalls are not giving laterally and the struts are not flowing enough through their internal valving, I will be simulating a "no suspension" condition. That said, all of the small lateral forces thrown into the tread will be nearly completely transmitted to the chassis thus requiring steering correction. The faster I go, the more quickly these effects take place and more skittish the car feels.

BTW, the reason I discount the aerodynamics is because of two things......

First, intuitively, it seems the increase in rear downforce due to the Gemballa rear wing appears to be comensurate with the increase in front downforce due to the GT2 nose, especially considering the proper radiator mount and corresponding exhausting of radiator flow-through air over the hood rather than under the car. That would lessen the under-car pressure and increase downforce. The extended mechanical wing with the Gemballa wing base in the rear appears to be roughly equivalent from a drag standpoint as the GT2 wing.

Secondly, the front end or rear end do not feel light at all. The car grips really well. At the higher speeds, it simply feels like it is pushed from side to side slightly, thus requiring steering correction and constant attention.
Incidentally, I have driven a previous girlfriend's BMW 3 series car with 0 rear toe (due to mal-adjustment after an accident). I assumed the car went into a + toe condition at speed and it was an absolute nightmare to drive. My car exhibits a completely different feeling.

 

KPV,

I will repeat. Have a knowlegeable mechanic check the subframe chassis where the adjusting screws are. Not easy to see damage I will try to get a picture for you tomorrow from my old parts. This is vulnerable in a stiff suspension setting. How are your roads btw. Any potholes? Your setting is really stiff.

 

I called the company that did the install and they said to check the tire pressures first. They recommended cold pressures of 36 psi front and 44 psi rear. This is Porsche's recommendation.

I checked mine and I am at 34 psi front and 36 psi rear.
Thoughts anyone? What would the effect of 2 psi low in the front and 8 psi low in the rear amount to?

 

I do not know if tire pressures could cause this but I have spoken at length a number of times with a fellow from Michelin's race tire group and his pressure recomendation for both Pilot Sports and Sport Cups is 34 front and 39 rear...cold.

 

Hi everybody,
I'll put in my guesses as to the causes to Ken's instability at speed, something I test for frequently in these cars.

1. Rake-Without at least some (at least a quarter of an inch is needed) front end lift will lighten the nose dramatically. The GT 2 specs have some range for front and rear. If you are in the high end up front and the low end of the rear spec, you may have a nose up attitude. The rake will create a low pressure condition in the center/rear, sucking the car down, as opposed to a high pressure in the front/center, lifting the car. The lift additionally unpresses the toe in front, causing toe-in.

2-Front Toe-The force of the road should force toe further in if the car has toe-in to begin with. The force of the road should force toe further out if the car had toe-out to begin with. On cars with older wheel bearings the only reason for the toe spec was to load the wheels either in or out to help maintain the toe setting. A zero toe setting allowed the wheels to flip-flop from toe-in to toe-out and cause a weaving behavior from the car. The choice of toe-in or toe-out will affect turning behavior, not straight line stability. With today's non-adjustable,sealed front wheel bearings(as used in the 964,993 and 996 very little if any toe is desired, or needed, at all up front.

3-Rear Toe-In the rear the suspension toes in when pressed. It must have some toe-in at all times. Rear toe setting only needs to be in 20' range, the rest comes under load to keep the rear end from passing the front.

4-Camber-good at the track not good at super high speeds. Ruf will tell you, it will overheat the insides of the tires. Especially in the back. No more than negative 1.5-2 degrees in back for Bonneville. On the old CTRs Ruf said -.4 max in back! Or else the tires would overheat at sustained speed.

5-GT2 Suspension pieces-These will have more of an effect on camber gain and handling in corners than alignment or straight line stability.

6-Damping-Run the PSS9s much softer unless the roads you are 'testing' on are very smooth. They need to soak up the road's imperfections. (around here, anyway)

Wait, let me get on my Nomex
PerfPow
www.perfectpowerinc.com

 

Ken
I am not in the same league as some of these guys making suspension recommendations but I have an observation to give.
I recently changed tires and wheels to get ready for an upcoming track event. I installed used Michelins with stock turbo wheels. I noticed the same 'twitchiness' that you initially complained about. It happens at much lower speeds. I made no other changes. My son just replaced his winter tires with new stock tires on his STI. He complained about a twitchiness also - independent of me. Your problem may be soley tire related.

Also, note that you are running 19" wheels. This may affect tire pressures as well as some wheel geometry.

David

 

PerfPow,
Thanks for your very in-depth response. Can you tell me what your credentials are? I ask,because you are new here and I wish to understand your background. I am a structural engineer and have extensive knowledge in physics, mechanics of materials, etc., etc. In general, you make some valid points. My comments corresponding to yours follow:

1. I know exactly what you mean regarding rake. In actuality, when the car was first done, it did have a almost imperceptible nose up attitude which drove my **** retentive visual receptiveness insane! I went back to the alignment shop and we raised the back to the highest margin of the spec. From a visual standpoint it corrected the problem. I believe the rear height increase was 1/4 inch.
2. I disagree with your comment The force of the road should force toe further in if the car has toe-in to begin with.. This comment would be valid only if the center of the tire contact patch (as viewed from above the car)was inboard the theoretical turning axis of the wheel. This is not the case. It is simply a yaw moment. The friction on the tire tread due to forward motion will always tend to induce more toe out from wherever the starting toe setting was. Toe change due to bump steer is a whole other matter. I generally agree with the rest of your #2 statement.
3. Same comment as #2, however, I agree with the need to have toe in.
4. This is very interesting since I now have additional negative camber and your comment makes a lot of sense. I will explore this further. Cary, where are you man???!!!
5. I am aware of the longer front control arms but nothing else. I agree they will impact camber. I am not aware of the link between camber and straight line stability beyond #4.
6. This will be my second trial. I will increase my rear pressure to 44psi first and then try going less stiff with the valving setting.

Disclaimer:
I am simply sharing my knowledge and opinions with you and do not mean any disrespect.

 

Ken:

I am enjoying this thread and can't wait for the masters input (Cary). I do have a couple of comments.

Where the hell in NJ are you going 170mph?

Also, I would be more concerned about the mismatch aero down force. You could start to see this around 60 mph normally (or slower if your driving into a stiff wind). Obviously, the higher the speeds the bigger the effect. This is precisely why I matched the TA GTS front and rear deck on my car. I’m hoping they knew what they were doing.

The other point is the car won’t track slightly as well in a straight line with high neg camber because you’re not putting as much rubber on the road but your numbers were fairly low so that shouldn’t be a issue.

 

Hi Ken,
My name is Sol Snyderman. I have owned Perfect Power since 1981 when I incorporated in Illinois. I have been tuning and racing 911s since before I started Perfect Power. Up until recently, when we started playing with the new Mini Cooper S, Perfect Power has serviced, modified, tuned and raced 911s and nothing else. This led to us becoming an authorized Ruf dealer from around 1984 through around 1995. I'm also an FAA-certified powerplant mechanic.
I test drive cars more often than I should and faster than I should. In doing so I've learned some things that I'm sharing with you now, and other things that I don't have time to share or discuss today, because too many experts will dispute them. I'll pull those things out in the future!
Rake is critical the cockpit floor must have some nose-down rake.
The toe issue was explained to me by one of Amco Tool's legendary engineers, Lennie Morrison. It's a tricky one to test, so I've gone with the explanation.
Also the 19 inch tires are a big factor. Don't underestimate the change in overall spring rate when you change to these lower profile tires. Softer compression settings will help but not cure their tendency to be more easily influenced by road imperfections. Try other wheels/tires.
Finally, I'm a huge advocate of corner balancing. Is it necessary? You'll find out after you put it on the scales! If it is good then you didn't need to do it. If its bad, then you were lucky you checked it. I've seen factory cars 'out' by 130 pounds. I'm happy to discuss the numbers. Most shops won't because they don't know how to calculate their target weights. Give me your weights and I'll send you your targets. I have no secrets....well, maybe a couple.

Sol

 

Tim,
I'll never tell!!

Your negative camber comment weighs heavily on me....both from a stability and wear standpoint.

I really do not have reason to believe the aerodynamics are at fault. The front or rear do not feel light at all and the car remains extremely responsive. It simply seems VERY susceptible to lateral movements due to inconsistencies in the road surface. Steerin input is dead nuts on. A small input equates to instantaneous response by the car. This brings me back to stiff, small sidewall + stiff shocks = bad for certain "less than perfect" roads.

Sol,
Thank you for the CV! Very impressive!
Regarding rake, when I went back to fix the rake due to my **** retentiveness, he did verify the "nose-down" rake with a level on the door sill. I try to cover all the bases whenever possible!! Educate oneself!!

Regarding corner weights, here they are:
LF 670#
RF 678#
LR 1105#
RR 1060#
The shop I went to is very active with PCA and has been in business many, many years and does racecar setups as well as street setups for Porsches. They are Eurotire in NJ. I have used them for many years and they are very capable and very knowledgable.

 

Sol, You’ll be seeing my car when I get it to get the suspension set up right. See you in May time frame.

 

Ken, I know exactly what you mean as we had discussed this once before. I feel the same exact problem with my car and I am now about 99% sure that it's the 19" wheels. On my 19's at high speeds the car gets loose and bouncy, jittery and requires too much attention. When I ran my car with GT2 wheels and Michelin Pilot's the car felt way better. If you have your stock wheels give it a try. And raise the settings a little on your PSS9's, NJ roads are horrible and the suspension needs to absorb the imperfections.

 

Ken,
Your weight distribution, 38.37 % front, and 61.63% rear yields targets of

681 LF 667 RF


1094 LR 1071 RR

By my method your car is 'out' by 11 pounds. It is 11 pounds low on the left front and right rear corners. And it is 11 pounds heavy at the right front and left rear corners.

These numbers are for a 1/2-2/3 of a tank of fuel and driver in the seat with the seat in the correct position.

Check what our set up sheet looks like on our website if you're interested.

www.perfectpowerinc.com

Am I shamelessly promoting yet?

Sol

 

Hi. Those corner weights give the cross weight (LF+RR) - (RF+LR)
of 53 lbs. Ideally it'd be zero, but it's not a *huge* diff. In order
to get good corner weights you should ensure the tire pressures
are right, and even disconnect one end of the sway bars. If the
scales aren't at exactly the same height, you'd be off too...
As I mentioned in another thread, I think it's important to
take corner weights with two different car loadings because there
can be two problems (like a weak spring *and* a higher preload)
that can compensate and show a neutral corner balance with
one load, but under a second load the problem would show up.
In the example I gave, under a second load the weake spring
would still travel more according to the new load than it would
if it were at full strength, so would contribute to unequal cross
weights in the second loading.
Joe

 

Joe, makes sense. Could you just do it with 1/4 tank and then full tank or would the weight distribution change screw things up? Could you put weights on the front and back of the car?