I have been struggling with a handling glitch recently and I figured I would post some information in an effort to collect some ideas.

The Problem
When I am in a fast corner (60-80mph) OR at very high speeds (130mph+) in a straight line, I find a need for constant steering correction.

In a corner, I find myself continually correcting for any slight undulations in the road surface. Constant attention is required. I tested this the other day and found, in a constant radius turn, the lift-throttle oversteer occurs as predicted and the effect is diminished when cornering while accelerating (Read unweighting front suspension while corning)

In a high speed straight line, I find the car to be very twitchy as if the car was caught in the truck tire ruts that commonly occur on highways. The steering requires constant attention.

My Theory and Some Background

I believe the effect I am experiencing is bump steer. For those that don’t know, bump steer is the steering effect that results from changes to the suspension geometry as it goes through its range of motion (generally compression). It becomes more and more prevalent in lowered cars. The reason for this is that the geometry of the lower control arms is altered by lowering. As an example (actual values are different) if the factory ride height USA TT has control arms that angle down 10 degrees towards the wheel when the car is at rest, their range, through compression of the suspension, may bring them to a horizontal position. Conversely, a lowered ride height USA TT may start with the control arms horizontal and at 10 degrees above horizontal at full compression. The tip of the control arm at the wheel will either be moving away from or towards the chassis as a result of suspension movement as it follows its arc of movement. Since the control arms are located behind the steering rack tie rod ends the following should theoretically occur (please keep in mind these are generalizations):

Factory downward sloping control arm
Undergoes a positive toe change through suspension compression

Lowered (assumed) level control arm
Undergoes a negative toe change through suspension compression

In my old 280Z set up for the track, we actually shimmed beneath the steering rack to preserve the control arm angle geometry.

When I lowered my TT with the Euro version PSS-9’s, I told the shop to set it up per the GT2 ride height and alignment spec. Before I get flamed, my choice and direction were simply because I figured same chassis, same ride height, same geometry rather than arbitrary values for each. At the time, I was unaware that the control arms were different lengths as well as other aspects of the suspension geometry. I confirmed all of the final settings met spec.

Remedial Plan

I have an appointment with the alignment shop for next Tuesday, April 27, to check and change the geometry. I am thinking of going to strict TT spec. (USA or ROW not sure)

The alignment geometry for the three cars are as follows:

Front Axle

Toe unpressed (total)
USA TT: +5’ +/-5’
ROW TT: +5’ +/-5’
GT2: +5’ +/-5’

Toe difference angle at 20 degree lock
USA TT: -1D20’ +/-30’
ROW TT: -2D20’ +/-30’
GT2: -1D50’ +/-30’

Camber (with wheels in straight ahead position)
USA TT: 0D +/-15’
ROW TT: -30’ +/-15’
GT2: -1D +/-10’

Max. Camber difference, left to right
USA TT: 20’
ROW TT: 20’
GT2: 15’

Caster
USA TT: 8D +/-30’
ROW TT: 8D +/-30’
GT2: 8D +/-30’

Max. Caster difference, left to right
USA TT: 40’
ROW TT: 40’
GT2: 40’

Rear Axle

Toe per wheel
USA TT: +10’ +/-5’
ROW TT: +10’ +/-5’
GT2: +10’ +/-5’

Max. Toe difference, left to right
USA TT: 10’
ROW TT: 10’
GT2: 10’

Camber
USA TT: -1D25’ +/-15’
ROW TT: -1D25’ +/-15’
GT2: -1D50’ +/-10’

Max. Camber difference, left to right
USA TT: 15’
ROW TT: 15’
GT2: 15’

Vehicle Height

Front Axle height with 18” wheels (from road surface to lower edge of hexagon-head bolt(a/f 18) of the tension-strut screw connection to the body)
USA TT: 158mm +/-10mm
ROW TT: 138mm +/-10mm
GT2: 118mm +/-10mm

Rear Axle height with 18” wheels (from road surface to locating bore in the rear axle side section (between toe and camber eccentrics))
USA TT: 158mm +/-10mm
ROW TT: 148mm +/-10mm
GT2: 133mm +/-10mm

Request from you guys

So, with all of that, my questions are:

1. Where is Cary? I really need to talk to you on the phone!!
2. Does anyone have any advice or insight?
3. Should I follow the USA TT spec or ROW TT alignment spec since they are based on same parts only lower springs (ROW specs seem more appropriate in my opinion)?

 

Ken: Have you considered bringing the car to have the suspension fined tuned and corner balanced??? DeMan Motorsports probably has the equipment and experstise to dial (optimize) in your suspension setup. Just a thought.

 

The car was corner balanced. I have gone to Eurotire for the past 20 years and I am very confident in their abilities.

 

Does Eurotire tune race cars??? With the speeds you are concerned with, someone with race tuning experience and expertise might be your best bet...Best of luck getting it ironed out...

 

Ken, 40mm would be considered a significant lowering of your car. You list the front toe as 5' +/-5'. With that spec, you could also have a 0' toe (or even toe out) situation in the front. That will certainly make your car wander much more than bump steer would. I have driven cars with noticable bump steer, and I find that it manifests itself mostly in undulating corners, where the steering "loads and unloads" due to toe changes during suspension movement, and on one wheel bumps, where it tries to rip the steering wheel out of your hands. If you want it to go straight, first make sure that it has at least 1/8" toe in. Then, have it bump steered by your shop to see how bad it actually is. I am not familiar enough with the 996 series (yet) to advise you as to the best method to reduce bump steer (rack spacers, higher/lower tie rod ends, etc).

 

Larry,
Thank you for the thoughtful response.
I looked at my specific alignment output.

FRONT:
Left Camber = -1D 0'
Right Camber = -1D 0'
Cross Camber = 0D
Left Toe = +0D 3' 0")
Right Toe = +0D 2' 24")
Total Toe = +0D 5' 24")

REAR:
Left Camber = -1D 54'
Right Camber = -1D 48'
Left Toe = +0D 7' 48"
Right Toe = +0D 7' 12"
Total Toe = 0D 15'

All of these values meet the spec for the GT2 setup. As a matter of fact, I could not be more pleased at how close they got everything to what I told them. I am very happy with Eurotire.

So, now what?????????

 

Chris,
Eurotire is and has always been involved with Porsche Club Racers for as long as I have been going to them (15-20 years). They know their stuff.

 

Ken, perhaps the best way to diagnose the problem would be to bring the tuner out for a "hair blowing back" ride up to speed where these handling issues occur...As Larry said, maybe it's just a toe camber adjustment...Having the identical adjustments as a GT2 may not be the cure since the GT2 is rear wheel drive so perhaps the awd suspension geometry behaves differently and the GT2 specs wouldn't be optimal...just a thought...

 

You have a great deal of info on set up there, but this is my set up on the GT3, it feels like it is on rails, and is a total joy on road and track, but this is a track and twisty road set up, not a high speed autobahn set up.

Track settings for GT2/3 from Germany.

Front axle:
Camber: -2.5ΒΊ
Max camber difference, left/right: 5'

Toe total: +5'
Toe difference at 20Β° lock: -1Β°20' +/- 30'

Caster: 8Β° +/- 30'
Max caster difference, left/right: 30'

Front height: 115mm
Swaybar – 1 from hard

Rear axle:
Toe total: 35’ +/- 2'
Max toe difference, left/right: 5'

Camber: -2.3Β° +/- 5'
Max camber difference, left/right: 10'

Rear height: 128mm
Swaybar – 1 from hard

 

ken,

the set up for 130 -190 mph should be quite different than your cornering setup -- very neutrAL...

i had the same prob;em in my SY GT2 and Tom now Lucent our sponsor fixed it, making the car almost O degrees camber and the car then tracked at very hi speed, he is worth talking to.

 

So now get it bump steered. Or you could raise it back up to US height & see if the problem goes away with your current GT2 alignment specs (only change being ride height). I would hazard a guess that the GT2 has completely different spindles, and so the tie rod arm angle is correct at it's factory height, while your's is correct at the US TT height, and will need to be modified if you lower your car to GT2 height. A proper bump steer exam will tell you what you need to do. Of course you don't have to re-invent the wheel, and you could go to a shop that has previous race experience with lowered TTs.

 

Larry,
I am totally with you here. I did some further research and will be having them check the bump steer. I believe it to be the problem. I also agree that the wheel carrier's (hub) tie rod alignment is different on the GT2 compared to the TT as you said. I had a long conversation with Steve at Rennsport and I believe I am on the right track. The results will be in later this week.
I am not trying to reinvent the wheel but rather educate myself. I need to understand the car. I cannot just simply hand it to someone and say fix it. It is my nature to explore all aspects of my interests.

Cary has been away at a race this past week and I am still hoping he responds to my emails and voicemails.

Race shops in NJ..........hmmmmmm................any recommendations??

 

Where are you in Jersey? I know of a couple of excellant shops west of Phila that can help you. One has an ex-Penske Race Engineer on staff who was crew chief for a car that won at Indy. Talk about credentials.

Also, I feel the same way. If I have to pay to fix something that I don't understand, at least I want to know what it was, and what it took to fix it.

Just checked your picts. dOOd, was that you at Hershey? Your Gemballa Biturbo? If it was, I saw you at the end of the day when you opened the door for the young guys who were drooling all over it. I was the tall, older guy in the light blue Maui Tee shirt, with the close cut beard.

 

Larry,
I am in central/west Jersey. I would appreciate the names and contact numbers of the shops you are speaking of.
As an engineer, I am forced to scrutinize the information that freely flows from many sources. I have to look at the credibility of the source and then make a judgment as to its validity. Call it a curse.
Yes, that was me at Hershey. I really wish you would have introduced yourself. I would have loved to meet you. Incidentally, I took first place in the Concours for my class. Up until my arrival, I was on the fence about even entering!! It was a nice surprise. My first show too!

So, here is the latest and greatest concerning the suspension:

I went to Eurotire yesterday and we decided to start our forensics with an alignment check.

A little background is necessary here. Eurotire installed my PSS-9's and aligned it at the time (July 2003). Shortly after that, I returned to have them raise the rear slightly to give the car a visual downward rake since it looked like it was squatting slightly. In my infinite wisdom, or is it ignorance, I told them not to realign it at that time.

Well, yesterday's alignment check revealed a slight TOE OUT at the rear!!!!!! They attributed it to a combination of springs settling as well as the height adjustment. Nonetheless, that is now corrected. After a drive, the high speed stability issue is gone. One problem solved! Classic textbook stuff....I love it!!!

Now, for the harder, or should I say softer (as you will see), issue.......the handling through a turn.

They said my suspension appeared to be much stiffer in the front as compared to the rear. That is surprising since my dampers were set at 3 front and 2 rear. I originally did this to try and induce some oversteer in the otherwise understeering car. (Remember, the lower the number the stiffer the damper setting)

Since all of the alignent specs were checked and tweaked, and the bump steer was reviewed and is not an issue, he recommended we set the fronts a little softer and see if it helps. His premise was that the combination of stiff 19" tire sidewalls and stiff suspension wasn't working for me.

Well, I tried a long sweeping left hander at speed after leaving the shop and it was not good. Really not good! The front end seemed to pogo and with each unweight of the front (pogo up) it would sort of drift out and each reweight of the front (pogo down) it would turn in!! Now keep in mind, this is occurring at a rate of roughly 2x a second or so. Completely unmanagable. It was far better before the change!!

So, as a final trial, they directed me to try the dampers at full stiff.....numero uno!! I will set them there and try it out. I haven't done so yet.

I tend to agree with this last direction. I am starting to think that the 19" tire sidewalls are so stiff, the dampers need to be compatibly stiff. If they are not, the suspension will oscillate too much and create a tire effect of "grab-release-grab-release" as the suspension is weighted and unweighted by the oscillating suspension.

It certainly is frustrating but I am learning alot.

I would welcome any comments in light of the above.

 

Ken, first congratulations on the concours. Your car is certainly beautiful. And yes, I would have liked to talk more about it with you, but at that time, you seemed quite harried. It was certainly a busy affair.

Second, toe-out will get you every time.

Anyway, with respect to your "new" problem, you do not say what kind of shocks you have, and what the adjustment does. I have experienced problems with single-adjustable shocks (konis) where all the adjustment did was tighten the rebound. Over-tightening the rebound leads to a situation where the suspension compresses on small bumps, but cannot rebound quickly enough to allow the tire to follow the road, and so the front tire is loading & unloading around bumpy corners. Sound like your problem? A shock that is too stiff in compression (jounce) will cause the car to chatter over these small bumps, but it certainly doesn't feel floaty.

I prefer a shock that is stiffer in compression than in rebound. It makes the car more responsive on initial turn-in, and once again, allows the suspension to rebound to follow the road. Remember that the shock over has to be stiff enough to control the movement of the springs (stiffer springs need stiffer shocks). I had Bilstein re-valve a set of sport shocks to match the torsion bars in my old car. What a difference. The car was much more solid and cornered flat (see picture on left) like a go cart. It also rattled your fillings, but such is the price for performance.

Finally, I feel that the front of the car should "feel stiffer". It is lighter and has less mass for the spring to suspend, and so it will appear to react to bumps more. It also has greater weight transfer under braking and turn-in, than the rear does under acceleration. If you compare suspended weight to spring rate, proportionately, it should be stiffer.