Crown

I am going to have a play with my 3 settings GMG sways in a week or so with my tuner.  I am currently middle stiffness setting both front and rear.

I want to get the car more stable over rough terrain and I think it's too stiff right now so it's losing traction and feels 'jittery', even a straight line.

Anyone have any recommendations?

GMG recommend stiff rear and soft front settings.  Also I know Cannga did some investigations of his own:

I have tried so far 3 different combinations.

1. Medium front, medium rear, stock drop links
2. Soft front, stiff rear, Tarett drop links (keep in mind the Tarett drop links make the suspension feel stiffer and you'll understand why I had to move to the next step)
3. Medium front, medium rear, Tarett drop links

Making front sway as soft as possible and rear sway as stiff as possible is the setting recommended when one is trying to reduce understeer. However there were consequences when I did this:

1. When I set the front sway to soft, it made the car lean more (as expected) AND the steering response feel loose. For example, when you wiggle the steering back and forth on the freeway, the car wouldn't lean/respond immediately, and would continue to lean a touch after steering wheel is stopped. This is similar to behavior of stock sway.
2. Making rear sway stiff: Might be ok with stock drop links, but I wouldn't advise this with the Tarett drop links. The Tarett drop links make the suspension feel stiffer to begin with, using Tarett with the stiff rear sway setting makes the ride way too stiff for me.

My observation is that the rear sway setting affects your ride quality quite a bit, more than the front. And the front setting affects steering feel. As far as understeer, hard for me to feel the difference, perhaps because I don't track the car and therefore don't corner at speeds high enough to feel a difference.

So bottom line is: keep in mind that when you change sway setting, the ride gets affected quite a bit, particularly in the rear. With stock drop links, I would suggest you try medium front, and medium or hard rear. With Tarett drop links, I would suggest medium front, and medium or soft rear.

Alex_997TT:

I am going to have a play with my 3 settings GMG sways in a week or so with my tuner.  I am currently middle stiffness setting both front and rear.

I want to get the car more stable over rough terrain and I think it's too stiff right now so it's losing traction and feels 'jittery', even a straight line.

Anyone have any recommendations?

GMG recommend stiff rear and soft front settings.  Also I know Cannga did some investigations of his own:

I have tried so far 3 different combinations.

1. Medium front, medium rear, stock drop links
2. Soft front, stiff rear, Tarett drop links (keep in mind the Tarett drop links make the suspension feel stiffer and you'll understand why I had to move to the next step)
3. Medium front, medium rear, Tarett drop links

Making front sway as soft as possible and rear sway as stiff as possible is the setting recommended when one is trying to reduce understeer. However there were consequences when I did this:

.....

So bottom line is: keep in mind that when you change sway setting, the ride gets affected quite a bit, particularly in the rear. With stock drop links, I would suggest you try medium front, and medium or hard rear. With Tarett drop links, I would suggest medium front, and medium or soft rear.

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Hello Alex,

First is this something new, or has it always been like this?

If it's something new, then obviously something is out of place, and the problem is unrelated to any component, but to SOME component not working right.

If it's always been like this, then I cannot rule out that the rear upper control arm and/or rear toe control arm are the culprits, as this is the only difference between my car and yours. (I have the Tarett but if anything, it makes the car stiffer.)

I most certainly could be wrong, but if your setting now is GMG sways with STOCK drop links, then I would be extremely surprised that the sway bar is causing such stiffness. This is the very first thing I did to my car, so I had excellent test condition: All parameters were same, with the only change being GMG sway with stock drop link in med/med positions. And my impression of this: Very very subtle change, hardly any change in stiffness or car leaning. Mostly the steering just feels a tad tighter.
If you change the setting of the sway and all of sudden your car does become soft, then I would think there is something out of place with the current setting.

To summarize, with stock drop links, no change in stiffness at med/med. With Tarett, stiff ONLY at the hard, not at the medium setting. Good luck and hope this helps.

BTW, what are your car's tire pressure? Are you setting the Differential reading at 0 throughout.

Regards,
Can
997 Turbo + Bilstein PSS10 (Review) + Cargraphic Exhaust (Heavenly Race Car Noise Review)

Alex_997TT:
Tyre pressures are if anthing a little on the high side Don't know what you mean with your last sentence
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"BTW, what are your car's tire pressure? Are you setting the Differential reading at 0 throughout."

Refers to the underlying TPMS "differential reading" which can be called up with the left-hand steering column stalk, and which should be used to set tire pressures. (These readings are temperature-compensated, unlike the first-level "atmospheric" pressure TPMS readings which vary with tire pressure).

after 'warming up'(relative in this weather):

34 - 34

42 - 42

Alex_997TT:

after 'warming up'(relative in this weather):

34 - 34

42 - 42

Those are the non-compensated readings displayed when the car is moving. 

The temp-compensated underlying readings can only be called up using the stalk when the car is stationary, and show the "real" difference between the actual tire pressures corrected to 20°C and the factory-recommended pressures for the tire size fitted to the car.Values "should" be 0 for road use, but can be + or - 1, 2, 3 or whatever. This display mode should be used for checking and adjusting the tire pressures, not the normal display mode.

Tire type  (18" or 19", summer or winter) has to be input after each wheel change so that control unit knows what datum values to compare the compensated measured values with.

fritz, once again you have very good info. I've been looking unsuccessfully for the ambient temp of that recommended 33/39 by Porsche. So it is 20 degrees centigrade? (It's not indicated in the manual.)

Alex, as fritz mentioned, the differential reading is the one you get by using the stalk, called TPC (don't know what this is the abbreviation for, must be German?). Because tire pressure is affected by temperature (Charles's law I think  for the nerds among us? ), just saying your tire pressure is x/y doesn't accurately tell you whether you have correct pressure. For example, at 10-15 centigrade here in Southern Cal this morning, my pressure reading is 31/36. But this does NOT mean I have under-inflated because pressure drops with temp.

To make long story short, the temp-corrected differential reading by using the stalk is the only thing I follow. And that's how we probably should compare notes. Mine is set at zero. As the car warms up, even though absolute pressures may vary a lot (31 to 34 psi for example), this differential reading stays more or less constant between -1 and +1, never more.

Other thing, although warm temp is important, for the purpose of comparing, it's better to get cold temp. Warm temp goes up according to how much you've driven, and fluctuates very quickly as soon as you stop, so it's a variable you don't want to have when comparing notes.

In my 17 miles daily drive to work, I start at 31/36 and end at 33/39 or so. Since the temp in California is 10 degree centigrade, I would guess that your tire pressure is on the high side, unless you've just terrorized the neighborhood at 100 plus mph for one hour non stop. Just kidding.

The reason is I ask about pressure is that I think it is a critical issue (that many are not paying attention to). I could tell if my car's is over by +1, most definitely by +2. Dealers always like to hyper-inflate car because it feels firmer and "better" in a superficial way, and when I have that soft-ass (hmm, rennteam has no vulgarity filter!) stock suspension, I over-inflate to compensate for the softness. But in general, tire grip and ride are much better at zero differential.

(The next question is of course, how do you know the TPMS is measuring accurately. The answer is, you don't. Sigh. I don't want to confuse the issue further so let me stop there for now. Suffice it to say, you need one of these to know for sure http://www.longacreracing.com/catalog/item.asp?id=982&catid=8
It's in my other thread http://www.rennteam.com/forum/thread/458942/Inventory_of_my_last_modsOr_latest_whateverLOL/page1.html .)

Regards,
Can
997 Turbo + Bilstein PSS10 (Review) + Cargraphic Exhaust (Heavenly Race Car Noise Review)

I saw Chris only a month or so ago and he set my tyres to 34/40 with a calibrated air pressure system.  Ambient temp at the time was around 5 degrees C in the workshop I recall.  So given it's been about 7 degrees C when I have been noticing the jittery-ness I am not sure tyre pressure is the answer.

I will go out to the garage today and check the TPMS figures again.  Won't be taking the car out this weekend probably unless all the snow and ice melts - no fun otherwise.

It's more likely that my Michelin PS2's just aren't getting warm enough and I just need some winter rubber on!

cannga:

fritz, once again you have very good info. I've been looking unsuccessfully for the ambient temp of that recommended 33/39 by Porsche. So it is 20 degrees centigrade? (It's not indicated in the manual.)

Alex_997TT:

I saw Chris only a month or so ago and he set my tyres to 34/40 with a calibrated air pressure system.  Ambient temp at the time was around 5 degrees C in the workshop I recall.  So given it's been about 7 degrees C when I have been noticing the jittery-ness I am not sure tyre pressure is the answer.

Since tire pressures fall by about 1.5 psi per 10°C temperature drop, your 34/40 measured at 5°C corresponds to about 36/42 when scaled up to "standard" temp of 20°C. You'll notice that increase over factory recommended values when you are driving a Porsche. (I generally have my pressures at 0.1bar / 1.5 psi above factory recommendation and find that any more than that is at the expense of comfort and with no benefit in feel and handling).

If you use the TPMS press-diff mode to set your pressures, it does the temp compensation for you without you having to think about it. A lot of people seem to have problems getting their heads around that.

Alex_997TT:

I saw Chris only a month or so ago and he set my tyres to 34/40 with a calibrated air pressure system.  Ambient temp at the time was around 5 degrees C in the workshop I recall.  So given it's been about 7 degrees C when I have been noticing the jittery-ness I am not sure tyre pressure is the answer.

I will go out to the garage today and check the TPMS figures again.  Won't be taking the car out this weekend probably unless all the snow and ice melts - no fun otherwise.

It's more likely that my Michelin PS2's just aren't getting warm enough and I just need some winter rubber on!

(Seeing fritz's post gave me a chuckle. This was mine that I wrote last night but did not post as I was waiting for your differential reading.)

I agree that the cold temp has something to do with your observation -- assuming what happens in your car is not any more than other Porsche's in the same environment. Also...

My (humble) opinion is that 34/40 -- cold temp, right?-- at 5 centigrade is too high and is a contributing factor. The gain in pressure is estimated to be 1.5 psi for every 10 degrees, meaning if I were to correct your pressure to compare to mine, at 15 degree centigrage, you are at 35.5/41.5 versus my 31/36!
I am very curious to see what the differential reading is in your car. As I have mentioned, +2 to my derriere-shock-dyno is borderline unacceptable. It gives my car the character you described: loss of grip & jittery.

Hope this helps.

Regards,
Can
997 Turbo + Bilstein PSS10 (Review) + Cargraphic Exhaust (Heavenly Race Car Noise Review)

Car not moved an inch this week due to the snow - so tyres completely cold

Alex_997TT:

Car not moved an inch this week due to the snow - so tyres completely cold

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PS: I must confess that, from your description of how they came about, I would have expected your "+" values to have been even a little higher than they now are. You might not experience so much difference in "feel" when you next drive the car with reduced pressures.

fritz:

Alex_997TT:

Car not moved an inch this week due to the snow - so tyres completely cold

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Good! This is your chance to "correct" tire pressures to differential pressure of +/- 0 psi all round and test the setup again as soon as road conditions allow.

 

PS: I must confess that, from your description of how they came about, I would have expected your "+" values to have been even a little higher than they now are. You might not experience so much difference in "feel" when you next drive the car with reduced pressures.

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fritz

Alex,

I seem to have a twin in Germany who keeps taking the words right out of my mouth.
Yes I too was expecting something like plus 4. Still this is a good start: +2 on a normal car is ok, but +2 on a Bilstein car IMHO is definitely too much. I don't know if it would take away completely the jittery, but it WILL help.
In general +0 might feel a tad soft the first 5 minutes of driving. But once warmed up, it becomes firmer and to me +0 is the "proper" setting.

Next question: Is your car TPMS system set to "Partial Load"?

Note that so far we are *assuming* that your TPMS is accurate. This may or may NOT be the case.

Regards,
Can
997 Turbo + Bilstein PSS10 (Review) + Cargraphic Exhaust (Heavenly Race Car Noise Review)

cannga:

fritz:

Alex_997TT:

Car not moved an inch this week due to the snow - so tyres completely cold

Good! This is your chance to "correct" tire pressures to differential pressure of +/- 0 psi all round and test the setup again as soon as road conditions allow.

PS: I must confess that, from your description of how they came about, I would have expected your "+" values to have been even a little higher than they now are. You might not experience so much difference in "feel" when you next drive the car with reduced pressures.

fritz

Alex,

I seem to have a twin in Germany who keeps taking the words right out of my mouth.
Yes I too was expecting something like plus 4. Still this is a good start: +2 on a normal car is ok, but +2 on a Bilstein car IMHO is definitely too much. I don't know if it would take away completely the jittery, but it WILL help.
In general +0 might feel a tad soft the first 5 minutes of driving. But once warmed up, it becomes firmer and to me +0 is the "proper" setting.

Next question: Is your car TPMS system set to "Partial Load"?

Note that so far we are *assuming* that your TPMS is accurate. This may or may NOT be the case.

--

Regards,
Can

I'm on my second TPMS car now and have never had any problems as a result of working on the assumption that the system is accurate. At least, not since I took the trouble of reading and understanding the manual, and then using the "pressure difference" display to set the tire pressures.

fritz:

I'm on my second TPMS car now and have never had any problems as a result of working on the assumption that the system is accurate. At least, not since I took the trouble of reading and understanding the manual, and then using the "pressure difference" display to set the tire pressures.

1. If you try to adjust the car to what you think feels right, then I agree, it does not matter what "true" tire pressure level is.

However, what I was trying to do is hold the tire pressure at "reference" level -- what Porsche and Michelin think it should be -- so that I could adjust the rest of my suspension components.  I want to set it at true 0 and the compulsive me feels a true 0 requires a reference tire gauge. http://www.longacreracing.com/catalog/item.asp?id=982&catid=8

*Assuming* the Longacre Racing is more accurate, then I did find that TPMS in my car is off by 1 psi in 3 tires (TPMS reads 37, my Longacre gauge reads 36), and off by about 1.5-2 psi in 1 tire. I didn't want to confuse the issue for Alex earlier, but actually in my car the differential setting varies between 0 and minus 1. I hope my thinking is correct here!

2. There is a second way to test accuracy of TPMS, without using reference tire gauge. If you have a tire gauge that is reasonably accurate from reading to reading (and most tire gauges are very much so): Set, say, the 2 rear tires so that TPMS read the same, at for example 37. Now using the tire gauge, measure each of the 2 tires. If the tire gauge reads different values between the 2, then you've proven at least one of the TPMS device is wrong.

I think I am a little nutty.

Regards,
Can
997 Turbo + Bilstein PSS10 (Review) + Cargraphic Exhaust (Heavenly Race Car Noise Review)

Was meant to go see Chris today however last min change of plans meant we ony got to speak on the phone.

He thinks having my pressures a little over the recommended values should be fine.

I am still struggling for grip even in a straight line though.  I joined an empty motorway on Sat and accelerated in 2nd gear and the front 2 tyres were REALLY struggling for grip.  It was 7 deg C, dry, smooth surface, and I had been on a different motorway just before so tyres were as warm as they were going to get.  I was also in normal mode and I did NOT have my gas pedal flat to the floor.

Shavester on this forum saw Chris for a similar issue earlier this week with his 700bhp Martini wrapped Turbo.  Apparently Chris made some good progress with his straight line stability and grip so he should have a tried and tested setup for me when I reschedule my visit.  Nick also runs Bilsteins and aftermarket sways.