SundayDriver

Cool - this is exactly the kind of info Mitch needs to make the decision. I would also ask the question as to whether they think the shock/chassis setup will handle those stiffer springs on a very bumpy track - that is where you will push the limits of the system.

RedlineMan

Hmmm...The difference between a Cup car (if that is what you are refering to, Ed) is night and day compared to a stock 996. Not relevant data for Mitch, I don't think.

RedlineMan

Hah...

Just what someone else already said here! Pretty good intuition, eh?

Mitch, I think you need to go back to square one and re-establish a baseline. Pop the stock springs back on and do some tuning. Get it to work... or not, with the original PSS9 springs. Once you have it where it is optimized as best it can be, take careful note of the car's attitude re: push, loose, bobbing at either end, porpoising, etc.

Normally I would suggest popping up an even amount in rate if you find a good balance with the stock springs. If you do not find that balance, you will have to determine which end needs a larger rise to balance what is actually happening. Normally in a 911 this is the rear, but the 996 is a different animal altogether than previous models.

If you don't establish a good baseline, you will be chasing your tail and buying a lot of uneeded pieces.

Now, you said initially that the PSS9 setup caused a lot of understeer. Theoretically, you would like things to be balanced even with no sways working, as this would represent a "natural" state of balance through effective spring choices. If the car could not be tuned with small to moderate sway bar adjustments, I would then say that the front spring rate was too low. It is often helpful if you think of grip in terms of the spring pushing the tire into the pavement instead of weight shift pushing the car down on the tire. In other words, you need the right amount of spring to resist this chassis lean. Not enough and you get no grip. Too much and you get no compression, weight shift, or grip. Same result, but opposite cause.

If the PSS9 setup started with 300F/550R, and you could not tune out the understeer with the sways, I would look to the spring rate as at least part of the problem. I would then say that the front rate is a little too low. One might imagine that an off-the-shelf 911 suspension system might be designed to understeer... given certain well earned reputations. The only direct experience I can offer to back this up is from a system I installed on a '77 930. Not the same car, of course, but the same family at least.

I set that car up with 350F/500R, which is 100 pounds different in balance to the PSS9 setup. I also installed Tarrett sways on this car, setting the front one .5" from full soft, and the rear at full soft. Leda damping set at 10 clicks front, 6 rear. The car was so perfectly balanced that the driver never even had to adjust anything until he went to Lime Rock, where he needed a bit more rotation.

This is all saying that I think you might try popping on the 550 rears and bumping the front rate to 375-400. Assumng all of the other attempts listed above to balance it have been tried, and failed, this may indeed give you increased grip in the front you are looking for without loosening your teeth.

Premier Motorsp

Does anybody else think it is strange that the kind of information Mitch is asking for simply does not exist?

The best we all can do is offer anecdotal advice with no mathematical back up. That just does not seem right. There are so many books on vehicle dynamics, and only one, Milliken&Milliken, even starts to deal with choosing spring rate etc. Problem is that that book is old, and does not address the specific needs of a modern Porsche at all.

Here is how I look at a car like Mitch's:

Start with damping. As Sunday and others pointed out, damping is more important than springs in determining how the car works over bumps. If you race for real, you need Moton triples. For a DE car Club Sports are acceptable. I have no experience with PSS-9s on the track. They seem fragile enough on the street, I can't imagine them holding up for long at Sebring.

Springs. Stock is around 200-300 lbs per inch, GT3RSR is around 2000-2200 lbs per inch. Mitch will be somewhere in this range, right? Mitch's car (on MPSCs) will corner about 75% as well as the GT3RSR (1.2 vs 1.5g) and it is 20% more massive. This makes a good case for springs in the 1500 lb/in range. Rates this high will not be acceptable for the street, but should work very well at Sebring. Note that dual rate springs are REQUIRED once spring rate exceeds about 50% of the corner weight. The helper springs are chosen to complement the lifting force of the dampers which is adjustable thru canister pressure. Helper springs are complicated.

Now you have to figure out the spring split. Everyone uses a 200 lb split which makes no sense because 200 vs 400 is a major difference (2X!)and 1800 vs 2000 is almost no difference (1.11X). Personally I favor a small spring split, especially if the car is low enough that the front roll center is too low. Stiff fronts help prevent so much roll center movement, and help the rear with traction.

Sway bars. For Sebring I would run very light bars. Mitch's only real options are stock or GT3 bars, and since stock isn't adjustable that leaves GT3 bars. GT3 bars are soft enough to be ok at Sebring, as long as the front bar is run nearly all the way soft.

Now you have to consider how all the roll steer and toe change will affect the balance. It has a very big effect. On a 996, this is not very adjustable so you pretty much have to live with what you have, but if you understand it, you can then set the static toe intelligently.

Finally you have to consider what Mitch likes. Is he going to be comfortable in a soft car or s stiff car? Does he like understeer or oversteer?

This is all experience and intuition on my part. Not much math to back it up. I don't know if the math would help much anyway because we know next to nothing about the tires.

Chris Cervelli
Premier Motorsports

Larry Herman

Chris, you'd better use your schooling to become a racecar engineer, because engineering a better widget for some miscellaneous componant for a useless thing-a-mabob will be a tragic waste of your talents. Thanks for the input.

Geo

For the sake of clarity, what you are referring to are tender springs. Helper springs keep the spring seated at full droop. Tender springs are used for dual rate set-ups. I'm not sure I agree your assessment of helping the dampers with lifting force.

Premier Motorsp

You are right, I use the terms interchangeably and it is not technically correct.

The damper's extension (ie lifting) force depends on the can pressure and the rod diameter. If the helper (tender!) springs are chosen correctly, the lifting force can have an effect on them. It moves the knee point of the system. If the knee point is within the amount of suspension travel that occurs in roll, there will be an obvious effect on balance.

Chris Cervelli
Premier Motorsports

mitch236

Now I am really confused. If I put springs at 1500lbs, I can't see how the tires will stay on the ground after hitting the kind of bumps that exist at Sebring. Even with racing dampers, there wouldn't be enough compliance to not get airborne.

For now, I am staying with the PSS9's and lowering the rates to the 350/550 F/R range and then will see how that works. Unfortunately, my next event is at Homestead which is much smoother and won't give me an indication about how well it will handle the bumps.

Adam@Autometrics

Using GT3 RSR spring rates to determine 996 street car spring rates is not really valid. The suspension geometry is radically different from a 996 street car. Roll centers (which are defined by suspension geometry) have MUCH more impact on spring rates than weight of the car or even grip of the tires.
And I'm confident that you won't see 2200lbs/in springs on any fast GT car at Sebring.

Tender springs are very necessary for GT cars, but optional on street weight cars, in my experience.
And you bring up a very good point about spring split. You can not use a finite split (ie 200lbs/in), but you also can not use strictly percentage (ie, 10% stiffer on the rear) because sway bars are typically not changed with springs and they also add significant roll resistance.

You can buy kinematic software (Mitchell is popular) that will give you mathematic answers, but in many cases, it is most efficient to try something and see if it works.

With PSS9, I expect 350 and 450 or 500 would work (200 is likely too big a split with springs that soft). With competition shocks, you could get away with 1000 lbs/in at Sebring easily. Possibly as much as 1500 in a street weight car.

penguinking

are there any *known* bad setups or *good* setups or does it all depend on the drivers preferences?

AudiOn19s

This brings up something I've noticed but dont understand. Standard 996's equipped with sport suspension have a <100 pound spring rate split front to rear (170F / 260R). and use sway bars that are 23.6mm F / 19.6 mm R

Now...GT3 rates have a large jump in the rear yielding nearly a 300lb spread between front and rear (280F / 550R)..BUT in looking at the change in sway size the rear barely goes up at all while the front goes up by a good bit over the standard sport bars. 26.8mm F / 20.7mm R

It's almost as if they tried to save some ride comfort in the front of the car by not making a big spring rate jump on the GT3 but tried to band-aid it by jumping a good bit up in size with the front sway bar. From what I've read the car is pretty well balanced this way.

Now...The problem I see is that if you go back to very little split in spring rate you'd almost have to either go back to the sport front bar or go very soft on the GT3 bar as balance would probably be off with that big GT3 bar on the car no?

Am I looking too much into this...hoping the technical folks can help me better understand the large spring rate split on the GT3 vs. the standard 996.

Andy

Larry Herman

I think that the GT3 spring selection was oriented towards a more sporting feel. They obviously needed stiffer rear springs to cope with the additional power (causes suspension squat) and seemingly chose not to increase the front springs as much to reduce understeer. Mine had no understeer at all, and more than a few GT3 track drivers I know of have softened the rear bar to reduce the oversteer. If you want to get deeper into the dynamics of it, like roll centers and stuff, you'd better talk to the experts.

RedlineMan

I think you are on top of it. They obviously did try to increase performance without trashing the ride completely. Controling the rear weight bias with stiffer springs, and balancing that with a big front bar. If you did send the spring rates Northward, reducing the differential while keeping the same front bar, you might indeed create a balance problem.

Having adjustable everything really comes in handy then. Suspensions are indeed a system.

333pg333

Hi guys. I'm new to this side of the 'List and I happened upon this thread which is fascinating if not mostly over my head. I'm in the process of sorting out new suspension for my car and just had a couple of questions. I have an '89 951 which is also a street car over short distances. I notice that most spring rates are stronger in the rear than the front. Is this in reference to the rear engined cars you guys are dealing with or is this across the board? I am getting custom made '2 way adjustable' found here: http://www.kw-suspension.com/en/30_P...tion/index.php
and my rates are 110 N front (approx. 615lb/in) and 50 N rear (285lb/in). Add to the rear the torsion bar 25.5mm (175lb/in) which I have been informed combines to about 660lb/in. I know this doesn't add up but Karl from racers edge said you have to factor in the wheel rate and the spring rate and work backwards with the known motion ratio. Anyway do you think that this will be a reasonable place to start? I am also looking at upgrading some/all bushes and bearings but have been told that if the suspension is too stiff for a road car that solid bushings can place too much stress on other parts of the car? Wheel 18" tyres 255/35 285/30. R-spec Toyo R888's. We only do very short Supersprints. 3-5 laps.
Sorry Mitch to hijack thread. It was that you all sounded like you know what you're talking about.
Thanks for any advice.
Patrick

AudiOn19s

Larry and Redline:

One more question. It seems as though I'm getting two contridicting views from this thead on what the spring rates are going to do to the car.

Larry you state that they left the GT3 spring rates relatively soft to reduce understeer from the standard 996...but wouldn't you want more front spring to reduce understeer? I realize that to a certain degree you can get TOO much front spring and TOO much front sway at which point you'll cause the front end to wash out due to lack of compliance...BUT until you get to that point the stiffer one particular end of the car the better it will stick? I'd think that 280lb springs would be nowhere near that limit...and again with that big of a front bar change compared to the rear possibly that's making up for the balance issues that were caused by the large rear spring rate jump and the relatively mild front rate jump.

Of course...individuals like Mitch and myself already have GT3 bars for our 996's, and for that matter all of the aftermarket companies model their sizes after the GT3 bars so I'm guessing that due to this when choosing spring rates for my suspension this winter I'll still go with a split...but I'd rather do a 100-150 lb split and run a softer front bar if I could than do a 200-300 lb split and need to run the front bar near full stiff where it comes from the factory on the GT3.

Andy