pete95zhn

200-225lb/in, spring rate.

Skip Wolfe

A quick way to tell is to put a tie wrap around the shock shaft and do some hard laps. You will most likely find the tie wrap embedded into the bump stop.

Also, you have way too little camber for Toyo RA1's - you need to be running neg 2.8 - 3.0. At only neg 1.3 you will beat the hell out of the outside edge of the front tire and you will understeer because you are not using the whole tire effectively - you will notice a huge difference in front grip. Get your self a set of camber plates if you want to run on the street. A competent alignment shock can dop a dual street and track alignment, although camber is not the real issuee with tire wear, toe is. Lots of negaticve alignemtn will cause the car to tramline, but as long as that doesn't bother you, I would go with -2.8 and 0 toe.

Also, the hot setup with stock 16" wheels is to run 8" - 245's all the way around. 245's is the absolute minimum width I would run on the track.

The rule of thumb for is to run 1.5 times front wheel rate than rear wheel rate. So a 28 mm bar has a 254# wheel rate. 254 * 1.5 = 381# front. So, before the major hassle of changing your rear tbars, I would run 375# springs up front, change you front alingment (biggest issue), and go with wide front wheels/tires.

I ran 400# fronts with 29mm rears with Koni Yellows for years, and with 255 & 275 x 17 RA1's I found it to be quite neutral and could easily induce throttle oversteer in corners.

pete95zhn

So would you please explain to me why the factory did just the opposite with 968 CS M030 suspension, when adding auxiliary springs to rear axle??

Skip Wolfe

What are the front spring rates, rear tbar size and helper spring rate? I have no idea what they are so I cannot comment. I also don't know how pertinent what Porsche did for a street suspension is for a track setup. My recommendation is based on 7 years of experience with tracking a 951 and talking to others in the 944 racing scene. I can tell you that a 200-225# spring rate with 28 mm tbars would be very evil handling - you would switch back and forth between severe understeer from riding on the bump stops to drastic oversteer by having such a higher rear wheel rate. 200-225# wheel rate might be close - have to dig up the calcs but that would be in the 400# spring rate range.

IanM

Wow, I may need to throw what I thought I knew about spring rate bias out the window here! It's amazing that I haven't heard a concensus on this after the 20 years that these cars have been around. My Turbo S came stock with about a 1:1 front to rear rate bias (150-175# front springs w/ 25.5mm t-bars @ 175# eff. rate). These cars understeer in stock form, so I would expect you'd want a slightly higher rate in the rear as opposed to the front. Pete is suggesting that, but Skip's experience is telling us the exact opposite.

My current setup (270# front effective and 254# rear effective) already has a slight front bias, and it understeers. It seems counter intuitive to me that I should be stiffening the front even further to correct that. Now I hadn't considered the "riding on the bump stop" theory. I've just been reviewing track video from Thunderhill where my buddy was following behind me in his 996TT. My car doesn't lean or compress much, I'd be surprised if I'm riding on the bump stops. I've also got lots of track photos taken of me in mid-corner, and it doesn't appear that the front wheel is up into the wheel well enough to be hitting the bump stop (I know, that's not a scientific method, but it's all I have at the moment). My insurance just expired for the winter, or I'd do a test with zap straps. Unfortunately, I had been hoping to revamp my suspension during the off-season, but now I'm not sure which way to go.

A good friend of mine is running 250# front springs w/ 28mm t-bars (225# eff front and 254# eff rear). I've driven his car in autocross, and it's amazing how neutral it is. Where my car plows, his drifts very neutrally. I've also sat in the passenger seat when we were at Thunderhill together, and it rides neutral to slight oversteer in steady state corners like turn 2. This also goes against the suggestion of having the front stiffer than the rear (by a whopping 1.5). If my car w/ 300# springs is riding on the bumpstops and understeering as a result, shouldn't a similar 951 w/ 250# springs be even worse?

Another friend of mine is running 300# front springs w/ 29mm torsion bars (i.e. 270# eff front and 292# eff rear). I drove it during a rain autocross a few weeks back, and it felt more neutral than mine.

I think it's back to the drawing board for me. I obviously respect the opinions of experienced track junkies like Skip, I sure wish his experience wasn't 180 degrees different than mine.

Skip - did you ever autocross w/ your 400#/29mm setup? What would the steady state handling of that setup be on a skidpad or steady state corner (running similar swaybar settings front and rear)?

Regarding those other issues. I do have camber plates (forgot to mention it in my first post - have now added). It's tough to find a happy medium when it comes to street and track alignment settings. I don't think I'd be willing to go to 2.8 or 3 degrees negative camber at this point. Toyo recommends a minimum 2.5 degrees. Yes, I do see a bit more wear on the outside, but it's not drastic. My plan was to go another 0.5 degrees for my next alignment, or 2.0 degrees all around. Maybe I'll try 2.5, see how it goes. BTW, I had the alignment guy check the change in toe for every 0.5 degree change in camber on my Racer's Edge camber plates. My results showed that for every 0.5 degrees of negative camber dialled in, total toe would decrease by about 20 minutes (toe out). I decided that it's not practical to make camber changes with the plates between street and track settings unless you change the toe as well.

I do plan on running 245's all around in the future. I just picked up a second set of club sports for this purpose. However, I know others who have managed to dial-in a neutral handling 951 w/ 225's on the front, so I know it can be done. Wider is obviously better though for ultimate grip.

Thanks again. I do appreciate hearing about everyone elses experience. Keep it coming! Maybe we'll figure something out in the end!

Rich Sandor

Ian, go with more camber.

Skip Wolfe

Check out this thread regardiong front vs rear spring rate as well as spring rate vs wheel rate.
https://rennlist.com/forums/showthre...&highlight=van

Also, I would suggets posting this question on the Racing forum - you'll get the racer crowds experience/recommendation. Keep in mind, these aren't "my obverservations" I started with 300# w/ 29 mm tbars and had many racers, as well as Eric Steinel, who is a well know onwer of a Porsche race prep shop specializing in 944's, and he was one of Fred Bakes mechanics when Fred one all sorts ob SCCA, etc championships in a 951. I listened to these guys, tried the tie wrap trick etc., and came to agree with their recommendation through my own trial and error. So this is not my own personal theory, this is something I have gotten from the 944 race crowd who have been there. I went from 300# w/ 29 mm tbars to 400 # w/ 29 mm tcars, and found the car was better balanced. Also all my experience is based on track experience an not on AutoX - I have very little Autocross experience.

You really need to get your camber to >2.5 or you will be trying to tune your suspension to compensate for less than optimal camber/tire contact. More camber will probably be one of the biggest factors in eliminating your understeer.

IanM

Thanks Skip. Interesting thread you provided link to. Still lots of variation on front to rear ratios. There were some reporting good balance with front to rear ratios less than 1.0 (stiffer in rear), and others (most people) like yourself recommending stiffer in front - with your ratio of 1.5 and Adrial recommending 1.2.

I hadn't heard of using 0.85 to calculate effective rate of front springs, I have always used 0.9. That does make a bit of a difference. Which is correct?

If I use 0.85, then my current 300#/28's give a ratio of exactly 1.0, which goes up to 1.06 using 0.9. Your 400#/29 combo gives a ratio of 1.13 using 0.85 and 1.2 using 0.9. Note that neither of these get to your recommended 1.5.

So, keeping my 28mm torsion bars - I would need a 305# effective front spring - or about a 350# front spring for a 1.2 ratio, or a 381# effective or 425 to 450# front spring for a 1.5 ratio. The 375# spring you suggest I try with my 28mm bar gives a ratio of 1.25 to 1.33 (depending on whether 0.85 or 0.9 is used to calculate front effective rate).

I'll go for more camber to start. What about rears, same as front?

pete95zhn

Oh Gosh, here we go again, this is my favourite subject...

968 CS M030:

F: 160lb/in x 0.9 = 144 lb/in ( wheel rate )
R: 25,5 mm t-b = 175 lb/in ( wheel rate ) +
Rh: 100lb/in x 0,56 = 56 lb/in
Rtotal: 175+56 = 231 lb/in

Front to rear bias: 1:1,6!!

Tires: 225/255-17".

My friend and I are running LEDAs with 400lb/in front and 850lb/in rear coil-overs. According to some opinions this set-up should be extremely dangerous and impossible to handle. Well, this friend of mine ( I'll refer his accomplishments, he's better driver than I am ) runs with this set-up ( and 350 fwhp 951 ) 8:25 BTG at the 'Ring, equal times with GT3s at shorter tracks and he's local AutoX champion...

Although I spec'd our LEDAs, I don't take the credits, because I imitated CS' bias and spring rates & bias of one commercial kit, H&R's RSS-37-827-1/1. http://www.h-r.com/katalog_download_de/Porsche.pdf
Porsche Club-racers use this set widely in Germany. It includes custom valved, non-adjustable Bilstein shocks. Bolt-on, race ready...well, you have to remove t-bars first.

So are Factory and H&R engineers totally wrong?

We have 968 CS M030 anti roll bars, Tommy's tires are 225/265-18" and mine are 235/295-18". Stiffer bushings and I also have camber plates. Wheel alignment is slightly track oriented.

You can always try to compensate improper spings with radical wheel alignment or tyre selection ( like equal widths F&R ), but why bother?

Rich Sandor

what is your alignment pete?

8:25 in a 951?

333pg333

I have 615 f and 706 r rates. 2nd last time at the track I was getting some resistance to turn in on some corners, so for the next track day we increased the neg in the front to something massive like -6 and got some nice turn in but it wanted to rotate the rear. We slacked off the rear sway and this helped a little, but we are thinking of increasing the rear neg out to -2.5 or -3.0 ish. Maybe we're getting a bit too crazy on the neg and should dial back both f & r. Car used to handle great for a while...hmm, what's changed?
More power, 80% lsd, lower S2 final drive and not a lot of time on the track to test. Right, there could be something to look at there...

IanM

Well, I knew I would learn something, and I have! There's only one explanation for the wide variation in experience here - it's an over-simplification of a complex system. There are way too many variables involved to be able to target one factor (spring rates) to explain the balance of a car.

In a perfect world, if everyone was running the same compression and rebound damping, same alignment settings, same swaybar settings, same tires and tire pressures, and had the same driving style, then this type of analysis could be possible.

I think the message here is that I could probably get my car to feel perfectly balanced without changing spring rates at all. I will definitely be going a bit more aggressive with my camber settings, and will do the zap-strap check to make sure I'm not riding on my front bump stops. Perhaps my Koni rebuilder did not revalve the compression damping adequately, even though I specified for them to be valved sufficiently for 300# springs on the track.

I can't explain why my experience shows that having a bit heavier springs on the back provide a more neutral car - perhaps the inherent balance of the car changes when you get up to the very stiff end things on spring rates. It seems that lots of the North American racers running very high rates find good balance when running stiffer in the front. But then they've also stripped a lot of weight from their cars and may have a different weight distribution than me. I've also noticed that the European philosophy regarding suspension tuning on lightweight cars is to run high damping and softer spring rates, whereas North Americans tend to run the other way around.

951and944S

Not really.....there are just two different schools of thought, I tend to agree with Pete but it's hard to argue against setups that seem waaay out of proportion but work perfectly fine on track.

The front-rear balance as used by Porsche in 944/51/68 competitive racing agrees with Pete's explanations but the reason (for all I can figure) for the much stiffer sprung cars that you see here or read about elsewhere is a corrective measure at dialing out initial turn-in body roll.

The cars are so stiff that they don't have the delay until a spring or sway bar loads.....they just turn......now....!

The following is only my opinion but these setups tend to limit suspension travel so much that the tire sidewall becomes more a part of the feel and initial damping. These stiffer, imo, out of proportion setups also are way more track specific.

I'm just back from our local PCA yearly where one of my friends raced an S2 he just bought from KP at Racer's Edge. I believe this car's current suspension is specifically tuned for a place like Road Atlanta or Road America but it seemed way too stiff for a place like No Problem Raceway, maybe like Sebring where it's said that 10-20% softer than places like the tracks mentioned above is the norm.

I also know guys that did the math, followed the factory front/rear proportions and ended up eventually changing to similar rear biased setups, it's hard to explain but on the car I'm building personally.....I'm following along the guidelines per Porsche, as outlined in Pete's posts.

T

Skip Wolfe

Gee Pete - I am so sorry I put you through this....again. You obviously use "if Porsche did it, it must be right" should be the absolute guiding principle.

I tend to do silly things like look at trends of front running Stock and GT3 class 944 and when there is a definite trend of a given setup then it is worth taking notice. So if 8 out of 10 front runners run the way I describe, and 2 out of 10 follow your 968CS example, which would you follow? Personally I am going to follow the trends as well as listen to guys like Jason at Paragon, Eric Steinel, Chris Cervelli, etc. One key reason to look at the trends is it balances out personally driving styles, and guys who just drive what they were given. I have found that I tend to adapt my driving style to whatever setup I am driving. I have also found that I can be pretty quick with a crappy setup. I get used to it quickly and don't always realize how bad it was until I changed it to a better setup. I can't tell you many times I have talking to someone about their setup, and asked them why it is setup like that and get "that's what XYZ guy told me I should do" or "I based it on XYZ factory setup" and they have never tried anything else. Personally I like to tinker and have driven multiple setups. I started with 300# springs with 29 mm Torsion bars (wheel rate of 292), and thought it was great until I went to 400# springs - and wow it was a lot better and my lap times dropped. Car was better balanced and I could carry more entry speed and get on the gas harder, sooner. I also have tried 968 M030 sways, Tarret sway, almost every R-comp tire on the market, rubber bushing, sphericals, and the list goes on. Most of my changes were from talking to a lot of guys, looking at trends and then most importantly, testing it out for myself - lap times don't lie.

Personally, I don't put much stock in what Porsche setup for the cars 15-20 years ago - technology has evolved and guys with a lot more money and talent than us have been testing, testing, and testing and have found that there are faster options. If the Escort Cup or CS setup was the end all be all setup then guys would still be running their progressive fronts with tbars+helper springs and not have ditched them for more modern setups like the Leda setup you are running. Chris Cervelli was running 1000 # springs in arguably the most successful GT1R 951 ever - that is 3 times stiffer than the Escort Cup cars, why would he do that. Because after extensive testing he determined it was faster and a lot had to do with how much shock technology has advanced.

All that said I do have some corrections with your original post. The rear spring rate to wheel rate multiplier is 0.42 not 0.56 (per Porsche Motorsport), so the revised math would be

F: 160lb/in x 0.9 = 144 lb/in ( wheel rate )
R: 25,5 mm t-b = 175 lb/in ( wheel rate ) +
Rh: 100lb/in x 0,42 = 42 lb/in
Rtotal: 175+42 = 231 lb/in

Front to rear bias: 1:1,5

That said you are not following you're own advice with your setup:
F: 400 lb/in * .9 = 360 (wheel rate)
R: 850 lb/in * 0.42 = 357 (wheel rate)

Front to rear bias: 1:1

So you yourself are running almost a 1:1 ratio, which is not bad. I personally don't think it is optimal, but is certainly not "extremely dangerous and impossible to handle". I have no doubt your buddy is fast witth this setup, but I bet he could be faster if he increased his front spring rate to 600#. Now if you followed your example of Porsche 1:1.5 ratio you would need 1300# rear springs which would be impossible to handle. So I think you proved your own theory wrong.

Skip Wolfe

Ian, my recommendation would be to methodically start adjusting setting - camber, spring rate, shock setting, etc. - and document as much as possible. if you can swing it get a datalogger - DL1, GX2, etc., and if you can't do that then monitor times. Front springs are relatively cheap and easy to change, so try some 225# springs and some 375# spring, and see how they feel - you can sell what you don't like. The last thing I would change is the torsion bars because they are such a PITA to do. I would also recommend talking to Jason at Paragon, Karl at Racers Edge, and Eric at Steinels - all are nice guys who will give you free advice, and all have lots of 944 experience. I would also reiterate the suggestion to post on the Racing forum here on the Rennlist - lots of guys with track experience. The key is to gather as much info as you can, keep an open mind, and experiment.