Rear Suspension Mods
07-25-2002, 10:17 AM
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Rear Suspension Mods
I'm looking to upgrade the stock suspension on my 86 944 T. I want something that's streetable, but firmer than stock for better control during DEs. It's clear to me what to do about the front end, but as for the rear, I've been told that the more reliable setup is to install larger torsion bars instead of adding auxiliary springs a la Turbo Cup.
It seems to me that with springs in increments of 25 lb/in, and a wheel/spring rate ratio of 0.44, you can adjust the rear rate in around 10 lb/in increments, allowing super fine tuning of the balance of the car. Also, ride height adjustment/corner balancing is a snap, once the stock torsion bars are reindexed appropriately.
Plus, given the limited life of torsion bars, one can easily increase the aux spring rate as the torsion bar rate drops off.
However, I'm told that aux springs somehow behave strangely at the tail ends of their ranges of motion. Also, if you have to reindex the bars anyway, the additional work to simply install the appropriately-sized torsion bars (to match what's happening in the front) is probably minimal. Then you don't have to go to the extra expense of a coil over setup at the rear, plus perhaps the additional stress on the chassis.
Any ideas?
It seems to me that with springs in increments of 25 lb/in, and a wheel/spring rate ratio of 0.44, you can adjust the rear rate in around 10 lb/in increments, allowing super fine tuning of the balance of the car. Also, ride height adjustment/corner balancing is a snap, once the stock torsion bars are reindexed appropriately.
Plus, given the limited life of torsion bars, one can easily increase the aux spring rate as the torsion bar rate drops off.
However, I'm told that aux springs somehow behave strangely at the tail ends of their ranges of motion. Also, if you have to reindex the bars anyway, the additional work to simply install the appropriately-sized torsion bars (to match what's happening in the front) is probably minimal. Then you don't have to go to the extra expense of a coil over setup at the rear, plus perhaps the additional stress on the chassis.
Any ideas?
07-25-2002, 09:59 PM
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I can't give you any experienced advice, but I'm planning on going through this in the near future, and have thought about it along the lines of what you're saying- I've had people agree and disagree, but the way I see it, I want to replace my T-bars if I replace the front springs (I intend to, but am not sure how far to go) and I feel that if I do so, and decide I want stiffer, I can add helper springs then- I really don't want to do the t-bars more than once (in teh next 10yrs anyway). So, I'm planning on Bilsteins and 225 - 275lb (probably 250lb) springs w/28mm t-bars- Chris Cervelli tells me to use 29mm bars w/the 250's, but I'm still a little leary of that- hoping for input...
good luck- let me know what you end up doing- we'll compare notes....
good luck- let me know what you end up doing- we'll compare notes....
07-26-2002, 12:28 AM
#3
Burning Brakes
I run Bistein coil overs all round with #350 springs. the ride is about as firm as I would like on the street, the performance is exceptional on the twisties, this thing has grip galore.
I have pushed the car to MY limits and never managed to loose it, the car is way better than me
The cost is about the same if you pay someone to swap out the torsion bars, as with the stiffer bars you will still need to improve your shock.
the good thing is I can change spring rates easily and It does not stress the chassis as a full coil over would.
in hind sight I would maybe go to a #450 in the rear, but I am not sure this would improve things much more, the car is pretty nuteral,M030 sways, -1.5 neg camber on the front an 17" twists with falken 451 tires, there is not much push at all now, trully a blast to drive.
I have pushed the car to MY limits and never managed to loose it, the car is way better than me
The cost is about the same if you pay someone to swap out the torsion bars, as with the stiffer bars you will still need to improve your shock.
the good thing is I can change spring rates easily and It does not stress the chassis as a full coil over would.
in hind sight I would maybe go to a #450 in the rear, but I am not sure this would improve things much more, the car is pretty nuteral,M030 sways, -1.5 neg camber on the front an 17" twists with falken 451 tires, there is not much push at all now, trully a blast to drive.
07-26-2002, 01:15 AM
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Robby,
I have the 29mm torsion bars on my 951. 250lbs. w/adj.coil-over Bilsteins in the frt. Bilsteins in the rear. Along with the M030 sway bar frt. In the rear is the Weltmiester adj. bar.
It doesn't seem to stiff on the street. It's personal taste.
The 944 n/a race car has 250lbs springs(cut) , adj. konis, adj. Weltmiester bars frt/rear and 30 mm. torsion bars. The corner balancing/ALIGNMENT really made a huge difference.
Neither car is a daily driver so that may skew my judgement rather than if I drove them every day. Chris has done both cars and I have not been disappointed yet: )
Are you in the Phoenix area?
Paul
I have the 29mm torsion bars on my 951. 250lbs. w/adj.coil-over Bilsteins in the frt. Bilsteins in the rear. Along with the M030 sway bar frt. In the rear is the Weltmiester adj. bar.
It doesn't seem to stiff on the street. It's personal taste.
The 944 n/a race car has 250lbs springs(cut) , adj. konis, adj. Weltmiester bars frt/rear and 30 mm. torsion bars. The corner balancing/ALIGNMENT really made a huge difference.
Neither car is a daily driver so that may skew my judgement rather than if I drove them every day. Chris has done both cars and I have not been disappointed yet: )
Are you in the Phoenix area?
Paul
07-26-2002, 03:07 AM
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Paul,
I'm sorry, what I meant was that I'm afraid 29mm t-bars may be a little stiff for 250lb springs. I'm considering 225's and 28's or 250's and 28's, but am kind of leary of 29's unless I were in the 275 to 300 range- yes, Chris knows what he's talking about, but he's also one of the best 951 pilots in the country- he can handle the oversteer if it occurs a lot, and probably enjoy it- you may be able to as well- I'm just not able to play around w/various set-ups before I buy, so I kind of have to GUESS at what I THINK sounds right. I'm planning to do the T-bars though, so that if I DO decide to go even stiffer w/the rates, I can always add helper springs to the rear and just change the fronts- that might not be the best idea though, and I certainly don't
WANT to have to do it- best to get it right the first time...
No, I'm across the country, unfortunately- Knoxville TN- another problem- there aren';t many modded 951's here...
I'm sorry, what I meant was that I'm afraid 29mm t-bars may be a little stiff for 250lb springs. I'm considering 225's and 28's or 250's and 28's, but am kind of leary of 29's unless I were in the 275 to 300 range- yes, Chris knows what he's talking about, but he's also one of the best 951 pilots in the country- he can handle the oversteer if it occurs a lot, and probably enjoy it- you may be able to as well- I'm just not able to play around w/various set-ups before I buy, so I kind of have to GUESS at what I THINK sounds right. I'm planning to do the T-bars though, so that if I DO decide to go even stiffer w/the rates, I can always add helper springs to the rear and just change the fronts- that might not be the best idea though, and I certainly don't
WANT to have to do it- best to get it right the first time...
No, I'm across the country, unfortunately- Knoxville TN- another problem- there aren';t many modded 951's here...
07-30-2002, 07:53 PM
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Sorry for the confusion, let me clarify:
Is there additional work in installing new torsion bars vs. the reindexing needed when installing coil overs at the rear?
Regarding spring rates, etc., I've done a lot of analysis on this and, while it is certainly a matter of personal taste, to me the highest spring rate you'll want is one that generates a natural frequency of around 2 Hz. For a stock 951, this is about 250 lb/in. Beyond 2 Hz, ride quality will deteriorate quickly, and you'll probably have difficulties with traction on anything but an entirely smooth track.
I know some people have had success with 350 lb/in (2.33 Hz) and even 450 lb/in (2.65 Hz), but this is really in the realm of pure race cars, and your ride quality will suffer.
Regarding the front/rear balance, I have no documented proof from Porsche regarding the actual wheel to spring rate ratios on the 944. Porsche Motorsports in Germany, where I lived for 3 years, assures me that this information is not available. So here are my WAGs:
The front wheel to spring rate is about 81%. The rear torsion bar gives about 4.13E-4 D^4 lb/in per mm of diameter (D). I'm told the coil over spring/shock at the rear gives 44% wheel to spring. So to keep the balance at stock levels, a 250 lb/in front spring (203 lb/in at the wheel) needs a 26.4 mm diameter torsion bar, or a stock 23.5 mm bar plus 173 lb/in aux spring.
Most people agree that the stock car understeers, so you'd want to increase the rear rate a bit -- say 10% -- to dial out understeer (you could also do this with anti roll bars, which would then improve straight-line ride). 10% more rear rate gives 222 lb/in at the wheel in the rear, requiring a 27 mm diameter torsion bar, or the stock 23.5 plus 218 lb/in spring.
I've been told 250 front works well with 28 mm at the rear, but to my original point -- once you buy new torsion bars and get the rear set up, if you find the 28 mm is too stiff, then you're in for the same amount of work to get your car sorted out.
Alternatively, you could simply undo the coil over shocks, change the rear spring, and adjust the ride height accordingly -- a minimum of fuss to change the rear rate.
A 28 mm diameter rear bar gives a rear rate increase of 26% vs. a 250 lb/in front spring -- this might dial in quite a bit of oversteer, but again, with the right anti roll bars, it might not be so bad.
As a final note, while I don't have it in front of me, as I recally, according to Racing Vehicle Dynamics, an engineering text by Milikin and Milikin, F1 cars run natural frequencies in the 2.5-3 Hz range. This is mostly because their aerodynamic packages require a VERY stiff suspension. In addition, F1 tires's construction is very different from passenger cars, with generally VERY stiff sidewalls (so stiff that they don't change shape significantly when deflated, despite fairly low pressures). At some time in the future I might justify much stiffer springs (i.e. 350 lb/in, etc.), but as a first step I don't see it happening.
Is there additional work in installing new torsion bars vs. the reindexing needed when installing coil overs at the rear?
Regarding spring rates, etc., I've done a lot of analysis on this and, while it is certainly a matter of personal taste, to me the highest spring rate you'll want is one that generates a natural frequency of around 2 Hz. For a stock 951, this is about 250 lb/in. Beyond 2 Hz, ride quality will deteriorate quickly, and you'll probably have difficulties with traction on anything but an entirely smooth track.
I know some people have had success with 350 lb/in (2.33 Hz) and even 450 lb/in (2.65 Hz), but this is really in the realm of pure race cars, and your ride quality will suffer.
Regarding the front/rear balance, I have no documented proof from Porsche regarding the actual wheel to spring rate ratios on the 944. Porsche Motorsports in Germany, where I lived for 3 years, assures me that this information is not available. So here are my WAGs:
The front wheel to spring rate is about 81%. The rear torsion bar gives about 4.13E-4 D^4 lb/in per mm of diameter (D). I'm told the coil over spring/shock at the rear gives 44% wheel to spring. So to keep the balance at stock levels, a 250 lb/in front spring (203 lb/in at the wheel) needs a 26.4 mm diameter torsion bar, or a stock 23.5 mm bar plus 173 lb/in aux spring.
Most people agree that the stock car understeers, so you'd want to increase the rear rate a bit -- say 10% -- to dial out understeer (you could also do this with anti roll bars, which would then improve straight-line ride). 10% more rear rate gives 222 lb/in at the wheel in the rear, requiring a 27 mm diameter torsion bar, or the stock 23.5 plus 218 lb/in spring.
I've been told 250 front works well with 28 mm at the rear, but to my original point -- once you buy new torsion bars and get the rear set up, if you find the 28 mm is too stiff, then you're in for the same amount of work to get your car sorted out.
Alternatively, you could simply undo the coil over shocks, change the rear spring, and adjust the ride height accordingly -- a minimum of fuss to change the rear rate.
A 28 mm diameter rear bar gives a rear rate increase of 26% vs. a 250 lb/in front spring -- this might dial in quite a bit of oversteer, but again, with the right anti roll bars, it might not be so bad.
As a final note, while I don't have it in front of me, as I recally, according to Racing Vehicle Dynamics, an engineering text by Milikin and Milikin, F1 cars run natural frequencies in the 2.5-3 Hz range. This is mostly because their aerodynamic packages require a VERY stiff suspension. In addition, F1 tires's construction is very different from passenger cars, with generally VERY stiff sidewalls (so stiff that they don't change shape significantly when deflated, despite fairly low pressures). At some time in the future I might justify much stiffer springs (i.e. 350 lb/in, etc.), but as a first step I don't see it happening.
07-30-2002, 11:57 PM
#7
Burning Brakes
You are not required to reindex the rear torsion bars to add a helper spring, it would help to preload the spring and achieve a better ride height, but it's not essential.
You can drop the rear down about 1/2-3/4" with the spring plate eccentrics, this is workable and what I have done.
then just adjust the coil over so as to just load the spring slightly, but not raise the rear.
The work involved in reindexing is the same as replacement.
Your best try to determine what spring rates would suit you is to find someone in your local PCA that has redone the supension and get them to take you out for a ride.
If you were near me I would glady do this.
hope this helps somewhat.
You can drop the rear down about 1/2-3/4" with the spring plate eccentrics, this is workable and what I have done.
then just adjust the coil over so as to just load the spring slightly, but not raise the rear.
The work involved in reindexing is the same as replacement.
Your best try to determine what spring rates would suit you is to find someone in your local PCA that has redone the supension and get them to take you out for a ride.
If you were near me I would glady do this.
hope this helps somewhat.