Spring rate
 

I just got a set of Bilstein struts for my 951, but they don't have springs so I'm going to have to get some before I can install the struts. I was thinking about 275 lb. springs, but I wanted to get some feedback before I buy springs. Obviously, I want to improve my handling, but the car will be driven on the street, so what is a good compromise between handling and ride comfort. I know this is a Porsche, so I don't expect it to feel like a Cadillac, but I at least don't want it to rattle my teeth loose. The guy I got the struts from has an extra pair of 350 lb. springs, and although he says he didn't think they were too stiff when he had them on his car, I'm still not sure if I want to go with springs that stiff. Anyone have any thoughts on the subject (1.2gees)?

I will try to give you an idea...

Stock 944 NA has about 175lbs springs, and that's the stock rate, I don't know about sport susp.

The 951 has just around 210lbs springs, and that's standard, (and a guess, I only know the 944 NA for sure). Both are normal springs, the rate doesn't change, the first inch of compression takes exactly as much force, as the very last inch.

I'm running 250-350 progressive weltmeisters on my car, and they're pretty hard. The softer beginning rate is supposed to make the car ride a bit better, over minor imperfections on the road, but I think it's overly hars for even the "avarage" Porsche driver. Than again, the biggest rim approved for early 944s were 15 inchers, and I had 17s, with twice the spring rate, poly bushings, 255 tires (versus 215, the widest factory approved width).

However, with the turbo's later susp. set-up, namely harder bushings stock, higher spring rates, bigger wheel bearings, aluminum (much lighter susp. components), and so on, I think you could go a bit higher, and have the same ride harshness I've had. Hence a 944 turbo rides better than a stock early NA, even though it's got harder bushings, higher rate springs, stiffer shocks, and heavier 16 inch rims. I think this is because of the improvements I've mentioned earlier.

Ok, let's try to sum it up:

I'd recommend about 1 inch drop, dropping the car makes a big difference in handling, without compromising ride, (given that the spring rates are the same) but you need harder springs, so the car/suspension don't bottom out. I do not recommend more than 1 inch of drop on a 951, because that would be too low, and would cause bump steer. (I consider it excessive at 1.75 inches drop). Should be just fine around 1 inch. As far as the rate of the spring? I suppose you could run 250-350 progressives, they're excellent to my taste, but this is very personal. (at least you can go pretty high with bilsteins, unlike the kyb sports, I run through a set at about 20k, if that).

If I were to do it over? (and I will), my car would be dual purpose, I'd get the lightest 18 inch wheels I can find, billet control arms up front, and have the spring rate, right around 460lbs-500lbs. I'm sure that would be too much for most people.

Oh, two notes:

The control arms were updated at some point (the aluminum ones), and it's common for the earlier type to break, with little to no warning. I've heard that even the late ones seperate, potentially, very dangerous. I'd run to the dealer, and make sure, if you think you're keeping the car, I'd recommend some FABCAR billet control arms for the front, far stronger, far lighter, larger replacable ball joint...

Second, the shocks (struts) bilsteins are excellent, and some of the best shocks for (relatively) the money. However I've heard that you need Bilstein SPORT shocks, if you want to go to shorter springs. However they should be able to handle pretty heavy springs, even if they're not bilstein's sport type.

Let me know if this helps at all,
Ahmet

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It's all the driver...

These are Bilstein "street sport" struts, as opposed to the "sport" struts which are intended for racing application. They have adjustable ride height, so I can play with that to get it where I want it. The springs I'm looking at are not progressive, they're constant rate springs, made by Eibach.
The FABCAR control arms are steel, not billet aluminum, they are heavier than the stock cast aluminum arms, but they are stronger. The billet control arms sold by Automotion, Paragon Products, etc. are made by Jon Milledge Engineering.

Fabcar...

The name must've stuck from somewhere, sorry about that...

so what rate are you going to go with?
Ahmet

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It's all the driver...

Todd,

Ride harshness is not totally determined by spring rate, it is mostly determined by damping rate. Some Bilsteins are digressively vavled, This means that short fast piston rod motions (like when you hit a bump) have less resistance than long slow movements (like body roll) so ride quality is better and traction is better and handling is better all at the same time.

However as I said before the damping rate is really the most important thing. If these shocks are set up to properly damp 350#/" springs then those springs will give the best ride quality and handling. If your springs are too heavy or too soft then the suspension will be either over damped or under damped. Both conditions will result in poor ride quality.

Question, are these struts yellow, with a silver grey adjustable sprng perch? Or are they green with a silver grey adjustable perch?

BTW I have a friend running a Honda Prelude with 400#/" in front and 300#/" in the rear and because his Konis can handle it the car rides better than whe he had the weak Neuspeed lowering springs in the car.

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The shortest distance between two points is in my car.

Pilot, the front struts are yellow and the rear shocks are unpainted. What spring rate do you think would work well with these?
I've also been thinking about replacing the rear torsion bars. There's a pair of 28mm torsion bars on eBay right now, but I don't know if that would match the 350# front springs, or if I would need 29 or 30mm torsion bars. Any suggestions?

Todd,

If you were to go with 350# fronts then 29 or 30 mm torsion bars are probably what you want for the rear. However if the car is predominantly street driven 28mm will be better for you as well as 275-300# front springs.

You have a problem in that you don't know what the damping rates are for those particular struts. If these are custom built Bilsteins then the damping rates were specified by the buyer not by Bilstein. If they are off the shelf Bilstein Heavy duty or Sport shocks then Bilstein should be able to provide you with the damping rates and acceptable spring rates.

More questions:
Do the front struts have adjustable spring perches, or are they fixed? (if they are adjustable then these are likely custom vavleved)

Did the previous owner of the shocks purchase the springs and shocks as a matched set? (If so I would use those spring rates)

Lastly I have never seen unpainted Bilstein rear shocks. Actually that may not be true, do the rears have threaded bodies for adjustable spring perches?

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The shortest distance between two points is in my car.

The struts and the rear shocks both have fully threaded bodies for adjustable spring perches (as opposed to a collar that slips over the body). I believe, though, that they are off the shelf parts, "street sport" struts and shocks, Part numbers: Strut Left AK1110-A, Strut Right AK1111-A, Rear AK37354.

Ok, I just called Bilstein and got the damping rates for the struts and shocks. The rate for the front struts is 380/150, and the rate for the rear shocks is 565/218. Now what size springs and torsion bars should I be using with these?

Okay the fronts sounds about right for streeable Bilsteins (a little soft in rebound IMO), you should go about 350# on the front, maybe with a progressive rate like 1.2.

The rears seem way overdamped in compression, but generally with a street/track/autox setup you would go with 28-30 mm rear torsion bars.

EDIT: Is the rear damping rate a typo?

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The shortest distance between two points is in my car.

[This message has been edited by Pilot_951S (edited 07-10-2000).]

No, that's not a typo. The valving for the rear shocks is 565/218. And yes, I agree that it seems too stiff in comparison to the fronts. The rears are brand new, never been installed, so I'm going to see if I can exchange them for the lighter valved (400/200) shocks.
I also think I'm going to go with 350# linear springs. The guy I bought the struts from was using 350# springs with them and said it felt just fine.