burgass

Hello
I am changing my suspension and going for the Koni Sport adjustable shocks. I have managed to find them after a lot of looking around Europe. I tried to find Adrian’s springs which should be a bit stiffer than RS spec. but that proved to be impossible.
Now I have to choose springs and I would really need some advice on spring rates and products. I am looking for linear springs with RS or a bit higher rates. The car is a C4 89 with 20 f / 21 r sway bars and 225/ 265 18” R tires. Wheel alignment and ride height is as the RS. The car is on the track 70% of the time so if it is a bit stiff in town does not really matter.
Doing some simple calculations I ended up with 35 N/mm front and 60 N/mm rear. Does this make sense or is my math copletely wrong?
Would that be good for the track or should I go for higher rates?
If I go for higher rates would the shocks cope with the forces or do they need revalving? Should I stay with this relation between front and rear or should I change it?
Does anybody know the shock rates of the Koni adjustables?
Any suggestions would be warm welcomed!!!
Thanks in advance!

Bill Gregory

For reference, here are some spring rates (sorry, don't have the nm equivalents):

Stock Spring Rates (lb/in) Front/Rear:

89-91 C2/C4 158/189
92-94 C2/C4 169/189

91-94 Option M030 169/263*
93-94 RS America, ditto
91-92 Turbo 3.3, ditto

1992 Carrera RS (European) 247*/440*
1992 Carrera Cup USA, ditto

Carrera Cup Racing Car 200-600/240-800

Note: * Progressive spring working rate

There are a number of us running 600-650 lb/in in front and 750-850 lbs/in in the rear. FWIW, I run 600/750 lb/in, wanting to retain some ability to drive on the street vs 100% track. Also matched to these spring rates are custom valved Bilstein shocks with digressive valves. Most Bilsteins for Porsche tend to be linear valved, however, Bilstein can revalve with the digressive valves, which, with the right rates, makes a 600 lb/in front and 750 lb/in rear, although stiffer, still driveable on the street as well as good on the track.

Can't help you with Koni adjustable shocks - do they have a web site with technical assistance, perhaps for racers? Again for reference, below are the various standard, not custom, Bilstein valving rates (rebound/compression), front;rear:

964 to 1990 332/104; 274/122
964 1991 on 332/104; 277/124
964 RS 259/84; 415/237
964 Cup 220/67; 379/172
964 RSR 237/96; 402/216

It's been said that to use much stiffer springs, a rollcage should be installed to stiffen the chassis.

In terms of spring availability, Hypercoil is one brand in the US that has quite a choice of linear spring rates. Their website is at www.hypercoils.com (note, Firefox doesn't work with the Hypercoil site, you need IE). Eibach and Faulkner also make a variety of linear springs.

joey bagadonuts

The conversion factor from Nm to Lbs/In is 8.85666. So, Burgass, the rates you referenced seem close to the maximum rates used for the RS springs.

35 Nm x 8.85666 = 310 lbs/in
60 Nm x 8.85666 = 531 lbs/in

The RS progressive spring rates are:
Front 250-308 lbs/in
Rear 375-508 lbs/in

BTW, the valving specs Bill graciously provided above are also in Nm's.

In terms of a dual set up, I like my RS coilovers and found them to be a tremendous improvement over my stock shocks and Eibach lowering springs (obviously ). The stiffer setup grips the road very well and performed marvelously at the track, although I wouldn't mind some adjustability.

As for using a stiffer setup for street and track, I'll defer to Bill's judgment on that subject. He also ran the RS coilovers for a year and eventually transitioned to a stiffer, linear setup without much problem. The key is to make sure the shocks and springs are well-matched.

burgass

Joey how did you get this conversion factor? I get 5,7101 factor?!?!?!
Regards

joey bagadonuts

I simply went online and found a metric conversion table like this one. It wasn't derived. Unfortunately, I couldn't find the one I used before but the conversion factor for torque on this "linked" chart is 8.8507458.

8.85 is the number I usually use to convert Nm to Inch-Pound. Am I way off on this?

burgass

Joey, I looked up H&R site for special springs. Check the page for 60mm ID race springs. There they have rates in both systems. If you divide the lb/in with N/mm you get 5,7118. By the way maybe the site you checked (swedish) did the conversion by 1kg=10N which is not exactly right.
Cheers
PS

joey bagadonuts

Okay, now I went to a more reliable source, Fred Puhn's How to Make Your Car Handle and there is a table at the back of the book which includes a conversion factor for Nm to pound-inches of 8.851.

Any engineers out there able to help us out?

tonytaylor

http://www.onlineconversion.com/torque.htm

seems to agree with Joey

burgass

Man, I am CONFUSED!!! I diid the conversion myself the long way:
1 in= 25.4 mm
1 kg = 2.204 lb
1kg = 9.8065 N
This way you get the same result as H&R
The big question is "lb" - is it "avoirdupois" or "troy"?
The "troy" lb is 2.679 per kg and it still does not give 8.851?!?!?!?

I think we need an axpert opinion here!

burgass

Joey I have the same book. The author takes "torque" which is Nm. I think that is different about springs. There is N / mm which I guess means amount of wiegt that is needed to compress the spring 1 mm. Am I making sense?

joey bagadonuts

Try it like this (from the Puhn book):

1 Newton = 0.2248 pounds
1 Meter = 3.281 feet or (mutliplying by 12) 39.372 inches

1 Nm (Newton x Meter) = 0.2248 x 39.372 = 8.851 pound-inches

gjvander

N.m is torque (force multiplied with distance). N/m is a spring rate (force divided by displacement).
Not sure which one you guys are talking about. The springs will be in N/m.

Regards,

Geo

NO-H2O

Hey Burgass..
Do you have some picturs of your car you can e-mail me ??
eric.mckenna@huffybikes.com

Thans SWEET car

Red rooster

1N = 0.2248 lb and 1 inch =25.4 mm

1N/mm = 0.2248 / 1/ 25.4 lb/inch = 0.2248 X 25.4 = 5.7 lb/inch

996 C2 front spring is 29.5 N/mm = 168 lbs/inch

996 rear is 40.3 N/mm = 230 lbs/inch

Hope I got this right.

Geoff

Red rooster

After the defening silence I got worried that I had got this wrong ,big time ,so decided to do a bit of detective work.Bill Gregory states that rear spring rate is
189 lbs/inch for std set up and 263 lb/inch for M030.I found that the Nmm equivalents are,according to Paul Frere , 33Nmm and 46Nmm.
189/33 =5.7 and 263/46 = 5.7 .Faith in simple sums restored UNLESS you know different !!
Geoff