mikey_audiogeek

One more thing: 30mm torsion bars (which a lot of people run in combination with about 350-400lb/in on the front) give 335lb/in wheel rate, which is equivalent to a 790lb/in coilover...

IanM

I ran a lot of spring rates looking for the best setup. Car was dual-purpose street and track. I ended up with 425# front / 600# rear on Bilstein Escorts, Weltmeister bars about mid-stiff and equal size tires all around. Handling was amazing.

docwyte

I'm going to start with 450# front, 700# rear springs and see where I go from there...

What do you guys think my current Turbo S suspension is worth? Front Turbo S coilovers with heavier front springs, koni's converted to double adjustables. Rear 30mm torsion bars and turbo S stock koni's...

dmcampbell

Yes that is what I am running, but with 425# up front, stock turbo s Konis set at about 70% rebound, 968 030 front and rear bars with rear in softest setting. I also have Delrin sway bushings, some poly suspension bushings ans stock spring plate bushings - which will soon be changed for poly bronze. Very neutral even on a rough, multi surface track like Sebring.

IanM

450# total rate * 0.9 = 405# effective rate front

700# total rate * 0.56 = 392# effective rate rear

front / rear eff. ratio = 405/392 = 1.03

That's a good ratio if you're planning to run a staggered tire setup, but may shift your balance towards oversteer if you plan to run same size tires all around. If you do plan to run equal size tires all around, I would use a ratio of about 1.1 to 1.2.

zerMATT951

I shouldn't confuse things with my questions... I'll start a new post .

docwyte

Ian, so that would be a 500# front spring then to get to 1.14. Hmm, I may be able to play with the swaybars some to dial out the oversteer...

IanM

1.14 is the ratio I ended up at with my 425# / #600 setup, worked very well with no stagger. I played with all sorts of rates, and ratios, and that's where I ended up. In the end the balance was very neutral. 500# / 700# would be a good track setup with wide front tires, but might be a bit rough if you're running a lot of street miles. Actually, I'm looking to put 700# / 900# on my GT3, so I shouldn't talk...

I actually have a bunch of springs in my garage if you're interested in any of them - 400# front, 500# front, 550# rear 650# rear. Three of the sets are Hypercoil, very low mileage on them.

mikey_audiogeek

The ratio for the rear is 0.4225 (=0.65^2). That's as per the Porsche Motorsport datasheet for the Cup cars.

333pg333

"The info on Paragons site is stuff they got from me a few years ago. In any
case, the 47% is a number that I have calculated By taking measurements. I
am now pulling out one of Porsches own Motorsport sheets that shows all the
rates of their Turbo and "Cup" cars back when they ran competitively.
Porsche lists the 25.5 mm bar as 31 N/mm which is 177 lbs/in. They also then
give the variable rate coilover helper spring rates at 34-65 N/mm which is
194 lb/in - 371 lb/in. They then give the total Rate at the wheel(T-bar plus
coilover) as 45.4 - 58.5. Back out the rate at the wheel due to torsion bar
which they list as 31 and you have 14.4 - 27.5 at the wheel due to the coil
over. So take your pick, 14.4 / 34 is approx 42% or 27.5/65 is 42%.

Bob is right about them being inboard but his numbers are off. Actually they
are correct I think in that the motion ratio is about 65%. But when
calculating wheel rates from spring rates it is the motion ratio squared
that is uses. So 0.65 ^2 is , guess what,... 42.25% which is the number that
Porsches own sheet claims as I outlined above. So your torsion bar is 177 at
the wheel, and your helper spring rate is a 285 which is 119.7 lb/in at the
wheel. So working backwards 177 plus 119.7 is 296.7 pounds per inch at the
wheel. Divide this by .42 and that is you equivalent coilover, or 706 lb/in
coilover (initially I had 661 lb/in which is attributable to my measurement
error - I had 47% and Porsche lists it at 42%).

So it may be a bit stiffer in the rear than I might run but with the ability
to tune sway bars etc... you should be Fine. Your setup is actually much
stiffer in the front than the Porsche cup setup which ran progressive front
springs (200 - 371 lb/in) with the rear setup I described above which is not
that far from what you have( yours is 296 at the wheel and theirs was 259 -
334 lbs/in at the wheel).

Hope this make sense, but believe me, what is above is 100% correct. I can
fax you the Porsche motorsport sheet if you think it will help."

Van

I think you might be sweating the petty stuff here... There are other variables that will effect the oversteer/understeer balance that aren't being taken into affect - like rear down force, weight balance of the car, camber settings and driving styles.

Additionally, when I physically measured motion ratios on my '88, I came up with slight different numbers - I got .91 for the front and .63 for the rear. Which, using Ian's spring rate numbers, yields 410/448 = 0.92.

I think the key here for Doc is that some trial and error (and perhaps some data acquisition) will help you dial in what works best for you.

wjk_glynn

Very informative thread...

Karl.

IanM

I knew someone would pull out those other conversion values. Because I did so much trial and error testing of different spring rates myself, I didn't sweat the absolute values when calculating front to rear ratios. For my particular setup and driving style, and using the 0.9 and 0.56 values, I found that a front to rear ratio of 1.0 worked well with staggered tires and about 1.15 worked well with wider front tires. If you use different conversion values, then your ideal front to rear ratios will probably be a bit different.

IanM

Yes alignment is important. I spent a lot of time on track and in the alignment shop getting it right - good temperature profile and even wear patterns. I'd be happy to share my numbers, would have to dig them up. Nothing too earth shattering - something like -2.8 camber front, 8 minutes total toe-in , max caster, -2.5 camber rear, 5 minutes toe-in rear each side.

Van

Interesting you run max caster - I run min caster, otherwise there's more friction lost due to the increased resistance while turning. I also have manual steering and min caster requires less effort. (Again, more effort means the car is using more energy to turn.) I believe my caster is about 4.5 or 5 degrees.

I run about the same camber, but about zero toe in the front and a little bit of toe-in at the rear.