Sweet 944

I have one questions regarding the spring rate for the rear springs and I was hoping for some help.

I am using 300# springs up front.
I have a 23.5mm Torsion Bar ( 126# - effective rate).

And stock sways. ( I will upgrade once I know how the car rides with the stuts/shocks).

When calculating the rear spring rate should I use a value of 300# for the rear rate I am trying to achieve or do I use the effective rate of the front springs?

EXAMPLE: (300-126)/.56= 310.71 or 300# rear spring
-or- (effective FRONT spring rate-126)/.56 = ???

ALSO..... What is or how do I determine the effective rate of the 300# front springs?

Any advise would be greatly appreciated.

Mongo

I don't think there is a way to figure out the effective rate with that formula for the front. I think it only applies to using a torsion and spring in conjunction.... hmmm.


The formula would apply to the rear of the car so the front would be ruled out to answer your question. Some of the guys around here have rear coil over shocks. I'm curious to know also if a 300 lb rear effective rate would be on the car. Seeing that the car is near 50/50 balance stock, would such a stiff rear effective rate cause the car to dramatically oversteer?

Manning

If you feel like doing a search, there are a few threads that cover this, though you will have to sift through a ton of junk to find them, or the correct combo of words for the search. IIRC though, effective rate for the front springs is something like 80-85% effective. So 300lb/in springs have between 240 and 255 effective rate.

Feel free to correct me if I did this wrong

Mongo

Manning how are you calculating the effective rate as a percentage? What's the formula to use?

Manning

300*.80=240

Is that incorrect? I don't have to use math too much so it tends to get FIFOed.

Sweet 944

As far as I know the first number (300-xxx) is the effective spring rate you are trying to achieve in the rear.

In other words.. I'm using 300# spring on the front.. I think I want to achieve a 300# effective rate on rear (300-126)=174/.56 = 310.71 or approx. 300#

My question is do I use the 300# spring rate I am trying to match or is there an efective spring rate for 300# front springs?

Ken D

I think this explains it well:

http://www.tech-session.com/kb/index...x_v2&id=22&c=4

M758

Oh here is goes.

300 lbs front. Based on my expeirece 300 lbs effetive in the rear is good.
I use 350lbs front and 335 lbs rear effective. (30 mm t-bar)

So shoot for 300 lbs effective rear

300- 126(effective rate for 23.5 mm t-bar) = 174lbs

So you need a coilover with the effective rate of 174 lbs-in. so 175/.54 = 322lbs-in coil over combined with the stock t-bars will get you 300 lbs-in in the rear.

Simple enough Except that you can't get a 322 lbs-in in spring. So settle for 300 to 325.

Note you can also go with 275lbs in if you want a bit more understeer. This maybe good for a street car.

Also not sure if the ratio is .54 or .56?

Sweet 944

Thanks, I already have this tech sheet and it was very informative...
I am trying to figure out how to determine the spring rate I want to achieve.(front spring rate of 300# that I want to match at the rear) do I use the 300# spring rate -or- is there a caculation to determine the EFFECTIVE rate of the front springs....

Manning

A spring that is listed as 300 lbs/in is not taking into consideration the suspension geometry. SO if you install a spring listed as 300lbs/in on the front of your car the position of that spring in the suspension renders it 80ish percent effective, or 240 lbs/in. If you install that exact some spring on the rear it is 56ish percent effective, or 168lbs/in

M758

There is, but that won't help you much. It is question of balance and feel how much spring you want in front vs in the rear.

I can say from experience..

250lbs front/ 254 effective rear was nice combo

350 front /254 effertive rear had too much understeer

350 front /335 effective rear seems quite nice.

I have know people to use 400F/335 Effective rear and 450F/335 Effective rear as well as 250F/335 Effective rear.

There are lot of factors that come into play.

I think stock fronts are 125 to 140 lbs-in actual spring rate.
So with 23.5 mm t-bars and 126 lbs effective rear you can do the math on what rear effective springs you need to get a stock balance with 300lbs-in front springs.

All my examples used bigger t-bars so i can't say what coilover shoudl be used with the 23.5 t-bar other than to calculate it.

You do know that you will still need to re-index the t-bar after you add the coil over. Of course reindexing is the same work as putting a 29 mm t-bar (perfect for 300lbs front springs) and not using coil overs at all.

Sweet 944

Thank you Manning.. This is what I am looking for as far as the correct way to calculate the rates... I do realize that driver preference has a lot to do with setting up that car.. I am new at working with suspensions, so in my mind information is the key.

So if I have this correct it looks like this:
(300# x .80 = Effective rate of front spring or rate I am trying to achieve which is 240#-126# 23.5mm Torsion Bar=114/.56= 203.57 or 200# rear spring..)

Yes..... I think I got it..

Manning

That looks right based on what I have read before, though the effectiveness of the front springs is still a question as I can't recall the exact number. It is something in the neighborhood of 80 to 85% effective.

M758

I heard closer to 90%

Even so 300 bls front and 240lbs effetive in the rear will cause understeer. You can probably dial it out with bigger rear sway bar.

Sweet 944

I know and I am searching for posts refering to this.
I am going to use the full coil-over set-up (Bilstein Turbo cups) I need rear shocks anyway & the Bilsteins were given to me.. FREE.. ZERO.. ZILCH.. NADA....so I will use them as they only have like 5 track days on them.. I have already sent them out to Bilstien for re-valving and expect them back soon...

The original set-up (came off of a friends 89 - 944turbo race car) had 550# front & 300# rears.. I am using the 300# up front and going get spring for the rear...

Once the car is set-up I will add better sways..