Manning

Joe has an awesome calculator and instructions for setting t-bars up

M758

You can find it at http//944spec.com in the tech section.

Geo

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

this is yet another reason to buy from our friends @ paragon

Sweet 944

Yeah.. Thanks... I have been using that tech sheet so far to calculate the rear set-up.. It is very helpful.. and I plan to buy my springs and sways from Paragon...

I am trying to under stand which rate to use as the "Desired Effective Rate" which is part of the calculation in the tech sheet you linked to.... based on using 300# springs in the front..

joseph mitro

paragon's website is very helpful for suspension DIY. but, in all the posts i've read here on rennlist, i've never seen anything about the front spring rate only being 80-85% of the actual rate. maybe skip or someone with some knowledge can clear that up.

in the meantime, my setup is 350F, 275R coilover springs with reindexed torsion bars. by my calculation this gives 350F, 300R effective rate (give or take 20lbs). seems to work with my 27/19 mm sways.

don't forget that you can fine tune the handling with sway bars, tire sizes, tire pressures, and alignment settings. there are so many variables. if you're looking for handling biased towards neutral/understeer, keep the tire sizes the same and have it aligned by somebody that knows suspensions, then you can play with tire pressures to suit your needs.

Manning

There was a very long thread several months ago about suspension setup where front spring effectiveness was mentioned several time, as well as several old threads on the e-mail list. It may be closer to 90% but I doubt it is 100% effective.

Geo

What we are talking about is the motion ratio which alters the effective spring rate. I highly doubt the motion ratio is 80%. Most MacPherson strut cars have a motion ratio near 100% if not actually 100%. I have not measured it yet personally.

Oddjob

Ive read/heard approx 90% effective rate for the front and around 55% for the rear. I really doubt that anyone developed a suspension geometry model to calculate those effective rate percentages. Its probably based on empirical measurements.

One way to ballpark these figures would be to measure ride height, add a lot of weight and remeasure the ride height. That would tell you how much the springs compress for a given weight. I tried this and found that the listed rear effective rate seemed to be ballpark accurate - or I should say that my crude test came pretty close to the published %.

Leigh, what you are asking is probably the key question and what a lot of shops and track guys spend a lot of time and money trying to figure out. What ratio do you want front to rear, to balance the car.

So with your 300 lb/in front springs (90% = 270 lb/in effective), what do you want your rear spring rate to be, so the car handles well. Do you want the rear to be 270 lb/in effective, which is a 1 to 1, front to rear effective rate? Or do you want a little stiffer rate in the front versus the rear, say a 1.3 front to rear effective rate ratio?

Most of the guys I know in racing that setup their cars with very high spring rates, use about a 1.3 to 1.5 front to rear effective rate ratio. I think that with an n/a and with lower spring rates you can lower the ratio and the car will still be balanced, so somewhere between a 1.0 and 1.3 ratio should be pretty safe. 1.0 would be closer to oversteer, 1.3 would be closer to understeer.

Manning

Wheel rate, that's what I have been looking for. This site explains it way better than I can. It's that math I was telling you about. FIFO, it ain't there in my tiny brain anymore.

http://e30m3performance.com/tech_art...e/eff_rate.htm

In any case, there is no way the effective rate on the front suspension of our cars is 100%. May be close, but not that close.

Oddjob

If you model the suspension as a very simple force diagram, only considering the weight of the car acting through the angle of the strut: the effective rate in the true vertical direction - the normal force (opposite of gravitiy), will be the cosine of the angle between the strut and the vertical. So if the strut is at 15 deg. from vertical: 300 lb/in spring x cos(15 deg) = 289 lb/in.

Not sure of the actual strut angle, but that could be the methodology used to come up with the theoretical 90% effective rate. The rear suspension geometry would be more complex to model.

Leigh, one other thought: take a look at the height of the springs. If you are taking the 300 lb/in spring off the rear coilover to use on the front, the length/height of the spring may be too short for the application. I do not use the same length springs on my rear coilovers as I do on the fronts - the rears are shorter.

Geo

I have absolutely ZERO doubt that someone has done it. There was a time when professional teams raced these cars. I'm quite sure they have done this. I'll bet if you call Don Istook or Jon Milledge, either could probably tell you off the top of their head what the motion ratio of these cars is.

Geo

Bingo! You got it exactly. That site is an awesome site BTW. I've been referencing it for years.

You're right, it won't be 100% for our cars. I'll bet the later cars are closer than the early cars. Again, I haven't modeled this, but my guess is that when the offset changed the scrub radius changed. This would put the motion ratio much closer to 100%.

Sweet 944

Originally, When I first received the coil-overs, I was thinking the same thing. BUT, I have measured the front & rear springs and they are exactly the same.. 9" length 2.25" ID..... Fronts are 550# HR's.. with approx 5 track days on them (anyone need them?) the rears are 300# EIBACH which I am going to use up front and order some EIBACH 250# or 275# for the rear...

I can't wait to get this set-up in the car.. I bought the car with the shocks & struts about 75% shot.... so this will be the first time I drive the car with a stiff set-up...

Sweet 944

I have to say that RENNLIST is such a great tool...
It is so awesome to get so many responses to the questions I have...
I love that there are so many people interested in the same thing and willing to offer help to others..
Thanks so much....

joseph mitro

just out of curiosity, what coilover setup are you using that allows a 2.25" ID spring? most that i've heard of allow 2.5" ID.

rennlist helped me rebuild my engine. yes, it's a great community.