GlenL

Looking for progrssive rate springs, maybe 250lb/in, for an '87 N/A. I've also got struts from an '86 Turbo to use.

I've been looking around and found only single rate springs.

Which of the two struts need the extra spacer? The car is an 8v model.

M758

I think 250 lbs weltmeisters ARE progressive. The set I had did not have constant winding. Maybe not.

GlenL

I was wondering that. The pics of Weltmeister springs show a constant spiral and they aren't described as progressive rate, from the materials I've seen.

Danno

Yeah, true progressive springs have continuously variable space between the coils so that as the coils bind gradually, the spring-rate goes up. Most of the "progressive" springs on the market are actually "dual-rate" springs with a lower-half that coil-binds all at once. Then the spring takes a big jump in rate. These are typically something like 150-350lbs or 175-425lbs:



A true progressive-spring has spacing that gradually changes between each coil so that they only bind one at a time. Like the lower motorcycle-spring in this picture:



There are numerous reasons why I wouldn't use a progressive spring in the car, unless you do it both on the front AND rear at the same time. I'll just point out one, and that is lateral weight-transfer under cornering.

What happens when the lateral-weight transfer occurs in a corner is that it's split between the front & rear outside tyre. Depending upon the front & rear roll-stiffness, this weight can be split evenly between front & rear tyres, or it can be biased more towards the front & rear. This delicate juggling act can be adjusted by spring-rates and sway-bar stiffnesses.




What happens with a progressive spring is that under initial cornering at sub-maximum speeds, you may get neutral steering because the initial spring-rate is similar to stock. BUT, as cornering speeds increase and you get closer and closer to the cornering-limit of the tyres, the weight-transfer will concentrate more and more on the front tyre (because more weight is transfered to stiffer end of car). So as you get closer and closer to the cornering limit, the car will understeer more and more.

Skip

Glen, I know of no direct-fit progressive spring for your model of 944. The only commercially available progressive spring for this series is from H&R but is only made to fit the 944S2 and 968. The Weltmeister 200# and 250# spring is not progressive, although I agree it does look the part. The closely spaced coils on the Welmeister setup simply go into bind once the car is lowered - it is, essentially, a tender spring used to keep the spring in constant tension whilst the car is lifted. Also, the 87 944 and 86 951 struts are identical except that the Turbo strut have a integral brake duct mount - you can use either with the same spring. The 1" spacer when used with 200# or 250# springs simply restores the original ride height. Most folks forego the spacer and simply lower the whole car (rear is normally lowered using the ride height eccentric located on the spring plate.)

There are some other options for front spring rates and ride height adjustability. Let me know if you'd like to know more about that. Further, a custom progressive spring setup using two shorter main springs is available but too cost prohibitive for most to consider.

Good Luck!

GlenL

Thanks M758, Danno and Skip!

Like the graphics. As the plan is to do street, autoX and track with the car I was hoping to finesse the issue with a variable rate spring. Thinking about it, maybe that breaks down a bit when tracking the car at the limit. Although I know more serious tracksters that do it.

I have looked at every web site and catalog I can and couldn't find them. Sounds like some regular 250# springs coming up. Want it lower, but down a full inch, I'm gonna need a new jack!

Geo

I think most racers now stay away from progressive springs. Dual rate compound spring set-ups were all the rage a few years ago and still in some use, but that's something you don't really want to play with unless you know pretty much what you're trying to do and why.

Serge944

Dual rate? Care to enlighten me?

Geo

Sure. It's actully a compound spring. You use a conventional coilover spring and a tender spring. The tender spring is similar to a helper spring (that keeps the spring seated at full droop), but it has a high rate. The spring coil is flat, not round, so when it coil binds it lays flat. When the tender spring coil binds, the conventional spring rate is what you are left with. But, before that you have a compound spring with a spring rate very different from the rate of the two springs individually.

The formula is: R=[1/(1/t)+(1/c)] where:

R = Compound rate
t = tender rate
c = conventional spring rate

Once the tender coil binds, R = c

[edit] Oops. Screwed up the formula. Fixed it now.

Serge944

Thanks!