Auto-X coming up

[Crazyswede]

The 99 will sit higher on any given set of springs when compared to a 900 with the same set in them. This has been my experience at least. The springs from my old rally car are in my 900 at the moment and I had to put spacers in the front and the rear to get the same ride height that the 99 rally car had without any spacers.

[Luke]

I agree, my 99 sat high enough for smoother rally use (stpr) with spg springs and no spacers. My 900 with them sits low.

[Jordan]

Two things...

It looks like they have updated the schedule to be April 15th which is good since it will give me more time ,especially with RNY coming so soon.

Speaking of springs I also have the OMP springs i sourced a while ago. I think they are model SA2490, but I don't know/couldn't find any of the specs. What I do remember is that they are very short, and very thick. Maybe I'll pull some measurements on those too. I think Geoff mentioned he had a pair a long time ago, but i might be wrong.

[Geoff]

You guys are comparing stripped rally 99s to stock 900s. Thats probably an 800 lb difference between the cars, of course they sit higher!

Jordo,
I have the OMP springs and the calculated spring rate data for them at home. The fronts are so short that I put them in my 99 last weekend without using a compressor. Those would make a good autoX spring. I'll post the specs when I get a chance in the next few days. Apparently they don't make them anymore. So... if you want to sell them....

[Jordan]

Yes I have installed them in a 900 squeezing them in without a spring compressor. In fact they kept falling out before i put the shocks in. I think I will wait to put the OMP in the 99... im going to run stock class and I think bright red springs might give that away.

[Crazyswede]

You guys are preparing stripped rally 99s to stock 900s. Thats probably an 800 lb difference between the cars, of course they sit higher!

Jordo,
I have the OMP springs and the calculated spring rate data for them at home. The fronts are so short that I put them in my 99 last weekend without using a compressor. Those would make a good autoX spring. I'll post the specs when I get a chance in the next few days. Apparently they don't make them anymore. So... if you want to sell them....

Not so,
I put the stock 99 springs in my 900 that were in my 99 5 door. In the 5 door the car sat at normal ride height or even a little taller. in the 900 the car sat lower then it does with spg springs.

[Jordan]

That is what he is saying... just in reverse

[max]

I might have to heed the calling, and do some AutoX... I'm definetly in G-stock unless they changed things. Too bad once I put on a pulley, I'll be in street prepared.

[Jordan]

Ok here are the spring measurements I took.

Total Coils (including top and bottom): 9
Coil thickness: 20/32in or .625in
Outter Spring Diameter: 5 13/32 or 5.40625in
Space between coils (just in case) 1in

ok I'm going to try to run it through some forumlas...


---------------- Results --------------------------
I ran it through a programming language...the ** means ^

wire_diameter = 0.625
mean_spring_diameter = 4.78125 #(Di + Do)/2
active_coils = 7 #9-2?
mod_material = 79.3 #steel?

spring_rate = ((mod_material*wire_diameter)**4)/(8*(mean_spring_diameter**3)*active_coils)

I got 985.826377202098

so help me out....

More --------------------

I ran it through a calc on iroczone.com (go figure) and got 280.453 which sounds more like it.

[Geoff]

You're using the wrong modulus. The shear modulus of steel is 11,240,000 psi (not 79.3)
also note there is a mistake in the top half of your formula, it should read:
(shear mod)*(wire diameter)**4

Using the proper formula and the proper shear modulus I get 280 lb/in as well.

Those should actually be very close to SPG front springs. I looked at my spreadsheet last night and the theoretical spring rates for SPG fronts were calculated at 272 lb/in This is actually very close to what Luke measured (310lb/in I think?) considering the factors of: diameter measurements (effected by paint thickness), material composition (not all steel has the exact same shear modulus), exact number of coils, etc.

[Jordan]

Ah ok... well you didn't specify sheer modulus was in psi and not Gpa . Thanks for your help.. so if calculated spring rate is greater than that of the SPG spring, then actual rate might be a little more as well?

[Geoff]

Its understood, otherwise you're mixing units, metric and english.

Not necessarily. It depends on how accurate the measurements were taken and other factors. Since two different people measured the wire and coil diameters there might be slight differences between their measurement methods. Was the wire diameter actually 0.625" or could it have been 0.624" (or something else)? How thick is the paint/powdercoat on the springs? One spring might have a paint thickness of 0.005" and the other might have a paint thickness of 0.015". Were there precisely 9 coils or were there 8.9 coils? Are both springs made of the exact same material from the exact same vendor from the exact same material lot? All of these factors influence the calculation which is why the calculated spring rate for one SPG spring was 272 lb/in while the measured spring rate for a different SPG spring was 310 lb/in.

So overall, that spring probably has a rate that is very close to that of SPG springs but the only real way to tell is to measure them and a bunch of SPG springs and compare.

hey, what are you running for rear springs? 99s and 900s behave best with stiffer springs in the rear than in the front. This probably applies to most FWD cars...

[Jordan]

Well see, that's why I'm not an ME and you are!

For the rears I'm running the stock EMS springs... I have the OMP set thats on my 900 but right now and thats the only other choice I have. From things I've read , the 76EMS had stiffer rear suspension (springs +bilsteins). With the rear sway bar and new bilstiens, it seems pretty tight. What else would you recommend?

I guess the only way to compare the spring rates would be just to measure it where Luke measured his. On a side note, what ever sheet that you have been compiling with spring rates would be nice to have on the site.

[Geoff]

Yeah, I got home past midnight last night and didn't have my memory stick with me so I didn't grab the Excel file. I'll probably post the info here over the weekend as a picture image.

Luke, did you measure both the fronts and rears or just the fronts? It would be nice to be able to compare between calculated and measured.

If I were you I'd put the front springs in and see how it works. You can manipulate the handling of your car with tire pressures. Go with the lowest pressure you can in the front to avoid tire rolling and then adjust the rears to make the car handle right. Higher in the rear tends to help (especially when you find the front end plowing). If you can't make it handle right then think about rear springs. On my '80 GLi (when it get it running again) I'll be using the OMP springs (front and rear), Addco swaybars front and rear and a stock 900 rear swaybar becuase the addco rear is smaller than the front. Bilsteins all around. We'll see how it handles.

[Luke]

Hmm, I'll have to pull out the specs for the spg springs tonight. I remember calculating them at around 400 and was suprised when they were only 310, they don't have many active coils so they should be theretically significant stiffer than the saab catalog ones which I remember had many more coils but the same guage wire. Also a couple things to keep in mind. We have to run a really high rate on the back springs because our motion ratio is so high, not because we actually want such a high rate at the wheel. Remember that the important thing is actually the rate at the wheel. FYI the 325 I use at the front of the rally car for most rallies work out to 200 at the wheel which is the generally accepted wheel rate for midsize rally cars. You will see other cars like VW using very light rear springs some where on the order of 150-180 lbs but they mount right at the axle and the effective rate ends up being similar to what we have. This is also important to take into consideraton with shocks. Our front shocks are mounted way inboard which gives the dampening less overall effectiveness at the wheel.