Spring Rate Mystery

Ok, I've got 3 sets of springs and 2 sets of shocks, and trying to figure out which to put on the wife's '95 Miata and which to put on the '93 track rat. I've got 2 sets of shocks, a set of Tokico HP and a set of KYB AGX. I've got 3 sets of springs, and I have no idea what their rates are. One set of H&R Sport springs, one set of Vogtland Springs, and a set of Rod Millen Springs. I've googled as many different variations as I can think of to learn what the spring rates are, but I've had no luck. The H&R springs are on the track rat right now, but I need something stiffer. The '95 is on stock 201k mile shocks & springs, so I figure anything is an improvement. Just figured I'd cherry pick the best for the rat and leave the 2nd best for my wife I do try to be a good husband!

Well, you could do some rough estimates yourself using a scale, something relatively heavy (like an extra wheel), and the springs themselves (weigh object - use to measure difference in spring length). You might not be super-accurate, but it'll at least work for comparison purposes.

Other than that I'm no help. I'm surprised no one in the Miata community has gathered a list of rates for the common aftermarket springs.

http://www.pontiacracing.net/js_coil_spring_rate.htm

IIRC, SpecMiata spring rates are 700/325 for an NA. If you use Zomby's calculator you may be able to see how close you can come to these rates for the track rat. I'd be tempted to put the AGXs on the rat so you can try and match damping to whatever springs you end up using.

Common aftermarket spring rate info has been collected a couple of times by various folks, but approach with caution. When a vendor changes their spring rates, they don't hunt down everyone with a spreadsheet to get out the new info. Also, some companies simply don't release their rates, preferring to fudge or hide behind "30% stiffer with a 1.5" drop". 1.5" compared to what, a 1990 or a 1996?

I wouldn't call the Rod Millen springs "common aftermarket", either

I'd recommend measuring the springs. A lot of aftermarket springs don't necessarily last well, and you may have some softened, saggy ones. That will show up in the calculations as the free length will have dropped.

The AGX can't handle springs much over 300 lbs. You could put the Spec Miata rates on, but it would be terrible. The HP can barely handle stock Miata springs for a short period before they wear out, so I'd recommend keeping the stock springs and putting on some better shocks instead.

Thanks for the info guys. I had not seen a website that gave the modulus of spring steel, so that was throwing in a monkey wrench for me. For the mean coil diameter, I understand that to mean the diameter from the center of the wire to the center of the wire on the opposing side? Anyone else have better knowledge on that?
Keith - thanks for the info. That's one of those things that I hate about aftermarket springs, you can get a guesstimate by adding 30% to stock, but again, you don't know which model. I had no idea that Millen even made springs, found these, figured I'd give them a shot, maybe they'd be firmer than the H&R that are on the car now. It leans way too much in the long sweepers, so looking for something to firm it up. All of the springs look nearly new, just a little bit of road grime on them, so I'm hoping they're still pretty good.
I knew the HP were pretty much just a stock replacement, but thought the AGX would be better than that. I'm wanting a set of Illuminas, had them on the old supercharged Miata, and I loved them. I may end up breaking down and getting a set, and keep the AGX for the Crow Mountain Hill Climb, the road is rough and I like a slightly softer spring rate so that the car doesn't end up getting bounced all over the place.

Know any race shops that have a spring tester ?

HoserRacing wrote: Thanks for the info guys. I had not seen a website that gave the modulus of spring steel, so that was throwing in a monkey wrench for me. For the mean coil diameter, I understand that to mean the diameter from the center of the wire to the center of the wire on the opposing side? Anyone else have better knowledge on that?

Modulus of elasticity-30x10^6 psi

And I am fairly sure that is the correct about the mean coil diameter.

Just measure them. Put 100 lbs on them and see how far they move. Do the math. Win races.

HoserRacing wrote: I knew the HP were pretty much just a stock replacement, but thought the AGX would be better than that. I'm wanting a set of Illuminas, had them on the old supercharged Miata, and I loved them. I may end up breaking down and getting a set, and keep the AGX for the Crow Mountain Hill Climb, the road is rough and I like a slightly softer spring rate so that the car doesn't end up getting bounced all over the place.

The Illumina is just a better shock than the AGX. You'll get a better ride and you can handle more spring rate with it. Typically, by the time you've turned up the AGX enough to control a set of springs, you've picked up a bunch of harshness.

Don't know anyone with a spring tester, tried the 100# thing, but couldn't get my son to stand still long enough to get an accurate measurement Looks like I'm whipping out the calculator & micrometer tonight :) I'll post up my results for rates later.

Ok, working on the set that's not assembled at the moment, using the formula

                                                    modulus of spring steel X wire diameter^4

spring rate = -------------------------------------------------------------------- 8 X number of active coils X mean coil diameter^3

modulus of spring steel = 11,250,000 pounds/inch^2 = 78,500 newtons/millimeter2

I am coming up with a front spring rate of 187 lbs/inch^2 and a rear rate of 127 lbs/inch^2. I am working with the thought that active coils means the number of coils that are still at an equal distance apart, and not counting the amount of coil from where it starts to flatten out. Can anyone tell me if this is proper or not? Also, found this website while doing a search for stock spring rates for a comparison

http://www.puppetmaster-robotics.com/Miata/miata_springs.htm

This tends to make me think my figures are pretty accurate. And disappointing :(

Had a vender respond, the Vogtland rates are 235# front, 140# rear. Looks like I'm going to go with these until I can save up some money for some Illuminas. As soon as I take off the H&R springs, I'll measure them and see what their rates are.