Anybody any good at figuring spring rates form measuments?

[dennis68]

I have some numbers like spring wire diameter, free height, and number of coils. Does anybody know how to figure out rate? I know Puhn has a formula, is it accurate. What I'm getting at is the coils now have a free height of 15" and it almost sits too low at rest. I wanted to run AFCoils, but they have a free height of 9.5". I realize that the rate will go up quite a bit will reduce the amount of drop but there is still a 5.5" difference. I would run adjusters but they only run like 2.5"-3.5" of adjustment. Any coments?

[dennis68]

Front coils are .625 wire, 15" free height, 8 active coils, and 5" diameter. Using Puhn's formula I get 215 lbs, even if I drop active coils to 7 I still only come up with 240 lbs. This just doesn't sound right, I have solid/delrin bushings and no sta-bar hooked up, it feels like a whole lot stiffer than 240 lbs???

[Norm Peterson]

Since spring rate is a 4th power function of wire diameter and a cube function of mean coil diameter it's kind of important to be as accurate as possible when making those measurements. I'd suggest the use of a vernier caliper with 0.001" resolution to measure the wire, use the caliper as a depth gauge for coil OD, and subtract one wire diameter from the OD of the spring coils to get mean diameter.

There is a correction for heavy closely coiled springs. If I'm following the equations in Marks' "Standard Handbook for Mechanical Engineers" correctly, there could be about an 18% correction in the stiffer direction with the dimensions you've given. BTW, the term 'heavy closely coiled' refers to the ratio of wire diameter to coil diameter and has nothing to do with the spacing between adjacent coils.

On the other hand, 12,000,000 is a slightly stiffer value for G than I've seen used most everywhere else (11.5E6 is more common, and values down around 11.1E6 are at least justifiable for some steels).

Norm

[Fuelie Fan]

norm's info is correct. The equation is quite simple but accuracy is paramount

k = d^4 * G/(8*N_a*D^3)
where d = wire diameter, D = is the mean diameter (NOT the OD or ID), N_a is the number of active coils, and G is the shear modulus, in psi. I'd also use 11.5 for most steels

N_a tpyically is about N_t-2 (I've seen anything from 1.5 to 2, depending)

[Norm Peterson]

There is another effect to the apparent stiffness if you're simply bouncing one end of the car to estimate spring stiffness. That's the resistance offered to vertical motion by the effect of lateral scrub. You feel it as an effective stiffness in parallel to the springs, as it's a geometric consequence of suspension arcs and lateral tire stiffness (and it goes through the suspension linkage instead). Since the force involved can be more than enough to hold a car over an inch too high when lowered off a jack, the effective rate is significant. It's why you roll the car to settle it and one of the reasons why alignment racks have turn plates.

Norm

[dennis68]

That is the formula I am using and my numbers are acurate. I come up with 240 lbs. I am not using the bounce on the fender method but the road test method. I have driven a few IROCs that I have installed 600 lbs Hotchkis springs in and my elco seems to have more spring stiffness than those, even with the sta-bar disconnected. Bushing preload obviously doesn't factor into this as I don't have any bushings to preload.

Biggest concern at this point is will the much stiffer springs from AFCO result in less spring drop than what is in there now. For example my 15" free length springs with a 240 lbs rate (I think, though it doesn't seem right) have a installed height of x, what will the installed height of 700 lbs 9.5" free height coils be? Is there a way to guesstimate the free to installed drop without having accurate corner weights? Notice I asked for guesstimation, if I can get within an inch or so that will work for determining what parts to order and then I can trim to desired height.

[dennis68]

What I'm getting at is the coils now have a free height of 15" and it almost sits too low at rest. I wanted to run AFCoils, but they have a free height of 9.5". I realize that as the rate goes up quite a bit, it will reduce the amount of drop but there is still a 5.5" difference. I would run adjusters but they only run like 2.5"-3.5" of adjustment. Any coments?

Is it true, has my grasp on this subject gotten so good that I now come up with questions that stump all? I think not, somebody has an answer for me.

[Norm Peterson]

That is the formula I am using and my numbers are acurate. I come up with 240 lbs. I am not using the bounce on the fender method but the road test method. I have driven a few IROCs that I have installed 600 lbs Hotchkis springs in and my elco seems to have more spring stiffness than those, even with the sta-bar disconnected.

Spring motion ratios (all 3 components), sprung corner weights, and shock bump damping (and the shock motion ratio terms) will affect the dynamic wheel rate and its perception. How do those differ between the two chassis?

Counting inactive coils to deduct from the total is a little vague, especially for those at the chassis side seat. And the 18% correction would put a 240 spring rate up around 280-ish.

Norm

[dennis68]

Thanks Norm, still seems awfully low compared to the 700+ springs I was going to run. Also seems low due to the fact that the OE rate is 250+ and these have had 1 1/2 - 2 coils hacked off.

[Norm Peterson]

You made me put together a spreadsheet for all of this, so it just may have become rigorous enough to find the likely culprit. You need to use the mean coil diameter in the formula instead of outer diameter. Mean diameter is 5.000" - 0.625" = 4.375", and that gives 403 lb/in if I assume 1.5 inactive coils (327 lb/in if all 8 are active). The heavy closely coiled correction goes up to 21% for the same (mean vs outer) diameter reason (so the *actual* spring rates go to 489 and 397 lb/in, respectively). Does 489 seem more likely, or at least more within reach of the other things that differ?

Norm

[dennis68]

I love this place. Where else would somebody gp through the trouble of building a spreadsheet to find hacked up coil rate to an unknown spring? Yeah Norm, that sounds better.

Next question, I am no stranger to harsh ride-hell I may have invented it. Harsh does not equal good handling however and that is the ultimate goal. I noticed today that over abrupt bumps the suspension doesn't seem to "work", it just unloads all at once and is very harsh. I don't beleive solid bushings is the answer because it was acting similiar prior to the change and I attributed it to 35 year old bushings and ball joints. I am running KYB gas-a-justs, maybe too harsh a shock for this combo.

*If* actual rate is high 490ish I may just leave it for awhile, though I am still wondering about finding how much free vs. installed height would drop on a spring of double the rate in that same application.

In any case, thanks Norm.