Spring Rate Measurements

[Jetaway]

LeonV:

Thanks for the formula, though I'm not sure how to count spring turns. So, I spread and came up with estimates of 140 lbs (if 9 turns) to 197 lbs (if 7 turns).

John Coffey:

I took your advice into consideration and remeasured. By now I had everything disconnected (last time the steering knuckle was still attached). I used wedges under the scale, the wood platform (used to spread out the force) and the bottle jack until I had the jack's piston movement lined up exactly with the struts stroke. I taped up my scale and marked 1, 1 and 1/2 and 2 inches of spring compression. The scale is auto-on and auto-off once it has a stable weight, so I couldn't stop and note the exact intermediate weight, but I did repeat the process several times and am confident with the figures.

The first inch required 240 (once), and 250 (twice), with 1 and 1/2" readings of 320, 330, and 330. I couldn't continue on to 2 inches because the scale maxed out at 396 lbs. (Odd capacity to say the least. But it does measure in pounds, kilograms and ... stones.) I extrapolated that the second inch would be 160 lbs.

I matched the angle of the coil to the jack, detached everything connected to the coil/strut assembly, and doubt that the scale would be as much as 5% inaccurate at 83% of rated capacity. That's pretty much in the sweet spot for solid-state scales. Think that about covers your points.

Interestingly, my estimate of 270 lbs using the simpler wheel on scale approach (and with the steering knuckle attached) isn't that far out of line with my new first inch estimate. By spec, the knuckle adds between 9 and 34 lbs and the mechanics I had give it a push thought it was still serviceable. Toss in half the difference and you get 250 lbs.

My biggest mistake, by far, was my ignorance of the first inch effect.

160 lbs seem more reasonable to you?

Chris

[LeonV]

LeonV:

Thanks for the formula, though I'm not sure how to count spring turns. So, I spread and came up with estimates of 140 lbs (if 9 turns) to 197 lbs (if 7 turns).

Take the total coils and subtract two, if the spring has squared ends (flat top and bottom). If your spring has 9 coils, then the calculation with 7 active coils (197lb) should be close.

[John Coffey]

160 sounds like the typical rate for aftermarket front coil springs for the S30. Did you continue jacking up the front until the chassis just started to lift? Noting the total amount of spring compression and "assuming" that corner of the car weighs 650 lbs. should give a ballpark check on your 160 lb. in. measurement.

[Jetaway]

160 sounds like the typical rate for aftermarket front coil springs for the S30. Did you continue jacking up the front until the chassis just started to lift? Noting the total amount of spring compression and "assuming" that corner of the car weighs 650 lbs. should give a ballpark check on your 160 lb. in. measurement.

I meant to write this in my follow-up post: I would have tried the method you suggested earlier, but with the suspension 3/4ths dismantled I would have had to reverse a lot of work already done. I'll give it a shot when everything is back together.

Chris