Car Analogy

I posted in another thread about the homebrew diy spring rate tester I built. There was a request to test some oem springs, to compare to the calculated spring rates from twohoos dissertation on stock springs. I tested the following springs I had on hand:

Blue-Orange-Blue
Diy - 304
Calc - 309

Blue-Orange-Pink
Diy - 280
Calc - 278

Blue-Yellow-Pink
Diy - 256
Calc - 262

I think the results say more about the inaccuracies of the home brew tester than anything about the calculated rates.

The tester uses a boom that allows testing higher rates, but this amplifies any inaccuracies. Also, since the boom swings, it gets less and less accurate the farther from level its positioned. Since testing requires starting half inch below level, and ends half inch above, this introduces inaccuracies.

But all things considered, I think the diy measured numbers are close enough to the calculated numbers to give further evidence of the validity of the calculated numbers.

The tester:

I built the rig in the photo. My design. It uses a regular analog bathroom scale, and the boom multiplies the rate by twice what the scale reads. So the 300 lb scale can measure up to 600 lb.

There is a ruler stuck to the side to measure how far the spring is compressed. The means of measurement was to jack the jack a little to initially compress the spring, then carefully jack it 1" farther, and record how much the weight on scale increased from previous reading.

The rig uses the stock top hat and stock lower spring mount removed from a stock shock. These cradle the spring, and immobilize one coil top, one coil bottom, just like real world. Twohoos likewise took this into consideration when he calculated the stock spring rates in his excellent dissertation.

twohoos

Awesome! Agree that we're within experimental error of each other.

Glad you had an '02-'03 rear (blue-orange-blue). Before my paper, that's the one that had been mis-measured as softer than the '00-'01 rear, and (as tends to happen on the interwebs) the error persisted for 8 or 9 years, essentially unchallenged.

thomsbrain

Great idea! Just realized I could probably do something similar with my shop press.

Car Analogy

Yes, a shop press would work perfect, without the error my rig has from boom angle (I don't have room for a shop press). The only problem is you could only measure springs up to scale max.

Bathroom scales are cheap (mass produced), and fairly accurate. You can find inexpensive (?$20) digital bathroom scales that go up to 440#. The only problem is they are virtually all made with tempered glass!

The only ones I found that aren't have ths hump in the middle between your feet, which is right where the jack would go (and they are more like $40). Maybe make a bridge out of 3 short pieces of 2

thomsbrain

I figured I could just put a piece of 2x6 wood across the glass to spread the load (measure the weight of that first to subtract it from results). I'd probably end up shooting for like 1/2" or even just 1/4" spring deflection to avoid overloading the scale, then multiply the result to get the lb/in spring rate.

Car Analogy

You could probably get away with using a glass scale with care and spreading the load. Tempered glass is tough. But be careful bumping it or dropping it or putting pressure in one spot.

The beauty of testing this way is you don't have to worry about taring off the weight of the 2x6 or any other brackets or anything.

Just compress the spring a little, take the initial weight reading, which includes the springs weight, the springs tension, the weight of any brackets or whatever. Then you carefully compress one additional inch, take that weight reading, subtract weight 1 from weight 2.

It doesn't matter how much you initially compress the spring. It doesn't matter how much anything weighs. It just matters that you measure those two weights accurately and compress it exactly one additional inch.