Does anyone know what those el cheapo eBay special coilover springs are rated at? I've seen a handful of posts on other forums that say 350-450. Can we get an amen on this? Or, does anyone know a spring shop that tests rates? No one in central Oregon does.
Figure it out yourself. You know your weight, yes? Stand on the spring and measure how much it compresses, so Spring rate = Your Weight / Amount it compressed.
You can get a rough idea by measuring coil diameter, number of free coils and spring ID. Although that does assume certain metallurgical consistency.
So I can use my own weight? Ok, I just didn't see how I was going to get 450lbs centered over a 2.5" spring and then safely measure it.
So I weigh 220lbs. If I compress it 1/2" that means 440lbs would be a full inch right? What if I can't compress it 1/2" what's a simpler formula?
Edit- spring rate= my weight divided by amount moved.
Theoretically the spring should behave the same in tension or compression.
So you can measure how much the length increases when you hang your weight on it if that is easier to do.
Hydraulic shop press, ruler and a load cell is how I do it.
And by load cell I mean I usually use a pad off the longacre corner weight scales, but I suppose a bathroom scale that registered high enough would do the trick.
I chuckle at the thought of someone trying to simultaneously balance on a coilover spring whilst kneeling over and attempting to read a tape measure. If you can successfully do this I want to see a video 
In reply to Jumper K. Balls:
Even better if you can do it with a beehive shaped coil.
A friend of mine recently tested some off-brand ebay springs and BC coilover springs on a real spring tester and got some wacky mismatched progrssive rates (and one pair had reverse progressive rates where the spring got softer the more it was compressed...oof). Back to back testing on the same rig with Eibach race springs as a control showed it was the eBay springs that were junk, not the test procedure.
So for low quality springs I'm not sure I would put a whole lot of stock in the rate observed from only the first 1/2" of travel using your body weight.
The 'correct' way to check spring rate is to preload the spring and then add a know weight and check the compression. The amount of weight to do this for typical car springs means lots of weight OR a simple lever setup. Build lever that has a pivot just below the free height of the spring and place the spring 1 foot from the pivot. Next add some weight at the max end of the lever (3-4 feet) and check the initial length. Add some more weight and check the length again. The rate is then figured out with the change in length and the lever effect of the weight (for 4 foot lever, 4 times the weight added, if the spring is at 1 foot form the pivot). This method gives you better control of the spring and the need for less weight (or more compression with more weight).
Keith Tanner wrote: You can get a rough idea by measuring coil diameter, number of free coils and spring ID. Although that does assume certain metallurgical consistency.
That's also missing one critical element, the helix angle.
I made mine. I suppose now that I have some time out of work I should write a article but herd to type with one hand.
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The heart of the spring rate checker is a 1.125 bore hydraulic cylinder. 1.125 bore has surface area of 1 Square inch thus add a pressure gauge with PSI markings the gauge will read pound for pound no converting needed.
If you don't have an hydraulic press, you could use leverage. That is, if you have a long enough piece of steel.
bravenrace wrote:Keith Tanner wrote: You can get a rough idea by measuring coil diameter, number of free coils and spring ID. Although that does assume certain metallurgical consistency.That's also missing one critical element, the helix angle.
Or more typically, the free length. Thanks for reminding me.
I wouldn't try to get a spring's stiffness from its dimensions, that stuff is rocket science and you could easily do it wrong and cost yourself a lot of money. Ask me how I know...
Weight vs compression measurement is the way to go, the bigger the weight you use, the better.
Even linear springs are not linear in their first and last ~10% of travel. That's why stafford1500 mentions that the spring should be preloaded (beyond that) before taking the measurements to determine spring rate.
Is it possible then to do this with springs installed in the car? Just set the car on the ground, measure fender to ground. Sit on corner of car. Measure again.
Too quick and dirty?
Too dirty, your load will be spread between different springs (and possibly sway bars, and bushings) even if you sit right on the strut tower. If you can get the car's corner weights, you'd be better off using those. That's how I spotted my expensive mistake.
Edit: I put the springs I was testing on both sides of an axle BTW. Putting a different spring on just one corner for testing would be a very bad test.
In reply to Robbie:
That gives a fair approximation of the wheel rate, IF you load both wheels across the axle evenly. Otherwise you have some effect from anti-roll bars. You will have some effect from the opposite ends axle rate too, but it is small enough to ignore for a quick check. Again this get wheel rate and you would have to go back thru the motion ratio to get spring rate.
Most spring raters tell you compress one inch "zero" then compress another inch take reading so you've preloaded 1 inch. On my unit I don't zero but subtract the 1st inches pressure form the second reading I will normally go to 3 inches as well if the spring has room if not 2.5 to see how linear thing go.
stafford1500 wrote: Again this get wheel rate and you would have to go back thru the motion ratio to get spring rate.
Don't overlook that one, it'll make a massive difference.
Keith Tanner wrote: You can get a rough idea by measuring coil diameter, number of free coils and spring ID. Although that does assume certain metallurgical consistency.
Steel be steel as far as elasticity is concerned. The different types of steel mostly related to how much they can be deflected without yielding, which is why you gotta pay the big bucks for lightweight low coil count springs - they deflect more per unit of travel so they have to be made of better materials.
Ran into this headon with the RX-7. I found that cheap 14" springs could end up coil-binding in less travel than mid-grade 12" springs. (I think the numbers were, generic no-name 14" were binding in 6" of travel, 12" QA1s were binding in 6.5", for 300lb springs)
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