I know how much everyone loves Jap "Coilovers"

[JohnL]

Stir,
Have you tested spring pairs for equality of rate? I have to say that I haven't, but I'd be surprised if there would ever be any significant rate difference between one spring and another if:
1) The wire OD were the same.
2) The number of coils were the same.
3) The coil diameter were the same.
4) Neither spring had a crack.
5) The free length were the same (though this wouldn't actually affect the real spring rate, only how much static weight that spring, relative to the other spring of the pair, carried when fitted to the car).

The reason for this is that nearly all steels (and all that are remotely likely to be found in a car spring) and regardless of the heat treatment of that steel, the modulus of elasticity is the same to within an utterly insignificant degree. So, for spring A to have a different rate to spring B, there would have to be some significant difference in physical dimension, not in some hidden metalurgical property of the springs...

Just speculating, but I suspect that spring companies tend to say things like "tested to ensure equal rate" (etc) because it sounds impressive in the marketing bumph. I'm not suggesting that they claim to test the springs but don't, I'll bet they do test them but for a slightly different reason(?).

I suspect what they may do is to compress each spring to X length to see if this causes the spring free length to shorten (or less likely for the spring to break), i.e. they are more likely to be testing the elastic limit of the spring (to ensure adequate heat treatment) than to be testing the actual spring rate (though while they're at it they may as well check rate just so they can say they do...).

If a spring (A) is too soft (heat treatment not rate), then it will have a low elastic limit and shorten if compressed beyond that elastic limit. If a spring (B) is too hard then it will also have a low elastic limit, but may break rather than permanently deform. A correctly 'hardened' spring (C) will have a higher elastic limit than either A or B, but A,B and C will all have the same stiffness (rate) up to their individual elastic limits. Incorrect 'hardness' could be a result of poor heat treatment, or faulty material.

So considering that all springs made to X dimensions will be within a cat's whisker in rate, testing the spring by compressing it may be more to do with quality control for spring longevity.