I put together a simple way to measure spring stiffness today. Very useful, as seemingly identical springs can actually be quite different. In my case the reverse was true.. I found a 3.2 guage spring that was actually no stiffer than a near identical wind in 3.0mm..

Figured I'd share...

Use two spring guies, slightly loose fit. thread into each end of test spring. Get a 9" length of M10 stud, with one end threaded into a small metal plate to support the weight that will be stood on it. Thread rod through the spring / guides. Mount in a vice / clamp, secured vertically by the lower guide flange. Must be very secure. Stick a big weight on top and measure the compression.

I used a convinent spare car battery (I guess around 18 kg), and got useful differences. e.g.

225mm long springs, x3

Spring A 3.2mm wire, 18mm deflection
Spring B 3.0mm wire, 17mm deflection
Sping C (std TX mk3 2.9 wire), 33mm deflection

I found it quite handy, so I will gradually label up my springs so I can compare when looking for the "right" spring.

Another approach BTW would be to use a sash clamp with some bathroom scales between the end of one guide and the clamp., and measure the deflection at a certain weight, e.g. 30kg. Probably safer than my car battery balancing method But we've no bathroom scales in the house...