Isnt a spring just a coiled up roll bar? Its basicaly twisting a rod of steel over its length when you compress it.
So if you measure the wire thickness and the length of the wire (number of coils x Circumference =
π × diameter) you should be able to make a chart??
Lets pretend they all use the same gauge wire, the longer the length the more twist you can get out of it ie the softer the spring??
Hmm think that'll work