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Springs used for coilovers come in a couple universal diameters (2.25" and 2.5" OD). Racing springs like this are available from a few different manufacturers at almost any length. If you want to change rates on a set of coilovers, you don't need to buy the spring from the manufacturer... just measure the uncompressed length of the spring and its outside diameter and order new ones from any racing spring supplier. This way often gives you more spring rate choices. Japanese manufacturers typically step their spring rates in 1 kg/mm (56 lb/in) increments. US and European manufacturers step their spring rates in 25 lb/in (0.45 kg/mm) increments.
Springs of any length can be built to almost any spring rate because of how a spring works. When you compress a spring, what you're actually doing is twisting the wire. So spring rate can be changed two ways, while keeping the free length and diameter constant:
- changing the thickness of the wire. Thinner wire twists more easily, so a spring made out of thinner wire will compress more easily. This is a geometric relationship - a spring with wire that's twice as thick will have four times the spring rate.
- changing the number of coils. If you keep the spring length the same but add more coils, if you were to "unwind" the spring, the wire would be longer. The longer the wire, the easier it is to twist. Therefore, the more coils, the easier the spring is to compress. This is a roughly linear inverse relationship - a spring with double the coils will have about half the spring rate. Anyone who has seen progressive-rate springs will understand this.. the more tightly-wound end of the spring is easier to compress.
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