Depends mainly on two things, wire diameter (torsional strength), and wire length (ie number of coils). There are other factors involved but these are at the more technical end of the design such as the elastic and plastic points of the steel which govern how much the spring can be compressed before it wil not expand back to its original length, etc., etc..

My V1.0 springs from Ironman gave me almost a 2" lift but were the same physical length the the O.E. springs were, they just used heavier wire. My V1.1 rears have gone down a bit in wire diameter and increased the number of coils giving me good <img src="/forums/images/graemlins/kewl.gif" alt="" /> <img src="/forums/images/graemlins/smile.gif" alt="" /> articulation in the rear. <img src="/forums/images/graemlins/butwiggle.gif" alt="" />