Greg, just to clarify my thoughts; I know the springs aren't made of plastic, but steels behave elastically up to a stress, and plastically thereafter. If you get into the plastic regime, when you take the load off you'll have a permanent deformation. If you want something to take alternating loads then you need to design way below the platic deformation limit (AKA yield point) otherwise you'll get fatigue cracking. For a spring, which has to take millions of cycles without cracking, the stresses need to be so low that I'd be surprised if you manage to get permanent deformation, ie sagging of the springs. Havind said that, I'm wondering how the spring broke? What does the break surface look like? I'd be interested to know what the usual failure mechanism for car springs is; any ideas?