I bought a set of 39mm fork tubes from a swap and I need to replace the springs. I want to use progressive springs but I'm unsure of what set I need to buy. I know that xl, fxr and some fxd's use the 39mm tubes. How can I tell what they came from? I guess I can measure the springs but they are sagging real bad and thats the reason I want to replace them. I don't want to waste the time and money buying the wrong springs.