We discussed that before in this thread - it is measurable by seating it in a cylinder with a piston and pulling it with a spring scale. The argument was that brand new rings that hadn't been broken in vs. the used ones that have a nice shiny broken in edge wouldn't be a fair comparison of spring force. I can't think of a way off the top of my head to make it fair. Even if I could make it fair, there is no "spec" spring force that I've been able to find for anything let alone a G10, which leads me to believe it actually doesn't matter too much once the engine gets to operating temp. I got the heating idea form the site I
linked to in my previous post. However, this person was making custom rings from one raw material, not existing rings, so your bi-metal thing puts yet another wrench into the gears here because I believe I did notice that the stock XFi rings have one metal as a "core", "wrapped" in another metal.