For what it's worth, I have a comment and a couple of observations. If helpful, great, if not, please keep sailing, you're doing some great work. Personally, I tend to work through the numbers a fair bit, but I get tired of it pretty quickly, and once I think I am in the ballpark, I'm ready to fire up the welder. The fact is that not only do things usually get adjusted due to the realities of fabricating, but the assumptions used to get the numbers in the first place are only approximations of the real world. My observations are these: Your set up will definitely work. You may (or may not) have to fuss with finding an acceptable spring, but that is just a nut and bolt problem. The other thing, is that, even if the kinematics are ideal, how well the bike handles likely will be limited by how well the swing arm pivot behaves with the very different loads applied to it. It had comparatively little to do as most of the weight (and loads) it has to deal with now previously were fed into the twin shocks and old sub frame. I'm not saying this will be a problem, just noting that everything in any design is relative compared to perfection - even if you start with a blank sheet of paper and have a fleet of experts doing the design. If everything didn't have some sort of compromise, we'd have just one motorcycle chassis by now that everyone would use exclusively. If this were my project and I felt like I was ok with it as-is, I'd get it up to a rolling chassis and strap enough weight on it to simulate the final product. I'd install my shock, and, well, jump up and down on it to see what the spring was like. If it is anywhere close, I'd expect to get some idea as to whether or not the regressive rate of the linkage made much of a problem. I agree with you, and would bet it's not a real problem as is. But if you have second thoughts after such a scientific lab test, you can make the changes now without having to repaint the frame and basically build another motorcycle.