Thanks for the doc Bitt, but it's not really helping me figure this out. I don't see a point C' in the calculations.

Also, while the distance of C to B' (the length of the spring) should change, their distances relative to A should not. Not when in operation anyway.

Point B' also can't be at the very end of the line defined currently by AB' - it would take far too long a spring and it would get (more) in the way of the open sides.