One thing I would suggest to change in your wording Octo, back pain can be caused by more than what you suggest, even stuff that is directly riding related. I can get back pain very easily by not drinking enough. The first signs of dehydration for me is lower back pain. I know it's actually the kidneys rebelling but I feel it as lower back pain. I suck down a liter of water and the problem is gone in no time.

One stupid question I have though. When I was a kid I used to ride a ??? bike. I see everyone here on the forums always intertwining the terms FG and SS together. By reading what you have written Octo I tend to think they are actually two completely different beasts. I used to ride a single speed bike as a kid before I switched over to the front freewheel bike, yes...one where you could change gears even while coasting because the chainring never stopped moving. It kept moving even when you stopped pedalling.

I think I'm tending to understand what you've written, FG is where the pedals move anytime the wheels are moving??? Am I understanding that one correctly? If so, yeah that could make a lot of things interesting. Do 100 miles on a bike like that...definitely takes a bit of getting used to after riding a freewheel bike for eons.