Originally Posted by
chasm54
OK thanks. Speaking as a non-physicist, I'd speculate that the question is how the power transfer operates as the frame springs back? That is, when the stored power in the flexed frame is released, does it propel the bike forward as opposed (for example) to raising the rider upwards? In which case the energy isn't lost, but neither is it usefully employed?
You're exactly right. I mentioned my simplistic view has problems, and that is the main one.
Two things can happen when the BB flexes: the BB axis can rotate so the spindle ends move up and down, or it can move so the BB ends move forward and backward. The former is a result of the vertical force of the pedals, and the latter is the result of the chainring pulling on the chain, at the top of the chainring. That force, when resisted by the inertia of the bike, tends to compress the drive side chainstay. If it also elongates the other chainstay, we have the BB moving side to side as well as rotating. Considering the motion of the BB communicaitng to the downtube, this lateral motion would also rotate the BB up and down.
This is pretty complicated for a thought experiment, it really needs to be addressed using finite element analysis software. I don't have any.
This all assumes the BB shell and spindle are rigid, and that flex is limited to the tubes. Might be a good assumption, but I don't know. How to handle component flex is another of the problems.