Originally Posted by
Clyde1820
The implication being that when a frame flexes such that a rider's full energy is split between forward motion and downward flex, that upon un-flexing the frame returns that energy directly into forward motion. I suspect a spring could achieve this, aside from heat loss, as it's force is in-line with both compression and return. But a frame's flex would have to be spot-on equivalent and in-line, and occur at a time when the rider could actually use it, for the return, in order to recover that energy in the form of the forward motion that was previously lost. Wouldn't it?
Yep, that's the whole issue. The energy that goes into flexing the frame has to be returned at the right time and in a way that generates forward motion. Unfortunately, the mechanism by which that returned energy can generate forward motion involves the riders legs, and this introduces losses to the system. (Legs make terrible springs.)