Originally Posted by
ColnagoC40
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A frame that deflects due to lack of stiffness, essentially behaves exactly the same as a spring.
In practice, if a cyclist is exerting maximum force on the pedal at say 3 o'clock, the frame will absorb some of that work (energy) as it flexes, it will be lost to going through the chain to the back wheel. As the pedal stroke progresses though, at say somewhere between 5 and 9' O clock, the frame will lose its deflection and return to it's normal unstressed position. When that happens, it returns the work, or energy into the drive train.....
This is the standard answer but the model it's based on may be too simple to be useful. Or correct in practice.
Return flex from some directions could be transferred to muscle flex for instance. Wasted work.