Originally Posted by
Kapusta
Whether or not stiffness matters for efficiency, it can make a difference in handling and tracking. This is a big factor in mountain bikes, but I think it is also true to some extent for road bikes.
Regarding the effects on pedaling, in particular out-of-saddle efforts....
I don’t think anyone can deny that out of the saddle hammering on a stiff vs flexible frame feels different. So clearly something is different. Until you can fully explain WHY it feels different and give an explanation as to why that does not in fact matter, then you cannot categorically assert that stiffness does not matter.
Your last paragraph is a clever burden shift--nobody can prove that a non-defined parameter doesn't matter. That has absolutely no implication that the "feel" matters in any way related to performance. And feels different = is different is actually a non sequitur. People "feel" like they're going faster when they can perceive bumps, and interpret smoothness in ride as going slower even if there is no difference in speed from the bumpier ride.