One of the things I learned in mechanical engineering school was that structural analysis and testing aren't easy and the (correct) results are frequently counter-intuitive. Adding an analysis of power delivery as it relates to bicycle speed or efficiency and you've got a mighty imposing task on your hands. Here is an article by an engineer who's put substantial thought and effort into modeling a bicycle frame as it relates to power and deflection. A quick read and his conclusions sound well grounded to me. He doesn't violate any big ticket "first principles" that I can see. The analysis and conclusions make sense so I tend to think he's in the correct zip code.
Short version: I wouldn't be too quick to utterly dismiss reasonable frame flex as an aid to propulsion.
http://www.bikethink.com/
John Clay Tallahassee, FL
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