Great article! I guess the "planing" theory began well before Jan Heine??
I did like that Mr. Popadopolous ran an experiment in stiffness:
... he had a local framebuilder, Glen Swan, construct three steel bicycles that were identical save for the differences in the frame tubing; the variations in tubing gave one bike more stiffness. ... Papadopoulos found that the test group could not correctly identify which bike was different...
Too bad that there isn't more info on this.
How much stiffer was the one stiff frame? Even if the riders couldn't identify the stiff one, did they get better or worse results on it?
I imagine that to really explore this topic would require plenty of funding and time, unfortunately.
I did enjoy this observation regarding sources of flex....
At this point, I can say that if you want to be rational about the bike design and want to eliminate flex, then you should not spend all the money on the frame, but think about solving things like bottom bracket axle flex and flex in the handlebars.
The bike industry did go after bottom bracket axle flex by introducing various designs with larger diameter axles. I assume that larger diameter steerer tubes and modern bar and stem designs have reduced flex too. I wonder if there has been any effort to link the stiffness with better race results?
Steve in Peoria