Originally Posted by
Leisesturm
You and Anthony both need to get out on your bikes and observe gears, terrain and resistances in the real world. Not physics textbooks. Both of you make your assumptions from the rider outward. Reality is actually the reverse. Shorter cranks require shorter gears. Thats why mountain bikes have shorter gears than road bikes. They always have shorter cranks and shorter gears together and longer cranks and taller gears together. If you go shorter than the MTB standard of 160/165 you must made still another reduction in the gearing. This, however is never done. People are using 150/140, even 130 with the same cassettes and chainrings meant for use with 165 cranks and calling the results a success. I'm not sure how that can be but people are contrary and who knows what criteria they are using to define success anyway.
What research on the topic exists is very little but supportive. Moreover, in the real world, those who are testing the traditional paradigms are among the most athletic among us and their anecdotal evidence is supportive.
Mostly what seems most certain is that 165s for a shorter rider but not shorter cranks and 175s for even the tallest riders but not more than 180 is simple and in general does work but is not for that reason alone the most efficient combination of factors given gearing and considering RPM vs. pedal speed and what feels more natural.
Less than 160 even for longer legs may actually be more efficient setup. For some, as low as 145 may be better than the 175s that came on the bike. But, learning that is the case if not easy because changing crank size is not as easy as changing gears. Nevertheless, being ignorant of the facts doesn't make 175s the ideal crank size just because that's what came on the bike.