Big Endian wrote:
I think the question should be whether there is an advantage to having longer cranks on the road bike? Conventional wisdom is that there is no inherent disadvantage to shorter cranks, so why do people, including myself, default to the typical 172.5 on the road bike?
Research has shown that there is no innate optimal cadence from a cardiovascular efficiency standpoint and when it comes right down to it, the only time you really notice the limits of the crank system is when you're either in the lowest or highest gear combination. If you are expecting to encounter a real grinder of a hill where you will spend extended time in the granny ring and big cog, then it may be worthwhile to have a bit of extra crank length to maximize your leverage. 165 to 172.5 is about five percent, which is going to be similar to increasing the big cog on the cassette from 32 to 34 (6%) or dropping the small chainring by a couple of teeth (40 to 38 would be five percent).
At the other end of the speed range, a shorter crank will give you a bit more ability to attain high cadence because the foot speed will be reduced compared to what you'd have with the longer cranks.
Longer cranks are a disadvantage in cornering because for any given bottom bracket height they will reduce the maximum lean angle you can attain while still pedaling (cough Dan Martin cough).
A nice thing about 172.5's is that they are very commonly available.
Nice theory, but only that.
I have a quite noticeably higher cadence on 180mm road cranks over 175mm.
The spin is just more natural and fluid.
I used to run 175mm on tri bike for hip angles and also ride far forward, my preffered cadence is 83 rpm vs up around 100 for the longer arms on the road bike.
So I am completely opposite of your theory.