Crank length is a subject very close to my heart but is a bit off topic for CR. Crank length is like a lot of other things such as frame sizing subject to fashion. Those riding Ordinaries in the 1870s and 1880s always used far shorter cranks - 5in was about the norm. At the turn of the 20th century there was a fashion for extra long cranks for a short time - anything from 7 to 9 1/2 inches. It was deservedly short lived and from then on until the 1960s 6 1/2 inches (165mm) was the norm with 6 3/4 and 7 inch being available also. The Japanese 171mm crank length was their attempt at 6 3/4in cranks. However crank length should be determined as a function of inner leg length and probably as a function of upper leg length. Too long cranks cause an excessive bend at the knee which is inefficient and possibly damaging to the joint whilst too short seems to be less efficient. Here in England several of us - Chris Juden Technical officer for the CTC is another have arrived at a formula which seems to work well. Crank length should be between 20 and 22% of inner leg length, crotch to floor. This does give quite a wide variation but takes into account the other individual differences such as pedalling style and foot size. In my case I have arrived at 162.5mm with 165mm also being fine. It has also worked really well with children who are racing. My son Keir who is 11 is at present on 145mm cranks but started off at 6 on 115mm cranks and has gradually worked upwards. It is noticeable that the kids who are on the correct length cranks pedal much better - smoother with fuller circles and without their knees hitting their chests. However I would qualify this a little - with mountain bikes crank length is less important as you are out of the saddle for much longer periods of time and crank length is much less important then. However as Sheldon very eloquently put it longer cranks extra leverage should only be considered as a part of the whole gear system. It is normally just as simple to fit slightly lower gears with correct shorter length cranks. There is a simple formula to work out the change in gear needed;
New crank length / old crank length X old gear = new gear needed
Regards
Hilary Stone