This issue of crank length is a subject very close to my heart - but what the pros use is not necessarily a good guide to what is ideal. They ride what they are given.
Having worked with kids for the past five years or so and having designed frames for short and tall riders for the past 17 years I generally recommend about 20 to 21% of your inside leg length. Foot length is another factor to affect crank length as is pedalling style. This particular formula results for me in 162.5mm cranks - my son is at present on 150mms. On the correct crank length pedalling is far smoother - I do not think power output is necessarily badly affected by slightly too long or too short but comfort is. However seriously too short or long cranks I am sure will seriously affect pwer. Correct crank lengths also make the design of small frames far easier - toeclip overlap (foot overlap when using clipless pedals) is far less of a problem. The bottom bracket can also be lowered which is particularly of benefit on a touring frame or one used mostly in the city.
In Britain the standard crank length for many years was 165mm (6 1/2in) which I suspect was about right. A lot of experimentation with crank length was carried out at the beginning of the 20th century in England - cranks as long as 9in were used but the standard length eventually arrived at was 165mm.
Hilary Stone, Bristol, England
Sterling Peters wrote:
> Jan Heine made some interesting comments about crank length in Vintage Bicycle
> Quarterly ( A 400K Brevet ). He commented about the fact that with shorter
> crank arms you can lower the Bottom bracket height , get a better cornering
> more stable bike and have the additional advantage of being able to continue
> pedaling thru the corners without fear of kissing the pavement , not to
> mention you also save your knees from the stress of longer cranks. I remeber
> reading somewhere that in the 50's the Randoneurs typically used crank lengths
> of about 165mm and smaller gear ratios.
> I can also recall that this long crank thing has been tried before by the
> Professional racers again I think it was in the 50's or 60's and they moved
> back to more knee forgiving lenghts after a few years. What does anyone know
> about this?