Quote:
Originally Posted by bicycletricycle
I had some 185mm cranks for a while. Once I got used to them they were fine but then my other bikes felt funny so I decided to stay with 175. I had the same experience with 165 cranks.
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Power is applied by the legs through the cranks. So people have assumed that there must be some optimum crank length. Further, since people (and their legs) vary in size, people have assumed that the optimum crank length must somehow also vary with rider/leg size. I mean, this must obviously be the case, right?
Sometimes some ideas seem so obviously true, that people don't bother actually testing to affirm them. There are many cases of this cycling. Sometimes when somebody does actually get around to testing it, it turns out to actually be true. But just as often, the "obvious" ideas turn out to not be true.
In 2001,
Jim Martin at the University of Utah tested a group of trained cyclists of various heights with cranks ranging from 120mm to 220mm. Over most of the range of crank lengths there were only small variations in power output, with only the shortest (120mm) cranks having a modest drop-off in power output (4%). Interestingly, even the riders with the shortest legs were able to produce power with the longer cranks, and the riders with the longest legs were able to produce power even with the shortest cranks.
So why the push toward short cranks today? As usual, it's probably a matter of over-extrapolation. Racing cyclists are interested in going faster, which means either increasing power or lowering drag. Shorter cranks may allow a cyclist to ride in a more deeply crouched position, reducing frontal area and drag. At the same time, they can produce the same amount of power with shorter cranks, so shorter cranks may produce a net boost in performance for racing cyclists.
But what about the rest of cyclists? For cyclists who are interested in riding in extremely bent over positions, shorter cranks may have less benefit. Other cyclists should select crank length based on personal preferences and their own physiology, and now what a small population of racing cyclists are doing.
Over extrapolation is a problem with many scientific studies. The conclusions of a scientific study may indeed be true - for the specific population in the specific conditions tested. However, the conclusions may not apply outside of the population and conditions tested - none-the-less people will always be tempted to apply those conclusions far more broadly than they really should.