Cranking Out the Data
In this study, the "optimal" pedaling rate and pedaling speeds were defined as that which gave the highest power. Therefore, not surprisingly, the pedaling rate was inversely related to crank length. In other words, the longer the cranks, the lower the optimal cadence at which maximal power was achieved.
Also not surprisingly, pedal speed was progressively higher with greater crank length. Most of us may think that this sounds crazy, because shorter cranks should spin faster, This isn't as counter-intuitive as it may appear, however, if you recall that the gearing was different to achieve a similar pedal resistance, and also that the larger cranks meant that the pedal traveled a much greater distance with each revolution.
Of course, the main variable of interest here was maximal power output, and the results are rather surprising. Despite the subjects not really being adapted to the different crank lengths except possibly the 170 mm, the maximal power outputs were only minimally affected. The range was less than 4 percent, from 1149 W for the 220 mm, to 1194 W for the 145 mm. Overall, the 145 and 170 mm cranks were slightly but statistically greater for maximal power output than the 120 and 220 mm cranks.
Even if you're a really finicky type, it would be reasonable then to toss out the 120 and 220 mm cranks as clearly being less beneficial. However, this still leaves a huge range between 145 to 195 mm cranks, spanning far beyond what is commercially available for cyclists, where no real difference in maximal power output.
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So How Do I Size My Cranks?
The other interesting data emerging from Martin et al. (2) is calculating ideal crank lengths for their different subjects. In the end, overall leg length and tibia length formed the best predictors for optimal crank length for the subjects, at 20 percent and 41 percent of these two measures, respectively (e.g. optimal crank length = 0.2 * (overall leg length)). This resulted in a range of optimal crank lengths from 151 to 183 mm for the subjects. However, it is important to note that the "standard" 170 mm cranks produced, at most, an average of 0.5 percent lower maximal power output. Therefore, it can be argued that crank length does not really matter when it comes to maximal power generation.
So What Cranks Do I Use Then?
It sounds like we've just been beating a dead horse, right? Not so fast! The ultimate result from this study is that crank length doesn't seem to matter, but what it really means is that we can be free to experiment with different crank lengths for different applications! For example, this may indeed mean that we can swap around different crank lengths between a criterium and a time trial. Alternately, mountain bikers and cyclocross riders might not need to aim for longer cranks as a default, but may experiment with shorter cranks for greater cornering or log clearance.
Another application is to really get a handle on your optimal or preferred cadence, and then adopt a crank length that facilitates your riding within that preferred cadence. For example, I know from many years of riding and training diaries that I tend to be a lower cadence type of rider. Therefore, I might err towards a longer crank length that will naturally have me spinning at a lower cadence, whereas a short crank might force me to become biomechanically non-economical in a high cadence.
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