Crank Length: Does Size Matter?

Greater power and big wattage is what we're all seeking on the bike with training. One way to achieve that is strictly through improving our biomechanical connection to the bike through a better bike fit. And with cycling being all about pedaling, one avenue may be through optimizing our crank length. What is the state of our knowledge concerning optimal crank length? Does size really matter?

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Embrace the Off-Season

One big part of the offseason for top pros is testing out new equipment or bike positions. This really is the ideal and arguably the only time such major changes should even be attempted. In the midst of the season with heavy training, the risk of switching out something as seemingly innocuous as a different saddle, shoe, or a lower/longer stem can tip the body over the threshold of stress, leading to tendonitis or other overuse injuries.

For those of us without big lavish sponsors, the general rule of saving major equipment changes for the winter time still holds true, and there's at least the Christmas gift season and a lot of subtle and not-so-subtle hinting!

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Crank Call

What are some things to test out during the offseason? With the all-consuming importance of an optimal pedal stroke to cycling, one thing to explore may be optimal crank length. This has, for a large part, been the topic in bike fit and biomechanics that has been rife with myths and black art. Folk lore suggests that crank length has a huge effect on the power that you can generate, or the cadence that you're able to ride at.

Namely, the general view is that shorter cranks allow you to spin and accelerate quicker but at the cost of high torque or power. Theoretically, this is a boon to sprinters and crit riders. In contrast, longer cranks take more time to turn over and force a lower pedaling rate, but are able to produce higher torques. This should favour time trialists and climbers. At the same time, a number of equations and rules of thumb exists relating either overall height, inseam, or leg length to ideal crank lengths. Is there indeed an optimal crank length for power output, and how well do these prediction equations actually hold up?

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The Study

Such a question, for me, obviously leads to finding relevant scientific data. One of the influential studies over this past decade was performed by Jim Martin at the University of Utah (2). The goal of this study was not to look at time trial performance or aerobic capacity, but rather purely maximal power output and the effects of crank length. Several interesting features of this study included:

  • Rather than play around with "typical" commercial crank lengths, this study really went to extremes. Namely, five crank lengths were tested. Besides the "standard" 170 mm, the cranks were 120, 145, 195, and 220 mm! This may sound completely irrelevant to "real" cycling, but that's not really the point. The aim rather was to see whether real extremes would make a difference at all before looking at "fine" changes of 2.5 mm here or there. After all, if big changes do not make a difference, then why should minor ones?
  • Seat position was standardized to each subject's normal saddle height, and was set so that the saddle top to pedal axle (at its most extended) were the same regardless of crank length.
  • The goal was not to explore things like VO2max or time trial performance, but rather maximal explosive power output. This was accomplished using a previously validated protocol consisting of a very short acceleration and sprint lasting 5 seconds or less.
  • Importantly, previous studies that have examined similar ideas have not adjusted the gear ratios. This creates a situation where the pedal resistance is different across the different crank lengths. In turn, this confounds the experimental design because more force is actually required because of the difference in leverage. What Martin et al. do in this study is to adjust the gear ratios on the ergometer so that the pedal resistance were similar across crank lengths. In other words, longer cranks got bigger gears, and smaller cranks got proportionally smaller gears.
  • Subjects were 16 trained male cyclists. The week prior to testing, they were familiarized with the different cranks. No further details of what this entailed were provided, and it's obvious that they were not adapted to the different lengths. However, this might play a much bigger role with longer, more aerobic efforts than maximal power.
  • Each testing day involved five minutes of warmup at 100 watts, followed by four maximal sprints.

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