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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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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Macdermid 2010 Study

There is, of course, a severe caveat that I must emphasize again. This study only looked at maximal power output and NOT anything to do with aerobic performance that might be relevant to the vast majority of cycling. Therefore, while logistically really difficult because of the need for people to be trained and adapted to different crank lengths, the more appropriate study would be replicating this design with more time trial type of tests.

This leads to an interesting Kiwi-Aussie 2010 study that looked at more "realistic" crank lengths and endurance exercise, with the specific context and population of female cross-country mountain bikers (1). In brief, cranks of 170, 172.5, and 175 mm were tested with maximal sprint, a VO2max test, and also an isokinetic (constant velocity) test at 50 rpm. The latter is certainly representative of grinding up a big climb off-road. No differences in the isokinetic or VO2max tests were observed across the three crank lengths. What was really interesting, however, was that, while peak power was not different, the time required to achieve peak power was much lower with the 170 mm (2.57 s) than the 175 mm (3.29 s) cranks.

This has significant practical applications. The shorter cranks permitted you to generate high power outputs faster. This ability is critical in closing a gap, accelerating in a sprint, and also laying out that massive and quick burst of power to get you over an obstacle on the trail or out of a turn in cyclocross.

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Test Yourself

Therefore, the best thing to do appears to be a matter of testing and understanding your inherent style and the demands of your sport. One great way to do it is to give yourself time to try out different bike positions and crank lengths during the offseason or the "easy" base period of training. There are several systems available for altering crank lengths on the fly, with likely the simplest and most adjustable system that from PowerCranks.

The "basic" PowerCranks model can be adjustable from 145 to 182.5 mm, and the "long adjustable" version can be adjusted all the way from 85 to 220 mm crank lengths. PowerCranks can either be purchased for your own bike or for variety of indoor training ergometers, making it possible for you to test your own preferred or optimal crank length under different conditions. The important thing is to give yourself time (at least a few easy rides) to adapt to each different crank length before doing any specific testing like a five-minute effort.

Have fun and ride safe!

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