Your Guide to Crank Length
A blend of science and practical advice about crank length.
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Crank length. So hot right now 🔥
I get asked about crank length almost daily, in particular shorter cranks, because it seems to be everywhere on social media and cycling magazines at the moment, often portrayed as a new concept. To be honest, I sometimes jokingly say that “2008 called and asked for it’s crank debate back”, followed by saying ‘it depends’, proceeding to bore the poor athlete who asked with far too much information about the pros, cons, myths, stories and then finally the only thing they really wanted to know - A simple yes or no answer to whether they should change their crank length or not...
Let me backtrack to 2008 so you have the background of that comment… If you read the article about how I got started as a bike fitter, you’ll know that in 2008, I started working with several triathletes training at the national triathlon training center in Victoria. After the Beijing Olympics, I also started fitting a few Olympians (draft legal triathlon) and that is where the crank length discussion started, as a conversation with their coach. As a result of that, I was doing a lot of reading, sorting through every biomechanics and physiology paper I could find on crank length. In early 2009, a two time medalist, three time Olympian (at the time) scheduled a fitting appointment with me. I packed up my gear for a house call bike fit and no sooner than a few minutes after I arrived, I got asked for my opinion about crank length for triathlon. I felt like this was a make it or break it moment, as my career was just getting started - If I gave him an answer that did not resonate, I might mess up a great career opportunity… I guess my answers made sense because I continued working with him for the remainder of the Olympic cycle, up to the London Olympics (he retired after). Phew, right? In case you are wondering, he used shorter cranks than what was typical at the time.
The crank length discussion is one that I have been having with athletes for the better part of 14-15 years, in particular on shorter cranks, so when someone tells me that going for shorter cranks is a new concept, I find it really hard not to roll my eyes.
I have many crank length related stories, but we don’t have all day and there is a lot of ground to cover here, so I will save those for another time.
Crank Length Introduction
Cranks are measured from the center of pedal axle to center of bottom bracket axle. If you want to know what crank length your bike has, just look for the number on the inside of the crank arm. Pro tip - Always check both sides. I have seen two different lengths on bikes on several cases (not on purpose), which resulted in disasters.
Cranks are usually somewhat proportional to the size of the bike (in most cases) - Smaller bikes typically come with shorter cranks and the crank length increases with the size of the bike. In the past, it was pretty much impossible finding cranks shorter than 170 on even the smallest bikes, but now many brands equip their smaller bikes with 165 cranks, with the largest bikes coming with 175 cranks.
To be honest, I would be a much happier bike fitter if brands allowed the crank length to be customized when a rider buys the bike… I suppose that’s the beauty of building a bike from the frame up!
On the surface, the crank arm is a lever and as such, any increase in length can potentially provide the rider using it extra leverage. Of course, this is a simplified approach that does not hold in practice because of the human joints that are involved in driving each side of the crankset through extension and flexion.
Below is a bit of science to kick things off, with references at the very end of this article (including papers not directly mentioned but used to write this post). Lets start by going all the way back to 1983, when Inbar et al (4) measured the mean and peak power for maximal efforts using crank lengths from 125mm to 225mm. They concluded that 165mm cranks could be the optimal ones for both mean and peak power, but the differences between 150mm to 200mm were not significant.
So what about metabolic cost of cycling and crank length? In 2002, McDaniel et al (5) showed that switching between crank lengths of 145, 170 and 195 did not change the metabolic cost per se, but that the metabolic cost was dependent on power output and cadence.
In 2011, a study by Martin (2) demonstrated that changes between cranks ranging from 150-190 did not compromise maximal power or change the relative joint power contribution at the hip, knee and ankle. It is important however to control pedaling rate (ie. cadence), as when cadence is constant across all crank length, the longer ones can result in less relative knee flexion power and more relative hip extension power.
In 2017, Ferrer-Roca et al (3) looked into metabolic cost and pedaling technique with crank lengths ±5mm preferred crank length. There was no significant effect on heart rate and metabolic efficiency between crank lengths, but longer cranks increased flexion and the range of movement required at the hip and knee (but not ankle), which was not noticeable with the shorter cranks.
There is one specific circumstance where a longer crank will develop more speed compared with a shorter crank - A standing start with a fixed gear, over a very short distance, AKA sprints under 200m. I am assuming this does not apply to 99% of you currently reading this, or almost all the athletes I work with.
Why would you want to change your crank length?
If changing your crank length results in little difference to power output, the obvious question is: Why bother?!
Cycling is highly repetitive for long durations, at various loads, using a highly symmetrical machine (the bike) powered by a highly asymmetrical machine (the human body). The proponents of shorter crank length claim that they could:
Enable a more aerodynamic position, in particular for TT events
Help with a smoother pedal stroke, hip stability & cadence
Help with lower back, hip and knee pain
Improved run off the bike for triathletes (both long course & draft legal events)
There might be improvement with saddle comfort
Technical considerations like toe overlap, off road riding considerations, bike geometry, etc
Big claims, right?! Lets break each of them down in (a lot) more detail and explore some of the cons of going shorter too… If you are a free subscriber or a visitor here (welcome!), feel free to get a trial subscription to continue reading, as I get into more detail about crank length as it relates to aerodynamics, pedal stroke, power, cadence, hip stability, velocity, technical stuff, how to decide what is right for you and more! If you are already a paid subscriber, carry on…