How to Measure the Correct Crank Length
Mikael Hanson details how to measure crank length on a bike and the pros and cons of having a shorter vs. longer crank on your bike (first appeared in Curated's Expert Journal)
Like many of the components on one’s bicycle (stem, handlebars, seatpost, and even the frame itself), crank arms do come in different sizes, and finding the optimal crank length for your height and riding style might improve your effectiveness on the bike.
When I began racing in the 80s (yes, that long ago), bicycle fitting was still in its infancy, and thus most riders went with what came on their bike with a few exceptions. I did not get my first bike until maybe 2005. Later in my cycling career, when I started to compete in time trials (a solo rider versus the clock), my coach instructed me to swap to longer 175mm cranks on my time trial bike as the extra length would give me more leverage to push a larger gear and thus go faster. I would later learn this is not necessarily true (more on this below).
How Does One Measure Crank Length?
The length of a crank arm is measured from the center of the bottom bracket to the center of the pedal axle along the crank arm. Another way to determine the length of your crankset is to simply check the backside of the arm near where the pedal attaches, and you will usually find the size printed there. Measuring a crank arm (left) and size inscription on crank arm (below - photos by Mikael Hanson)
What Is the Correct Length? A great deal of research has been done on this topic, with the most widely viewed being the study by the University of Utah’s Jim Martin, who studied differences in power output among crank lengths measuring from 120 to 220mm long (even though the range commercially available is really 165 to 180mm). In his study, he found that when riders switched among lengths, the difference in maximum power was around only 4%! Common Crank Lengths Most cyclists will find that the crank length supplied on their stock bicycle is more than adequate for their needs. The three most common crank lengths are 170mm, 172.5mm, and 175mm. Bicycle manufacturers tended to equip smaller bikes with 170mm cranks, medium-size bikes (like the 56cm size I ride) with 172.5mm, and larger bikes with 175mm. In fact, Japanese manufacturer Shimano has stated that 170 to 175mm is the “Goldilocks” zone for crank length for most people and most bikes. Ironically, these are typically the two sizes found on mountain bikes (170 or 175mm). The crank arm lengths found on stock bikes stem from the normal distribution curve of riders’ heights and assume their leg length (or arm length) are proportional to their height.
Other Available Crank Arm Sizes Shorter 165mm cranks began to appear on very small frame sizes (XS or sizes under 49cm). One can even purchase 167.5mm or, on the longer side, 177.5 or 180mm lengths should they want. Some manufacturers have even created cranksets that allow the rider to adjust the arm length themselves, such as the Look Zed crankset. According to Look, to reduce production costs, they created only one size of ZED 2 cranks, with a clever rotating three-lobe insert providing options of 170mm, 172.5mm, and 175mm effective arm lengths—ideal for the full range of frame sizes it was equipped on.
Considerations So, choosing a specific crank length outside these norms will then depend on the individual rider, their riding style, and the discipline they are riding (road, time trial, mountain, etc.). A professional bike fitting can help you determine if a crank length change is needed. One way this is done is with a pedaling analysis, which helps determine the rider’s pedaling efficiency at differing cadences and even different crank lengths. However, if you are thinking of making a change, keep costs in mind, as the crank is one of the more expensive parts on a bicycle.
What Are the Disadvantages of Longer Cranks? I mentioned earlier that my coach once advised me to put longer 175mm cranks on my time trial bike, with the idea that a longer arm would act as greater leverage and allow me to push a bigger gear efficiently, which it did. However, while a longer crank arm does provide more LEVERAGE, a rider’s power output is a simple math calculation: torque or force applied to the pedals multiplied by cadence. Longer crank arms can lead to lower cadence, not necessarily faster speeds.
Another concern we did not think of for a time trialist was the rider’s knees hitting the chest or handlebars when in the aero position. The longer the crank arm, the HIGHER the knee moves into the rider’s chest at the top of the pedal stroke and essentially closes up their hip angle. If an athlete does not have the proper flexibility to hold this aero position, their torso will have to lift up to compensate.
What Are the Advantages of Shorter Cranks? We are seeing a trend of triathletes and time trialists moving to SHORTER crank arms for aerodynamic reasons (see picture below). In fact, a few years ago, bicycle manufacturer Canyon updated their Speedmax time trial bike but shortened the stock cranksets by 2.5mm (going from 172.5 to 170mm in the case of the medium bikes).
Looking at hip angle in a bike fit (Photo by Mikael Hanson)
Where might a different crank length be advantageous?
A shorter crank arm will provide greater ground clearance for mountain and gravel bikes
A shorter crank arm can help relieve joint pain (especially in the knees)
A shorter crank arm can assist a rider who is unable to pedal at higher cadences
A longer crank arm can help mountain bikers generate more power (torque) at lower cadences on steeper terrain
A shorter crank arm can help with acceleration (which would benefit mountain bike and BMX riders)
So as you can see from the list of advantages/disadvantages above, there are far more advantages to using shorter crank arms than longer ones, but this does not mean running out and buying new crank arms! If you are unsure, seek help at your local bike shop, or look into a professional bike fit.