What are They?
Cranks are essentially levers that are a part of a bicycle’s crankset. They are one of the most important parts of a bicycle. They allow you to convert the power you produce to move the bicycle forward. The are part of the bicycle’s drivetrain and are made by multiple manufacturers. Most cyclists are familiar with Shimano, SRAM, and Campagnolo, the three largest producers of groupsets. They are also made by multiple smaller brands as well.
What are the Standard Lengths?
The three main crank lengths that can be found on bicycles are 170mm, 172.5mm, and 175mm. Smaller bikes tend to come fitted with 170mm cranks, while larger bikes tend to come fitted with 175mm cranks. Those in the middle tend to be fit with, you guessed it, 172.5mm-long cranks.
These are certainly not the only options, however. The main component manufacturers offer cranks as long as 180mm and as short as 165mm, with smaller ones also being available from brands like Rotor, which makes a 150mm crank.
Why are These the Standard Lengths?
The claim is that cranks between 170-175mm provide the optimal balance between rotational speed and rotational inertia. What this means is that they are a balance between the torque required to turn crank and the cadence required to achieve the same power output. Cranks that are too long force joints to have too great of a range of motion. Those that are too short require too much torque.
How is Crank Length Measured?
Simply, the crank length is the distance from the center of the bottom bracket to the center of the pedal axle. Typically, this value is printed or engraved on the backside of your cranks, just under the medal axle. You can also simple measure it yourself.
What is the Optimal Crank Length?
In a study conducted in 2002 tested the affects of crank length on maximum cycling power. The researchers determined that out of a selection of crank lengths including 120, 145, 170, 195, and 220mm cranks, two were the most optimal. The 145mm and 170mm cranks were significantly better for power transfer, though do keep in mind, this is only for maximum power, not for riding at lower intensities. For sprinting, the recommenced optimal crank length is roughly 20% of one’s leg length or 41% of one’s tibia length, though finding the exact right fit is unrealistic for most riders.
Benefits of Longer Cranks
Longer cranks can be beneficial to mountain bike riders, as they can help the rider to generate more torque at lower cadences while riding up technical or steep terrain. The same is true for road or gravel riding if one lives in an area with steep climbs, though choosing the proper gearing offers more significant results.
Benefits of Shorter Cranks
Shorter cranks offer a benefit to those who have joint or hip flexor problems. Shorter cranks put less stress on the rider. The can also make a marginal difference in terms of ground clearance when riding, decreasing the likelihood of accidentally clipping your pedals on obstacles or while cornering.
Shorter cranks can also lead to a minor improvement in aerodynamics. It is recommended that in a more aggressive riding position on a race bike or time trial bike that you choose shorter cranks. With longer cranks, your hip angle would become closed, which can affect your breathing and negatively affect your power output. Many athletes such as Bradley Wiggins prefer shorter cranks in time trial events, opting for a rather short 170mm crank setup during his hour record, rather than his favored 177.5mm cranks for racing.
What Crank Length is Right for You?
Ultimately, the choice of what crank length to get is up to you. There are benefits of shorter and longer cranks. There are many factors to consider, and it mostly depends on the kind of riding you are doing and personal preference.