Having longer or shorter crank arms on your bicycle can make the difference between having a more efficient or less efficient pedal stroke. But getting the wrong crank length can lead to discomfort during your rides or even cause an injury.
As with other components, such as the seatpost or handlebars, the crank arm is one of the bicycle components that can be modified to achieve better performance on each of your rides and prevent injuries. The crank arms are responsible for transmitting the energy from each pedal stroke to the bottom bracket and from there to the wheels through the chain. In other words, the crank arms are responsible for transforming the force of pedaling into movement.
Choosing the correct crank arm length is fundamental. The improvement you can make to your cranks is determined by their size, specifically by the length of the arm, which is the section that connects the pedal to the bottom bracket. You can also achieve better performance by choosing cranks made of a lighter material, such as carbon, or with special features, such as hollow cranks. However, the weight savings between different cranks, averaging around 200 grams, will not be as significant as with other components like wheels or handlebars.
Why change the size of your crank arms?
Choosing crank arms of one size or another will provide certain improvements to your pedaling style, although it won't necessarily make your pedal stroke more powerful. The latest biomechanical studies related to crank arms, especially those by Professor Jim Martin from the University of Utah, have determined that the increase in power is very small with the change, less than 4% from one size extreme to the other (120 to 220 mm). This is a much wider range than the most commonly used sizes on the market (165 to 175 mm).
Pedaling with the ideal crank arm size prevents the appearance of muscle discomfort
The improvement, in practice, translates into an adaptation superior of your pedaling style to the joints involved, mainly hip, knee, and ankle. Pedaling with the ideal crank arm size prevents the appearance of muscle discomfort, by adapting the movement of those joints to your pedaling style. However, depending on the discipline (road or mountain bike), choosing one size or another will add extra improvements in aspects such as acceleration or passing over obstacles.
What crank arm sizes are available?
In today’s market, two crank arm lengths have become standard: 170 mm and 175 mm. The first is most common on size S and M bicycles, while the second is more typical on larger sizes, such as L or XL. However, you can also find crank arms of larger or smaller sizes, as well as other intermediate options (180 mm, 172.5 mm, or 165 mm). Thus, in general, the choice of one length or another is determined by the cyclist’s size.
Crank arm sizes according to bicycle size
In the following table, we can see a relationship of crank arm sizes according to bike size or the cyclist’s height (Source: Burke, Edmund R. High-Tech Cycling):
| Height / Bike size | Crank arm length |
| Less than 152 cm / XS | 160 mm |
| Between 162 and 168 cm / S | 165 - 167 mm |
| Between 168 and 183 cm / M, M-L | 170 mm |
| Between 183 and 189 cm / L | 172.5 mm |
| Between 189 and 195 cm / XL | 175 mm |
| More than 195 cm / XXL | 180 - 185 mm |
However, this size/crank arm length relationship is the traditional one, which brands have generally adopted to manufacture their models. But recent advances in cycling biomechanics, with the publication of various specialized studies on the subject, contradict this formula, giving more importance to leg length than to the cyclist’s height.
What is the ideal crank arm length for me?
According to these recent studies in biomechanics, the choice of the ideal crank arm size depends on several body segments that actively participate in pedaling, such as inseam height (measured from the perineum to the ground). This change of reference is based on the imbalances that each body may have and that are not considered when measuring height. Two cyclists with the same size may have different leg lengths, so they may not have the same recommended crank arm size.
Crank arm sizes according to inseam height
The following table from Biolaster shows how to choose the crank arm length based on how long your legs are. To do this, you should measure the length of your outstretched leg from the floor to your groin.
| Inseam height | Crank arm length |
| Less than 70 cm | 165 mm |
| Between 70 and 74 cm | 167.5 mm |
| Between 74 and 80 cm | Between 170 mm |
| Between 80 and 86 cm | 172.5 mm |
| Between 86 and 99 cm | 175 mm |
| More than 99 cm | 177.5 - 180 mm |
Choosing crank arms according to discipline
In addition to considering your leg length to find the perfect crank arm size, you should also take into account the special characteristics of the cycling discipline you usually practice. Although the measurements are practically the same across all disciplines (road, mountain bike, gravel or cyclocross), there are certain elements, such as the type of routes, distance, or terrain, that will make us choose one crank arm length over another. On the road, for example, a standard of longer crank arms, 172 or 175 mm, is becoming popular, especially for bikes more focused on touring, such as those with endurance geometry.
However, on aero bikes, focused on performance, 170 mm crank arms predominate, as they favor adopting a more aerodynamic position. In mountain biking, there is also a tendency to shorten crank arm size. First, because there are biomechanical studies that have shown certain improvements in acceleration, such as the one conducted in 2010 by William Macdermid (source), on the difference between using 170 mm, 172.5 mm, and 175 mm crank arms in cross country events. Secondly, because shorter crank arms are less likely to hit rocks or steps on many trails, thus preventing damage or causing crashes.