Most cyclists probably know a lot of their key bike-fit measurements: shoe size, saddle height, preferred handlebar width, etc. Crank length, however, seems to fly under the radar.
There are several reasons for this, not least of which being that many riders simply aren’t adversely affected by the crank length specified on their respective bikes. Commercially available crank lengths have hovered around 170 millimeters since the inception of the safety bicycle at the turn of the 20th century because they work well for most riders—they’re long enough to deliver effective leverage while keeping within the useful range of motion of the human leg. Product managers design bicycle frames with target rider height ranges in mind and select crank lengths accordingly; if a particular bike comes in six sizes the two smallest get 170mm cranks, with the two in the middle getting 172.5s, and the large sizes getting 175s. This range is generally sufficient for consumers, assuming most of them are of decent physical health, with distinctly average proportions, doing relatively undemanding riding.
When a rider presents with a physical limitation, or is focusing on a particularly demanding discipline with regard to either exertion or technique, we start to look beyond the usual scope of crank length selection. This could be as simple as an otherwise average rider who is carrying a bit of extra weight or lacks flexibility, particularly in the hips. Folks who sit at a desk for 40 or more hours a week commonly experience one or both of these scenarios. In this case we’ll usually reduce crank length slightly, but not beyond the range of stock offerings. This helps increase comfort by keeping the rider’s hip angle open and their knees away from their midsection and chest. This also promotes better technique, by reducing the size the pedaling circle and assisting the rider in maintaining a higher cadence.
When a rider presents with more severe limitations, such as a previous hip or knee surgery, reduced flexibility from muscular injuries or advanced degradation of their joints, we may need to explore custom orthopedic cranks. Cycling requires a rider to maintain a highly repetitive motion over long periods of time under fluctuating loads, and if the range of that motion is not sustainable for a particular rider the potential for injury is high. By reducing crank length (and therefore range of motion), we are able to reduce stress on the rider’s muscles and joints at the point of maximum torque in the pedal stroke which reduces this risk. We’ve gone as low as 130 millimeters here, there options exist to go even shorter if need be.
Reduced crank lengths can also be beneficial for riders focusing on time trial or multi-sport (duathlon, triathlon) events, regardless of their fitness level. These riders are subjecting their bodies to greater stresses and higher sustained loads than most other disciplines require. They’re also doing so without being able to draft other racers or otherwise use tactics to their advantage, making an efficient and sustainable position all the more important. For very fit and competitive riders looking for any edge they can get, we’ll work to get them in the most aggressive position they can sustain without impinging on their aerobic performance. A shorter crank will allow these riders to adjust their position forward and down for better aerodynamics while maintaining proper tracking of the knees at the top of the pedal stroke. Aerobically, it helps to keep their hips open for improved pedaling rhythm and breath control. For newer or more enthusiast level triathletes whose positions are focused more on sustainable performance than aerodynamics, we’ll still often utilize shorter cranks to this end—although they may not be as concerned with their times, by keeping their heart rate and oxygen consumption in check we’re able to set them up for greater success in the other legs of their race. In either case the goal is to deliver the rider to start of the run as efficiently as possible.
No matter the reason for an individual seeking a bike fitting, coaching on form and technique is almost always part of the solution. And the bottom line is, using the correct crank length cannot be decoupled from having a solid pedal stroke.
As always, the key here is enjoyment. Whether your goal is to post your own best times, step on a podium, or simply ride with more comfort and less risk of injury, having the right equipment for your body is essential to that process. We’d love to discuss your riding goals with you, and help you get onto the best possible setup to achieve them.