Points of Contact: Crank Length
Crank arms that are too long for your leg length will cause a greater range of flex in your knee and hips than necessary throughout the pedal rotation. With too much of a bend, your knees and hips are not working at their best efficiency, and prolonged riding in this position can cause fatigue and undue strain.
Unless you’re an elite athlete or racer, chances are the topic of crank length has never come up. Which is surprising, because not only is the crank a key lever through which your power is transferred to the drivetrain of the bike, it’s length is arguably the key to achieving the best combination of comfort and efficiency while riding.
Ideally, you should choose a crank length that would have your knees working within a tighter and more efficient range of motion. In the US, there are four commonly available crank arm lengths: 165, 170, 172.5 and 175mm. Stock bikes may be equipped with different crank lengths based on frame size, but don’t count on it. Even if that’s the case, you can’t be sure that the existing crank length is right for you even if you are on a correctly sized bike. For example, a hybrid bike may come in 6 sizes, accommodating riders from 5’ to 6’4”, but with only 170 and 175mm crank arms offered across the spread of sizes.
When you’re bying and entry level to mid-level bike, changing out the cranks would add considerable expense in terms of parts and labor, which is probably why the topic rarely comes up, and people simply get used to riding with whatever crank came stock on the bike.
However, if your pedaling cadence seems inefficient, talk to your bike mechanic about the possibility of changing to different length cranks. In cases where optimal performance is required, there are specialty cranks and pedal adaptations that can minutely customize the efficiency of the pedal stroke, and even offer customizations for uneven leg length. However, the crank range offered by those four sizes mentioned above will probably meet the needs of most utilitarian and casual riders.
Links to other parts of this guide: