Dr Jeroen Swart wrote this article for Bicycling Magazine What’s with different crank lengths, 175, 172.5? Optimal crank length is a topic on which there is still no firm consensus. The principles of a lever arm apply to crank arm length. As you will know from using a crowbar or a wheelbarrow; The longer the lever, the greater the load that can be lifted. Similarly, a longer crank arm requires less force to generate a similar torque than a shorter crank arm. This naturally provides the rider with an increased ability to accelerate and also allows the use of a higher gear ratio than would otherwise be possible. All good right? Not really. A longer crank arm requires each muscle to contract through a longer range of motion. As muscles produce peak forces at a specific length (normally in their mid range of contraction), the longer crank arm forces each muscle into a range on each end where it is weaker. This means that power is lost at the top and bottom of the pedal stroke even though it is gained in mid pedal stroke. Another down side is that the larger diameter of the pedal stroke requires a greater change in the inertia of each leg during each rotation which may offset any gain in torque by lowering efficiency. Thirdly, the greater range of motion needed in each joint when using a longer crank arm places the rider at a higher risk of overuse injury. The patello-femoral joint of the knee is particularly at risk when overly flexed at the top of the pedal stroke. Generally, longer legs negate some of these negative consequences which is why it is often recommended that taller riders ride longer crank arms. Mountainbikes generally have a crank length which is 2.5mm longer than the equivalent road bike due to the slow speeds and lower cadence and the requirement for a sudden increase in torque to maintain forward momentum over rough terrain. One particular school of thought that has changed 180 degrees is that of time trial bikes. In the past the “experts” recommended longer crank arms to allow the rider to grind along in a higher gear ratio. However, this forces the hip to flex more than it otherwise would. As the torso on a time trial bike is lowered to gain an aerodynamic advantage, the hip is already over flexed, robbing the rider of power as the crank reaches the top of the pedal stroke. A shorter crank arm reduces this effect and allows the rider to adopt an even more aerodynamic position while producing the same power output by using a higher cadence. Confused yet?
