Guest Post: Crank Length (and I mean pedal cranks) - Is Shorter Faster?

I have the luxury of getting to pick the brain of Dave Ripley often. Dave works for Zipp/SRAM and serves as a performance consultant for 51 Speedshop (www.fiftyonespeedshop.com). He is an extremely bright mind always, and especially when it comes to the science of bike performance, "free speed", and bike mechanics. I wanted to share some writing of his regarding crank arm length, as it is a hot topic, and especially relevant to triathletes. Enjoy!

Do You Need Shorter Cranks? 

By Dave Ripley

The great crank debate spins on, so to speak. And with good reason… discussions of crank length relative to absolute power on a bike remain fodder for fitters, and a crucial decision for all types of cyclists of all levels looking to optimize their performance.

Manufacturers have long been focused on designing componentry with the wide range of human ergonomics in mind. Although, those same manufacturers have been the ones deciding what crank length is best for youand specifying them on the bikes you buy at a bike shop.  You are not stuck with the crank length that came standard on your bike, you can change that.  But, which length is best for you?

When making this choice we’d like to provide some perspective to help you decide the best crank length for you. Different types of riding – triathlon, track and road cycling – each bring their own considerations when considering crank length. Many have conducted studies on the subject to determine if there is any loss or gain in your ability to generate power output on a bicycle, but none more notable than exercise science researcher Jim Martin’s work at the University of Utah.

Jim showed definitively that down to a length of 145mm, an athlete could maintain the same level of power output as one riding cranks up to 195mm within .5 percent. How can that be? A crank arm is a lever, and longer levers should theoretically create more torque at a constant force. But the torque created by the crank arm does not directly provide forward motion. This energy is transmitted through the crank arm to the spider then to the chainring. It is at the chainring where power is delivered to the rear wheel of the bike by the chain. So, a cyclist’s ability to make power is more a factor of his circular efficiency and foot speed. Here is where the length of the crank comes in. Biomechanically we have virtually no musculature to push the pedal straight forward at the top of the stroke, nor pulling straight backward at the bottom. A cyclist with less circular efficiency will have a larger dead spot at the top and the bottom of the pedal stroke/chainring. By shortening the crank arm, the cyclist’s feet have a smaller circle to travel thus moving at a higher rate of speed, limiting the time spent transitioning through the dead zones, and keeping power more constantly applied to the pedals/chainring.

Track racers have long since been fans of shorter cranks. The shorter levers have more pedal clearance on steeper banked tracks, but also allow them to ride a higher cadence more efficiently and still achieve high power outputs.

Triathletes Mirinda Carfrae and Jordan Rapp, both long course athletes and world champions, made the move to shorter cranks a few years ago. Other triathletes have begun to embrace this concept, too. To a multisport athlete, there are more benefits than merely increasing efficiency. A shorter crank arm will allow an athlete to ride with a more aerodynamic position without closing his or her hip angle or impinging hip flexor muscles, a major contributor to running speed and efficiency. Also, since the total distance of the pedal stroke is lessened, a common result of shorter cranks is an increased natural cadence in a given gear, which couldresult in an increase in power. Power on a bike being a calculated function based on torque multiplied by angular velocity (cadence).

Road cyclists are starting to test this theory with positive results. One area where the shorter cranks will start to limit an athlete’s ability to make maximal power, however, is when the cadence starts to drop and torque requirement goes up: steep gradients and large gears. Yes, outfitting a crank with compact gears will help transfer the torque more easily, but they won’t help alleviate pedaling inefficiencies that will tend to be amplified in a lower cadence/higher torque situation. Typically, athletes will ride with an average of 5mm longer crank arms on their road bike vs. their tri or TT bike due to torque requirements.

So, the question still hangs out there.  Which crank length is best for you?  Go see a good fitter, is the best answer we have.   We would suggest any of the fitters in the 51 SpeedShop family for starters (www.fiftyonespeedshop.com).  We are currently working on some definitive research, which should finally show some scientific evidence of the impact that crank length has on how we make a circle with our feet, and how to optimize that circle.  Or, SHOW you which crank length is right for you.  Stay tuned!