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A new spin on crank length
I've heard many triathletes and time trialists are switching to shorter cranks: 170, 165, even 160mm. What do you recommend?
It is true that many top athletes are switching to shorter cranks for timed racing such as triathlon and TT. This is relatively new trend because longer cranks were traditionally thought to provide better leverage. However, crank length is just one lever in a drive train composed of a system that transmits your foot's force on the pedal to your tire's thrust on the ground. The other levers in this system are the chain ring radius, cog radius and wheel radius. We vary two of these (chain ring and cog) at will whenever we shift gears. So a small difference in crank length doesn't affect leverage.
For many athletes, the idea that "longer is better" changed after the publication of Dr. Jim Martin's 2001 study, "Determinants of maximal cycling power: crank length, pedaling rate and pedal speed." It involved 16 bike racers of various heights performing sprint power tests, typically of less than four seconds in duration. During the study, they repeated their efforts while systematically testing the following crank lengths: 120, 145, 170, 195 and 220mm. Believe it or not, the tests showed no statistical difference in maximum power among the three middle crank lengths (145, 170 and 195mm). The saddle height (measured to the pedal) was maintained throughout and researchers did not adjust fore-aft saddle position or handlebar height despite changes in pedal-to-knee relationship and handlebar drop with the various crank lengths. For years crank-length tests had been inconclusive, and the general working knowledge came more from experience and intuition than science. Now athletes can choose the crank length they like without worrying about affecting power.
With the leverage-dependency myth debunked to a certain degree, it was the application of these lessons that really highlighted the value of this study. The graph above shows how the aerodynamic drag area (CdA) changed when four pro athletes tested multiple crank lengths in the wind tunnel. (Keep in mind that lower CdA is better.) Rider 1's CdA increased (from 0,27 to 0,28 m2) when he changed from longer to shorter cranks (from 180 to 175mm), but the other three riders' CdA stayed the same or decreased slightly when changing from longer to shorter cranks. The crank length and CdA data for each athlete is listed in the table below.
As you can see from wind tunnel test data, changing crank length by itself doesn’t always have a predictable effect on aero drag (CdA). But for each of these pros, the change to a shorter crank solved a range of motion issue at the hip that allowed them to comfortably make other changes to reduce aero drag without decreasing power.
With maximum power essentially unaffected by a wide range of reasonable crank lengths, athletes are now free to choose crank length based on other criteria. Convenience (you might already have a serviceable crank on your bike); comfort; pedal clearance (from the ground); toe overlap - all of these are affected by crank length. However, what is now understood is that, especially in an aero riding position, shorter cranks can sometimes alleviate a common fit problem: If the hip angle is too tight at the top of the pedal stroke, the athlete can be uncomfortable, or is unable to produce maximum power at the top of the pedal stroke. Even in athletes with no existing fit problem, some choose shorter cranks to drop the torso still further by lowering the arm pads. Perhaps this is not a surprise, but the hours of wind tunnel testing we've done with various Cervélo-sponsored pro athletes over the years confirms that for nearly all athletes, a lower bar means less aero drag. Keep in mind that hip angle isn't the only limiter to lowering the torso. Saddle discomfort, digestion and vision are other common ones. If an athlete is limited in these ways, then shorter cranks won't help them get any lower.
Some athletes keep their long cranks and still perform well. Some try short cranks, aren't happy with the results, and switch back again. Others keep the short cranks and tell us the following:
They pedal faster. The effort and foot speed is about the same, but the RPM is higher, typically about the same percentage higher as the change in crank length. For example, the difference between 165 and 175 is about 5 %; some athletes find themselves in a gear about 5 % easier than before, with a corresponding cadence about 5 % higher. Coincidentally, the difference between a "compact" 50-tooth chain ring and a 53 is close to 5 %. Likewise, 20 and 21 teeth are about 5 % different. They adapted immediately. The leg muscles operate over a slightly shorter range of motion with shorter cranks, so no "new" muscle training is needed. Also, the faster cadence doesn't need to be learned or trained, because the foot speed (and thus the muscle fiber shortening velocity) is the same as before. They feel more similar between aero and road bike positions. The typical practise is to rotate your road position into your aero position, but usually the torso rotates farther than the rest of the body. This closes the hip joint, and shorter cranks on the aero bike can maintain a hip angle more similar to that of their road position. They can run better. Triathletes say the initial part of the run feels better coming off bikes with shorter cranks.