A word from Cervelo about Crank Arm Length

Crank length  – Damon Rinard, Cervélo Engineer

engineers were asked how come there is a trend to go to shorter crank lengths.  Here is the answer.
For many athletes, the idea that  “longer crank is better” has changed in part because of Dr. Jim Martin’s 2001 study titled “Determinants of maximal cycling power: crank length, pedaling rate and pedal speed” (Eur J Appl Physiol (2001) 84: 413-418). Jim’s study involved 16 bike racers of various heights doing maximal sprint power tests, typically less than four seconds duration. During the study, they repeated the efforts while systematically testing the following crank lengths: 120, 145, 170, 195, and 220mm. Believe it or not, the test results 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 the 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 they’re affecting power.

 
What does the Wind Tunnel say?

 

With the leverage-dependency myth debunked to a certain degree, it was the application of these lessons which really drove the value of this study. The figure above graphically shows how the aerodynamic drag area (CdA) changed when four pro athletes tested multiple crank lengths in the wind tunnel. (Keep in mind lower CdA is better.) Rider1’s CdA increased (from 0.271 to 0.277 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.
Wind Tunnel Run Rider Crank, mm CdA, m2
LSWT 0908   Run 756  Rider1  180     0.271
LSWT 0908   Run 757  Rider1  175     0.277
LSWT 0908   Run 806  Rider2  170     0.270
LSWT 0908   Run 807  Rider2  172.5  0.272
LSWT 0908   Run 805  Rider2  177.5  0.277
LSWT 0908   Run 701  Rider3  175     0.213
LSWT 0908   Run 702  Rider3  170     0.213
LSWT 0908   Run 707  Rider4  170     0.207
LSWT 0908   Run 708  Rider4  165     0.205
Table 1 Crank and CdA data used to generate the Figure above. Only CdA pairs with adjacent run numbers are comparable; other position changes were made in between non-adjacent run numbers which make them non-comparable.

 

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 their aero drag without decreasing power.

 

What is the application?
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 (your might already have a serviceable crank on your bike), comfort, pedal clearance (to 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 in order to further lower the torso 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 lower aero drag.

 

Keep in mind that hip angle isn’t the only limiter on lowering the torso. Saddle discomfort, digestion and vision are other common limiters. If an athlete is limited in these ways then shorter cranks won’t help get them 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:
1. 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 matching 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.
2. 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.
3. They feel more similar between aero and road bike positions. The typical idea 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.
4. They can run better. Triathletes say the initial part of the run feels better coming from shorter cranks.

 

What does the Race Engineer say?
As Team Garmin-Cervélo’s Race Engineer, I advise athletes to choose whatever crank length they like. Those who are interested may try shorter cranks on the TT bike; in that case I usually recommend a 5mm difference: longer on the road bike than the TT bike. In all cases, regular training on the TT bike is important to promote adaptation to all aspects of the aero position. The main thing is to realize that the choice of crank length doesn’t significantly affect power, so any length is now free to choose for any other reason. This lets the athletes relax about crank length, knowing it’s not as critical as we used to think.

 

Dr. Martin’s results are not widely understood yet, so crank length is still controversial, and many athletes have strong preferences on crank length.

 

Cheers,
Damon Rinard, Cervelo Engineer

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