Crank Length

I’ve heard many triathletes and time trialists are switching to shorter cranks: 170, 165, even 160mm. What do you recommend?

  • Crank length

Crank Length

Great question. It is true that many top athletes are switching to shorter cranks for timed racing such as triathlon and TT. This is relatively new, because traditionally longer cranks were thought to be better since they give more leverage. However, crank length is just one lever in a drive train composed of a system of levers that transmit 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 we don’t need a small difference in crank length to change the leverage available to us.

What does Dr. Martin say?

Cranks 1

For many athletes, the idea “longer 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?

crank 1

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:

  • 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.
  • 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 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.
  • 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. Let me know what you think in the comments section below.

Cheers.

Comments (20)
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  • marcoaleite

    Hi Mr. Damon. Do you consider interesting a change of crank length in my case? Now I use a crank of 172.5 mm in my road bike and 177.5 mm in TT bike. I'm a cyclist with climber's profile, I have 1.77 m of height and 64 kg of weight and ride with 90-100 rpm in hard efforts without problems. This article is an interesting question, in my road bike I ride with 300 W with a good comfort in pedals but when in my TT I don't fell the same sensation. My cadence down a little and I suffer a bit more to maintain the same power. Cheers! Marco

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  • Damon Rinard, Cervélo Engineer

    Hi Jay, Concerning power output, there's no "wrong" until 145 mm. That said, most folks are happiest with TT cranks about 5 mm shorter than their road bikes. 165 is reasonable for almost everyone, even 6-foot riders. Cheers, -Damon

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  • Jay

    I ride 172.5s on my TT bike and thinking about going shorter..what would you recommend? Based on your article sounds like going to 170s for me is not a big difference...so do you recommend going to 165? Im buying new cranks so just wanna get it right.

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  • Jay

    I ride 172.5s on my TT bike and thinking about going shorter..what would you recommend? Based on your article sounds like going to 170s for me is not a big difference...so do you recommend going to 165? Im buying new cranks so just wanna get it right.

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  • valebike

    Thank you!! nothing TT For road bike would you recommend for me 170 or 172.5 for long distance and lots of climb?

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  • Damon Rinard, Cervélo Engineer

    Hi valebike, If I understand you correctly, you have mostly average height and proportions. The small change you are considering (only 2.5 mm) will not change your power output in a normal road bike riding position. Either crank length will serve you well. If you also ride a TT bike, then consider a crank 5 mm longer for your road bike. This difference helps muscle adaptation between the two different riding positions by making the range of hip motion more similar. Cheers, -Damon

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  • valebike

    Hello, sorry for the bad English ... I'm 1.72 cm x 67 kg horse femur 43.5 cm 81.5 cm nothing TT, only GF are not pure climber but I love the climbs ... cadence: plain: 88/102 easy climb 73/85 hard climb 63/75 mmmmm very very tough climb normally 55/65, normally> 60 I've always used the cranks from 170, do you think it might be better to use the 172.5? You would not change anything? In my place would you try to change it or not? Thank you very much, V.

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  • Damon Rinard, Cervélo Engineer

    Hi SteveI, yes, to the bottom of the stroke. Also yes, saddle height was adjusted with each crank change in the wind tunnel, but bars were not touched at that time, so drop did change. (Later we adjusted the bars, often as a result of the increased range of motion we'd earned with the crank length change.) Cheers, -Damon

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  • SteveI

    Damon, when it says that saddle height to the pedal was preserved, do you know if that is to the pedal at the bottom of the pedal stroke? And for the wind tunnel data, was the same saddle adjustment made each time the crank length was changed? If so, were the bars moved up and down with the saddle, or was saddle to pad drop changed with crank length?

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  • Bernard

    The crank length is a mystical affair, more or less. It could actually be that simple. If we understand it as a five bar linked system, one can easily calculate the optimum. However, since our system is composed of outer and inner levers whose transmission depends additionally on the movement and excitation velocity of muscle forces, the thing is computationally complex but solvable. First now it gets complicated. A neurological and genotypic component also affects the propulsive performance. In addition to our individual adaptive mechanisms. It seems it is mystical, but there is hope in the world of simulation. Have you ever thought about it?

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  • triguy2k

    Hi Damon, Can you confirm if the Cervelo 51 P5 2014 come specced with 167.5 mm cranks. I purchased the 51 P5 in December and wanted to know if it would be possible to exchange the crank with 165 mm?

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  • Ian Thomas

    One of the benefits of shorter cranks is the ability to maintain a good cadence especially when climbing. Maybe 90 - 100 is ideal for triathlon but it varies from rider to rider. When running off the bike, a foot strike of 90 steps per minute is easier to maintain when the legs are conditioned to that rate of turnover.

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  • Damon Rinard, Cervélo Engineer

    Hi bstudenka, Crank length has less to do with climbing than plain old power-to-weight ratio. Your comment on cadence is a clue, however: consider changing gears (e.g., install a bigger cassette) to avoid very low cadence; a more normal cadence while climbing helps bring the pedaling frequency into a range (thus bringing pedal force into a range) you're more likely to have trained. Cheers,

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  • bstudenka

    Someone told me that shortening my crank length (I have 175 on my Scott TT bike right now) would help me on the hills, which I seem to be very bad at. Any idea of if or why this might be? My only thought was that, with a lower cadence I might be wearing myself out (only up to 60 rpms) at the beginning of a big hill and then I'm not able to maintain up to the top? I'm also really exhausted by the end of a ride if there is a hill. Thanks! Bree

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  • Damon Rinard, Cervélo Engineer

    Hi Orionblanco, Your Cervelo shop should be able to adjust your bike to your body, or you can start reading about it here: http://www.slowtwitch.com/Bike_Fit/Tour_de_Bike_Fit_649.html Sorry, it's all in English... Thanks, -Damon

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  • Orionblanco

    como ajusto la bicicleta a mi talla?

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  • Damon Rinard, Cervélo Engineer

    Hi Chris, Thanks for your comment. Dr. Martin's study did adjust saddle height, higher with shorter cranks, just as you suggest. I agree with this philosophy too, and it's what I recommend to our pros. Thanks for the suggestion for saddle positioning article - we'll add it to the list! Cheers, -Damon

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  • Chris

    Thanks for the great article! I'm left wondering, wouldn't you want to raise saddle position if your cranks are shorter to maximize your leg length/power? Isn't most of the power coming from the near full extension of the leg? I'd love to see another article explaining ideal saddle height/positioning for triathlon.

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  • Damon Rinard, Cervélo Engineer

    Hi David, your experience mirrors many good riders. Cranks a little longer on the road bike helps keep bocy angles more the same, so you're doing fine! Cheers, -Damon Rinard, Cervélo Engineer

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  • David

    Interesting , i ride 170s on my P3 and alomost always have since buying it, my bike leg results are very good and i tend to have a top 10 bike split overall in most races - my age group is ( 45-49) yet on my road bikes i have 172s. All of my roadie friends say i should ride the same however it does suit me - i am 5.7" , 65kilos .

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