Crank Length Article- Mtbr.com
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  1. #1
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    Crank Length Article


  2. #2
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    Yeah, it is interesting.

  3. #3
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    I think it speaks to how equipment-obsessed riders are that you still see so many crank-length questions when that study is over 10 years old now. Crank length seems to be just another part of bike-fit.

    For the record, across all my bikes, I have cranks ranging from 172.5 to 180 and I suck equally on each regardless of crank length.
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  4. #4
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    Quote Originally Posted by garage monster View Post
    I think it speaks to how equipment-obsessed riders are that you still see so many crank-length questions when that study is over 10 years old now. Crank length seems to be just another part of bike-fit.

    For the record, across all my bikes, I have cranks ranging from 172.5 to 180 and I suck equally on each regardless of crank length.
    I have absolutely no idea of the length of my cranks and don't feel that I am any worse off for not knowing.
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  5. #5
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    I was told at a bike shop that I was running the wrong length cranks on my road bike. I bought some nice carbon fiber cranks that I had transferred over when I bought a new bike. They told me that they were 5 mm too short. When I heard this, I thought it was a joke, but the guy was quite serious. I have been running those cranks for to seasons since. I wonder how much faster I could be......not really. Like Garage Monster, I would probably suck just as much on the longer cranks.

  6. #6
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    I'm oddly sensitive to crank length. I can ride around with my seat two inches lower and barely notice, but the 5mm shorter cranks made me feel like I was trying to run with my feet tied together.
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  7. #7
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    Quote Originally Posted by saturnine View Post
    i usually run 170mm cranks. whether that is "right" or not, who knows, but i put 175mm cranks on my dj bike and i don't know if i'd be able to tell the difference. for what it's worth, i believe you can achieve the same effect by running thinner or thicker pedals.
    I'm no engineer, but I think crank leverage would be from pedal axle to the centre of the BB, I don't see how pedal width would adjust the length of the lever.... Pedal width, perhaps, would be noticed with saddle height perhaps???
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  8. #8
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    i usually run 170mm cranks. whether that is "right" or not, who knows, but i put 175mm cranks on my dj bike and i don't know if i'd be able to tell the difference. for what it's worth, i believe you can achieve the same effect by running thinner or thicker pedals.
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  9. #9
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    Quote Originally Posted by ghettocruiser View Post
    I'm oddly sensitive to crank length. I can ride around with my seat two inches lower and barely notice, but the 5mm shorter cranks made me feel like I was trying to run with my feet tied together.
    I got a set of 170mm cranks from the LBS once without realizing they were 170s, and put them on a spare parts bike. I often felt a little weird on that bike, like I had to spin faster on it, but could never pin down exactly what was up, or why I felt that way. Some time later I realized they were 170s and I had been riding 175s for years on my other bikes.

    That was about 7 years ago, and those cranks are in the parts bin now, but I'm tempted to swap them back and forth a few times just to see what effect it has on me now with more power and better skills.

  10. #10
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    Most of what they talk about is aero drag area. Not that big a deal for MTB'ers, where mudrodynamics is more important.

    I've always thought crank length is less about torque and more about fit as it relates to the length of leg below the knee and the length of foot, as that combination lever of knee/ankle/foot should determine what the optimum crank length would be for a given seat height. They reference that a bit in this paragraph...

    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.
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  11. #11
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    I bought a road bike in 2006 after years of riding mountain bikes with 175mm cranks. I had a difficult time getting comfortable on it despite numerous tweaks to saddle and handlebar position. I felt like the pedals turned over "too fast" and I felt like I had no power/leverage, especially going uphill or during out of saddle sprints. I felt more comfortable and just as fast on my mountain bike. Frustrating!

    Fast forward a few years, and I've finally had enough and go for a professional bike fit. The physiotherapist noticed the shorter crank length (172.5) on the road bike, which completely escaped my attention. She made some recommendations regarding saddle and handlebar position, but none regarding the crank. I guess the crank length was within spec for my body measurements.

    I made the saddle and handlebar changes, which improved the comfort level, but made no discernible difference in what I was feeling in the pedals or in my overall speed. On a whim and somewhat out of desperation, I tried two different cranksets: a 175 and a 180mm. With both, I felt an immediate improvement. I was finally faster on my road bike. I found the 180mm gave me knee pain during long distance rides, so I settled on the 175.

    Maybe crank length has minimal impact on power output, but after nearly two decades of riding with 175mm cranks, perhaps my body has adapted to that length and doesn't like anything else? I dunno. I do know I'm not going back to the 172.5 crank.

    Also, for the record, I still suck at biking. I just suck a little less now on the road bike.

  12. #12
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    Quote Originally Posted by rockyuphill View Post
    Most of what they talk about is aero drag area. Not that big a deal for MTB'ers, where mudrodynamics is more important.
    My take away after reading that is there is no major power loss in running the shorter cranks.

    For mountain biking application this can mean you can run shorter cranks with a lower bb and not worry about loss of power. Lower cog and better handling.

  13. #13
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    I remember somewhere I had read in an article that optimum crank length is supose to be 2/3 leg length (length from knee to ankle) I can't find the article. The one thing that the article at the begining of this is that it doesn't account for metabolic use, perceived exertion and power is one thing but actual energy use is different.

    Just as a comparison a doctor with cervelo actually did metabolic useage tests while working on TT positions in the wind tunnal and found that some of the more aero positions were the most costly in energy expenditure. So in the big picture you need to look at both the mechanical view as well as the physiological view especially for cycling.

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  14. #14
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    An issue I can see with that study is that it seems to be focused on cadence and aerodynamics.
    There are no conclusions about very low cadences like we will see on a mtb - down to almost zero like on a single speed on a steep hill. If you look at the study in reverse, there is very little negative impact to efficiency running long crank arms.
    One of the comments mentions that there is a 12% increase in power moving from 160 to 180mm crank arm. I've always understood that it was this low speed torque, sheer power that is the reason for longer crank arms on mtb's.

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