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Explanation Please on Crank selection. For 125 vs 150mm

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  • Explanation Please on Crank selection. For 125 vs 150mm

    Hi all. I have dual 125/150 cranks on my 29" Nimbus and have been riding with the 150mm. It's 98% flat where riding (understand 150mm better for hills) and am trying to apply logic as to why I should transfer over to the 125mm.
    Can anyone rationalise the logic as to why I may be better off with the 125mm ?
    Some science analogy / example would be helpfull to help the penny to drop... ?? Thanx Jimmy

  • #2
    Originally posted by GideupJimmy View Post
    Hi all. I have dual 125/150 cranks on my 29" Nimbus and have been riding with the 150mm. It's 98% flat where riding (understand 150mm better for hills) and am trying to apply logic as to why I should transfer over to the 125mm.
    Can anyone rationalise the logic as to why I may be better off with the 125mm ?
    Some science analogy / example would be helpfull to help the penny to drop... ?? Thanx Jimmy
    The shorter crank length will allow for smoother cadence and reduce the size of the pedaling 'circle'. This can also allow for greater speed with less wobble and more efficiency, and because your legs are moving up/down to a lesser extent, there is also less likelihood of chaffing. So for riding on mostly flat I would recommend the shorter crank length, and for many riders, 110's would be optimum.
    Last edited by MuniAddict; 2014-04-17, 03:12 PM.
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    • #3
      110 is a popular size but is not necessarily optimal. It depends on a lot of variables, not least your level of experience.

      I have ridden most sizes from 80mm to 170 mm on a range of wheels and I have ended up settling for 150s on my 36, 29 and 24 (muni) and 125s on my 28 and 20 (which I seldom ride).

      Within certain common sense limits, a longer crank will give you more leverage up hill and - very importantly - downhill, and a shorter crank will allow you to pedal at a faster cadence (more rpm) because your feet don't have so far to travel.

      A guide is that a 10% difference in crank length will make about 10% difference - either to speed or to leverage/control. However, this only works within common sense limits. Imagine riding with 10mm cranks or 200mm cranks and you will see that very short cranks use only a small part of the available muscle movement, and very long cranks are unwieldy.

      If you have square tapers, a set of cheap cranks is a fun investment and it is the work of a few minutes to swap them over. (You need a crank tool though.) If you have a splined hub, cranks are available drilled for 2 pedal positions and these will give you versatility.

      Your original question was a choice between 150s and 125s. On most wheel sizes, it will be little more than a matter of preference.
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      • #4
        Why try 125?

        Why not?

        Just switch your pedals and see for yourself. It might not make sense from reading about it as much as feeling it for yourself. As your cranks get shorter, your pedal stroke gets smoother, as others have said your legs are pedaling smaller circles, but also if you think of your wheel as a flywheel, moving to shorter cranks effectively increases the size of the flywheel of your 'motor' (legs/body acting on the wheel). Smoothes things out.

        It completely changes the feel of your uni. Sometimes it takes a while to get used to it, you may feel a bit out of control at first, but sticking with it helps you get more out of your riding, instead of fighting the uni, trying to control it, you become more a part of it, you get much more efficient.

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        • #5
          Thanx

          Appreciate the replies. The below was the science was seeking...
          Naturally have since switched to the 125s and early days ... Kinda feels am travelling faster. Shofar so good. ....
          "The shorter crank length will allow for smoother cadence and reduce the size of the pedaling 'circle'. This can also allow for greater speed with less wobble and more efficiency, and because your legs are moving up/down to a lesser extent, there is also less likelihood of chaffing. So for riding on mostly flat I would recommend the shorter crank length, and for many riders, 110's would be optimum."

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          • #6
            People debate and discuss the merits of different crank lengths endlessly here. But the discussion must be driven by more than physics and theory. I often see people here on the forums applying oversimplified physics numbers to situations that are much more complex, and then defending their math, when its clear that the math is meaningless if it doesn't cover the whole picture.

            In other words, not only must one try the different sizes to really form an opinion, one must stick with them long enough to get used to them. When first tried, a new crank size usually feels disruptive, awkward or even scary. Give yourself time to get used to them, try them on various terrain, and be honest with yourself.

            If you're riding a 29" for distance, on mostly flat ground, I would recommend 102mm cranks. They will still work fine on easy hills. I even used 102s on a 36" in a (flat) Marathon race once, and they worked great. I also think 140mm is a great size for non-technical MUni; you can really move!
            John Foss
            www.unicycling.com

            "Who is going to argue with a mom who can ride a unicycle?" -- Forums member "HiMo"

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