Has anybody tried short cranks for SS? 165mm or less?- Mtbr.com
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  1. #1
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    Has anybody tried short cranks for SS? 165mm or less?

    Just curious if any of you have experimented with unconventionally short crank lengths on a singlespeed. 165mm or lower.

    I know this has been debated at length in other parts of the forum, but I'm specifically interested in those who tried them on a singlespeed.

    How did it go? Did you miss the length when climbing? Or when needing that extra push to get over chunky stuff? Did you feel you put down less power?

    Is the 170-175mm standard because it simply works best, or is it something we do just because we've adapted to what is readily available?

  2. #2
    Hardtail Steel Forever
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    Personally 170 - 175 is for availability sake. If you look at the physics and geometry of it, there's numerous papers and studies, they clearly show that short cranks are beneficial for almost all riders, both for the hips and joints and for the physics of spinning the wheels.

    But, I'm a bit interested if someone has done any SS specific studies, simply because I feel I tend to be out of the saddle more on SS than on my geared setups. I could see some benefits to the additional torque, at the same time, I could see it being a hindrance that would lead to a higher chance of spinning out.
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  3. #3
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    Quote Originally Posted by jestep View Post
    But, I'm a bit interested if someone has done any SS specific studies, simply because I feel I tend to be out of the saddle more on SS than on my geared setups. I could see some benefits to the additional torque, at the same time, I could see it being a hindrance that would lead to a higher chance of spinning out.
    I think it'd depend on the terrain and riding style. In my anecdotal experience, I noticed a big improvement when I switched from 170 to 175. My local trails are loose, rocky, rooty, and I'm frequently out of the saddle for short, punchy efforts. Seemed like more torque helped with that. On smoother, hard pack trails, maybe it'd be more beneficial to sit and spin a higher cadence.

    On my singlespeed, I'm infrequently able to maintain that ideal 90-100 rpm cadence, so to me it makes more sense to optimize for torque. When I hang gears on my bike, I do end up sitting and spinning a higher cadence, but not too much since it's a hardtail.

    My gut tells me that the shorter crank/higher cadence argument makes more sense for geared bikes, road, or hardpack trails, but I could be wrong about that.
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  4. #4
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    Quote Originally Posted by heartland View Post
    On my singlespeed, I'm infrequently able to maintain that ideal 90-100 rpm cadence, so to me it makes more sense to optimize for torque. When I hang gears on my bike, I do end up sitting and spinning a higher cadence, but not too much since it's a hardtail.

    My gut tells me that the shorter crank/higher cadence argument makes more sense for geared bikes, road, or hardpack trails, but I could be wrong about that.
    Yes, this is what gives me pause about the shorter cranks. When I'm trail riding the SS, it's mostly standing, and a lot of low cadence grinding, versus the sit and spin, road-centric style of riding that seems to be the prevailing style of geared riders.

    But theoretically, if you apply the general rule (1 cog tooth per 5mm of crank length), shouldn't the shorter cranks produce equal torque?

    Also, when grinding up a tough climb where it gets to be a fight just to get the cranks to turn over, perhaps the shorter arc could be a benefit?

    How does standing with low cadence grinding feel different with short cranks? I imagine it makes the entire, whole body movement more compact, and I'm not sure this would be a good thing.

  5. #5
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    I have 165mm arms on my SS MTB and have had even shorter on a CX/gravel bike sense sold (it was either 160 or 162.5) From what I can see on geared study's it doesn't really effect power. Arm length is about fit and feel preference/balance of torque feel and ability spin.

    I don't miss the longer arms at all. Partially because I am a odd fit with really short legs and the shorter arms might work better for me generally. Which is likely why most don't use them. They fit and prefer what's readily available.

    The other reason is because it is mostly flat where I live. So if I gear for the occasional hill I spend a bit more time spinning. The ability to hold a higher RPM is a lot easier for me. I don't really notice the lack of leverage and if I do it is a much smaller portions of a ride then the time spent at higher cadence. Just a few rotations at the top of the occasional hill.

    I find that I spin at a much higher cadence with SS as I try to get one gear to cover a wider range of speed. Where with gears I stay at a more efficient cadance 90 to 100 and have a really wide range of speed I can maintain that RPM.

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  6. #6
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    Quote Originally Posted by heartland View Post
    My gut tells me that the shorter crank/higher cadence argument makes more sense for geared bikes, road, or hardpack trails, but I could be wrong about that.
    Crank length is really about fit, not one being better than the other.
    I'm short, and bought 165mm cranks with a new frame this fall, because I assumed it would be run geared all the time, shorter cranks would minimize pedal strikes, etc. I've since run it both geared and SS with 165 and 170mm cranks. According to some fit calculator thing I found online awhile back, 167.5mm was "right" for me, which put me between those sizes. After trying both, I'll be sticking with 170mm, regardless of setup. I can't tell the 165s make any real difference when spinning, but they definitely didn't help mashing a SS.
    A crank's length either works better with your body mechanics, giving you more leverage *against the cranks* at whatever degree of knee flex its length results in, or it doesn't. By contrast, a longer crank gives you more leverage *against the drivetrain*.
    So you want the longest crank that you can comfortably use to give yourself every possible advantage. I'd think that's especially important for SS, where your biggest efforts are standing and mashing.

  7. #7
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    Crank length affects the mechanical advantage of your bike. It is similar to changing gear ratios. A longer crank effectively lowers gearing by increasing torque and reducing top speed (same as using a smaller chainring). A shorter crank effectively increases your gear ratio by reducing your torque and increasing top speed (same as using a larger chainring). Sheldon Brown's gearing calculator has a 'Gain Ratio' metric that takes into account crank length so you can find equivalent gearing for different crank lengths.

    You don't have to worry about loss of torque with shorter crank arms, because you can always lower your gear ratio to make up the difference. Since you can spin a higher cadence with shorter crank arms, the lower gear ratio won't limit your top speed (as long as the gain ratio is the same).

    I've not tried using a shorter crank on my single speed personally. I am 6 feet tall with long legs and currently ride 175s. I wonder if a shorter crank would help me spin faster on the flats (I live in the midwest). Currently my legs burn out fairly quickly above 120 rpm. My only reservation is that I may need to lower my gearing to climb steep grades and lose the benefit at high speeds on the flat. If this were the case, there would be no net benefit.

    I think crank length is a fit issue, but the studies I've seen show that crank length (within commonly available sizes) doesn't affect cycling power. I suspect that as long as you have a crank that fits you reasonably well you won't benefit much by changing it. Cranksets are expensive, so this could be an expensive experiment!

  8. #8
    psycho cyclo addict
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    Nope.

    I'm 6'1" or so and ran 180mm cranks on a couple of bikes (where the geometry seemed to be more appropriate/comfortable to pedal- Niner SIR 9 in particular) for a few years. 175mm's are on everything now: two (custom steel frame) SS's and geared hard tail and full suspension 29er's.

    A buddy of mine went crazy last year and swapped a bunch of (expensive carbon) 175mm cranks out for 165mm. Haven't noticed that he is any faster on our weekly hammerfests and joke with him that he should be because his wallet is lighter
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  9. #9
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    Well thank you for the interesting replies. Thus far, I didn't hear anything above that was unfavorable enough to extinguish my curiosity. What has me most intrigued about short cranks on a SS is how it may make that whole body, standing and grinding motion just a bit more compact; could there be energy savings there? After all, I seem to spend a lot of time in that position on my SS. And I love it!

    So I just pulled the trigger on some Canfield 155mm cranks. I'm 6'0"! We'll see how this goes. I'll report back once I finish the build and have a few rides. I figured worst case scenario I can put them on my 9 year old's bike.

  10. #10
    Trail Ninja
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    Quote Originally Posted by MidnightFattie View Post
    Yes, this is what gives me pause about the shorter cranks. When I'm trail riding the SS, it's mostly standing, and a lot of low cadence grinding, versus the sit and spin, road-centric style of riding that seems to be the prevailing style of geared riders.

    But theoretically, if you apply the general rule (1 cog tooth per 5mm of crank length), shouldn't the shorter cranks produce equal torque?

    Also, when grinding up a tough climb where it gets to be a fight just to get the cranks to turn over, perhaps the shorter arc could be a benefit?

    How does standing with low cadence grinding feel different with short cranks? I imagine it makes the entire, whole body movement more compact, and I'm not sure this would be a good thing.
    That general rule checks out, at least when going from 32t with a 175 crank to 30t with a 165 crank, in terms of achieving the same torque. Shortening the crank is essentially like making the gearing taller. There shouldn't be any extra difference in speed or work if you have the same gain ratio. This is especially true considering the bicycle drivetrain is a series of simple machines. Torque/leverage at the crank is like torque/leverage of the rear cog on the rear wheel--a longer crankarm is like a bigger rear cog.

    I suppose it all depends on how a person prefers to move their legs. If two people raced each other up a variety of staircases, time trial style, which technique would they use? Bounding up 2-3 steps per stride, using their arms to muscle up, or doing quick steps up 1-2 steps at a time focusing on their rhythm, depending on the height and length of the steps. I'm sure people will get better results with one or the other, and might perhaps feel more comfortable with one or another. Make the staircase really really long (e.g. skyscraper or statue of liberty) and check the results of each technique again.

    Personally, I don't recognize either as superior outright. Just a matter of taste. I'm sure there's a lot of conformity going on, where one person sees the technique of someone better, and gains the bias that it's a more proven way to go. All I know is that I'd hesitate going any longer than 175mm, but am willing to try shorter. I only expect ground clearance gains and a lightened wallet though, with no other significant performance difference.

    Here's some data on the subject:

    Has anybody tried short cranks for SS?  165mm or less?-emvfgyj.png

    Done on a monark cycle ergometer, 75d seat angle, recumbent. These guys (25yr old, 1.8m tall, 82kg) can friggin spin and put out some crazy power... M=mean, SD=standard deviation

    Hmm, curious that they tested peak power with various angles and seat orientations. They found that a 75d seat angle with a 90d backrest was ideal. I tried to align it to make it equivalent to a 75d upright bike, but my image editor froze, so all I have is a screenshot of it:

    Has anybody tried short cranks for SS?  165mm or less?-d8hqrnc.png
    - I basically first rotated the pic until the guy's seating position was directly 90d vertical, laid a line, then rotated the image back 25 degrees and laid a perpendicular line at perceived crank center and attempted to measure the angle. I wanted to see how much pelvic tilt the guy had compared to an upright cyclist.

    Source: http://www.ihpva.org/HParchive/PDF/hp51-2001.pdf
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  11. #11
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    Haven't run short cranks on my SS, it got destroyed by a SUV, but run short cranks on all my newer bikes. No need to change gearing.
    Attached Thumbnails Attached Thumbnails Has anybody tried short cranks for SS?  165mm or less?-fullsizeoutput_540.jpg  


  12. #12
    Stateline Falls, Watauga
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    My gain ratio calculator tells me the general rule of thumb should be: 1T on the cog equals 10mm of crank arm length, not 5mm.
    It never gets easier, you just go faster. -Greg LeMond
    I'm not as fast as I think I am. -JeffL

  13. #13
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    General ratios....I went from 170's to 135mm....been riding for yrs...no change in gearing, and don't want to change anything. My road bike has 125mm.

  14. #14
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    Quote Originally Posted by MidnightFattie View Post
    Well thank you for the interesting replies. Thus far, I didn't hear anything above that was unfavorable enough to extinguish my curiosity. What has me most intrigued about short cranks on a SS is how it may make that whole body, standing and grinding motion just a bit more compact; could there be energy savings there? After all, I seem to spend a lot of time in that position on my SS. And I love it!

    So I just pulled the trigger on some Canfield 155mm cranks. I'm 6'0"! We'll see how this goes. I'll report back once I finish the build and have a few rides. I figured worst case scenario I can put them on my 9 year old's bike.
    any feed back on how it went with the shorter cranks

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