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  1. #1
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    Short on Legs, Long on Problems

    I read an article about fitting bikes. Based on their measurements, my wife needs 150mm cranks.
    Does this seem right?
    If it is right, where the heck am I going to find these things?

  2. #2
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    Depends on your BB. I run a square taper BB and have had no problems with Sugino cranks.

    Looks like they come in 152mm and 165mm as well.

    http://www.universalcycles.com/shopp...&category=2637

    You're welcome. ;-)

  3. #3
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    If you can't get all the way down to 150mm, something close will be fine. That Sugino posted above would work.
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  4. #4
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    Sounds way too short to make torque. Ever ridden a beach cruiser or kid's bike with cranks that small? Try 165s.
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  5. #5
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    Quote Originally Posted by Lelandjt
    Sounds way too short to make torque. Ever ridden a beach cruiser or kid's bike with cranks that small? Try 165s.
    That's what gearing is for. You can generate just as much power with 150mm as 175mm. Power is the measure of importance, not torque which is simply an intermediate term.

    The question should be why the article recommends such short cranks and what reason is there to believe that advice. Such a short crank would raise CS considerably. On a stock frame it's unlikely that would bring good results. I would suggest ignoring the recommendation in favor of a more common size, especially if she has positive experience with something longer. 150mm could work with an abnormally low bottom bracket or a custom frame, but you'd also need to carefully consider lower gearing.

  6. #6
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    Quote Originally Posted by craigsj
    That's what gearing is for. You can generate just as much power with 150mm as 175mm.
    You'd have to shift a LOT. If you ride a kid's bike or beach cruiser with cranks this short you'll feel why. The rev range over which you can make power is really small.
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  7. #7
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    I've run into a lot of articles for and against smaller cranks for shorter riders. If you take the idea of too long a crank to its logical extreme, the result would be unridable. (Imagine trying to ride a 700mm crank arm.) It seems like when it's tested, though, nobody can get conclusive results, or they find that the pedaling style of the rider has more to do with the crank they want than leg length. I think they usually only look at riders within a relatively narrow height band, though. If your wife is particularly short, she probably falls outside the range of leg lengths tested.

    Leanard Zinn does custom cranks down to 130mm. He also has yet another formula for figuring out a good crank length. His are five-arm with the old 94/58 BCD, which is nice - you can get a 20t granny and as small as a 29t middle ring, which will help with the less torque problem.

    Does your wife have problems with the current cranks? Finding something smaller than 160mm is going to be expensive and require her to raise her saddle, which will make a higher-riding bike as some other posters have noted. I think that's better than a knee injury, but if this is something you've invented and she's fine, it's probably better to leave well enough alone. You also might see if you have a well-equipped shop handling fits or custom bikes in your area. They often have adjustable-length cranks, and your wife could figure out if they make a difference for a sample set of the one person who really matters.
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  8. #8
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    I bet you didn't even KNOW you had a problem

    until you read that particular article. There area a lot of article out there.

    FWIW I don't think you have a problem. The teen girls on my racing team, as short as 4' 10", ride on "normal" cranks and race. They have for 9 years. Let's not overthink this.

  9. #9
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    Quote Originally Posted by Lelandjt
    You'd have to shift a LOT. If you ride a kid's bike or beach cruiser with cranks this short you'll feel why. The rev range over which you can make power is really small.
    Nonsense. The difference is constant, you have to shift no more or less often.

  10. #10
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    Quote Originally Posted by craigsj
    Nonsense. The difference is constant, you have to shift no more or less often.
    Like I said, your efficient rev range becomes very small with such a short crank. You can rev a little higher but you lose a lot on the low end. If you'd ever ridden such a short crank you'd have felt this immediately. Unless you live around a lot of beach cruisers or kid's bike though you probably haven't.
    Keep the Country country.

  11. #11
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    Quote Originally Posted by Lelandjt
    Like I said, your efficient rev range becomes very small with such a short crank. You can rev a little higher but you lose a lot on the low end. If you'd ever ridden such a short crank you'd have felt this immediately. Unless you live around a lot of beach cruisers or kid's bike though you probably haven't.
    Yes, you did say that. If only there was some reason to believe it.

    You can "rev a little higher" in proportion to the "lot on the low end" that you lose. The ratio of the two will be the same to the extent you could measure them. Testing has shown that 150mm is well within the range that adult riders of most sizes can adapt to, it is not so short that biomechanical efficiency is lessened. Yours is nothing more than a different perspective on the commonly held belief that long cranks make power for free. They don't.

  12. #12
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    Quote Originally Posted by craigsj
    Yes, you did say that. If only there was some reason to believe it.
    2,000+ posts doesn't give me instant, irrefutable credibility? I thought that's how the internet works
    Quote Originally Posted by craigsj
    Your's is nothing more than a different perspective on the commonly held belief that long cranks make power for free.
    More like an observation from riding such short cranks that there is a lower limit to how short a crank can be and make efficient power. If you are comfortable spinning really fast you could adapt to a 150 but my legs didn't want to go much faster than with 175s and that wasn't enough to make up for losing 20 rpm or so at the low end. Of course useful crank length range will depend on how fast the rider likes to spin and how long their legs are (mine are VERY long) but everyone would agree that there is a limit to how long/short an efficient crank can be, just not exactly what those lengths are. My advice to anyone curious about this is to keep an eye out for bikes with really long or short cranks and try them out.
    Keep the Country country.

  13. #13
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    Quote Originally Posted by Lelandjt
    More like an observation from riding such short cranks that there is a lower limit to how short a crank can be and make efficient power.
    Yes, but that is much shorter than 150mm even for large adults. Perhaps you are over 7 feet tall.

    Quote Originally Posted by Lelandjt
    If you are comfortable spinning really fast you could adapt to a 150 but my legs didn't want to go much faster than with 175s and that wasn't enough to make up for losing 20 rpm or so at the low end.
    Cadence is limited by footspeed and is self-adjusting for different crank lengths. If you think your maximum cadence is the same regardless of crank length then you suffer from confirmation bias. I suspect you don't really know. Yes, I've done the tests with variable length cranks, not down to 150 but down to 160 and I've read scientific tests that have gone down to 140. The problem here isn't that I lack the "experience" that you have riding beach cruisers.

    Just as an example, if your maximum cadence is 90 rpm on 175mm cranks, the equivalent on 150mm cranks is 105 rpm. It's not that big a deal for even that drastic a change. Footspeed in either case is the same, as is torque with equivalent gearing. The real problem with such short cranks is finding equivalent chainrings and a bottom bracket low enough so that handling isn't screwed up.

  14. #14
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    My usual riding partner has very long legs. He also has 175 cranks. He also powers up hills with a lot fewer revs than me because of all that leverarm.
    My wife has shorter legs and I see her really gearing low and pedaling like crazy to get up hills, her cranks are 170, a small fraction of an inch difference to the 175s.

  15. #15
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    I think your riding partner is stronger than your wife.

    On a side note I recently rode a friend's singlespeed with 180 cranks. They really helped (in concert with a really wide stiff handlebar) in those singlespeed situations where I was in way too high a gear for a climb. The did feel a little funny coasting and made you not want to rev out as much.
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  16. #16
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    Sounds like your wife likes high cadence.

    If she had shorter cranks, she'd probably pedal even faster. I don't see that as a problem.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  17. #17
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    Quote Originally Posted by Lelandjt
    They really helped (in concert with a really wide stiff handlebar) in those singlespeed situations where I was in way too high a gear for a climb. The did feel a little funny coasting and made you not want to rev out as much.
    A lower gear would have really helped, too, and wouldn't have the side effect of lowering maximum cadence which you are now acknowledging. All the crank length is doing in that situation is changing the gearing.

    Quote Originally Posted by Lelandjt
    If you are comfortable spinning really fast you could adapt to a 150 but my legs didn't want to go much faster than with 175s...
    So your max cadence is constant up to 175 but drops off precipitously once you hit 180? Sure.

  18. #18
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    Quote Originally Posted by craigsj
    So your max cadence is constant up to 175 but drops off precipitously once you hit 180? Sure.
    Yup, the 180s felt long. Felt like more of a difference than the 175 & 170s I have on my bikes. Remember this is about feel and what our legs like, not hard numbers. I also noticed how far apart my feet were when coasting and it felt like my balance and control weren't as good. More than 1/2 a mile on them and I probably would have adjusted more.
    Quote Originally Posted by craigsj
    A lower gear would have really helped too.
    Yes, singlespeed MTBs are dumb unless you live somewhere flat. We were riding the Monarch Crest Trail, not flat. The quietness (his is a belt drive) and weight are nice but there's no replacement for always being in the right gear.
    Keep the Country country.

  19. #19
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    Quote Originally Posted by Lelandjt
    Remember this is about feel and what our legs like, not hard numbers.
    Funny, I don't remember it being about that. I can see why you would like it that way, though.

    I bet if I put you on a trainer you couldn't tell if you were on 180s or 175s. I cannot. On the road I can but that's because handling changes. This talk about legs liking certain sizes is without merit and has been disproven in research.

  20. #20
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    I haven't given much of the research a close read necessarily. I got the impression that individual riders might have different performance in terms of power output and knee pain over longer distances from different crank arm sizes. My reading was that there's just no correlation to the rider's leg length within the range of "reasonable" crank arm sizes. (130 - 210, IIRC)

    It seems like in order to be able to do anything useful with that, though, we'd all have to spend a few days recording power numbers with different crank arm lengths, in different orders, on a trainer and after all that, the difference between mean power outputs at whatever heart rate might be less than the standard error of each mean - as in it might exist, but the difference is too small to be useful data.

    If someone had a particularly non-normative crank arm length they liked, they'd probably also have to get a custom frame to make that fit right. D'oh!

    It wouldn't stop me from trying to design an experiment if I had adjustable cranks and one of those funky fit cycle things. But absent a few thousand dollars' worth of fitting equipment, I think I'll just keep riding my stock frame and the 170mm cranks that are already on all my bikes. I have heard that switching lengths between bikes is disconcerting (although I've also heard that it only takes a few minutes to adjust. So go figure.)

    Kind of a counter-intuitive result and it flies against my desire to be able to make crank arm length an optimization problem that has an answer. Oh well.
    "Don't buy upgrades; ride up grades." -Eddy Merckx

  21. #21
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    Quote Originally Posted by AndrwSwitch
    I got the impression that individual riders might have different performance in terms of power output and knee pain over longer distances from different crank arm sizes. My reading was that there's just no correlation to the rider's leg length within the range of "reasonable" crank arm sizes. (130 - 210, IIRC)
    I've not seen any research on knee pain. Research shows that differences in power are minor over a wide range of crank sizes. The range you list seems reasonable.

    Quote Originally Posted by AndrwSwitch
    If someone had a particularly non-normative crank arm length they liked, they'd probably also have to get a custom frame to make that fit right. D'oh!
    I think that's the key takeaway. Crank length is part of fit but it depends on frame construction. Unusual crank lengths will require unusual frames.

    Quote Originally Posted by AndrwSwitch
    It wouldn't stop me from trying to design an experiment if I had adjustable cranks and one of those funky fit cycle things. But absent a few thousand dollars' worth of fitting equipment, I think I'll just keep riding my stock frame and the 170mm cranks that are already on all my bikes. I have heard that switching lengths between bikes is disconcerting (although I've also heard that it only takes a few minutes to adjust. So go figure.)

    Kind of a counter-intuitive result and it flies against my desire to be able to make crank arm length an optimization problem that has an answer. Oh well.
    I agree.

    I've had an adjustable crank on a road bike that offered from 165mm to 180mm. On a trainer it was difficult to tell changes of 5mm. On the road you could sense the changes due to the difference in saddle position and CG.

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