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  1. #1
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    shorter cranks = higher gear?

    The esteemed Mr Sheldon Brown argues that crank length should be part of the gear calculation:
    http://www.sheldonbrown.com/gain.html

    If true, this means that switching to 135mm cranks should give a 20" wheel equivalent gearing to 175mm cranks on a 26" wheel.

    Has anyone experimented with this as a supplement/alternative to changing sprocket sizes? I say it's baloney.

  2. #2
    Bicycle Repair Man !!! Sixty Fiver's Avatar
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    Have you ever tried to ride on 135's ?



    With a much shorter crank you will lose leverage which will defeat the small gain you may get from the shorter crank and I can feel the difference between a 165 and 175 on the same bike and prefer the longer crank on this basis. From a comfort perspective I can run either and over time one would adjust to running a different length of crank but there are limits to that.

    My CCM Mustang has a 150 mm crank with 20 inch wheels and with the 3 speed I find I have to spin like a crazed hamster to maintain speed as low speed pedalling is not fun at all due to the lower amount of leverage.

    The longer cranks on my touring bikes and mtb (175's) are very nice for lower cadence riding and climbing while the rest of the bikes run 170's.

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    Quote Originally Posted by Sixty Fiver View Post
    With a much shorter crank you will lose leverage which will defeat the small gain you may get from the shorter crank
    But that's exactly the point. According to Sheldon Brown's gain ratio theory losing leverage can't defeat the "gain" because it is the gain!

    However, it doesn't make any sense to me because, as you say, shorter cranks promote higher cadence, which is exactly the opposite of higher gears which promote lower cadence. So how can the prolific Mr Brown have come to such a conclusion? Low gearing a pedal strike can both be problems for folding bikes and if Sheldon Brown's gain ratio theory were correct then shorter cranks could help with both, but it just seems wrong on the face, which is a difficult pill to swallow given how knowledgeable and experienced he was.

    Now I know 135mm cranks is an absurd exaggeration, but the real question is can shorter cranks be a suitable substitute for larger chainrings? For example, if one wanted to reduce pedal strike and moderately increase the gear, would a sensibly shorter crank kill both birds with one stone?
    Last edited by itsajustme; 05-08-09 at 11:51 AM.

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    Bicycle Repair Man !!! Sixty Fiver's Avatar
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    I never subscribed to gain ratios.

    The gearing of a bike is determined by the chain ring to cog ratios and the wheels have to be factored in as their rate of travel changes according to their size... it is obvious that a 20 inch wheel spinning around at 1 revolution per second will not take you as far as a 28 inch wheel turning at the same rate and gearing adjustments have to be made so a person driving a 20 inch wheel does not have to spin the cranks at ludicrous speed to maintain the same speed as someone on a bigger wheel.

    The crank and it's length are just the mechanism by which power is delivered... it is a lever which reduces the amount of force that is required to turn the chain wheel.

    Calculating gain ratios is something I do not do as it makes no sense (to me) and feel that gear inches are a very efficient way of calculating how far a bike will travel per revolution of the chain ring.

    The delivery of power is not a factor as in the end, it's always about how fast the chain ring turns in relation to the rear cog or cogs and the wheel size that is being used.

    The crank lengths in use (and our wheel sizes) are there because these they are what work with a broad spectrum of riders and an average physiology / body structure.

  5. #5
    rhm
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    Oh, no, here we go again! Let me start out by saying that, after over a year of extensive experimentation, I have determined crank arm length does not matter very much. Though people range widely in size, almost all of them seem pretty happy with 170 mm crank arms. In the abstract it seems obvious that crank arm length should be proportional to leg length, but there doesn't actually seem to be much evidence for this.

    That said,

    Quote Originally Posted by Sixty Fiver View Post
    Have you ever tried to ride on 135's ?



    ...
    Yeah, yeah, don't laugh! I haven't tried 135's; they're not readily available. But I have 127's on my Strida, 140's on the folder I rode today, 145's on my recumbent, 148's on my Downtube Mini, and so on. 127's feel very short. 140's are fine. I am pretty comfortable on all of them, but think best would be somewhere in the range 150-160 (see below). I too can tell the difference between 165 and 170, but anything over 170 feels pretty much the same to me: too long!

    For reference, I'm 6' tall.

    By the way, I have read the article, but am not sure I followed Sheldon's reasoning, so I am not going to pretend I am representing his view here.

    If you take pedal speed into account when calculating gearing, then crank arm length makes a big difference. A longer crank arm means a larger pedaling circle, so at a given gear ratio and a given speed, the rider's foot must be going faster if the crank arm is longer. With me so far?

    But if that is obvious (and I think it is), it is not necessarily obvious that pedal speed has any relevance to anything. The muscles pushing the pedal are in the thigh, not the foot, so pedal speed may be completely irrelevant. With the longer crank arm, the thigh has to move a little farther, so a little faster; but whatever movement it makes, it makes that movement at the same frequency regardless of crank arm length.

    It is also obvious that longer crank arms give better leverage; but this is only relevant at low cadences. If you have a wide range of gears to chose from, you can maintain a high cadence, so you don't need that extra leverage at all.

    Short crank arms make it possible to spin at higher cadences, while long crank arms make it possible to mash at low cadences. While it is possible to mash a short crank arm, it is painful (and presumably bad for your knees); and while it is possible to spin long crank arms, this is initially tiring and painful in the long term.

    My impression is that the chief disadvantage of a longer crank arm is the unnecessarily wide range of movement of the thigh and both hip and knee joints (which may in turn be bad for those joints). When I ride long crank arms, my butt gets very sore. You may argue that this is only because I'm not accustomed to the longer arms; and you may be right. I don't know.

    My current thinking is that crank arm length is an easy way to find a balance between aerobic and anaerobic effort. If, after a long ride (100 miles, for example) your legs feel like jelly but your chest feels fine, then your crank arms are too long and your cadence to low. But if, at that point, your legs feel fine and your chest hurts, then your crank arms are too short and your cadence is too high.

    A couple Saturdays ago I rode 140 miles on my touring bike, with 152 mm crank arms. I was really tired by the end of that ride, but I must admit my chest felt worse than my legs. So maybe I'd be better off with something a little longer....

    Quote Originally Posted by itsajustme View Post
    ...
    Now I know 135mm cranks is an absurd exaggeration...
    No it's not!
    Quote Originally Posted by itsajustme View Post
    ... but the real question is can shorter cranks be a suitable substitute for larger chainrings?
    Not exactly!
    Quote Originally Posted by itsajustme View Post
    . if one wanted to reduce pedal strike and moderately increase the gear, would a sensibly shorter crank kill both birds with one stone?
    It will definitely reduce pedal strike and toe strike. It will not increase the gear, but it will enable you to go a little faster. On my Strida, where the speed tops out somewhere around 16 mph and my feet are spinning insanely fast, my super short crank arms (127) help; I can spin them a little faster, and I can therefore go a little faster. But while I do get a rather neglibible increase in top speed, it comes at a significant decrease in accelleration, especially from a stop. On the Strida it has the additional benefit of increasing knee clearance; for a person of my size there's a constant danger of hitting the knee on the handlebar, which can cause erratic steering!
    Last edited by rhm; 05-08-09 at 12:30 PM.

  6. #6
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    I wonder if SB's gain ratios is one of those things that is based on sound physics but in the real world is only a fraction of a difference? Kind of like the difference between spinning at say 80rpm vs 85 rpm on a given gear. You also have to factor in the physiological differences to really make gain ratios work. Things like femur to tibia ratio to overall height.

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  7. #7
    Bicycle Repair Man !!! Sixty Fiver's Avatar
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    If the chain ring on my folder is turning over at 90 rpm and is in the middle gear I will be moving at 26 kmh and in 3rd gear I will be travelling at 35 kmh.

    The crank length only affects how that power is delivered and maintained and does not affect the gearing ratio...the crank and pedal revolution remains constant no matter their length.

  8. #8
    rhm
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    ^
    Of course it doesn't affect the gearing ratio; that's a function of wheel size, cog size, chain ring size, and internal gearing (if any). But if crank arm length affects how power is delivered, it likewise affect how much power is, or can be, delivered; and of course it has biomechanical implications on wear-and-tear on the knees &c. But we quibble.

    By the way, if you want to give short crank arms a fair trial, a three speed is not the best way to do it. Short crank arms work much better if you have a lot of gears to chose from, so you can maintain your 90 rpm cadence at almost any speed.

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    Quote Originally Posted by rhm View Post
    On my Strida, where the speed tops out somewhere around 16 mph and my feet are spinning insanely fast, my super short crank arms (127) help; I can spin them a little faster, and I can therefore go a little faster. But while I do get a rather neglibible increase in top speed, it comes at a significant decrease in accelleration, especially from a stop.
    Is the pedaling rhythm perhaps confusing the issue here? If we forget about cadence for a moment then if you are going faster for a given amount of foot travel (in inches, not revolutions) that means you are pushing harder; it's just that more revolutions are going by as it happens.

    Is it possible that it's just difficult to think about it as a "higher gear" in the face of so many revolutions? Because increased top speed and hampered acceleration sure sounds like a higher gear to me! What would your top speed and acceleration be like with an equivalent "gain ratio" created with sprockets and normal cranks?

    Quote Originally Posted by wahoonc View Post
    I wonder if SB's gain ratios is one of those things that is based on sound physics but in the real world is only a fraction of a difference? Kind of like the difference between spinning at say 80rpm vs 85 rpm on a given gear. You also have to factor in the physiological differences to really make gain ratios work. Things like femur to tibia ratio to overall height.
    Well, "gain ratio" compares the distance traveled by the foot/pedal to the distance traveled by the bicycle.

    However, I'm thinking the mistake is that it doesn't account for the fact that the deadspot of the pedal stroke is wastes part of that foot travel and, thus, must be subtracted. So perhaps shorter cranks do increase the effective gearing, but they do so in a highly nonlinear fashion which is different from changing sprocket ratios in two important ways:
    1. Calculation is only approximate because the deadspot of the pedal stroke is difficult to estimate and cannot be computed with only simple mathematical operations.
    2. The relative size of the approximation error compared to the "gain" is always large because, unlike sprocket ratios, large changes in crank length quickly produce untenable deadspots.

    In other words perhaps slightly shorter cranks increase gearing and slightly longer cranks decrease gearing, but to maintain efficiency the change in effective gearing has to remain small and even then no one can say exactly how much it is.

    I think rhm's experiences bear this out...they're just all tangled up with biomechanics and the cyclical point of view, but maybe he can comment.
    Last edited by itsajustme; 05-08-09 at 01:23 PM.

  10. #10
    rhm
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    Quote Originally Posted by itsajustme View Post
    ... increased top speed and hampered acceleration sure sounds like a higher gear to me! ....
    Forgive me for just responding to small portions of your post! But no: Shorter crank arms make it easier for your legs to do one thing, while longer crank arms make it easier for your legs to do something else.

    The effect may sound like higher gearing or even feel like higher gearing, but they are not the same thing. So when you talk of "effective gearing" I know what you mean, but don't like the term.

    More later! It's time for me to shut this PC down, hop on my folder with 140 mm crank arms, and forget about this discussion until Monday.

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    Quote Originally Posted by rhm View Post
    Forgive me for just responding to small portions of your post! But no: Shorter crank arms make it easier for your legs to do one thing, while longer crank arms make it easier for your legs to do something else.

    The effect may sound like higher gearing or even feel like higher gearing, but they are not the same thing. So when you talk of "effective gearing" I know what you mean, but don't like the term.
    Ok, leverage then. It's not gearing but since the purpose of gearing is to influence leverage it accomplishes the same purpose.

    But my question to you is when does it feel like higher gearing? That is, if you can even tell; I know that fatter tires change the gearing too, but I'd be hard pressed to say I can feel it and, as I conjectured above, I'd imagine the pure leverage ratio effect of crank length would only hold true under small variations.

    Quote Originally Posted by rhm View Post
    More later! It's time for me to shut this PC down, hop on my folder with 140 mm crank arms, and forget about this discussion until Monday.
    Ha, ha. See you when you come back to work!
    Last edited by itsajustme; 05-08-09 at 01:51 PM.

  12. #12
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    Quote Originally Posted by rhm View Post
    My impression is that the chief disadvantage of a longer crank arm is the unnecessarily wide range of movement of the thigh and both hip and knee joints (which may in turn be bad for those joints). When I ride long crank arms, my butt gets very sore. You may argue that this is only because I'm not accustomed to the longer arms; and you may be right.
    +1

    Quote Originally Posted by Sixty Fiver
    I never subscribed to gain ratios
    A longer crank allows you to apply more torque. If you disagree with this, then find a mechanic and ask him to demonstrate the use of a breaker bar. If you still disagree with this, take it up with Issac Newton.

    The reality of gain ratios are moot for most people since it is not something you can specify unless you're doing a custom build. Even if you are able to, you should be specifying crank length based on what's the most comfortable for the length of your legs. Whatever gains or losses from different crank length can be compensated for by gearing selection.

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    Quote Originally Posted by sqynt View Post
    Whatever gains or losses from different crank length can be compensated for by gearing selection.
    ...unless you're already pushing the limits of gearing technology (which is why this is in the folder forum).

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    Quote Originally Posted by sqynt View Post
    Even if you are able to, you should be specifying crank length based on what's the most comfortable for the length of your legs. Whatever gains or losses from different crank length can be compensated for by gearing selection.
    Ideally this would always be the case, and in most cases it is since bikes have a pretty wide range of gears available. But two cases come to mind where there are limitations to the gearing and in those cases it can be beneficial to adjust the crank length to compensate.

    The first has already been mentioned above: a small-wheeled bike may not have sufficiently high gears available to reach a reasonable top speed at a comfortable cadence. In that case reducing the crank length can allow the rider to comfortably pedal at a higher cadence than he/she could otherwise and thus partially overcome the limitation of a low top gear. OTOH, many small-wheeled bikes manage to have pretty standard gear ratios through other means. E.g. my Bike Friday uses a 60t chainring and 11t small cog to get about the same gearing as my Cannondale gets with 52t in front and 13t in back.

    The second is in the case of a traditional tandem where both partners must pedal at the same cadence. Usually that works ok, but sometimes one member of the pair prefers to spin in a lower gear while the other one has a masher-style, preferring a higher gear at a low cadence. Giving this second person a shorter crank will make it easier for them to spin at a higher cadence and give them the added resistance force they're used to feeling. Then they can both comfortably pedal at the same cadence despite their different preferred styles. This assumes you want the two cyclists to remain synchronized since otherwise the timing-chain rings can be made different sizes to allow for a different cadence for the front vs. rear rider. I did do this when I had the tandem set up for my daughter as a 'kid-back'. Her cranks were way up high so pedal strike wasn't an issue and her weight was small enough that having her pedaling out-of-synch with mine was also fine. Wouldn't recommend that arrangement for two adult cyclists though unless the bike is setup for independent coasting and shifting.

    Most people are pretty adaptable to a range of crank lengths as long as it doesn't get too far away from their optimum. On my bikes I have 165 mm cranks on the tandem, 170 mm on the Friday, and 175 mm on the Cannondale. They all feel comfortable, but I do notice that I tend to spin faster on the tandem and use a bit higher gearing at lower cadence on the Cannondale.

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    rhm
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    Quote Originally Posted by itsajustme View Post
    ...
    But my question to you is when does it feel like higher gearing? That is, if you can even tell; I know that fatter tires change the gearing too, but I'd be hard pressed to say I can feel it and, as I conjectured above, I'd imagine the pure leverage ratio effect of crank length would only hold true under small variations.
    I notice it the most when I'm on my Strida, starting from a stop. There are several reasons for this:

    --it's a single speed, and the only single speed bike I ride; so I'm used to starting in lower gears.

    --the crank arms are unusually short, even for me: 5" or 127 mm (I installed them; OEM was 170).

    --the Strida's design makes it hard to press really hard on the crank arms anyway; I can't ride it while standing up.

    It does not help that Strida's design makes it especially unstable at its slowest speeds. Having unusually poor leverage is especially evident when first pushing off from a stop, when just taking one foot off the ground.

    Once I'm moving and reasonably stable, acceleration to normal speed is also somewhat compromised. I feel it in the knees. Once I'm really going, at or near top speed, the pain goes away.

    Am I really faster now (with short cranks), than I was before (with normal ones)? I don't have any data, but my impression is: YES and NO. YES, because the super-short crank arms enhance my ability to sustain a high cadence, but NO because they compromise my ability to achieve that high cadence.

    On my other bikes, where I have an adequate range of gears to chose from, this issue does not come up.

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    Quote Originally Posted by itsajustme View Post
    ............
    Now I know 135mm cranks is an absurd exaggeration, but the real question is can shorter cranks be a suitable substitute for larger chainrings? For example, if one wanted to reduce pedal strike and moderately increase the gear, would a sensibly shorter crank kill both birds with one stone?

    Will 140 mm. do?

    Here's a M3L with 140's I fitted recently.

    I rode a similar Brompton earlier in the morning with the standard 170's, then this one later, & I felt a considerable difference.

    It seems there's a "changeover point" for me at 140, as my 150, & 155's don't feel as different when going from 170.

    My shortish test ride was about 2 mph faster than the bike with 170's, & my cadence was much higher.

    I imagine, but don't know, that my power output was also better, & the area above my knees (don't know the anatomical details) felt like it was working harder.

    I'm keen to see what happens after I've ridden for say 50 miles, & the longer term effects.

    I don't think I'll experiment with smaller sizes (eg. 127 mm. as rhm has tried) unless I drop on some cheap cranks, but I may try 145 mm's next.

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    Senior Member bhkyte's Avatar
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    My memory of this is that shorter cranks are harder to push, and due to the reduced distance reguired to make a reverlution act like a harder gear.But they will still spin out at maxium rpm more or less the same. So its not baloany as reguards effort, but it does not result in a difference to gearing in inches. So not a substute for a larger chainring ,but some powerfull riders find shorter cranks better for their sytle.

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