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  1. #1
    bt
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    Do longer cranks arms make for a taller gear?

    Or shorter or no effect on gearing?

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    They make for "different."

    You get more leverage. So that would imply a shorter gear.

    Most people pedal a slower cadence, though.

    Yet, they have to have higher pedal speed - basically, your foot travels further in one rotation than it would with a shorter crank arm.

    The range of acceptable sizes for cranks is really, really big. There was a study a while ago that found that 120-220 were all fine. Crazy!

    There was another study that found that while they couldn't find strong correlations between crank length and sustained power output, acceleration was better with one size over the other.

    My takeaway from all of this has been that it matters a whole lot less than it seems like it should. Certainly differences like 5mm are not particularly significant given the range that seem to work fine. Also, that if you're going to base your decision on anything, it should be your pedaling style. Which way to go in either case is a little hard to figure out though.

    Suppose I have a high cadence. (I do.) In terms of rpm, I'll have an even higher cadence on smaller cranks. High cadence tends to correlate with athletes who are better over distance than in a sprint. So maybe I should have longer crank arms, so that I exert less force, and emphasize my aerobic abilities more. Analysis paralysis! Lucky for me, I'm a cheapskate, I think my 170s work fine, and my knees are a little flaky, so going longer is out; going shorter requires a much less common product.

    Suppose someone has a low cadence. Should he go with it and get longer crank arms, or should he suspect it correlates with a preference for exerting a lot of force at a lower pedal speed? Oh, noez!

    Whatever it means in terms of physiological demands, though, lower cadence means pushing a higher gear for the same speed. So while the effect on your efficiency is debatable, you might find you use a little different part of the cassette when you change arm length.
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  3. #3
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    Well, put Andrew. I ride 175mm cranks on MTB and road, despite me being under 5''7" with a 29 inch inseam. I'm totally used to it and it feels good to me. YMMV...
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    as stated above, mostly. Gearing does not change, Longer cranks allow you to create more leverage, and, if you are able to continually provide that leverage, you will go faster. Will you notice the difference? In time, your knees will be the first to notice!

  5. #5
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    Longer = More levarage
    Shorter = Knee saver
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    Quote Originally Posted by Ace5high View Post
    ...
    Shorter = Knee saver
    Or not.

    My knees suffer when I use shorter (under 180) cranks, and I am not overly tall.
    I need the greater range of motion in my joints.
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    And for me, pedal strikes (actually, avoiding strikes) trumps the leverage v. cadence reasoning. I have enough of 'em on 175s with my style and terrain, so longer is out. At 6 and a half bucks plus, I don't want to go shorter.
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  8. #8
    craigsj
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    Quote Originally Posted by dumper View Post
    as stated above, mostly. Gearing does not change, Longer cranks allow you to create more leverage, and, if you are able to continually provide that leverage, you will go faster. Will you notice the difference? In time, your knees will be the first to notice!
    This is not true.

    Leverage and gearing are the same thing. That's what gearing does, it allows you to choose your leverage in exchange for speed. There is no free or extra leverage that enables you to go faster. You go faster when you produce more power.

    AndrwSwitch is right. A longer crank has more "leverage" on the crank spindle but it requires a slower cadence for the same foot speed. A slower cadence means a slower speed for the same combination of gears, so you get more "leverage" but slower speed. Just like a lower gear. Crank length is part of gearing.

    The overall gearing of the bicycle is how fast it travels for a given amount of foot speed. Sheldon Brown called this gain so as to avoid confusion with common understanding of gearing. A longer crank lowers the gain without changing the gearing ratio.

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    Quote Originally Posted by craigsj View Post
    This is not true.

    Leverage and gearing are the same thing. That's what gearing does, it allows you to choose your leverage in exchange for speed. There is no free or extra leverage that enables you to go faster. You go faster when you produce more power.

    AndrwSwitch is right. A longer crank has more "leverage" on the crank spindle but it requires a slower cadence for the same foot speed. A slower cadence means a slower speed for the same combination of gears, so you get more "leverage" but slower speed. Just like a lower gear. Crank length is part of gearing.

    The overall gearing of the bicycle is how fast it travels for a given amount of foot speed. Sheldon Brown called this gain so as to avoid confusion with common understanding of gearing. A longer crank lowers the gain without changing the gearing ratio.
    So let me see if I get this right:

    Cliffs--for a given crank speed, foot speed with a 180mm crank is greater than foot speed with a 175mm crank so in effect, the 180mm provides lower gearing. If foot speed for a 180 matches foot speed of a 175, the result is a slower crank speed and slower forward speed of the bike.
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    Quote Originally Posted by bt View Post
    Or shorter or no effect on gearing?
    The make the gear feel a little bit lower...


    For example stand and hammer up a hill...you have a bigger lever arm with the longer crank.

  11. #11
    craigsj
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    Quote Originally Posted by Malibu412 View Post
    So let me see if I get this right:

    Cliffs--for a given crank speed, foot speed with a 180mm crank is greater than foot speed with a 175mm crank so in effect, the 180mm provides lower gearing. If foot speed for a 180 matches foot speed of a 175, the result is a slower crank speed and slower forward speed of the bike.
    Yes.

  12. #12
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    A concept that ties all this stuff together well is power. Cyclists throw the word around a lot, but I think it's worth getting a little technical.

    power = force * distance / time = force * speed = torque * angular velocity

    Speed's maybe not a great word to apply to the crank because it's the same word being applied to my foot. My foot translates. It happens to translate in a circle around the crank because the motion is restricted, but it's still translating. The crank rotates.

    Basically, the tradeoff is that for most riders, a longer crank arm means more torque and less angular velocity. The loss in angular velocity is because they're maintaining, or even slightly increasing, the same foot speed, but not as much as the longer crank arm increases the circumference about which the rider's foot would need to travel to maintain the same angular velocity.

    Ultimately what it comes down to is which motion is most efficient for a given rider. The studies found that people can adapt, at least within the constraints of the study, to a really huge range of sizes. I think they didn't adequately address people with mobility problems - to my mind, a reduction in necessary force to get the same power is great for some kinds of knee problems and a reduction in range of motion is great for others. For someone with healthy knees, there's some idea that there might be a useful way to match crank length to pedaling style, but it's hard to figure out what that would actually mean in practice because of the foot speed vs. cadence thing. (As in, once you start messing with crank length, they're not the same.)
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    Quote Originally Posted by AndrwSwitch View Post
    A concept that ties all this stuff together well is power. Cyclists throw the word around a lot, but I think it's worth getting a little technical.

    power = force * distance / time = force * speed = torque * angular velocity

    Speed's maybe not a great word to apply to the crank because it's the same word being applied to my foot. My foot translates. It happens to translate in a circle around the crank because the motion is restricted, but it's still translating. The crank rotates.

    Basically, the tradeoff is that for most riders, a longer crank arm means more torque and less angular velocity. The loss in angular velocity is because they're maintaining, or even slightly increasing, the same foot speed, but not as much as the longer crank arm increases the circumference about which the rider's foot would need to travel to maintain the same angular velocity.

    Ultimately what it comes down to is which motion is most efficient for a given rider. The studies found that people can adapt, at least within the constraints of the study, to a really huge range of sizes. I think they didn't adequately address people with mobility problems - to my mind, a reduction in necessary force to get the same power is great for some kinds of knee problems and a reduction in range of motion is great for others. For someone with healthy knees, there's some idea that there might be a useful way to match crank length to pedaling style, but it's hard to figure out what that would actually mean in practice because of the foot speed vs. cadence thing. (As in, once you start messing with crank length, they're not the same.)
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  14. #14
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    Lemme try this again. You develop a certain amount of power with your legs. If your drivetrain were perfect, you'd transmit exactly the same power through your chain.

    Changing crank arm length changes a bunch of things, but for most riders it's going to be kind of a wash. If you happen to be more efficient pushing the crank harder and moving your foot slower, or moving your foot faster with lower force, or your metabolism prefers a certain cadence, there'll be some advantage to a crank that facilitates that.

    It can also free you to choose a crank that facilitates something else. Better pedal clearance or more leverage for lofting your front wheel, for example.
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  15. #15
    craigsj
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    Quote Originally Posted by AndrwSwitch View Post
    If you happen to be more efficient pushing the crank harder and moving your foot slower, or moving your foot faster with lower force, or your metabolism prefers a certain cadence, there'll be some advantage to a crank that facilitates that.
    While that is true, there is no evidence that people have these unique distinctions nor is there any reason to believe so biomechanically. There is plenty of research regarding peak power, biomechanical efficiency, and even pedal stroke. There is nothing that suggests that crank length variation helps anyone significantly unless they have some kind of disability of fall outside the 90th size percentile that the reseach covers.

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    Quote Originally Posted by Malibu412 View Post
    So let me see if I get this right:

    Cliffs--for a given crank speed, foot speed with a 180mm crank is greater than foot speed with a 175mm crank so in effect, the 180mm provides lower gearing. If foot speed for a 180 matches foot speed of a 175, the result is a slower crank speed and slower forward speed of the bike.
    This might be true but doesn't "change the gearing" as it was stated. Leverage in not the same gear ratio.
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  17. #17
    craigsj
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    Quote Originally Posted by Ace5high View Post
    This might be true but doesn't "change the gearing" as it was stated. Leverage in not the same gear ratio.
    The "gearing" that doesn't change with crank length doesn't matter. It's the overall effect that matters.

  18. #18
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    Quote Originally Posted by craigsj View Post
    It's the overall effect that matters.
    We can agree on this ^^^

    I tend to see cranks and drivetrain gearing as two separate items. Since one revolution will always be one revolution at the crank I view the chainings back as the "gearing".

    Anything that happened before or "in front" of this to me has more to do with rider comfort and correct fit in terms of stride and inseam measurement.

    My reason behind this is because its just not enough to say longer cranks provide an easier (shorter gear ratio) because this would only be dependent on the rider. Is a 240mm crank arm really an "easier" pedal stroke than a 165mm one? It is more leverage but is it easier to spin continuously? Maybe only if your a giant
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    Quote Originally Posted by craigsj View Post
    While that is true, there is no evidence that people have these unique distinctions nor is there any reason to believe so biomechanically. There is plenty of research regarding peak power, biomechanical efficiency, and even pedal stroke. There is nothing that suggests that crank length variation helps anyone significantly unless they have some kind of disability of fall outside the 90th size percentile that the reseach covers.
    I was too lazy to go beyond reading the studies referenced in the article on Pez Cycling.

    Crank Length Does Size Really Matter?

    I did read them, though. IIRC, one of them found occasionally better power output when people rode with their usual crank arm length, although it was marginal. None of them compared performance for people who spent an entire offseason training on different arm lengths.

    We've all seen that different cyclists, even different strong cyclists, have different pedaling styles. I tend to spin pretty high - around 105 last I checked. I had a teammate who always looked a little push-push to me, and had a lower spin. But he was a lot faster than I was, probably still is.

    My takeaway from all this has been that arm length matters a whole lot less than it seems like it should, and there can be some combinations of pedaling style, degree of mobility (incidentally something that applies to me) etc.

    I have to admit I was too lazy to track down the study that found that 90 rpm is an optimal cadence for reasons to do with muscle fatigue, and that that rate correlates to proportion of fast-twitch vs. slow-twitch muscle, muscle vascularization, and capacity to clear fatigue products. That's something else I've run into, though, and people are incredibly variable about those proportions.

    I think we all know someone who took to cycling like they were born to do it, and other people who appear to be quite dedicated and consistent about training, but can never seem to get out of their plateau. The evidence that people are different from each other is all around us.

    The results certainly suggest that differences are pretty marginal. But their marginal and the marginal that throwing money at other parts of a bike yield are different "marginals." Like 4% vs. half a percent. I think it still makes more sense to spend some money on "the right" crank than it does on a fancy stem. Given the standard deviation in most people's performance, really, neither makes sense. But we all do it anyway, and if you have to buy a stem or a crank anyway, why not get the one with a possible, if slight and unprovable, advantage over the one with no advantage.
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  20. #20
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    Quote Originally Posted by Ace5high View Post
    This might be true but doesn't "change the gearing" as it was stated. Leverage in not the same gear ratio.
    Sure it is. Gearing is just another way of adding leverage.

    Imagine a seesaw. You are sitting in one end and I have to hoist you up. The longer my side of the seesaw is, the easier it is for me to hoist you. I can keep adding length to my end to make the task easier. Another way to get the job done is to saw off my end of the seesaw, mount a big cog on the axle of the seesaw and a smaller cog with a handle which I can use to hoist you. The smaller the cog, the easier the task, but the tradeoff is that I have to keep winding for longer time.

    Basically it doesn't matter if I lift you by pushing down my end of the seesaw or wind you upwards spinning a handle on a cog. I can add more length to my side of the seesaw or mess around with the cog sizes, it's all the same..

    Back on the bike, you can add longer pedal arms to get more leverage for climbing up a hill easier but slower. Or you can change the front cog to one slightly smaller to get more gearing for climbing up a hill easier but slower. Same, same..

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    why would shorter cranks be a knee saver? i would think less effort required due to more leverage of longer cranks would save my knees. this has always confused me.

  22. #22
    bt
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    Man I'm sorry I can't absorb all the above but appreciate ya'lls efforts!

    LET me ask it a different way. If I replace my 175mm cranks with 180mm cranks on my singlespped am I making it a taller geared bike?

  23. #23
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    Quote Originally Posted by bt View Post
    Man I'm sorry I can't absorb all the above but appreciate ya'lls efforts!

    LET me ask it a different way. If I replace my 175mm cranks with 180mm cranks on my singlespped am I making it a taller geared bike?
    No you are making it shorter geared. You will have a slightly easier time climbing but your maximum effort speeds may be slightly less. Odds are you will find the difference difficult to tell.

  24. #24
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    Quote Originally Posted by AndrwSwitch View Post
    I was too lazy to go beyond reading the studies referenced in the article on Pez Cycling.
    Pez referenced the important article by Martin. Martin has other articles, too, that discuss submaximal efforts, efficiency, and pedal stroke effects. He also done a study on standing efforts and crank length. The results there are quite interesting.

    The key thing with Martin is that he let his test subjects self-select cadences and pedaling styles that suited them. There is variation among riders with regard to pedal style and cadence, that is definitely true. What's not the case, though, is that crank length has some preferential effect based on those preferences. It doesn't matter if a rider is a spinner or masher, crank length effects him the same way, which is not much at all as you know.

    His results on pedaling style are even more interesting. We all know the common wisdom of how a proper pedal stroke is important and how riders work so hard to train themselves to have it, yet Martin has shown that it makes no difference. He's proven that riders naturally and quickly learn the most efficient way to pedal and that other techniques doesn't improve it. To me it's entirely reasonable since our bodies are naturally adept at learning that sort of thing.

    In case anyone is wondering, Martin is an accomplished racer highly respected in tri circles. It shouldn't matter of course, but he a better cyclist than most any of us hope to be.

  25. #25
    bt
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    Quote Originally Posted by craigsj View Post
    No you are making it shorter geared. You will have a slightly easier time climbing but your maximum effort speeds may be slightly less. Odds are you will find the difference difficult to tell.
    yeah I get the not being able to tell the difference part.

    interesting though, I had assumed that longer crank arms meant taller gearing.

  26. #26
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    Quote Originally Posted by Sandrenseren View Post
    Sure it is. Gearing is just another way of adding leverage.

    Imagine a seesaw. You are sitting in one end and I have to hoist you up. The longer my side of the seesaw is, the easier it is for me to hoist you. I can keep adding length to my end to make the task easier. Another way to get the job done is to saw off my end of the seesaw, mount a big cog on the axle of the seesaw and a smaller cog with a handle which I can use to hoist you. The smaller the cog, the easier the task, but the tradeoff is that I have to keep winding for longer time.

    Basically it doesn't matter if I lift you by pushing down my end of the seesaw or wind you upwards spinning a handle on a cog. I can add more length to my side of the seesaw or mess around with the cog sizes, it's all the same..

    Back on the bike, you can add longer pedal arms to get more leverage for climbing up a hill easier but slower. Or you can change the front cog to one slightly smaller to get more gearing for climbing up a hill easier but slower. Same, same..
    Again, gearing and leverage are not the same thing. If drivetrain has a 1:1 gear ratio and you have 170mm crank arm how many rotations of the crank take to one full turn? 1. If your gear ratio is 1:1 and your crank arm is 200mm long how many rotations does it take to complete a turn? 1. Gear ratio is always the same, leverage is different.

    What your explaining about the seesaw is in fact a gear ratio change (regarding the mounted cogs)

    Quote Originally Posted by G-Choro View Post
    why would shorter cranks be a knee saver? i would think less effort required due to more leverage of longer cranks would save my knees. this has always confused me.
    Not so, this has to deal with biomechanical efficiency. The large majority of us find the shorter arms to be easier on knees, it doesn't mean there is no exception to that. Its the same reason guys with Knee issues in the Gym will not do full range squats they will shorten their range of motion which puts less stress on the knee joint.
    Last edited by Ace5high; 02-28-2012 at 04:15 PM.
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    I have stumpy legs, my 175's are wearing on me. Feels awkward to pedal real slow over technical stuff. Everything else be damned, im going back to 170s for feel alone.

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    Quote Originally Posted by craigsj View Post
    This is not true.


    The overall gearing of the bicycle is how fast it travels for a given amount of foot speed. Sheldon Brown called this gain so as to avoid confusion with common understanding of gearing. A longer crank lowers the gain without changing the gearing ratio.
    I think you just disagreed with what I said, then restated exactly what I said. In any case, I think we can almost all agree that the difference in crank length (on a bike!) would be difficult to notice.

  29. #29
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    Quote Originally Posted by dumper View Post
    I think you just disagreed with what I said, then restated exactly what I said. In any case, I think we can almost all agree that the difference in crank length (on a bike!) would be difficult to notice.
    I quoted what you said that was wrong and I most certainly did not restate it. You said "Longer cranks allow you to create more leverage, and, if you are able to continually provide that leverage, you will go faster." Longer cranks do not "allow you to create more leverage" nor will they make you faster as you have to make more power to go faster. There is no free power in a longer crank.

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    Quote Originally Posted by bt View Post
    yeah I get the not being able to tell the difference part.

    interesting though, I had assumed that longer crank arms meant taller gearing.
    Longer crankarms spread the force of pedalling a constant wattage out over a longer area. The effort per degree of the circle will drop, but you apply that lower force over a longer time now. This won't affect the gearing, only your perception of how easy/hard it is to turn based on whether you find spinning or cranking easier.
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    Once it gets technical, and you're not spinning anything near a consistent cadence, foot speed stops mattering. The foot speed difference between half a crank turn with a 175 and 170 to get over a root is completely irrelevant, but the 175 gives you more leverage by definition of the word, and gives you more power. If you can comfortably and effectively ride longer cranks (I cant), you're better off.

    Roadie cadence stuff translates extremely poorly to mountain biking.

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    Quote Originally Posted by Ace5high View Post
    Again, gearing and leverage are not the same thing. If drivetrain has a 1:1 gear ratio and you have 170mm crank arm how many rotations of the crank take to one full turn? 1. If your gear ratio is 1:1 and your crank arm is 200mm long how many rotations does it take to complete a turn? 1. Gear ratio is always the same, leverage is different.
    Lets say you're a single speeder running a fixed gear ratio. Your pedal arms are 180 mm. On your training route there is one particular tough hill and you can just make it up putting in your best effort, but if it was any steeper you would have to get off and walk up. Then for the fun of it, you modify your pedal arms to be only 50 mm. Do you still think you'll be able to get up that hill or did your overall gearing in fact change?

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    Don't confuse torque and power. They're related, but not the same thing.

    It wouldn't "cost" the singlespeeder any less energy than if he had 130mm crank arms and lower gearing, and if the 130mm crank arms meant he could sustain a higher cadence proportional to the change in gear ratio to get up that big hill, we're back at "it doesn't matter." I do suspect that there's a different enough way that we use cranks in technical situations for it to matter for some people, as suggested by OnePivot, but if you want to talk about something that's near-impossible to test and prove...

    While it's unproveable that it matters, I think if someone has a preference for a certain length, he should go with it. Just pick the gear ratio that gives the right cadence at a reasonable power output, and be happy. At worst, it's a completely unfounded preference, but he hasn't actually made anything worse either.
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    Quote Originally Posted by Sandrenseren View Post
    Lets say you're a single speeder running a fixed gear ratio. Your pedal arms are 180 mm. On your training route there is one particular tough hill and you can just make it up putting in your best effort, but if it was any steeper you would have to get off and walk up. Then for the fun of it, you modify your pedal arms to be only 50 mm. Do you still think you'll be able to get up that hill or did your overall gearing in fact change?
    No, and no.

    The gear ratio has not changed. One revolution of the cranks still moves the bike forward the same distance. The short crank arms have reduced the mechanical advantage to be able to turn said gearing, and I would not expect to be able to pedal up a steep grade.
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  35. #35
    craigsj
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    People are choosing arbitrary definitions for "gearing". Saying that crank length doesn't effect gearing is neither right nor wrong until there's an accepted definition of the word. Changing the wheel size doesn't change gearing by some definitions either.

    Changing crank length doesn't change the "gear ratio", that is the ratio of the front ring to rear cog or equivalent. It won't change the "gear inches" which is simply the "gear ratio" times the wheel diameter. It will change the "overall gearing" of the bike if you define gearing as everything that happens after the pedal.

    If gearing is everything after the crank or everything between the crank and the wheel, then crank length won't effect it, but so what?

    Leverage is the mechanical advantage a system provides when doing work on an object. "Gearing" is one means of implementing that leverage. The total leverage a rider has on accelerating the bicycle is effected by the crank length whether you want to call that part of the "gearing" or not.

    There are 4 wheels that make up a common bicycle drive, the drive wheel, the cog, the chainring, and the crank. These 4 collectively determine the leverage and leverage is all that matters. Argue endlessly which subset is "gearing" all you want, it doesn't change what's going on.

    Defining gearing to not include the crank wheel encourages people to believe they can get free power from a longer crank.

  36. #36
    bt
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    so would it be easier to get up that hill with longer or shorter crank arms?

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    Quote Originally Posted by bt View Post
    so would it be easier to get up that hill with longer or shorter crank arms?
    I guess its kinda like saying is it easier to get up the hill with a 29" or 26" wheel. Some would argue either side. I personally have never noticed any difference in power by changing a few mm length on a crank arm but I guess some might...?
    I do all my own stunts, but never intentionally...

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    Quote Originally Posted by bt View Post
    so would it be easier to get up that hill with longer or shorter crank arms?
    longer crank arms make it easier to get up hill...

    180mm/175mm=1.0287 time easier....

    That is about equal to the difference between a 34tooth sprocket compared to a 33 tooth sprocket on the rear.

    34/33=1.0303....

    Most people will notice that much difference.

  39. #39
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    I understand that perceived difference will be negligible, but for the sake of argument, which one would make it up the hill easier?
    Last edited by bt; 03-02-2012 at 06:08 PM.

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    Jeff, your argument is simplistic. The extra leverage only makes a difference for initial acceleration. The larger distance your legs must travel to make 1 rotation will counter the increased torque you get from the longer arm. Also, the personal traits of the rider have an effect too, making it a much more complex deal than just the application of a formula.
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    Quote Originally Posted by shiggy View Post
    The gear ratio has not changed. One revolution of the cranks still moves the bike forward the same distance. The short crank arms have reduced the mechanical advantage to be able to turn said gearing, and I would not expect to be able to pedal up a steep grade.
    So if I modify my bike and make it harder to get up a hill, I'm not allowed to call it a change in the overall gearing? I think Craigsj is spot on.

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    Quote Originally Posted by AndrwSwitch View Post
    Don't confuse torque and power. They're related, but not the same thing.
    Power is how fast you can apply the torque. If you look at the power and torque curves of an engine, power = torque * rpm provided that you translate the power to W, the torque to Nm and the rpm to radians/sec.

    As already indicated in this thread, longer crank arms allows you to make bigger torque but at the same time your feet has to travel longer and it probably lowers your rpm. From a pure power perspective the trick is to find the optimum compromise between how long your crank arms can be vs. how fast you can spin them.

    In the real world though, I'd say crank arm length is a matter of finding something that feels good to pedal and fix the workload afterwards by swapping cogs. I can't imagine short legs and long crank arms can be very nice to run, having to "hit you chin with your knees" on every round.

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    Quote Originally Posted by Sandrenseren View Post
    So if I modify my bike and make it harder to get up a hill, I'm not allowed to call it a change in the overall gearing? I think Craigsj is spot on.
    You can call it a flying pig if you like but it doesnt make it true

    Quote Originally Posted by Sandrenseren View Post
    Power is how fast you can apply the torque. If you look at the power and torque curves of an engine, power = torque * rpm provided that you translate the power to W, the torque to Nm and the rpm to radians/sec.

    As already indicated in this thread, longer crank arms allows you to make bigger torque but at the same time your feet has to travel longer and it probably lowers your rpm. From a pure power perspective the trick is to find the optimum compromise between how long your crank arms can be vs. how fast you can spin them.

    In the real world though, I'd say crank arm length is a matter of finding something that feels good to pedal and fix the workload afterwords by swapping cogs. I can't imagine short legs and long crank arms can be very nice to run, having to "hit you chin with your knees" on every round.
    Whats getting kinda ridiculous about this thread is not "does it effect gearing". I think the people arguing that it does are looking for some sort of cheat to create free power and want someone to tell them this is an easy way to get it. The real point here is that anyone who would put on some extra long crank arms just because they want to climb better is silly and most liking doing themselves an injustice in the process. Match your crank arm length to your inseam and what is comfortable on your knee's and if you need to climb better buy some lighter wheels for cryin out loud
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    Last edited by asphaltdude; 03-02-2012 at 06:21 AM.
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    Seems to me that part of the problem is that people are trying to stretch the term "gear ratio."

    In a mechanics context, gear ratio has nothing to do with how the power got into the shaft driving the first gear, or even what's happening when it comes out of the last gear. It's just a ratio of the sizes of the gears. Hence "gear ratio." One of the things that's cool about it is that you don't have to measure anything. All the teeth in a gear system, or at least all the teeth that are in ratios with each other, are the same size. Just count 'em, and you've got an answer.

    The lever comparison is of limited use because a person moves a lever through a limited range, usually once. A cyclist has to spin the cranks for an extended period. Actually, I think there may be something to the idea of using longer cranks on a mountain bike because it would mean that a mountain biker could push a little higher gear and loft the front wheel with a smaller amount of rotation at the crank. So, maybe useful for technical situations and trails where you have to ratchet anyway. But there's no free lunch - in steady pedaling, that bigger circle has all the effects people have already been talking about.

    If you're going to insist on thinking that arm length as part of gear ratio, think of what's driving it. That now has to move something with a larger diameter too. In a gear system, that would mean sacrificing torque again.
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  46. #46
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    Quote Originally Posted by AndrwSwitch View Post
    Seems to me that part of the problem is that people are trying to stretch the term "gear ratio."
    Yes, but at the same time "gear ratio" and "overall gearing" aren't the same.

    It's all just semantics. Crank length doesn't contribute to the "gear ratio" but it does change the overall leverage the rider has. Ultimately it's what happens end-to-end that matters and what people call the "gear ratio" is just a piece in the middle.

    I'm not convinced a longer crank is better in the situation you describe but it will be sometimes (but worse other times). It has been shown that longer cranks help when standing and there are some riders who care greatly, even primarily, about standing performance. For them I'd say longer makes sense, but at the same time I'd call them more trials riders than MTB riders. Just another example how it's important for everyone to speak the same language.

    We could look at the drivetrain as two gearing stages in sequence. The first is the ring/crank and the second is the wheel/cog. In a penny farthing there is only one stage, that being wheel/crank. It's easy to see in this approach that changing crank length does change "gearing" but only if you accept that the crank and wheel can be called gears. They are gears, they just have different kinds of teeth. Again, it's only the language we choose.

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    Sheldon Brown's page has a good discussion of this:
    - Sheldon Brown's Bicycle Glossary G

    The distinction is made between "gear ratio" and "gain ratio". The first is just the simple numerical ratio with tire size added in, the second includes the effect of crank length and tire size.

    I do find a difference between a 170 and 175 crank on my touring and road bike. The touring bike is 170 with a 24 front and 28 and 34 rear cogs and the road is 24 front and 28 rear cog w/ 175 cranks. The 24-28 on the road bike is somewhere in between the 24-28 and 24-34 on the touring bike in terms of pedaling effort up a similar grade. It is only a 3% crank length difference, but the difference seems to be more than that. Some of that is no doubt differences in the bikes. I guess the rear test would be to swap different length crank arms on the same bike and test.

    So yes, I find that a longer crank makes it easier to turn a given gear, at least uphill.
    Last edited by 4Crawler; 03-02-2012 at 01:23 PM.

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    Semantics has some value here.

    If you don't degrade the specificity of the term "gear ratio," it's an easily calculated number that expresses how many revolutions happen at the driven gear for one revolution of the driving gear.

    Being restrictive about that means it's easy to figure out that a standard mountain bike drivetrain has gear ratios running from 4 at the top end to 0.65 at the low end, with a little stretching of both ratios possible with bigger chain rings, 36t cogs, etc. Flip the number over and you get the way the torque relates. Regardless, it's a unitless number. Which makes it a kickass tool. As soon as you multiply it by something with a unit (175 mm, the radius of your wheel, whatever) you've got something that's a lot harder to work with.

    Now if you want to mess with different crank arms and wheel sizes, great. Do it. If you've kept gear ratio a unitless number, it's easy to figure out what gearing you need to go with a 29" wheel instead of a 26" wheel, to maintain the same gear inches at your low end, high end, whatever you're trying to keep constant.

    If you're better with longer crank arms, that means you're actually developing more power. So you need a different gear ratio, you don't have a different gear ratio. A lot of aspects of bicycles, especially the less tangible or less easily quantifiable ones, have to do with making the person powering the bike more efficient. You wouldn't try to sell me on the idea that someone with a better-fitting bike and the same size crank arms has changed his gear ratios. But, he's going to be more efficient so he'll probably develop more power at the same effort. That's the real same-same.
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  49. #49
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    Quote Originally Posted by AlexRandall View Post
    Jeff, your argument is simplistic. The extra leverage only makes a difference for initial acceleration. The larger distance your legs must travel to make 1 rotation will counter the increased torque you get from the longer arm. Also, the personal traits of the rider have an effect too, making it a much more complex deal than just the application of a formula.
    Actually no it is bang on....

    The effect of longer cranks makes it feel like a lower gear by about 1 tooth on granny gear.

    By easier I mean less force needs to be applied to the pedal...simple.

    That force must act through a longer distance and hence the energy is equal...again quite simple.

  50. #50
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    Quote Originally Posted by AndrwSwitch View Post
    If you don't degrade the specificity of the term "gear ratio," it's an easily calculated number that expresses how many revolutions happen at the driven gear for one revolution of the driving gear.
    Right, but that doesn't do a rider any good. I don't degrade "gear ratio" though, I say the crank length effects overall gearing.

    Quote Originally Posted by AndrwSwitch View Post
    Being restrictive about that means it's easy to figure out that a standard mountain bike drivetrain has gear ratios running from 4 at the top end to 0.65 at the low end, with a little stretching of both ratios possible with bigger chain rings, 36t cogs, etc. Flip the number over and you get the way the torque relates. Regardless, it's a unitless number. Which makes it a kickass tool.
    So is gain a unitless number and it includes the crank length. The only term that includes units is "gear inches" and the equivalent and that term possesses units because it neglects the effect of crank length.

    Quote Originally Posted by AndrwSwitch View Post
    As soon as you multiply it by something with a unit (175 mm, the radius of your wheel, whatever) you've got something that's a lot harder to work with.
    Only when you do it wrong.

    Quote Originally Posted by AndrwSwitch View Post
    Now if you want to mess with different crank arms and wheel sizes, great. Do it. If you've kept gear ratio a unitless number, it's easy to figure out what gearing you need to go with a 29" wheel instead of a 26" wheel, to maintain the same gear inches at your low end, high end, whatever you're trying to keep constant.
    Oh ouch! I guess terms with units aren't that hard to work with! You just proved that gear ratios aren't as valuable as gear inches.

    Quote Originally Posted by AndrwSwitch View Post
    If you're better with longer crank arms, that means you're actually developing more power.
    Research has shown that doesn't happen.

    Quote Originally Posted by AndrwSwitch View Post
    A lot of aspects of bicycles, especially the less tangible or less easily quantifiable ones, have to do with making the person powering the bike more efficient.
    Research has shown that crank length doesn't impact rider efficiency.

    Quote Originally Posted by AndrwSwitch View Post
    You wouldn't try to sell me on the idea that someone with a better-fitting bike and the same size crank arms has changed his gear ratios. But, he's going to be more efficient so he'll probably develop more power at the same effort. That's the real same-same.
    I have never tried to "sell" the idea that anything changes the "gear ratios" other than the gears. There is more to gearing, however, than the chainring and the cog. I'm happy to accept that the "gear ratio" is = (chainring / cog) but I'm not that crank length doesn't effect the total gearing of the bike.

    The gain of the bike is the units of distance a bike travels for one unit of pedal distance. That's the number you need to keep the same, not gear inches.

  51. #51
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    In answer to the OP's question does a longer crank seem like a slightly smaller gear, probably yes, it requires less Force to generate torque to achieve than a shorter one. so it will appear to the rider that he is supplying less force even though the gear ratio remains the same - I am not convinced by JeffScotts quantification - slight changes in the moment of inertia and angular momentum (due to differing geometries, angular velocities and mass of cranks) etc could possibly reduce the observed benefit - it would involve running a lot of calcs and probably empirical data from physical tests
    But this is hypothetical BS, any kind of consideration of optimum crank length needs to consider the engine achieving maximum power in watts generated and then transferring this as efficiently as possible to the drive train . I have never seen any study results from this but you cannot get away from cadence here, regardless of MTB, tour, road racing etc. The legs generate power (Watts) to apply a force Netwons per metre which is converted into torque and angular velocity by the pedals and crankset. We all know that a 6' guy with 30mm cranks is going to require a huge force to generate a huge torque at ridiculous slow angular velocity / cadence. The same six foot guy with 400m cranks will be generating a miniscule torque at an impossibly fast angular velocity / cadence.
    In both cases there will be a low power generated by the legs. It stands to reason that there will be an 'ideal' Force / cadence torque / angular velocity somewhere in the middle for the rider where he is generating maximum power in Watts from his legs- this will be the optimum crank length. Even though theoretically longer cranks may seem like a good idea, if it reduces power output in watts by a greater amount - the hill will seem harder to pedal up. I have no empirical evidence but I believe that optimum power generated by the legs would be a better gauge of how 'easy' it was to pedal up a hill than torque - This is a very similar argument to the endless horsepower vs torque arguments that petrolheads have been going through for years.
    This may make an interesting PHD thesis for somone

  52. #52
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    Quote Originally Posted by SimpleJon View Post
    ...any kind of consideration of optimum crank length needs to consider the engine achieving maximum power in watts generated and then transferring this as efficiently as possible to the drive train . I have never seen any study results from this...
    Then you didn't read the article linked to in post #19.

    Quote Originally Posted by SimpleJon View Post
    It stands to reason that there will be an 'ideal' Force / cadence torque / angular velocity somewhere in the middle for the rider where he is generating maximum power in Watts from his legs- this will be the optimum crank length.
    There is such a ideal. What doesn't stand to reason is the assumption that this ideal has a strong peak. It doesn't. It's basically flat for the bulk of riders over the entire range of common lengths.

    Quote Originally Posted by SimpleJon View Post
    Even though theoretically longer cranks may seem like a good idea, if it reduces power output in watts by a greater amount - the hill will seem harder to pedal up. I have no empirical evidence but I believe that optimum power generated by the legs would be a better gauge of how 'easy' it was to pedal up a hill than torque - This is a very similar argument to the endless horsepower vs torque arguments that petrolheads have been going through for years.
    This may make an interesting PHD thesis for somone
    If it wasn't answered conclusively a decade ago. You might find that article interesting.

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    Lets look at the original question again.

    Quote Originally Posted by bt View Post
    Do longer cranks arms make for a taller gear? Or shorter or no effect on gearing?
    One way to answer this is to say: "Neither the chain rings or the cogs change their number of teeth, so no, the gearing stays the same". However the original question doesn't make much sense if you look at it that way, I'm pretty sure the OP knows that changing the length of the crank arms doesn't change the number of teeth on the rings or cogs.

    What I read in the OP's question is: "If I change the length of my crank arms, what kind of difference will I feel?". The answer to that is: "Longer crank arms will make it easier to go up a hill, but at the same time you'll go slower - similar to choosing a lighter gear".

    If you compare the distance your feet travels to the distance the bike travels, changing to longer crank arms will force your feet to travel longer in order for the bike to cover the same distance as before, just as picking a lighter gear will do.

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