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  1. #1
    Alfredo Contador |3iker's Avatar
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    Crank length 170mm, 172.5mm, 175mm

    I am currently using 172.5 right now and am looking to replace the crankset. The differences is at max 2.5mm on the two ends of the spectrum. Does it really truly matter in real life?

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    About 1.5% worth either way. Spinners tend to be more sensitive to changes in the pedal circle than pushers, but you'll adapt pretty quickly. Possibly a more important consideration is bottom bracket height. If you have a fairly low BB, and do lots of hard cornering you may not want to lose more pedal clearance with a longer crank.

    All in all, though the difference is small, don't change if you don't have a reason to. Meaning if you can't decide whether to go longer or shorter, than what you have is probably just right.
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    coprolite fietsbob's Avatar
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    No, I interpret 'real life' as non performance/race high cadence riding,
    just riding the bike for pleasant travel, and transportation.

    I have a 170, 175 and a 180mm on different bikes , they are just a little different,

    but interchangeable..

    I got a feeling of a little , desired, setback by going to the 180,

    on a zero setback seatpost equipped bike
    Last edited by fietsbob; 11-16-10 at 10:13 AM.

  4. #4
    AEO
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    the taller you are, the less it matters.
    if you're of average height, about 180cm, then those should all work for you.
    Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
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    Old fart JohnDThompson's Avatar
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    On a road bike you're unlikely to notice the difference much. On a fixed gear bike, shorter arms give you more pedal clearance when cornering. If you have problems with pedal strike going around corners you might consider 165mm arms.

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    |3iker, Except in a case where there's some joint problem I think we're reasonably adaptable to crankarm length, in particular to adjacent sizes to what a cyclist prefers. I notice a difference between 172.5 mm and 175 mm if I ride back-to-back, but within a minute they feel the same. Cadence is unaffected, but might be if I tried 180 mm crankarms.

    Brad

    PS I prefer 172.5 mm crankarms.

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    Quirky Grifter LesterOfPuppets's Avatar
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    I like 170s on the flats and 175s on the steep hills. 172.5 is a good compromise.
    1980ish Free Spirit Sunbird fixed * 1996 Mongoose IBOC Zero-G * 1997 KHS Comp * 1990-ish Scapin * Lemond Buenos Aires Triple

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    Senior Member Monster Pete's Avatar
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    I've used cranks between 170mm and 140mm (from a kids' bike) and can't tell much difference, so 2.5mm either way shouldn't change much at all. The gearing changes for a given chainring though, so it's another factor to consider when designing a drivetrain. For example, the 38 tooth big ring on my 140mm crank is like having a 46-and-a-bit tooth ring on a 170mm crank. When you're playing with crank length it's worth working out the gears in gain ratios rather than gear inches.

  9. #9
    commuter TimeTravel_0's Avatar
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    anyone use peter white's 18.5% method? when I calculate my femur length, I get 164.9...so that would put me on 165mm cranks which seems a bit short.

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    Alfredo Contador |3iker's Avatar
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    I'm 5'8" with an inseam of 31". According to some of the articles found via Google, I should use 170mm. Cool. That should shave a few grams.

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    Senior Member DannoXYZ's Avatar
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    Personally, I think bike manufacturers size cranks on small bikes way too large and on big bikes, the cranks are too short.

    A 5mm difference (170 & 175) is about 3% and makes a difference in rear-wheel torque if you're climbing a technical off-road hill. Going the other way helps you spin faster. I get an extra 10rpms (220->230) on my track bike when I use 165mm cranks instead of 170mm. A lot of TDF racers have custom cranks made in 180mm+.

    However, unless your paycheque depends upon your finishing results, I wouldn't worry about it.

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    we be rollin' hybridbkrdr's Avatar
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    For what it's worth, I read a message from someone saying once that longer cranks hurt his knees.

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    coprolite fietsbob's Avatar
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    The crank arm is just a thing, it does not hurt your knees , the way you ride the bike with them on it may .
    by pushing too high a gear , in a quest for speed , for example .

    Mend a few of your own broken bones in your life, and the need for speed is moderated

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    Senior Member Monster Pete's Avatar
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    Quote Originally Posted by DannoXYZ View Post
    A 5mm difference (170 & 175) is about 3% and makes a difference in rear-wheel torque if you're climbing a technical off-road hill. Going the other way helps you spin faster..
    Though the same effect can be obtained by adjusting chainring or sprocket sizes. Ideally, you'd size the crank to your leg length, then use chainrings and sprockets to get the desired gearing.

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    Old fart JohnDThompson's Avatar
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    Quote Originally Posted by fietsbob View Post
    The crank arm is just a thing, it does not hurt your knees , the way you ride the bike with them on it may .
    by pushing too high a gear , in a quest for speed , for example .

    Mend a few of your own broken bones in your life, and the need for speed is moderated
    A longer crank arm causes the knee joint to move through a wider flexion-extension angle over each pedal cycle, which could be a issue for someone with knee problems.

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    Senior Member DannoXYZ's Avatar
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    Quote Originally Posted by JohnDThompson View Post
    A longer crank arm causes the knee joint to move through a wider flexion-extension angle over each pedal cycle, which could be a issue for someone with knee problems.
    Yes, IT-band rubs more when the knee is bent more.

  17. #17
    I'm Carbon Curious 531phile's Avatar
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    I have ridden 165, 170, and 175 and I do notice a difference. Maybe b/c I have short legs. 165mm just feel right for me. 170mm is tolerable, but 175 just feels too big for me.

    Quote Originally Posted by avner View Post
    I loled. Twice. Then I cried. Then I rubbed one out and cried again, but thanks for sharing.

  18. #18
    Gear Combo Guru Chris_W's Avatar
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    I've found that it is the diameter of the crank's circle that matters, not the radius, so you need to double the differences when considering the effects. Threrefore, 172.5 vs. 170 is actually a 5mm difference.

    I have 172.5mm cranks on the front of our tandem, and to get the extension at the bottom of the pedal stroke correct I find that there is too much compression at the top of the pedal stroke, because the pedal is then 5-10mm higher up than I'm used to (I'm using 170 and 167.5mm cranks on other bikes). Because of this, I really don't like the crank length on the tandem, but tandem cranks come in an even more restricted range of lengths than cranks for single bikes, and so it would not be easy to change it.

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    This question is a bit like "which saddle do you like?" Some riders are sensitive to crank length and have a definite preference and others (I'm one of them) are pretty much indifferent within a reasonable range. I have 170 mm cranks on most of my bikes and 175 on one. I've ridden the two lengths back to back and barely noticed the change.

    Several years ago Lennard Zinn did a fairly extensive study using a wide range of crank lengths (150 to 220 mm IIRC) and a large range of rider heights and leg lengths trying to develop a correlation of "optimum" crank length as a function of rider leg length. His conclusion was that it didn't matter and his riders all adapter to a huge range of crank lenghts with little to no effect on power output or other parameters. He was disappointed since he makes custom length cranks but he reported the results honestly.

  20. #20
    we be rollin' hybridbkrdr's Avatar
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    Quote Originally Posted by HillRider View Post
    This question is a bit like "which saddle do you like?" Some riders are sensitive to crank length and have a definite preference and others (I'm one of them) are pretty much indifferent within a reasonable range. I have 170 mm cranks on most of my bikes and 175 on one. I've ridden the two lengths back to back and barely noticed the change.

    Several years ago Lennard Zinn did a fairly extensive study using a wide range of crank lengths (150 to 220 mm IIRC) and a large range of rider heights and leg lengths trying to develop a correlation of "optimum" crank length as a function of rider leg length. His conclusion was that it didn't matter and his riders all adapter to a huge range of crank lenghts with little to no effect on power output or other parameters. He was disappointed since he makes custom length cranks but he reported the results honestly.
    As mentioned before in this thread, it may have an impact depending on your knees. I'll clarify for those who never had that issue. I spoke to a doctor who told me I had a mild case of condromalacia in my knees because of the competitive running that I did years ago. He told me that anything that lifts the knees could provoke pain there. I adjusted my seat as high as I reasonably could but in reality, it's the same recommended distance from the middle of the bottom bracket that the calculator on the Colorado Cyclists website suggests. I usually use 170mm with almost no problem ever. However, I want to build a "light touring" bike next year and I already bought the cranks which are 175mm because I wanted to go faster. I'll only be able to tell next year if it makes a difference to me. Like I said in my previous message, I was talking about someone else's message although I can believe what he was saying.

  21. #21
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    I'm a believer in the thought that the length DOES matter- I once had a collection of similar bikes, of which one felt 'funny' and inefficient to pedal. I was using the same pedals, same shoes, etc on every bike. At some point, I swapped cranksets between bikes (just because I like to tinker). After a while, I noticed a different bike was uncomfortable to ride, when it had been fine before. I finally put together that all of my bikes had 170mm cranks, except one set of 175mm cranks, that had been on the two uncomfortable bikes.

    Though less than perfect science, that's about as close as I can get from my own accidental experience.

  22. #22
    AEO
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    Quote Originally Posted by hybridbkrdr View Post
    As mentioned before in this thread, it may have an impact depending on your knees. I'll clarify for those who never had that issue. I spoke to a doctor who told me I had a mild case of condromalacia in my knees because of the competitive running that I did years ago. He told me that anything that lifts the knees could provoke pain there. I adjusted my seat as high as I reasonably could but in reality, it's the same recommended distance from the middle of the bottom bracket that the calculator on the Colorado Cyclists website suggests. I usually use 170mm with almost no problem ever. However, I want to build a "light touring" bike next year and I already bought the cranks which are 175mm because I wanted to go faster. I'll only be able to tell next year if it makes a difference to me. Like I said in my previous message, I was talking about someone else's message although I can believe what he was saying.
    The crank length doesn't matter when it comes to going faster, that's what gears are for. It's mainly how much power the engine can output.

    The cranks only change the leverage, or torque you can put down, particularly when you're going uphill. Crank length also affects the rpm to a certain degree. Shorter cranks are easier to spin and longer cranks are harder to spin.

    Put the two together and it's either "more power, less frequently", or "less power, more frequently". The net effect is the same, but the variable is in the engine. If the engine has longer legs, it will be easier to for them to give the whole range of motion required by the cranks without excessive deflection of the knee. A rider with shorter legs or limited flexibility wouldn't be able to give that motion range and thus require a shorter length.

    It's all proportionally scaled, just like kids bikes using 24" wheels with 6" cranks, or how 700c road bikes cannot get any smaller than 52.5cm, because the wheels wouldn't fit without bad geometry. The thing is, however, not all riders fit within the average proportions for their height and not everyone has the same level of flexibility.

    If one size fit all, then there wouldn't be all this hocus pocus with bike sizing or fitting.
    Food for thought: if you aren't dead by 2050, you and your entire family will be within a few years from starvation. Now that is a cruel gift to leave for your offspring. ;)
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  23. #23
    afraid of whales Mr IGH's Avatar
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    It depends on the rider, some will notice, other will not. I notice a 2.5mm change but many riders will report it makes no difference to them. Buy a different sized set and see which camp you're in.

  24. #24
    Faster but still slow slowandsteady's Avatar
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    Quote Originally Posted by JohnDThompson View Post
    A longer crank arm causes the knee joint to move through a wider flexion-extension angle over each pedal cycle, which could be a issue for someone with knee problems.
    Having longer crank arms is in effect the same as lowering your saddle in terms of what it does to your knees.
    "Ride lots." -- Eddy Merckx

  25. #25
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    Quote Originally Posted by slowandsteady View Post
    Having longer crank arms is in effect the same as lowering your saddle in terms of what it does to your knees.
    Not quite. That's what happens at 12 o'clock, but at 6 o'clock the effect is like raising your saddle.

    Longer cranks increases the distance from top to bottom, not often an issue for the long legged, but much more of an issue for those with shorter legs. The impact of the short difference invloved depends on how close to critical any particular rider is. It's like what a dollar is worth. If you have dough, not much, but id it's your last dollar it's like gold.
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