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  1. #1
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    Crank Arm Length

    Is there a noticeable difference in performance/spin/power transfer with regard to crank arm length? What are some factors I should consider in deciding which 170/172.5/175 is correct for me? Can a professional bike fit identify the ideal length or is it a rider preference issue? I'm about ready to fork out some hard earned cash for a new bike and I want EVERYTHING perfect.

  2. #2
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    Do what you like

    The "logic" of "crank length should be proportional to leg measurements" has been around for a LONG time, and lots of people have turned that "logic" into a formula for determining crank length. Only one problem: the research doesn't support it. Don't you just hate it when the data doesn't support the theory? One key feature that is often ignored in these discussions is the duration of muscle contraction that is controlled by cadence. It just may be that there is an optimum here, which is why there is a fairly narrow range of cadence for optimum performance. Longer cranks mean lower cadence, moving you out of that optimum range. Crank length has been a point of debate since the introduction of the "safety" bicycle in the late 1800s, and there have been all sorts of fads in that regard. Do you think that we have standardized on this narrow range because of some sort of global conspiracy, or because well over 100 years of experience (and testing the limits) have repeated shown that the 170-180 mm is really what works for human beings?

    There is no reliable formula for predicting crank length. There ARE lots of formulas out there, but they are just figments of the imagination of their purveyors. No one has ever done a study that shows how crank length should relate to anything. Probably the best work done on this VERY difficult to research topic was by Lennard Zinn. He unintentionally showed how our adaptability was more important than our size or riding style. He's sure that the results of his study are wrong, but he just can't seem to find any data to support his pre-conceptions.

    You will find no high quality data to support any particular crank length as being better than any other. This is true whether or not you correct for leg length, femur length, etc. On the other hand, you will find lots of anecdotal or low quality data to support all kinds of conclusions, and more theories than you can shake a stick at. A rider's response to changes in crank length is 1) highly individual, 2) dependent on riding style and the event (TT, climbing, crits, track racing, etc.), and 3) most important, highly adaptive. This is why it is so hard to study the effect of crank length.

    It is generally the case that longer cranks make it harder to spin, and high cadence is the best way to minimize knee problems. That said, an extra 5 mm in crank length may only take away 3-5 rpm of spin, so it is not a large effect. Spinning is more physiologically efficient, all else equal.

  3. #3
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    If you're 5' 08" or above, get 175's
    If you're 5' 2" get 172.5's
    175's may slow your spin for two or three weeks, until your body gets used to them.
    What size cranks do you have now ? How tall are you ? What type of riding do you do ?
    The only major mistake you could make, would be to buy 175's if you are 5' 0".
    There are many people who can "spin" a 180 crank.
    If your opinion differs from mine, ..........Too bad.
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  4. #4
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    2 points

    First, if you have more than one bike make sure all of your bikes have the same length cranks. I've never heard of any research backing me up, so I may be talking out of my a$$, but I can notice a difference. When I put the same length cranks on my road, TT, cx,and mtb bikes I was definitly more comforatable on all of them.

    Second, and again this is my observations, I think crank length should be based on femur length. It just seems to make sense to me. I have long femurs for my height (5'9") and I like longish (175) cranks on all my bikes for my height. It seems easier to set up my cleats in the right position and I can climb the steep stuff better. According to my SRM, more watts in the saddle at the same given HR. Who knowes?

    Justin

  5. #5
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    huh?

    Quote Originally Posted by MR_GRUMPY
    If you're 5' 08" or above, get 175's
    If you're 5' 2" get 172.5's
    175's may slow your spin for two or three weeks, until your body gets used to them.
    What size cranks do you have now ? How tall are you ? What type of riding do you do ?
    The only major mistake you could make, would be to buy 175's if you are 5' 0".
    There are many people who can "spin" a 180 crank.
    Here's a rhetorical question: From where did you pull these figures?

    I'm 5''7", and after using a 170 crank, put on a 172.5. After 9 months, I can still feel the difference, and they feel too large to me. If I buy a new crank, I'll go back to 170. It's not math--it's feel. Either it's comfortable or it's not.

  6. #6
    srf
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    IMHO it has a lot to do with what you're comfortable with. Without trying them out, I don't know that you'll ever know. I'm 5'10" and have one bike with 175mm and one with 172.5mm. I've never been able to come to terms with the 175mm and will probably move that bike to 172.5mm. It probably has a lot more to do with if you prefer lower or higher RPMs (I prefer slightly higher). Supposedly longer cranks help you on hills, though that doesn't seem to hold true for me.
    Last edited by srf; 10-23-2005 at 09:14 AM. Reason: typo

  7. #7
    NeoRetroGrouch
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    Quote Originally Posted by 2faced
    Is there a noticeable difference in performance/spin/power transfer with regard to crank arm length? What are some factors I should consider in deciding which 170/172.5/175 is correct for me? Can a professional bike fit identify the ideal length or is it a rider preference issue? I'm about ready to fork out some hard earned cash for a new bike and I want EVERYTHING perfect.
    I have everything from 165 to 175mm and cannot tell the difference. Any blind testing (even from those who KNOW it has to be) that I have seen has been zero difference. - TF
    "Don't those guys know they're old?!!"
    Me, off the back, at my first 50+ road race.

  8. #8
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    Quote Originally Posted by 2faced
    Is there a noticeable difference in performance/spin/power transfer with regard to crank arm length? What are some factors I should consider in deciding which 170/172.5/175 is correct for me? Can a professional bike fit identify the ideal length or is it a rider preference issue? I'm about ready to fork out some hard earned cash for a new bike and I want EVERYTHING perfect.
    I think in all honestly it's a very personal and individual decision, there is no right or perfect number. I'm 6'0 even barefoot and I am on the other end of the spectrum with sizing. For what it's worth, my 40K PR is 56:08 and that was done on 170's, but I digress that was 16 years ago....I'm so old!

    Now that I'm recently back into the sport I am trying out some 172.5's and while I don't have a good starting point reference with my layoff, I can say I have yet to get comfortable. I may very well go back to 170's and much of the conventional wisdom says I should go UP to 175's. My knees won't take the added stress I know for sure, but I cannot turn 'circles' as well with 172.5's and fight turning 'squares'. I know how to pedal a ring properly and I'm struggling to get the old feel back with the 172.5's.

    It's personal......and with a 34.25" inseam I go against conventional wisdom.

  9. #9
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    I'm 5'10", but have the leg length of your average 5'8" person.

    I've tried 175s... they were too long for me.
    172.5s? Ahhhh. Just right.
    .
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  10. #10
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    I am 6"2" with a 33" inseam and I have 5 bikes, 3 with 172 and 2 with 175 and I ride them all interchangeably (no racing). Compatibility is really a personal thing and from what I have read, this specification is one of the least critical as far as proper bike fitting goes.

  11. #11
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    Quote Originally Posted by chas0039
    I am 6"2" with a 33" inseam and I have 5 bikes, 3 with 172 and 2 with 175 and I ride them all interchangeably (no racing). Compatibility is really a personal thing and from what I have read, this specification is one of the least critical as far as proper bike fitting goes.
    Isn't that weird with body vatiation? You are an inch taller than me, but my inseam is 1.25 longer? Thus keeping bike fitters eternally busy....

  12. #12
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    Some people insist crank length is important; I'm 5'8", and use 175s on the mountain bike, 170s on one road bike and 172.5s on the other road bike and never think twice about any of them.

    One of my acquaintances used a 170 left crank with a 175 right crank and didn't notice for a year....

  13. #13
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    not to hijack...but, I'm 6'7'', does that mean I should use 180mm cranks?

  14. #14
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    Your length

    Quote Originally Posted by Britishbane
    not to hijack...but, I'm 6'7'', does that mean I should use 180mm cranks?
    No. Crank length is personal, and there is no way to predict the "right" length for you. If you like to spin, use shorter cranks, If you like to push, use longer. Shorter is better for your knees, but not an issue if your knees are OK.

  15. #15
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    I'm 5'9" with 32" inseam. Got 170s on the "fair weather" bike and 172.5s on the "going somewhere where it might get stolen" bike. Can't tell the difference really. Maybe I should ride more...

  16. #16
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    Thanks for the response Kerry. I do like to spin ( I think?). My preferred cadence hovers around 100rpm or so.

    Helpful thread guys/gals.

  17. #17
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    Quote Originally Posted by Britishbane
    not to hijack...but, I'm 6'7'', does that mean I should use 180mm cranks?
    At your height/probable leg length, I strongly suspect you should at least try some 180s.
    .
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  18. #18
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    I think Kerry nailed it in his first post. The later responses run the gamut from "cannot tell any difference and don't care" to people having very precise preferences. I've observed a similar range of responses amongst my acquaintances.

    FWIW, I'm 180cm tall, and have slightly shorter lower legs than average, otherwise I'd be a few cm taller. I've tried 165, 170, 172.5, 175, 178 and 180mm cranks, and could tolerate all except the 165 and 180mm. The 178mm cranks were only used for a short (1 hour), mostly uphill MTB ride and were (for me) great for climbing but wretched on the flat. I now use 172.5mm cranks for all except the fixed which has 170mm as these are less inclined to tear my legs off on fast descents. Sadly, I suspect I may be forced to revert to 175mm when I buy another MTB, as the only cranks available in a range of lengths seem to be XTR, which has an unacceptably high Q factor - for me. I find 172.5mm and low Q to be more comfortable, even for offroad use.

    Unfortunately, the only way to figure out what crank length is best for you is to try a variety; this is easier if you work in the industry!

  19. #19
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    The best thing regarding crank length I've heard is your optimum crank length is probably the one you've ridden the most. That said, if you're 6'7" and 175's feel better than 180's, you should probably ride the 180's and really give them a chance (the SRAM Rival crank is a reasonably priced crank available in 180mm). I'm 6'4" and I ride 180's. I rode 175's for several years and when I tried 180's, they felt better from the 1st pedal stroke.

  20. #20
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    I'm 6'2" with a 36" inseam (LeMond method) and I switched all my bikes to 180mm cranks (well, the RB-2 has 177.5). I am not, nor will I ever be, a spinner. I don't have the desire nor the discipline to be a racer, but I do like to ride.

    If it feels good, that can't be bad.
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  21. #21
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    I'm 6'5" with a 36" inseam. I've ridden 172.5, 175 and currently ride 177.5 crank arms. I never did get comfortable on the 172.5 arms. They just felt too tight in the rotational circle. Between the 175 and 177.5 arms I honestly don't feel much difference in climbing or ability to spin. I settled on 177.5 since they don't seem to impede my ability to spin, and I figure the additional leverage couldn't hurt when climbing. My cadence averages around 90 on the flats.

  22. #22
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    Quote Originally Posted by SystemShock
    I'm 5'10", but have the leg length of your average 5'8" person.

    I've tried 175s... they were too long for me.
    172.5s? Ahhhh. Just right.
    .
    same here 5' 11" longer torso shorter legs, I use a 172.5, the 175 and thr 170 feel wrong to me
    Quote Originally Posted by zank
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  23. #23
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    172.5mm=6.692in
    170mm=6.629in
    difference.063in = 1/16 of an inch

    preception vs. reality

  24. #24
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    Quote Originally Posted by tsull247
    172.5mm=6.692in
    170mm=6.629in
    difference.063in = 1/16 of an inch

    preception vs. reality
    Actually, every 2.5mm difference in crank arm length = a 16mm difference in the size of the pedaling circle.

    Some ppl may not be able to feel the diff, but I was certainly able to.
    175s felt big, and were hard to spin.
    172.5s were a nice blend of spin and leverage.

    But I agree that ymmv.
    .
    Monkhouse: I want to die like my Dad did, peacefully, in his sleep... not screaming in terror like his passengers.

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  25. #25
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    Huh? How's that? I can see that a 1/16 inch difference in crank arm length might equal a 1/8" (or double) diffence in overall crank length but I don't get 16 mm.
    Are you saying that the 1/16" longer crank gives a 16mm pedal circumference? I don't understand how pedal circumference would be relevant here. I'm thinking that only the minimum and maximum would matter
    Quote Originally Posted by SystemShock
    Actually, every 2.5mm difference in crank arm length = a 16mm difference in the size of the pedaling circle.
    .

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