Crank arm lengths

Discussion in 'Cycling Equipment' started by jeffgordon73, Feb 2, 2010.

  1. jeffgordon73

    jeffgordon73 New Member

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    Anyone have any good info on crank arm lengths? What determines the length? Are shorter/longer faster or slower/ more power or less? Thanks
     
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  2. gman0482

    gman0482 Member

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  3. tafi

    tafi Member

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    Mechanically speaking you tend to turn faster in smaller gears with shorter cranks and slower in bigger gears with longer ones. Changing crank length (all else being equal) will not yield more power. However, changing cranks often requires (or allows) changes to the bio-mechanics of the bike setup, some of which MAY be beneficial to power output.

    Some say crank length should scale with leg length (or some part of the leg), but this doesn't take into account other factors such as flexibility or how low/upright one's position is.

    Just as the changes made MAY be beneficial, they could just as easily be detrimental. Small changes can sometimes compromise your comfort on the bike so this has to come into the equation as well. A longer crank will require you to sit lower to reach the bottom of the stroke and you will then also close your hips up a bit more at the top of the stroke.

    In short, the only way to find out if there are benefits to be gained is to experiment.
     
  4. 531Aussie

    531Aussie Well-Known Member

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    How tall are you?

    I found that longer cranks have more leverage, BUT they're harder to push because you are relatively much lower, so it's kind of a wash. And by 'longer', I mean 5mm or more. The extra length means you have to lower the saddle to accommodate for the reach through the bottom of the stroke (assuming your seat was in the right position in the first place), therefore, the extra length is roughly (and effectively) doubled through the top of the stroke. For eg, if you get cranks which are 5mm longer, you'd end and sitting relatively (roughly) 10mm longer, in relation to the pedal through the top of the stroke.

    I'm 182cm with an 89.5cm inseam, and a few years ago I obsessed over this topic. I persisted with 180mm cranks for about a year (I even bought 3 sets), but ended up going back to 172.5s for most riding. I feel more powerful with the shorter cranks, coz I sit much higher over the pedal, and can, therefore, really pound the crap out of the 'shanks'. I still have some 177.5s on one bike that I sometimes use for early season criteriums when I feel that my fitness might be down, coz I can get off the saddle and 'lever' my way out of trouble a bit easier

    Arnie Baker has a good article about crank length, in which he discusses the conundrum between torque and muscle force.
    http://www.arniebakercycling.com/pubs/Free/Optimum Crankarm Length.pdf
     
  5. frenchyge

    frenchyge New Member

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    Difference in mechanical advantage between a 170mm vs. 180mm crank = 5.8%
    Difference in mechanical advantage between a 14t and 15t cog = 7.1%
     
  6. 531Aussie

    531Aussie Well-Known Member

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    ^ Ya know, I 'hear' this, :) and have read it on Andrew Bradley's site and Sheldon Brown's, etc, but I'm sure there's more leverage with a longer arm. But, the extra leverage is negated somewhat (or a lot?) by the biomechanical impact on position (extra hip and knee flexion) and the extra muscle torque required (see Arnie Baker's article). However, when stomping off the saddle, the impact on postion and muscle torque is pretty much gorn. Interestingly, Pantani sometimes used 180mm cranks for mountain stages, but spent a lot of his climbing off the saddle.

    I once accidentally put myself in a blind test. I was riding 170s all the time, then, one sunny day, I dusted off my old race bike, which I hadn't ridden for a couple of years. I totally forgot that this bike had 175mm cranks. When I got to my regular, short 'off-the-saddle' hills, I couldn't work out why I was flying. I even got off to check that I was using the same gears as were on my other bike. A few kms later I remembered that the long cranks were on this bike. This ride was a short commute to uni, so I was wearing jogging shoes, which may be why I didn't immediately notice the larger circle (coz of the arbitrary foot position when flopping my feet into the loose toe clips).
     
  7. roadhouse

    roadhouse New Member

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    we're talking millimeters folks, if you suck on a 160 crank arm, you're gonna suck on a 175 arm. quit the excuses and ride as if you were born to be out front or ride as if you think a few millimeters may be the cause of you sucking so noticably so bad off of the back.

    my two cents.
     
  8. alienator

    alienator Well-Known Member

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    Like a lot of others cycling, whether longer crank arms are benefit or not is wholly dependent on the rider, more specifically on the dimensions of the bits on his lower leg and on his/her musculature. It's like stems: some riders can't tolerate changes in stem length, sticking instead to one size, while some riders can tolerate changes of 10, 20, or more millimeters. Likewise with cranks some can run 175's on one bike and be comfortable spinning 165's on their fixie or track bike. Others, maybe not so much.

    As mentioned already, the issues with various crank arm lengths pretty much goes away when you're standing, and that's great if you stand for all of your climbs. If you have to sit a bit, though, turning those longer cranks over might not be so sweet.

    In any case, the effect of crank arm length on performance isn't something that can't be boiled down to a few hard facts, and it certainly can't be reduced to just being "millimeters." Numbers and measurements are meaningless without context.
     
  9. Sassonian

    Sassonian New Member

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    I agree with Alienator - It's definately its a horses for courses thing...

    I'm 2m tall and a 123kg ex rugby player.

    Check out my bike.

    [​IMG]

    It's a custom built Titanium Zinn with 210mm cranks. The whole geometry of the bike is built around the custom length cranks. So the bottom bracket is higher so I don't have pedal clearance problems or have to lower the seat as suggested earlier. It's also configured to give me the right hip angles etc. The added bonus is that the higher bb make the frame shorter from the bottom up - which is a big bonus in a big bike. (and - off this topic - but the front end is designed to eliminate shimmy that can plague big bikes - with a relaxed headtub angle giving more trail hence stability)

    The jump in performance I've had from this bike has been just mind blowing. I'm in a club and regularly race. I can get on top of gears far quicker and hence the accellerations that used to spit me out the back are now not so much of a problem. And I can now proudly say that I no longer expect to get dropped on hills like I used to - the climbing ability is incredibly better.

    I put this mainly down to the long cranks and having a frame that's configured to let me get my power down on the road more efficiently.

    I did have a 53/12 on it and was finding that I was running out of gears on downhills and at the sharp end of sprints because of the longer cranks - but I've now got a 53/11 and it's solved that problem.

    So I can confidently say from experience - that if you are a tall rider or a rider with a long inseam - long cranks will help you incredibly

    But they aren't for everyone. Stands to reason that they should be proportional to your size. (Zinn uses 0.21 of you inseam as a starting point in his designs)

    (I'm talking centimeters Roadhouse) :)
     
  10. tafi

    tafi Member

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    Sassonian, this seems to work well for you so keep going with it, but this appears to be a case in point as to what could be required to fit someone on a particular sized crank.

    0.21 x leg length takes no account of other physiological parameters such as musculature, flexibility, proportion of leg length above an below the knee etc, all of which will have an effect.

    Was any adjustment of the crank length carried out after the calculation?

    As i said it's the whole bike fit that makes the difference and that's the point, you can't test the value of longer or shorter cranks in isolation from all other parameters.
     
  11. Sassonian

    Sassonian New Member

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    Fair points tafi.

    That's why Zinn uses the formula as a starting point only - and he does discuss your body type - what sort of riding you do - flexibility - and takes a whole lot of other measurements into account as well to determine hip angles etc.

    I've seen lots of people decrying the formula approach - but it's not a hard and fast thing - it's just used as a guide to start the design discussion.

    He's been doing this stuff for a while and specialises in tall riders (which he's one) - so he knows what he's doing and what works for us giraffes :)

    I'm constantly surprised that riders I know are amazed at the performance increase I've experienced - yet they still question the validity of the bike that's facilitated it. It's weird that I have to constantly defend it's features - when it's obviously enabling me to perform at a level way above what I could previously.

    It looks a little different I know - and I think that's the rub.

    I even had one guy telling me that you couldn't race on a bike with such a long headtube - and his reasoning seemed to be that he hadn't seen anyone else doing it - so it must be wrong.

    He also told me that "time will tell if this approach is right". Time doesn't need to tell for me.

    :)
     
  12. tafi

    tafi Member

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    Don't worry i'm not trying to denigrate your bike.

    It's just an example of how it is impossible to answer the original thread question in a simple way. It always comes back to the idea that humans do not conform to geometric rules.
     
  13. Sassonian

    Sassonian New Member

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    That's cool - I didn't think you were. I was just highlighting some of the pitfalls of the arguments around this sort of issue.

    I agree totally with your assertation that it's more complicated than a straight crank length argument.
     
  14. jeffgordon73

    jeffgordon73 New Member

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    WOW! I did not realize the length of the cranks arms are such a complex issue. However, I do appreciate all the info and I do have a better understanding of the length of the arms. I thank you all for your replies
     
  15. frenchyge

    frenchyge New Member

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    There definitely is more leverage. In the case of 175's vs 170's it's about 3% more. So, even though you were in the same cog on your race bike, mechanically you were climbing about half a gear shorter than when you rode your 170's.

    I wasn't trying to imply that there's no difference, but that the difference in crank lengths is small relative to the range of available gears on a road bike. Trackies are obviously not the case, plus there's that whole banking thing. :)
     
  16. 531Aussie

    531Aussie Well-Known Member

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    ^ Oh, i thought you were talking about the Sheldon Brown thing:
    Bicycle Cranks
     
  17. 531Aussie

    531Aussie Well-Known Member

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    Yep, there are other issues. I even knew a guy who kept whacking the rear derailleur with his heel, partly because he had huge feet. I gather he must've pedalled with his heels down on the up-stroke

    Ya know what they say: 'huge feet, huge rear derailleur problems'
    Booooom Bom!
     
  18. Sassonian

    Sassonian New Member

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    That's the other beauty of a custom build...

    I've got size 15 feet to go with my 210mm cranks. No problem with heel striking the derailleur. Slightly longer chain stays solve the problem.
     
  19. Akadat

    Akadat New Member

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    Longer cranks will hit the ground sooner while cornering.

    Leg bones do count, and all else is simply a different cog front or rear.
     
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