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  1. #1
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    Crank arm lengths

    Is crank arm length determined by the size of your frame or just personal perference? I can't see 2.5mm or 5mm being a big difference in the amount of leverage, am I wrong?

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    It is a personal preference thing but it can make a big difference to how smoothly you ride. They often put longer cranks on larger frames as they expect these will be ridden by taller riders, the notion is that the longer your legs are the longer you will want your cranks to be. Even on larger frames it seems 170/175's are the norm.
    I am 6'7" and have had cranks ranging from 165 to 180 on my commuter bikes over the past few years and have determined that I like 175's the best after putting in about 8,000km per year. That 5mm change on either side can have quite an impact on the range your knee moves through. It also depends on what sort of riding you are doing, the 180's felt good on my mountain bike but on my commuter they started to gradually hurt the front of my knee.

    My son races BMX, it is really obvious when the crank length is wrong with little kids on BMX race bikes. When the cranks are too short they can't get the bike moving out of the gate unless the gearing is really easy, which means they really have to spin to get any speed on the straights. When the cranks are too long the pedal stroke becomes really choppy. It becomes a bit of a balancing act between the gearing and crank length.

  3. #3
    bac
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    Quote Originally Posted by andymac
    It becomes a bit of a balancing act between the gearing and crank length.
    Excellent post. It really comes down to comfort, and personal preference. Femer length is one of the big factors to think about if you want to. However, ride a few different sizes if you can. You'll know.

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    Ahhh...my old roadie has 170s and my cyclocross has 175s, I don't really notice a difference between them though. I just ordered an '06 LeMond CdF which has 175s, I was planning on getting an Ultegra crank so I guess I'll have to figure out soon which length I like the best.

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    There is a range of usable crank lenghts for any rider (probably +- 1cm) but the size should be proportional in some way to leg length. This is more of a problem for big or small riders; medium riders can ride most cranks without discomfort.

    Crank length interacts with frame and wheel size: the BB height should place the pedals to avoid grounding on corners so shorter cranks can have lower BB height (and visa versa for long cranks). This makes mount/dismount much easier for small riders. The BB height on most production bikes varies very little across the size range.
    Cranks can interfere with the front wheel so designers should match the wheel size and length of the bike accordingly.

  6. #6
    Air
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    Check out Peter White

    Crank Length

    Crank length determines the diameter of the circle that the pedals move in. The larger that circle is, the more flexion of your knee and thigh muscles will be needed to turn the cranks. Your thigh muscles cannot exert the same force throughout their range of motion. This is very easy to demonstrate. If you squat down so that your knees are fully bent and lift yourself up, say, five inches, it takes a good deal more effort than it would to squat down just five inches from standing straight and then lift yourself back up. At the full squat position, your muscles can't put out the same power as when your knees are just bent enough to drop you down five inches. So if you had to choose between a crank length that had your knees bending through their entire range of motion and a length that only required, say, 20 degrees of flexion at the knee, you would choose the shorter crank. That crank would have your muscles working through a more efficient range of motion. You would avoid having to flex your knees enough to bring you into an inefficient range of motion.

    So how long should the cranks be? Well, that's a good question. I wish I had a good answer but I don't. It should be obvious that a 5' 2" rider would not want to use the same length crank arms as a 6' 7" rider unless they somehow managed to have the same leg length (highly unlikely). Some research has been done to determine the optimum percentage of leg length to crank length. I doubt that there is an optimum percentage that would apply to all people. One writer in a major magazine article quite a few years ago claimed that after considerable testing with many different riders, 18.5% of the distance from the top of the femur to the floor in bare feet should be the crank length. You can find the top of the femur pretty easily. It's 5" to 6" below your hip bone, and moves rearward when you raise your knee. After reading this I promptly changed from the industry standard 170mm cranks for road bikes to 175mm cranks. There was an immediate improvement in power and endurance. I began using this formula when recommending cranks to my customers. So far, I haven't gotten any complaints. But of course that doesn't mean my customers wouldn't be as happy or happier with some other length. And I must admit that I have never tried still longer cranks than 175mm for enough time to tell if I would be even happier with them.

    The top of the femur measurement ignores differences in legs themselves. Differences in the proportion of calf length to thigh length should affect the optimum crank length. A rider with longer thighs and shorter calves would use a longer crank to get the same flexion at the knee as a rider with short thigh and long calf. Of two riders with the same body proportions, one might prefer to pedal at a faster cadence. That might favor a shorter crank length. And perhaps even two riders with identical skeletal proportions would find after testing that they required different crank lengths to achieve maximum performance due simply to differences in their muscles.

    Trying different cranks to find the optimum length would be time consuming and expensive, but I believe it is the only way to determine the correct length for any individual, assuming there is a correct length. It would be nice to have a crank with many pedal threads at various lengths to test. But I know of no such thing being made and I lack the ability to make one! Of course, some riders with multiple bikes report being just as happy on one crank length as another. Go figure! So, for lack of a better system, I'm staying with the 18.5% guide for my customers until something better comes along. It hasn't failed yet.

    In the United States, it has been difficult and expensive to obtain cranks shorter than 165mm or longer than 175mm. But a French company, Specialites TA has been making high quality cranks in lengths of 155mm through 185mm for many years. In order to offer my customers better fitting bicycles, I've decided to sell these cranks. See my web page; http://www.PeterWhiteCycles.com/carmina.asp for details.
    When I got new cranks on my mtb I put on 175s vs the old 170. Felt a pretty big difference, it felt like I had a few more teeth and a bit more power. When I ride my folder that has 170's on it I feel a bit fustrated that the rotation is a bit off. But it is pretty individual.

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    If you hunt around the net there is a computer program that can calculate optimal whatevers, good luck if you try it. For sure going from 170 to 175 drops the required torque 3% (170/175).
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  8. #8
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    I read a bit about it.....I am 6'2" so I assumed I needed 175 I guess because I got a large frome when I bought from a bike shop....well aparently I have a long torso....short legs....I have a 32 inseam (Pants) which is pretty short for a guy 6'2" so lately it has been a problem getting in to my drops....

    I ordered a set of 172.5 cranks. I will let you know how it goes....hopefully it will stop my knees hitting my chest so much and be good.

  9. #9
    velosipedist
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    Quote Originally Posted by biffstephens
    I read a bit about it.....I am 6'2" so I assumed I needed 175 I guess because I got a large frome when I bought from a bike shop....well aparently I have a long torso....short legs....I have a 32 inseam (Pants) which is pretty short for a guy 6'2" so lately it has been a problem getting in to my drops....

    I ordered a set of 172.5 cranks. I will let you know how it goes....hopefully it will stop my knees hitting my chest so much and be good.
    We're about the same size. I've been riding 172.5 cranks for ages and they feel great.

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    I'm about the same size too and I much prefer 175 on the road. It is, indeed, a matter of personal prefernce.

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    So I mounted my new cranks 172.5 from a 175....and I can't tell you the difference it made. It was pretty close to amazing. My right knee pain is gone. Climbing was way better....spinning was easier....and my knees were not in my chest when I was in the drops...just amazing. I am very happy I went with the 172.5 for me. I think it was the correct choice.

    I have a 32 inch inseam by the way....short legs for a guy that is 6'2"

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    Air
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    Glad to hear it!!

  13. #13
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    I recommend reading what Sheldon has to say: http://www.sheldonbrown.com/cranks.html
    Uphill or downhill; headwind or tailwind; Pavement or Dirt ... it's all good.

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    The Site Administrator: Currently at home recovering from a couple of strokes,please contact my assistnt admins for forum issues Tom Stormcrowe's Avatar
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    Quote Originally Posted by geo8rge
    If you hunt around the net there is a computer program that can calculate optimal whatevers, good luck if you try it. For sure going from 170 to 175 drops the required torque 3% (170/175).
    http://www.machinehead-software.co.u...rank_calc.html
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  15. #15
    Paste Taster Retem's Avatar
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    for me it is personal preference but I also ride different crank lengths on most of my bikes (different bikes different purposes) i.e. my touring bike with wide gears is 171mm
    my fixed gear conversion is 171mm also - made the transiion easy
    my track iron is 170mm
    and my other track bike_mostly for street use is 165mm (this is helping me to spin more evenly)
    on my dirt jumper 180mm
    and like others have said it also has some to do with your body structure
    I am dyslexic so bear with my posts.... [SIGPIC][/SIGPIC]

  16. #16
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    crank lengths ... WOW. I initially decided to put my two cents in.. ya know to be a big fat know-it -all..
    ultimately I learned more than I have to bring to the table .. I will add : I have a short femur length -23cm, a short anatomic inseam-33.25, I'm a tiny bit over 5'10' though. In consulting with many ..more astute than I..the concensus was..172.5 at MOST..some folkes thought 170 'd suit..the new bikes gettin' 172.5.. Not JUST an arbitrary decission either. I've had bike with 168's.. then 170 .. Size calculations have gone either way 172.5 or less. Maybe it's just me, I feel the dif. between the sizes big-time. I recently clicked-on to a site..by accident..a dude lists 30-40 famous riders in reguard to crank length..past and present..the onlt pattern was..there WAS no pattern. Taller guys had 170's for example. Mig of course used 180 (sometimes 190) .I've often read that.
    over the past 30 or so yrs. LOTS o' pros use 175's..shorter guys as well. Trends effct the sizes that prevail to a great extent, bikes sold in in the 70's and 80's had shorter cranks for sure..

  17. #17
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    A lot of it is simply pedalling style. Folks who like to spin tend to find happiness with shorter cranks, while folks who like to sit back and push through a bigger gear find happiness with longer cranks.

    As far as I am concerned the only problem that crops up is when folks insist that everybody do it their way, ie. "Short cranks rob your power!"... "Long cranks destroy your knees!" etc.

    When I was a junior, I started gravitating toward longer cranks as I reached my full height. Certain "coaches" told me 175s were certain to cause me knee problems, which turned out -- after 25 years of experience -- to be nonsense.

    IMO, if you have the money you can try a bunch of different lengths and see what feels best, while ignoring advice/cautions/dire predictions from "experts". If you don't have the money (or the desire, or the total lack of anything better to do) then ride what you've got and don't worry about it.

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