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  1. #1
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    How to know when to legnthen/shorten crank arm length

    Hi everyone, I've read up a good bit varying ways to calculate optimal crank lengths and some general fitment guides. I've been using 175mm cranks the entire time I have been cycling since the mid 90's. I'm not really interested in a formulaic suggestion on crank length for my next bike. My question has more to do with:

    A) what general effects lengthening or shortening the crank arms have

    and

    B) what symptoms should signal that a change in crank arm length would be beneficial

    I am in the early stages of considering a new bike and when I spec'ed out the cranks, I wondered to myself why I simply filled in a 175mm part when maybe changing things could could help. I do feel like my current stroke lacks fluidity, but I read shorter crankarms make it slightly harder to realize max power potential?

  2. #2
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    Unless you're below 5'5" or taller than 6'2", you'll be ok on 175's.

    Small people will find their legs move too far on long cranks, meaning they have a knee bend at the top that is too much for optimum power (over 90 deg). Tall people on short cranks will find they don't get to the necessary optimum power angle.

    I'm betting your poor pedaling stroke isn't to do with crank length.
    Rimmer - "There's an old human saying - if you talk garbage, expect pain"

  3. #3
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    B) what symptoms should signal that a change in crank arm length would be beneficial
    Any kind of pain in knees could be related to cleat position or to crank & bike fit.
    Majority of MTB riders seem to prefer longer cranks on the basis they can apply more leverage (power). Down side is spinning the pedals with longer cranks is harder.

    As for not being able to apply max power if your tall and use a shorter crank, not sure this is proven. Yes, there will be an optimum output but this is dependant on many other factors in the bike fit.

    Shorter cranks will also provide better ground clearance however they will also raise your centre of gravity.
    As the majority of power tends to be applied when riding up hill, it can be much better to go with a shorter crank and spin.

    Just my views from experience following a bike fit and switching to shorter cranks.
    MTB:
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  4. #4
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    I have recently switched from 175mm to 170mm crank arm length for a couple of reasons. I am 5'11", btw. I know some people may argue with what I have to say, but all I know is this is why I did it and it did work for me.

    First was because I like to spin more than mash, so the shorter length helped me achieve that. The second reason was because I was having slight knee pain at the top of the crank cycle. The pain did disappear after swapping the cranks.

    Like I said, this is just my experience and nothing more. I don't care if someone chimes in and tells me I am wrong for this or that reason because it won't matter to me. It worked and I am still riding.
    I always wanted a Wookie, but I found out they weren't real.
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  5. #5
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    How to know when to legnthen/shorten crank arm length

    I went in the opposite direction from 175 to 180 couple of years ago. I'm 6'2" and I have long legs relative to my height. I immediately felt my stroke had more optimal angles and I can spin just fine. I agree with you that it should be part of proper bike fitment.
    If you're really honest about it, they're all "cheater lines".

  6. #6
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    I was searching for crank arm length info and found this thread. I'm configuring a FS bike (first one), and I'm thinking I should go with 170mm. I'm 5'1". I have 170s on my hardtail, but since it was my first bike, it just came that way and I didn't question it. I'm looking at the Santa Cruz Blur TRc - it has a low bottom bracket at 13.1" on the size small.

    My question is - because I'm short, should I stay with the 170? Or would it be more powerful pedaling with a longer crank arm? I'm also wary of pedal strike, since the BB is low.

    Thoughts?

    -laine

  7. #7
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    go to a shop and try bikes with different length cranks is the best way to find out which suits you best, or get a bike fit.

    Do you want to spin or grind. If spin go short, if grind go long. As you say there is a compromise between length, pedal strikes, possible foot strikes with wheel during turns and also balance.

    There is some science out there but unless you can get on a power meter to see which one give you optimum leverage & spin-ability its a different call.
    MTB:
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    16" HotRock Custom

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    08 Jamis Xenith Comp
    13 Pinarello Dogma 65.1

  8. #8
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    Thanks for the advice griffter18, and I wish it were that easy. Sadly, it's not that simple. I'm 5'1". I'm lucky to find a shop with one bike in my size at all.

    Can you explain what you mean re: spin vs. grind?

    thanks.

  9. #9
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    "Can you explain what you mean re: spin vs. grind?"

    Watch most MTB riders going uphill and youll see them stood up on the pedals mashing (30 - 60 rpm) to get the most power down on the crank.
    Watch most Road riders going uphill and youll see they stay seated and turn the pedals over (80- 110 rpm)

    Standing on the pedals has its uses, but it uses body weight to turn the pedals over and isnt efficient. Try for yourself, find a hill that takes a couple of mins to climb and see how it feels to stand the whole way up. Leg burn within a few mins.

    Using shorter cranks means your feet dont travel as far to get around the circle so you can actually spin the pedals faster, assuming you've got the gears.
    Much more efficient and you can do it longer.
    You can still stand up when the time comes but the leverage is slightly less.

    I used to use long cranks until I went for a bike fit. I was able to put more power out & for longer by spinning so have now switched all cranks to shorter ones.
    In mtb it also helps with ground clearance but it does move centre of gravity up slightly (compromise)
    MTB:
    09 Stumpy Elite
    10 Enduro Pro Custom
    16" HotRock Custom

    Road:
    08 Jamis Xenith Comp
    13 Pinarello Dogma 65.1

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