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  1. #1
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    Choosing proper crank length

    If I'm going to order a 105 groupset I need to know which crank length to get: 170mm, 172.5mm or 175mm.

    Is this purely a question of my size, bike fit, and overall geometry? Or does it really matter that much?

    I never gave it a second thought on my mountain bikes, just bought the standard 175.

    Thanks.

  2. #2
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    Whats your inseam and how tall are you?

  3. #3
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    It's a feel thing. As studies can't make a firm, collective conclusion on matching cranks to leg length. If I were you, I'd stick with 175mm since you've been with it previously.

    I personally made a move from 175mm (on a hybrid) to 172.5mm on the replacement road bike. Didn't realize the difference. But the move to 170mm went from tolerable to crap over the span of months. Some riders use different lengths between road and TT bikes.

  4. #4
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    I'm 5'11" and my inseam is 34 I believe...

  5. #5
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    Lennard Zinn on crank length (Link via VeloNews)

    Lennard Zinn crank length follow up (Link via VeloNews)
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  6. #6
    A wheelist
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    It's a "feel" thing with broad generalizations. I'm 5' 8" but with longish (33") inseam. I use 165 for my track bike, 170 for two road bikes and my mtb and 172.5 for another road bike and they all feel good. 175 for my mtb just didn't seem right. I'd suggest 172.5 for you
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  7. #7
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    After much agonizing between 172.5 and 175, I finally made the move to 175. Way more torque. Highly recommended.

  8. #8
    pmf
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    I went from 172.5 to 175 because the cranks that I wanted were on sale, but only in 175. Frankly, I didn't feel any difference. Think about how long 2.5 mm is. It's roughly 0.1 inches. I'd bet cleats differ by more than that. Anyone who can tell you they feel a big difference probably is also enjoying that plush carbon seat post as well.

  9. #9
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    Quote Originally Posted by rubbersoul View Post
    After much agonizing between 172.5 and 175, I finally made the move to 175. Way more torque. Highly recommended.
    "Way more torque" from 1.5% increase in crank length. It's like magic.

  10. #10
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    Quote Originally Posted by JCavilia View Post
    It's like magic.
    More like "sleeping through Physics 101."

  11. #11
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    Quote Originally Posted by pmf View Post
    I went from 172.5 to 175 because the cranks that I wanted were on sale, but only in 175. Frankly, I didn't feel any difference. Think about how long 2.5 mm is. It's roughly 0.1 inches. I'd bet cleats differ by more than that. Anyone who can tell you they feel a big difference probably is also enjoying that plush carbon seat post as well.
    This. Between morning and evening, your feet probably swell more than 2.5mm
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  12. #12
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    Quote Originally Posted by rubbersoul View Post
    After much agonizing between 172.5 and 175, I finally made the move to 175. Way more torque. Highly recommended.
    And yet, we change gears, which cancels out the effect.

    From what I've discovered, it has very little to do with leg length, but the flexibility of the person, and a personal preference. Larger cranks make you work more, because you're flexing over a bigger circumfrence.
    As many will tell you, you won't be able to tell the difference between 170 & 175 on a test ride. Over a long distance, shorter cranks help my legs feel fresher though.
    My carbon footprint has cleats

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    Cadence

    Quote Originally Posted by Patterson View Post
    If I'm going to order a 105 groupset I need to know which crank length to get: 170mm, 172.5mm or 175mm.

    Is this purely a question of my size, bike fit, and overall geometry? Or does it really matter that much?

    I never gave it a second thought on my mountain bikes, just bought the standard 175.
    As others have noted, there is no reliable way to predict what crank length will work "best" for you, as if it would even be possible to define "best." Shorter cranks make it slightly easier to spin. That's about it.

    And whatever you do, ignore the "Way more torque. Highly recommended." comment from rubbersoul. It is pure nonsense.

  14. #14
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    You'll be happy with a 172.5 or 175. If you can, find a bike with 172.5, take it for a spin right after you've ridden your mtb. I can tell a difference between 170, 172.5 & 175 arms within a couple pedal strokes. As other folk have said, you can spin a shorter crank a little easier, and you'll get a bit more leverage on a longer set. I'm sure a 170 would not be your optimal choice.

  15. #15
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    Quote Originally Posted by Kerry Irons View Post
    Shorter cranks make it slightly easier to spin. That's about it. (longer cranks give) "Way more torque. It is pure nonsense.
    As a regular user of 165 to 172.5 cranks (and years of mtb 175 cranks in the past) I know that it's easier to spin shorter cranks. And as I'd rather pedal than push, I choose shorter cranks than longer.
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  16. #16
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    Isn't it (to a degree) independent of your leg length? I know some tall riders who use relatively short cranks....

  17. #17
    LC
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    I would also suggest 172.5 for you

  18. #18
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    The one negative experience I've had with crank length is just before a racing season (when I was younger , which was a long time ago) I changed from a 170mm to a 175mm and that was the only time I had any significant problems with my knees. Since then I use 172.5mm and have never had any other problems.

    Now this is not to say that it was just because they were 175mm and 175mm cranks will give you problems... the moral of this story is don't make a big jump in size and then pound big gears right away... I think it was because I jumped all the way from 170 to 175 and then over did it too soon.
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  19. #19
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    OK thanks everybody for your advice. My trusty local bike shop owner told me to go for 175 based on my height and what I'm used to.

    Thanks.

  20. #20
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    Quote Originally Posted by Patterson View Post
    I'm 5'11" and my inseam is 34 I believe...
    I think thats the "you can ride anything" height.. If you were 6'2 id tell you to go to a 175. If you were 5'2, id say 170, pretty solidly for both.

    Ive been riding my 170 crank'd roadbike all year now.. after hopping on my mtb with 175mm cranks, It felt very off. I used to ride 90% mtb and 10% road using the same two bikes and never really noticed the difference. Im 5'8. Im sure Id get used to my 175 cranks if I rode them more, but the 170 juuuuust a bit feels better.

  21. #21
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    One should consider that your knees lift TWICE the distance of the difference in crank length at the top of the stroke, so about 1cm more for 175mm vs 170. If you have knee problems this could be significant. I have torn ligaments in both knees and can only ride 170mm or shorter cranks.
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