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Thread: current thinking on crank length

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    Default current thinking on crank length

    Seems a lot of MTB folks are rethinking their crank lengths. For purposes of avoiding pedal strikes, but also for purposes of spin, muscle recruitment, ease on joints, etc.

    What is the consensus in this part of the MTB world?

    My current numbers:
    • inseam - 90cm
    • MTB saddle height - 79cm
    • crank length on '19 Trek Remedy - 170mm, pedal strikes lead me to this length
    • crank length on '21 Specialized Epic EVO - 175mm, but considering going to 170mm for purposes of spin etc; no issues with pedal strike

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    Default Re: current thinking on crank length

    I went with 170 on all my bikes some years ago, having ridden 165 on track to 175 in the woods. Never looked back.
    Jay Dwight

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    Default Re: current thinking on crank length

    Change is bad.
    I run 180s road and offroad.

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    Default Re: current thinking on crank length

    170 for spin and pedal strike reasons. I subsequently changed my road cranks to match.
    Dan Fuller, local bicycle enthusiast

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    Default Re: current thinking on crank length

    Whatever feels best.

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    Default Re: current thinking on crank length

    Quote Originally Posted by twowheels View Post
    Whatever feels best.
    Hippy.

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    Default Re: current thinking on crank length

    Quote Originally Posted by Too Tall View Post
    Hippy.
    More like a simpleton

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    Default Re: current thinking on crank length

    I think that what you're using for pedals has a lot to do with this. Like if you're using flat pedals with your foot farther forward than with clips, you'll want shorter crankarms. And a steeper seat angle.

    I'm sticking with 175's. After seeing that I could spin them fine in the woods, I switched to them on the road too.

    Does 5 mm of crankarm length really make a difference for pedal strikes in the woods? This push seems like a gimmick or fashion to me. And speaking of fashion, this is how the Stones dressed back when I made the decision to run 175's on everything.

    Trod Harland, Physical Educator

    Not everything that is faced can be changed, but nothing can be changed until it is faced. — James Baldwin

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    Default Re: current thinking on crank length

    Quote Originally Posted by thollandpe View Post
    I think that what you're using for pedals has a lot to do with this. Like if you're using flat pedals with your foot farther forward than with clips, you'll want shorter crankarms. And a steeper seat angle.

    I'm sticking with 175's. After seeing that I could spin them fine in the woods, I switched to them on the road too.

    Does 5 mm of crankarm length really make a difference for pedal strikes in the woods? This push seems like a gimmick or fashion to me. And speaking of fashion, this is how the Stones dressed back when I made the decision to run 175's on everything.

    My experience with going with 170mm cranks on my Remedy (a 27.5 bike with a lower BB height) I definitely have few pedal strikes.

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    Default Re: current thinking on crank length

    Quote Originally Posted by thollandpe View Post

    Does 5 mm of crankarm length really make a difference for pedal strikes in the woods? This push seems like a gimmick or fashion to me. And speaking of fashion, this is how the Stones dressed back when I made the decision to run 175's on everything.

    The whole shorter cranks = fewer pedal strikes might be technically true but I'm not subscribing. Maybe it is different on different terrain than what I ride but whenever I pedal strike it is because the pedal was in the wrong place. I'm not cursing my cranks for being too long but my skills.

    Oh, but I'm using shorter cranks than I used to. My knees seem to like it.

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    Default Re: current thinking on crank length

    @jscottyk, what pedals are you running?

    Flat pedals also present a bigger target for rock strikes, so that's another way pedal choice affects crank length.

    A wider Q factor also increases rock strikes, I've noticed this on the fat bike. Having a fat bike with a low BB is great in snow but those wide, low pedals are rock magnets. Luckily those monster tires allow you to adjust technique to ride over everything instead of trying to ride around it.
    Trod Harland, Physical Educator

    Not everything that is faced can be changed, but nothing can be changed until it is faced. — James Baldwin

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    Default Re: current thinking on crank length

    Quote Originally Posted by thollandpe View Post
    @jscottyk, what pedals are you running?

    Flat pedals also present a bigger target for rock strikes, so that's another way pedal choice affects crank length.

    A wider Q factor also increases rock strikes, I've noticed this on the fat bike. Having a fat bike with a low BB is great in snow but those wide, low pedals are rock magnets. Luckily those monster tires allow you to adjust technique to ride over everything instead of trying to ride around it.
    Was running OneUp flats. Now running CrankBros Mallet E. I think pedal thickness is about the same'ish.

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    Default Re: current thinking on crank length

    Quote Originally Posted by rec head View Post
    The whole shorter cranks = fewer pedal strikes might be technically true but I'm not subscribing. Maybe it is different on different terrain than what I ride but whenever I pedal strike it is because the pedal was in the wrong place. I'm not cursing my cranks for being too long but my skills.

    Oh, but I'm using shorter cranks than I used to. My knees seem to like it.
    Yeah, I am just an unskilled knob who likes to curse my equipment.

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    Default Re: current thinking on crank length

    The “shorter crank arm thing” for clearance comes with full suspension trail bikes with “modern” amounts of bb drop and the assinine stump pulling low gears widely available these days. Unless you have a bike with plenty of spring and phenomenal kinematics, any kind of pedal is getting closer to the ground more often these days.

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    Default Re: current thinking on crank length

    A 5mm difference is nothing, both physiologically and regarding clearance to avoid pedal strikes. I've had bikes with both 170 and 175mm cranks and would switch without realising there was a difference.
    --
    T h o m a s

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    Default Re: current thinking on crank length

    If you cant notice a 5mm difference in crank arm length on a full suspension mountain bike, it means you aren’t having enough fun in the corners!
    Clips or flats, 5mm is 5mm, but because its 5mm that swings 360 degrees and you can’t always predict exactly how a bike will squat, it makes a much more significant difference in terms of clearance than adding or subtracting 5mm of BB drop. I don’t want to waste my limited cognitive processing abilities worrying about ground clearance.

    Having your feet 10mm closer together clears up a bunch of swing room laterally as well as vertically and it’s times that you are swapping from left to right across a stump filled rut to flick a corner setup that pedal clearance really becomes worrisome. Same pedals, same suspension settings, I absolutely have to modify corner entry and exit strategies if i swap crank lengths, otherwise the pedals certainly drag on the ground and excavate, when parallel(with the fall line), on a line where previously the pedals would just barely kiss the ground.

    The steeper the terrain and the better the descending form(eyes up, heels down!)of a rider, the more likely the back foot will hook the ground in a rough chute or rut.

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    Default Re: current thinking on crank length

    Quote Originally Posted by suspectdevice View Post
    Clips or flats, 5mm is 5mm, but because its 5mm that swings 360 degrees [...] it makes a much more significant difference in terms of clearance than adding or subtracting 5mm of BB drop.
    I want to see your mathematical explanation.
    --
    T h o m a s

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    Default Re: current thinking on crank length

    Quote Originally Posted by sk_tle View Post
    I want to see your mathematical explanation.
    Is math going to buy me new crankarms or stitch up my knees?
    On the average i spend 10+ hours a week solely building and riding corners for my own amusement, and then i go to work and get paid to build corners for other people
    How much time do you spend cornering?
    How many corners did you build yesterday?

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    Default Re: current thinking on crank length

    Quote Originally Posted by suspectdevice View Post
    Is math going to buy me new crankarms or stitch up my knees?
    On the average i spend 10+ hours a week solely building and riding corners for my own amusement, and then i go to work and get paid to build corners for other people
    How much time do you spend cornering?
    How many corners did you build yesterday?
    What kind of answer is that ?

    You claim that a 5mm difference in BB drop makes less difference than a 5mm of difference in crank length. Just show me the magical equations or geometry schemas that back this extraordinary assumption. Maybe I am stupid and missing the obvious but it doesn't seem logical.
    Last edited by sk_tle; 01-28-2021 at 10:15 AM.
    --
    T h o m a s

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    Default Re: current thinking on crank length

    Quote Originally Posted by sk_tle View Post
    Maybe I am stupid and missing the obvious but it doesn't seem logical.
    I don’t think you are stupid, but ffs how can you not understand that center of mass and mass transfer has more of an effect on ride height than bb drop? We aren’t riding on a flat plain. The cranks are connected to your feet. your feet are connected to your hands. Those two contact points are all you have to drive the bike with. When your feet are closer to the ground because you are initiating a weight shift by loading and squatting the system, your back foot is inherently very close to the ground in the split seconds before you weight shift to the inside handlebar grip. The lateral translation that occurs in the cornering event happens while the back foot is temporarily unweighted- so both overall length and “swing length” come into play, as a shorter crank ratchets back up more quickly and is also farther from the ground... because it’s shorter... all the time.

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