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what the bike comes with is a good place to start. Or you could ask here and get a lot of different answers.

In the end it will boil down to can you really tell a difference and if so get the one You like.
 

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Do you realize the size of the can of worms you just opened? You'll get all sorts of opinions on crank arm length. I've seen everything from 115mm to 185mm - maybe longer? Some say shorter cranks are better for the knees, but I've talked to people who swear that the opposite is true...

If you like what's on your bike, I'd stick with that. If you have issues smacking pedals on trail features, shorter cranks can help, but there's give and take with everything.
 

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Awesome. Just looking through everything n saw this post. Thank you 4 posting that video it helped alot. Starting 2 look 4 a new one myself.
 

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Fat-tired Roadie
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Pez did a roundup of a few studies a while ago.

Crank Length - Does Size Really Matter?

It's pretty equivocal.

One of the takeaways is that sustained power doesn't change with any of the lengths that are in production. I think there're two big pros for everyone with shorter cranks: ground clearance, and one study did find a correlation with spinning up to peak power faster. And there's a pro with longer cranks for mountain bikers, which is that you don't have to rotate the crank through as much of an angle to develop the same power. Which is to say that for ratcheting or popping up the front wheel, longer cranks allow use of a higher gear, which can be advantageous.

There's a third big pro to shorter cranks for me. I don't have a great range of motion, in terms of knee angles where I can develop a lot of force, so it's harder for me to get a bike nicely set up with 175s, while shorter cranks are easier. So I'll probably just stick with 170s, since they tend to come stock anyway.
 

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How do you know what length crank arms to use?
I have ridden everything from 165mm to 225mm cranks.

Found that I prefer the feel of 182-185mm and the greater range of motion in my knees and hips.

Shorter than 180 and it feels like my shoe laces are tied together. Longer than 190 and I have trouble lifting my knees over the top of the pedal stroke.
 

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Longer than 190 and I have trouble lifting my knees over the top of the pedal stroke.
That's the biggest advantage to shorter cranks in my opinion, ie not having to lift your knees as high at the top of the pedal stroke. Just feels more comfortable, and less upsetting forces (if you will) to balance etc. Seems to me the trend is toward shorter cranks for mtb's and the old standard of 175 was just a throwover from road bikes. I'm 6'2" and recently switched to 170, like it so much better thinking of 165
 

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That's the biggest advantage to shorter cranks in my opinion, ie not having to lift your knees as high at the top of the pedal stroke. Just feels more comfortable, and less upsetting forces (if you will) to balance etc. Seems to me the trend is toward shorter cranks for mtb's and the old standard of 175 was just a throwover from road bikes. I'm 6'2" and recently switched to 170, like it so much better thinking of 165
And for me "shorter" is 185. It is about having the range of movement that suits you.

175mm cranks did not come from road bikes. 170mm was and still is by far the most common length on road bikes.
 
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