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I'm getting ready to build up a small 26" bike for my 12 y.o. and am thinking about the right crank length. My initial thought is to just go with a 165 and not to worry about it. Any thoughts?

I found this great chart on Dans Comp for BMX crank sizing which may or may not be relevant for mountain biking: Crankchart at Dan's Comp

Crank Length for BMX bikes.jpg
 

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when I went to 125mm on road bike and 135mm on the MTB and cross bike....much better....couldn't stand my 165mm....glad I went short....wish I had thirty years ago...that chart is crazy wrong....29" inseam...
 

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My initial thought is to just go with a 165 and not to worry about it. Any thoughts?
FWIW, I took the "don't over think it" strategy when I put 165s on my daughter's bike when she was 11. Seems to have been a good choice for her.

https://forums.mtbr.com/families-riding-kids/2005-cdale-f800-re-build-my-11-yr-old-851395.html



The only problem is that the turkey is growing out of the Size S already. I found a Size M for next year that I plan on putting 170mm cranks on.



Luckily, my youngest can move up to the Small next year. Then I'll sell (or part out) the 24" wheeled bike with 152mm cranks

https://forums.mtbr.com/vintage-retro-classic/1995-cannondale-mc400-youth-sized-killer-v-24-wheels-534377.html

 

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I have a 32'' inseam and wouldn't run a 185mm crank. I went with the 10% of height. From what I was able to read online, too long is worse than too short.
 

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you need take into account body position and possible gearing... plus rider skill level. A new rider is going to prefer their saddle as low possible. Body position is not going to be correct anyways and you are not going to get the benefits of the short arms. The short arms will allow you to raise your seat higher (allowing proper knee extension) and most young rider feel more comfortable when they can touch both feet on the ground. If that applies to your little one that like to be able to touch both feet on the ground, longer cranks will provide a more comfortable ride (except for hill climbs, will require them to be out of the saddle). If more advance rider, especially if doing long or a lot of hill climbs, I would be looking at shorter cranks to be able to raise their seat to put them in proper riding position.
 

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I just moved my son from 160mm to 170mm and it made a crazy difference in his climbing ability for the better. he is 60"
 
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