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  1. #1
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    Any issues swapping to shorter crank arm?

    From what I've been reading, the 140mm standard on kids' 20" bikes is on the long side for the kids. The bikes considered some of the best all have 127mm crank arms. Can I just swap out a 140mm crank arm to a 127mm crank arm on a bike without having to change anything else? Or am I setting myself up to have to change to a different sized front crank or different freewheel?

    The bike I'm looking at has 140mm crank arms with 36T crank, and 14/28 in the rear, 6-speed.

    The 127mm bikes I looked up are as follows:
    32T front, 12/32 rear, 7-speed
    36T front, 14/34 rear, 7-speed
    36T front, 14/28 rear, 7-speed

    I'm unsure if that larger rear on a couple of the bikes is better when you have a shorter crank arm? It seems the last bike I listed does it with the same 14/28, but it's a 7-speed and I don't know if that makes a difference.

    Or maybe I'm just making this too big of an issue and she'll be fine with the 140mm?
    Last edited by QueenMAUW; 05-07-2014 at 01:52 PM.

  2. #2
    I Tried Them ALL... Moderator
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    Don't even sweat it, mom. Kids are one of the most adaptable creatures on this Earth....
    "The mind will quit....well before the body does"

  3. #3
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    Quote Originally Posted by Zachariah View Post
    Don't even sweat it, mom.
    This does not match up with my personality.

    From what I'd be reading it seemed like I was going to be damaging my kids knees or making it impossible for her to pedal and making her hate cycling, so ya know, I had to freak out and try and make it perfect.

  4. #4
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    Shorter crank arms will create slightly less torque but will likely be more biometrically efficient for smaller rider so they will benefit overall. I wouldnt worry about changing the chainring and rear cluster gearing in response to a change in crank length. They will naturally be able to adjust for any slight difference just by shifting to a lower gear when needed, except of course when they are already in 1st gear. A 32/28 low gear on a 20" wheel is fairly low, I would only consider changing out to lower gearing if you find the kid is unable to ride up the hills that they commonly encounter, stalling out on hills will be very apparent and happen much sooner than any possible knee damage from being geared too high.

  5. #5
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    Thanks GrayJay. That's super helpful. So I had a typo in my first post (which I've now corrected), the bike we're interested in is actually 36 front and 14/28 rear. Does that change your opinion on the lower gear at all? We're not a hilly area, quite flat in fact.

    I'm ordering the bike in and I'm going to see if the local bike shop can swap out the new crank arms on assembly for me and save me any work.

  6. #6
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    36/28 low gear would probably be OK for flat riding but I also doubt that a 20" wheel kid would need a 38/14 high gear, if you can get a 32 tooth chainring on the new crank that would be better idea to give them option for a lower gear that they might actually use.

  7. #7
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    Quote Originally Posted by GrayJay View Post
    36/28 low gear would probably be OK for flat riding but I also doubt that a 20" wheel kid would need a 38/14 high gear, if you can get a 32 tooth chainring on the new crank that would be better idea to give them option for a lower gear that they might actually use.
    I can totally look into that. So stupid question here, does that mean I'll need a new chain too? Or can the current chain be adjusted to fit since I assume with the new amount of teeth it will fit differently?

  8. #8
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    My daughter does great with a 115m crank length. She used it on her 20" Hotrock, and I just swapped them onto her 24" Trek.

  9. #9
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    Manufacturers don't really pay much attention to crank lengths fwiw. That said shorter is generally better for kids.

    On my daughters Hotrock 20, I switched to Redline Microline 130mm crank arms with 32T front ring. I also switched the bike to 8 speed with a 13/34 freewheel. The Redline arms are pretty darn light compared to the available list of arms below.

    BMX PARTS - Cranks - Crank Arms - J&R BMX Superstore BMX Bikes, Parts and Gear- Custom Bikes and Wheels

    As far the chain, you might be able to get by. Chains are pretty cheap too so it won't hurt to get a spare just in case.

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