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  1. #1
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    Halp! Crank arms and chain rings confusion

    I just bought my son a Trek Superfly 20. It comes stock with what appear to be 150mm crank arms. His Spawn Banshee is currently 102mm... I think he is tall enough for 130mm though.

    I see I can get 130mm crank arms from Sinz and such, but they are 110mm BCD and I can't find any 32T chain rings I can use with these cranks, only 34T+.

    The other problem, is perhaps I could find 130mm crank arms in 104mm BCD that would let me run a 32T ring, but I have no idea what type of bottom bracket I have, all I know is that the arms are held on by hex bolts like this (Edit: I think it's a square taper):



    (sorry, my camera on my phone is broke)... I contacted Trek last week to ask what size bottom bracket it is but have yet to hear back.

    Does anyone have any insight? I'm not sure how much a difference pushing a 32T vs 34T chain ring would be for a 5 year old if I had to go that route, it is a 6 speed so maybe it isn't a huge deal?
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  2. #2
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    I emailed Spawn and Islabike to see if I could buy one of their 127mm cranks and a 32T ring.

    Alternately, what if I ran the 102mm arms and 28T ring from the Banshee for a little until I get this sorted out?
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  3. #3
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    running a set of these GT cranks on my oldest daughter's 24". Got em from my local shop for $135 with the External BB. He could get from 130mm-165mm. If you currently run a square taper this BB will replace it. I had to mount the Raceface Narrow wide 30 tooth chainring on the outside of the crank to get the chainline right. If you want the ability to run 104 bcd and 64 bcd then I'd go with BikeSmithDesign. He shortens Sram S600 cranks for $107. I currently have a set of these on my youngest daughter's bike. 32 tooth on the 104 bcd for cruising the hood. 28 tooth on the 64 bcd for the trail days.

  4. #4
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    Quote Originally Posted by XJaredX View Post
    Does anyone have any insight? I'm not sure how much a difference pushing a 32T vs 34T chain ring would be for a 5 year old if I had to go that route, it is a 6 speed so maybe it isn't a huge deal?
    My 5-year old daughter is riding a single-speed Redline Pitboss with 38/16 and she is pedaling just fine even uphill, and that was on 140mm cranks that were WAY too long for her. Much better now with the Sinz 115mm cranks.

    I would just run the 34t gear and not sweat it, especially if it's a 6-speed. JMO.

  5. #5
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    I would contact your local bike shop or even trek direct and see if you can get a set of their "grow" cranks to go on it. I have them on my sons trek superfly 20 and they are great. He is 5 and using the 120mm pedal holes at the moment and the 140mm holes are there ready and waiting. We were a bit worried about the crank ends hitting things but it doesn't seem to be a problem. I will try and post a photo or two.

    Halp!  Crank arms and chain rings confusion-colleens-win_20141016_133806.jpgHalp!  Crank arms and chain rings confusion-colleens-win_20141016_133735.jpg

    Sorry but his bike needs a bit of a wash.

  6. #6
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    Cool, so it looks like we're OK then with options... I'm gonna switch the Spawn crank from the Banshee onto the Trek in the meantime.

    I had him sit on it today and with shorter crank arms, a 40mm stem (maybe 50) and cutting the bars, he'll legitimately fit on it. I knew he was sort of small yet but that we could make it work.

    Thanks for the help!
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  7. #7
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    If anyone is interested I just checked and these trek "dialed fit" cranks come is two sizes. 120 / 140 mm and 127 / 152 mm. Santas LBS put them on before he picked the bike up as he got his superfly 20 when he was 4 1/2 so the stock cranks and stem were way too big. Very clever dude that Santa.

  8. #8
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    My advise - don't cut the bars. You can't make the longer again. Just put 130mm grips on and move everything in on the handlebars so he can hold in further. We put a 35mm kore repute stem on. Works well and looks great.

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  9. #9
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    Quote Originally Posted by silvascape View Post
    If anyone is interested I just checked and these trek "dialed fit" cranks come is two sizes. 120 / 140 mm and 127 / 152 mm. Santas LBS put them on before he picked the bike up as he got his superfly 20 when he was 4 1/2 so the stock cranks and stem were way too big. Very clever dude that Santa.
    The shorter trek dialed cranks are made of solid steel, very heavy.

  10. #10
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    Quote Originally Posted by mountaingoatepics View Post
    running a set of these GT cranks on my oldest daughter's 24". Got em from my local shop for $135 with the External BB. He could get from 130mm-165mm. If you currently run a square taper this BB will replace it. I had to mount the Raceface Narrow wide 30 tooth chainring on the outside of the crank to get the chainline right. If you want the ability to run 104 bcd and 64 bcd then I'd go with BikeSmithDesign. He shortens Sram S600 cranks for $107. I currently have a set of these on my youngest daughter's bike. 32 tooth on the 104 bcd for cruising the hood. 28 tooth on the 64 bcd for the trail days.
    Did you weigh them before installing? I was looking at those a few months back and contacted the seller, but they couldn't give me a weight.
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  11. #11
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    Quote Originally Posted by Dave88LX View Post
    My 5-year old daughter is riding a single-speed Redline Pitboss with 38/16 and she is pedaling just fine even uphill, and that was on 140mm cranks that were WAY too long for her. Much better now with the Sinz 115mm cranks.
    I guess "uphills" are very different in your area Our local places to ride are all notably flat, and even though 34/16 single speed racing BMX (140mm crank length) of a tiny 6.5 kg weight (14.4 lbs) possess real problem to my younger son (6 years old) and before that to his older brother (9 y. now) on those uphills that we happily have. And these boys have some strong legs! Oh, they also ride in clipless shoes! => I'd like to say that even 34/16 is rather high gear for kids on uphills, although marginally usable).
    As to the OP question, possible suggestions depend on which cassette (teeth numbers) is on the bike, final crank length and the ratio of a bike weight to a kid weight. In any feasible case I wouldn't go above 34t front. And there is no possibility to go below 34t for 110mm BCD cranks (as there is not enough space for the bolts). If the cassette is something like 28t-14t or wider, then 34t front ring would just be Ok.

  12. #12
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    Well today we went on a 6.5mi ride with the 102mm Spawn cranks (28T) and the 14-28T freewheel that came on the bike. He was in the hardest gear and basically spinning out on the flats. So that made me give up trying to find a crankset with 32T ring, we just ordered a Sinz 130mm crank and 34T ring.

    I think he'd probably be better with 115-120mm but he can grow into the 130. Had I bought him a Spawn or Islabike he'd have gotten 127mm cranks anyway.

    Halp!  Crank arms and chain rings confusion-_20141016_160033.jpg

    Halp!  Crank arms and chain rings confusion-img_20141016_163729162.jpg
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