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  1. #1
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    mods for kids bike

    I just got my son set up with a 20" wheel bike this year. While it's a good bike, it came with a few problems. The lowest gear combo was 36-28 which just isn't low enough to get up much of a hill. And, the front fork was a bit oversprung, somewhere in the 60-70 lb/in range if the sketchy documentation is to be believed.

    The stock single chainring was spotwelded to the crankarm so needed a replacement crankarm to get a lower chainring. I drilled a new hole on an old sugino crank and cut off the end to get a 140 mm length. Replaced the original 14-28 freewheel with the well-known 14-34 'megarange' freewheel. Combined with a 30t chainring, that will provide for 30-34 a big improvement climbing hills.

    Ordered some lighter springs and elastomers from mcmaster in the 30-55 lb/in range. I may have to try some different combinations to get something optimal. The springs come out the top of the fork crown so they're easy enough to change back and forth.

    We're riding the road apples trail and the beginner stuff at joyride. He seems to like it so far!
    Attached Thumbnails Attached Thumbnails mods for kids bike-xcrspring.jpg  

    mods for kids bike-crank.jpg  


  2. #2
    No. Just No. Moderator
    Reputation: Circlip's Avatar
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    Quote Originally Posted by Kay. View Post
    I just got my son set up with a 20" wheel bike this year. While it's a good bike, it came with a few problems.
    Nice mods. I remember going through a similar process trying to set up one of my kids with a decent rig without spending a fortune. I found a stock solution with very nice 3x8 gearing and very manageable quality drivetrain and other components, except perhaps for the fork which may as well have been rigid! Then again, at the weight of my kids they were more likely to see "suspension" out of tires inflated more relative to their very light weight, rather than the recommendations on the sidewalls.

    Unfortunately, that was a quite a few years ago from a company that no longer exists. Kids' bikes are a real head scratcher sometimes and options are often poor as even your mods are beyond the capabilities of the vast majority of people.

  3. #3
    Evil Jr.
    Reputation: garage monster's Avatar
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    I don't know why but I especially like the crank mod. Nice work!
    Please enjoy seeing this terrible collection of me - something wonderful is about to happy.

  4. #4
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    Got the build more or less finalized.

    1. The 1x7 megarange idea didn't work out so well. The stock derailleur didn't seem to be able to keep clear of the 34 and 25 tooth cogs which are side by side on megarange cogset. I thought B tension was the answer to this problem, but couldn't find any such adjustment on the stock derailleur. It was a low-end shimano item, similar to tourney, but notable in that is was a short-cage unit, which is what you want on a 20" wheel and all you need in a 1x7 configuration. I replaced it with a short cage road derailleur that has B tension, but never did get back to the megarange cogs.
    2. Instead, I just went to a 2x7 config now with 20/30 on the front and the stock 14-28 cog set on the rear. To get the chainline, had to swap out the stock bottom bracket for a shorter spindle. The LBS came up with a brand new 20t shimano chainring on 5 bolt 58 mm diameter... not bad at all. To replace the singlespeed 30t I started with, I ordered one of the last remaining NOS 94mm ramped rings on the open internet.
    3. The front shifting was a bit challenging. Didn't have any braze-ons for the cable, and the LBS had no clamp on cable stops in 38mm which I needed for the downtube, but one day I spotted a plastic clamping bracket on the ground that had fallen off someone's commuter bike, and it just so happened it was 38 mm size I needed. After that was fixed up, I promptly found out why 20" wheel kids bikes just don't have front derailleurs. The grip strength needed to upshift a front derailleur is just too high. I tried all three shifter modes, under the bar thumb (trigger), gripshift, and old-fashioned over the bar thumb (friction), but no luck. So, we do the upshifts with parental assistance for now. Mostly, the 30 tooth is used on the ride to the trail, and the 20t on the trail, so it's only 1 up and down shift per ride anyway.

    So the upshot is that with 20/28 gearing, it's possible to ride many of the ups in the don, and with the 30t ring, he can keep up with his friends on pavement who all have big ring bikes or bmx bikes. (It's been a fun hobby project for me too.)

  5. #5
    No. Just No. Moderator
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    Quote Originally Posted by Kay. View Post
    The grip strength needed to upshift a front derailleur is just too high. I tried all three shifter modes, under the bar thumb (trigger), gripshift, and old-fashioned over the bar thumb (friction), but no luck. So, we do the upshifts with parental assistance for now. Mostly, the 30 tooth is used on the ride to the trail, and the 20t on the trail, so it's only 1 up and down shift per ride anyway.
    The upcoming XTR Di2 will solve that for your littl'un in a jiffy. More seriously, sounds like a great project with problems to solve vs. modern higher end stuff which more often than not is just plug and play.
    Nuke the entire site from orbit. It's the only way to be sure.

  6. #6
    sheep in FOX clothing
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    Impressive mods.

    I gotta confess: I got son-of-ghettocruiser a 20" singlespeed, and tried to tell him that the reason other bikes had gears is because the weight of the extra gears made them go up hills slower.

    Which might have been a more plausible explanation if I actually owned a singlespeed myself...

    So yeah, he has a geared bike now as well. He did notice the gripshift was pretty stiff to turn, but he seems to be used to it after a few days. He goes way too fast already, so no mods are in the immediate future.

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