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I bought two used bikes for my children, both with 16 inch wheels. I was surprised to see that one child has to pedal much faster to go the same speed as the other child, and discovered the two bikes had different sprocket sizes.

Is is common and reasonable to replace a smaller sprocket with a larger one, or should I just try to find a different bike? I assume I will also have to replace the pedals and chain.

Thanks in advance for the advice.

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Can you clarify why you're thinking of replacing things? You seem to be implying that there is a problem but I'm not clear on what the problem is. Is one of the bikes uncomfortable for the child? That aside, if you do replace a sprocket, I wouldn't expect pedals to be affected, and wouldn't necessarily expect chain to be affected unless it had become too short. –  PeteH Jul 14 '13 at 20:04
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It probably could be done, but likely not cost-effective. It's pretty hard to find parts for the small bikes, and so you may end up paying more than for "big kid" parts. You may or may not need to replace the crank (which is likely a 1-piece version). You'll need a new chain (or new links from somewhere) if you make the sprocket much larger. I'd look around for another used bike. –  Daniel R Hicks Jul 14 '13 at 21:23
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PeterH, the problem is simply that child 2 has to work much harder than child 1 when my intention was for them to have the same thing. Daniel, thanks. That's what I was afraid of. I suppose I will just let the two boys out grow their bikes and be more careful when purchasing the next size bikes. –  Paul Jul 14 '13 at 23:56
    
Picture of the rear cog and the chainring for the two bikes? As noted, on some kid's bikes the cost would be prohibitive, but others use more common parts. With a pic or two we could give more specific advice. –  Ken Hiatt Jul 15 '13 at 6:20
    
It should be noted that it will be easier to climb hills on the "slower" bike. And if anyone gets into stunts it may be a better stunt bike. –  Daniel R Hicks Jul 15 '13 at 14:37

1 Answer 1

up vote 2 down vote accepted

You should only need to replace the sprocket, nothing else. If you put on a smaller sprocket to get higher gearing/slower pedalling you need a smaller sprocket, not a larger one. You may need to remove a chain link.

Those parts should be standard, the same as on an adult bike, and any bike shop should be able to make the switch. Coaster brakes are common in Europe on adult commuter bikes and in the US on "beach cruiser" type bikes.

You're looking at a cog like this:

coaster brake sproket and lock ring

Which is about $7 on Amazon, so probably $10 or so from a bike shop, plus labour. You could swap it yourself if you're a little confident - just take the wheel off, use a screwdriver to pop the lock ring, swap the cog and put the lockring back (beware: the lockring is a spring and likely to go flying). With an o-ring plier they're easy enough to reinstall or you can use a couple of screwdrivers to push it on. The only hiccup might be if you need to remove a chain link to get it to work, in which case you will need a chain tool as well.

The main issue is 15-20 minutes of the mechanics time, which will cost you. It will be cheaper if you pop the wheel off before taking it in, or at least make sure it's easy to do so (but if that's tricky you probably want the mechanic to do it all anyway).

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I'm guessing he wants to replace the front sprocket. –  Daniel R Hicks Jul 16 '13 at 11:19
    
Daniel, I assumed he wanted to solve "gearing too low" and would accept a cheaper way to get that result. –  Mσᶎ Jul 16 '13 at 22:03

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