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Purchased a used bike for my child a few months ago. It is one of those where to brake you pedal backwards. Anyways, I was noticing that she was having a hard time starting. Once she got going, everything was fine. Upon closer examination, I noticed that the sprocket on the back was slipping. It would turn, but the wheel wouldn't turn.

How do I fix this?

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This is a "coaster brake". Generally they are pretty trouble-free, but can get mucked up sometimes. (And, I suppose, there are cheap ones coming from China that fall apart.) If you're mechanically inclined, take it apart. (I was doing this when I was 6.) If not, take it to a bike shop. There are only 3-4 basic designs, and you can find diagrams for most on the Internet. –  Daniel R Hicks Dec 17 '13 at 17:25

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I guess that the spring isn't hold the sprocket well. The best is to bring the bike to shop to repair it.
But if you want to repair it all alone, first get your rear wheel off, get the sprocket off and check the 3 teeth inside the sprocket - they have to be perfect half-circles. If they aren't, you should replace it.
The spring must touch itself on its' ends. If it's not touching, put it pliers that way that the ends of spring will be out of pliers and squeeze it more than needed, so the ends will come round one on other, then turn the spring this way that the ends will be inside the pliers and squeeze it again. Check the spring, that it tight enough.
Put the sprocket on back. To return the spring you should use a screwdriver or something.
If the problem still exists (though it shouldn't), it's inside the wheel, and there is more complicated.

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You don't seem to be describing a coaster brake. –  Daniel R Hicks Dec 17 '13 at 20:21
    
Pretty much all brakes that are activated by back-pedaling are referred to as "coaster brakes". It sounds like you're describing some sort of freewheel -- a coaster brake is quite a bit more complicated. –  Daniel R Hicks Dec 17 '13 at 21:14
    
I didn't explain the inside pattern of it. Only about the sprocket. BTW here is good diagram: sheldonbrown.com/coaster-brakes/shimano-cb-e110.jpg . There are bikes that spring isn't hard enough to hold the sprocket, therefor the sprocket is slipping and wiping it's mounting holders. –  Alexander Dec 17 '13 at 21:20
    
I'm speaking about the "torpedo". A drum brake. –  Alexander Dec 17 '13 at 21:25
    
The brake in that diagram is called a "coaster brake" in the US -- that one is pretty much the Bendix design. A "drum brake" is generally activated by a cable or rod (and is an extreme rarity in the US -- only used on tandems and heavy touring bikes, prior to disk brakes). –  Daniel R Hicks Dec 17 '13 at 21:44

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