It is legal for bicyclists not to wear a helmet?

No single answer to that, the laws can be different from one state to another.

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Why does your bike chain get jammed between the gears?

Well, you might be running the wrong kind of chain, or your shifter isn't adjusted properly. www.parktool.com and www.sheldonbrown.com are two excellent sites that can help you figure out what's wrong.

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