Is your bike chain loose? Is it coming undone while you ride? Check your chain out. It should be tight enough that it only allows you to move it up and down about one inch. If it is sagging or much looser than that, you need to tighten that chain up.
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Chains often loosen when a bike does not have a derailleur. Derailleur is the gear changing mechanism towards the back of the bike. The derailleur helps to pull the chain and keep it tight. Bikes without a derailleur are single-speed bikes or fixies.
Tools: Grab a wrench that will fit your rear axles.
1. Loosen your tire. Loosen the bolts that hold your rear tire.
2. Pull back the tire. Move the tire back until the chain starts to tighten.
3. Find the appropriate tension. When you have reached the point where the chain is tight enough to only move about a half of an inch in either direction you need to tighten the bolts and your wheel into place. It may be easier to have someone help you so that they can hold the wheel in this position as you tighten it.
If the chain is not getting tighter after you have done this than it is stretched, and time to replace the chain.
If you have a bike that has a derailleur that is not creating chain tension, than it may need to be replaced. This often occurs when the derailleur loses some of its springiness and can not effectively create chain tension.