All Mountain Bike Parts Descriptions

#16) Bike Chains - The crank to drive-wheel link.
  The Chain folds around and through both the crank and the rear cog set and is able to ramp up and down the different gear combinations depending on your present speed. Chains come in varying widths, like fat for single speed (usually fatter style chain rings have longer life and better strength), or super narrow (to squeeze between up to ten tightly spaced cogs like on a road bike). There is a super fat BMX chain out now that would make great jewelry too.

Bike Parts Diagram