A rear derailleur is a device that attaches to the back wheel of your bicycle and changes gears for you. It allows all riders, from children to seniors, to enjoy cycling in any terrain without having a strain on their wrists or hands throughout the ride!
A rear derailleur moves with two pulleys which connect it into an open cable loop (a gear chain). The chain loops around one cog at a time while moving forward as you pedal. A wire connecting both pulley wheels must be pulled by hand during shifting so that tension stays constant across each cogs’ teeth before releasing them when they are aligned properly; this keeps pressure off of these shifts and prevents wear and tear