Although the bicycle rear derailleur looks complicated, it can be changed--even by an amateur--in just a few minutes. The derailleur is the mechanism that allows your bike to shift gears. Located on the bike frame next to the rear tire, the derailleur lifts the chain out of position to provide slack so it can be put into the next gear. Once the bike is in the next gear, the derailleur tightens the chain to keep you moving.

1. Remove the idler pulleyby pulling the chain backwards while you slide the pulley in the opposite direction (forward). The idler pulley is the bottom pulley of the derailleur and the lowest pulley to the ground. If you can't remove the idler pulley, you'll need to break the chain with a pair of wire snips.

2. Unscrew the derailleur from the hanger (the bracket holding the derailleur in place) by inserting a hex wrench into the bolt on top of the derailleur and turning counter-clockwise. This will allow the derailleur to pull away from the hanger.
 
3. Remove the derailleur cableby loosening the hex bolt on the clamp.
 
4. Pull the end cap off and slowly pull the cable through the derailleur.

5. Put the new derailleur in position. Be sure to keep the tension-adjusting screw slightly above the notch on the hanger.
 
6. Screw the mounting bolt into the hanger.
 
7. Place the chainonto the derailleur. You'll need a new chain if you broke it earlier when removing the idler pulley.

8. Pull the chain downward and set the idler pulley on top of it, making sure the chain lines up with the teeth of the pulley.
 
9. Push the derailleur cable through the hole on the back of the derailleur.

10. Thread the cable through the cable clamp and screw the bolt in place to tighten.

11. Lift the rear tire--or turn the bike upside down--and rotate the pedals to verify the chain and derailleur allow the rear bike wheelto turn.

12. Take the bike for a short ride and shift through all the gears to ensure the derailleur is installed correctly.