A smooth-shifting rear derailleur is key to get the most from your road or mountain bike experience. The derailleur attaches to the back of the bicycle frame and engages the chain to move it over the cassette. The cassette, also known as the sprocket, is the cluster of toothed circles in the centre of the rear wheel. Most rear derailleur problems can be attributed to loosening of the derailleur cable or a faulty spring in the derailleur housing. Springs stretch over time, as does the cable, making the derailleur miss shifts.
Place the bicycle into the work stand. Gripping a pedal, turn the bicycle's crank to rotate the rear wheel and use the rear shift lever to move the chain onto the wheel's largest sprocket.
Turn the barrel adjuster -- the small plastic tubelike cylindrical piece surrounding the cable at the back end of the derailleur -- on the rear end of the derailleur to the right. Use one-quarter-turn increments to tighten the rear derailleur cable, making for tighter shifts with the device. Turn the crank and shift through the gears with the rear derailleur. Check that the adjusted derailleur shifts through all the gears and the chain moves between each toothed wheel in the cassette.
Remove the derailleur from the bicycle frame dropout, the small piece of metal that meshes into the frame where the derailleur attaches to the bike.
Unscrew the derailleur plate with the small Allen keys. Take the plate off to find the spring. Pry the spring off the derailleur.
Grease the replacement spring by rubbing a coating of bicycle grease over each curve in the spring. Attach the spring to the derailleur housing. Screw the plate back on the derailleur. Attach the rear derailleur to the bicycle derailleur hanger.
Turn the barrel adjuster on the derailleur to the right to tighten the reattached derailleur and cable. Turn the pedals and shift through the gears to ensure smooth shifting between all toothed gears on the cassette.