How to Change a Rear Derailleur
Rear derailleurs are the metal arms that shift the chain of a multispeed bicycle from one back gear to another in response to changes in tension in a cable connected to shifters mounted on the handlebars. These gears allow cyclists to handle varying terrain and inclines by changing the ratio between the revolutions of the pedals and that of the back wheel of the bicycle. Derailleurs are finely tuned pieces of equipment that, if improperly maintained, can rust and seize up and must be replaced. With some basic tools and bike maintenance skills, this is possible to do yourself.
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Instructions
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Detach the idler pulley -- the small gear on a spring at the bottom of the derailleur -- by using a hex wrench (also known as an Allen wrench) to loosen and remove the bolt holding it to the rest of the derailleur. Set the pulley aside. This will allow you to pull the derailleur free of the chain.
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Detach the derailleur from the derailleur hanger on the bicycle's frame by inserting a hex wrench into the hexagonal hole on the bolt holding the two together and turning it counterclockwise until the derailleur comes loose. Note how the bike chain fits into the derailleur so that you can thread it the same way when you install the new derailleur.
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Pull the end cap from the derailleur cable using a pair of pliers, then use a hex wrench to loosen the nut holding the cable down.
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Pull the freed cable out of the derailleur.
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Carefully work the old derailleur loose from the chain and set it aside.
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Attach the new derailleur to the derailleur mount by threading the included bolt through the hole on the derailleur and into the frame. Hold the derailleur slightly above and back from its normal position, and tighten the bolt slightly with a hex wrench. This will make installing the chain easier.
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Route the chain through the derailleur, matching the arrangement you took note of when you removed the old one. It must go above and in front of the top pulley, then around back of the lower pulley and forward underneath the bottom one. Once the chain is threaded through, reinstall the idler pulley. This will keep the chain properly tensioned.
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Thread the derailleur cable through the new derailleur, pull on it to remove any slack, and tighten down the cable nut with a hex wrench.
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Set the limiting screws. These are small screws on the back of the derailleur that limit its motion so it lines up with the back gears. One limits the derailleur's motion at the lowest gear, and one at the highest. To adjust the high-gear screw, shift into the highest gear, then tighten it until the upper pulley exactly lines up with the smallest cog, then give it a 1/8 to 1/4 turn to move it inward slightly. Adjust the other screw by shifting into the lowest gear (with the chain on the big cog) and tightening the other limit screw until the pulley is just a hair inside of lining up with the biggest cog.
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Tighten the derailleur cable where it enters the shifters on the handlebars. To do this, shift the gears repeatedly while twisting the small metal barrel that tensions the cables as they leave the shifters. Adjust the tension until one click of the shifter moves the derailleur the exact distance necessary to place it over the next on the back gears.
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References
- Photo Credit a downhill mountainbike gear wheel and gearshift mechanism image by Stephen Gibson from Fotolia.com
