Join Date: Oct 2007
Location: Québec, Québec, Canada
Bikes: Fuji Touring 64cm, Peugeot Canada PB-18 61cm and winter beaters
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I've seen a few seized up derailleurs that simply needed oil.
Lots of oil.
Step 1: Take a rag and clean your derailleur as well as you can. Use a shoe-shine motion to clean areas that are difficult to access. You can also use an old toothbrush to finish the cleanup once you're done with the rag.
Step 2: Oil. Lots of oil. Light oil that isn't too viscous, something like triflow works great. You want to oil every pivot point (at least 6 of them) as well as the spring. If in doubt, add more oil.
Step 3: Work the derailleur to get the oil in everywhere.
Step 4: Take a clean rag and remove the excess oil.
Derailleurs need to be oiled regularly to keep working well. If they dry out, they can seize, squeal, or otherwise stop working well.
Rear derailleurs have even more points that need to be oiled - 8 pivot points for in the parallelogram (4 corners in front, 4 corners in the back), a spring, 2 rotating pivots (1 for the frame attachment, 1 for the cage), and 2 jockey wheels (oil the crack on both sides of each jockey wheel).
You should always clean a derailleur before oiling it, to avoid getting dust and grime inside the inner workings. You should always wipe it off afterwards, otherwise the excess oil will attract more dirt and grime.