Replacing the front derailleur is one of those jobs that seems impossibly complicated. But once you've done it, it seems ridiculously easy. The hardest part is buying the correct derailleur for your bike.
How do you know you need a new front derailleur? (1) The cage won't hold its shape. (2) The cage can be moved and forth more than a whisker when you wiggle it with your fingers. (3) The mechanism sticks or catches despite cleaning and lube.
How do you order a new front derailleur? You need the correct "clamp size" (the clamp is the ring that anchors the derailleur to the bottom of your seat tube). Clamps come in 1-1/8", 1-1/4", and 1-3/8". Most bikes will use the 1-1/4" size. If you're holding a ruler against the bottom of the seat tube to see what size you need, you'll probably underestimate the size. For example, if the outline of the tube falls exactly on 1-1/8 on your ruler, you probably need the 1-1/4 size. Front derailleurs can be activated from top or bottom. You need to get a "bottom pull" (if the cable comes into the derailleur from underneath the bike) or a "top pull" derailleur if the cable comes down to the derailleur from above.
If you have a "Quick Link" connector in the chain, that's easy. Otherwise, use your chain-breaking tool to push out a pin. See our chain repair section.
Loosen the cable-clamp bolt with a hex wrench until the cable comes free of the derailleur.
Remove the clamp from the seat post.
Check to make sure the cage of the derailleur will clear the large chain-ring when the hinge extends.
Set the limit screws so the inner wall of the derailleur cage is 2 mm (just less than 1/8 inch) away from the chain while in the small chainring and the biggest biggest cassette cog, and the outer wall is 2 mm from the chain while in big ring - smallest cog.
If necessary, loosen the clamp and turn the derailleur so it's parallel to the chainwheel.
Adjust the derailleur limiting screws and cable tightness as needed. See our derailleur tuneup section .