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Replacing the Front Derailleur

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.

First, you need to get the chain out of the way. (Unless you have a rivet tool and the correct rivet to reassemble the derailleur cage.)

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.

Remove the old derailleur.

Loosen the cable-clamp bolt with a hex wrench until the cable comes free of the derailleur.

Unscrew the hex bolt that tightens the clamp onto the seat post.

Remove the clamp from the seat post.

Now put the new derailleur into position, right where the old one was. Tighten the bolt on the clamp lightly -- only enough to keep it from slopping around.

Check to make sure the cage of the derailleur will clear the large chain-ring when the the hinge extends.

Take up the slack on the shifter cable and tighten down the bolt on the retaining clamp.

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.

Check the position of the derailleur cage. Shift the derailleur over to the largest chainwheel. The derailleur cage (the long thin "box" that moves with the chain) should be exactly parallel to the chainwheel.

If necessary, loosen the clamp and turn the derailleur so it's parallel to the chainwheel.

Check the height of the derailleur cage. It should barely clear the teeth of the large chainwheel when sitting directly over the large chainring. There should be about 1 to 2 mm or clearance. Adjust the height by loosening the clamp and pushing the entire derailleur up or down, then tightening again.

Now reassemble the chain and remove the stiffness.

Adjust the derailleur limiting screws and cable tightness as needed. See our derailleur tuneup section.

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