Front Derailleur Installation

The front derailleur is a crude mechanism that shunts the chain from one chainring to the next. These are not difficult at all to install, but they are very sensitive to adjustment. I've heard lots of bikes that have their front derailleurs completely out of adjustment. The chain will constantly rub the derailleur's cage, making a pretty obnoxious sound. If you ignore it, then this happens:

If you ignore it even more, then this happens:

The chain will eventually cut through the cage, rendering the derailleur completely useless (although I suppose the constant clattering noise will diminish once the cage has a hole in it). Anyway, this post is about setting up a front derailleur on your bike. I installed this derailleur on a bike with the chain attached by removing the little screw and bushing at the back of the cage:

Next I slid the chain through the cage and loosely clamped the derailleur to the seat tube:

Try to align the cage plates to run parallel with the chain. The next step is to bolt the cable onto the derailleur for fine tuning:

You'll notice two small screws on the derailleur, sometimes marked H and L. These are the limit screws, and they play a big role in whether the chain will rub the cage or not. To adjust these, first shift the chain into first gear (biggest gear in back, smallest chainring in front):

Now adjust the low limit screw (marked L) so that the inner cage plate is as close to the chain as possible, without actually touching it as you turn the pedals:

Next shift the chain into the highest gear combination (smallest gear in back, biggest front chainring):

Now you can adjust the high limit screw (marked H). You will screw it in until the outer cage plate is close to the chain, but again doesn't touch:

The derailleur's height is important too. If it is too high, the chain will likely rub the cage in the cross chaining gear combinations. It might also be sluggish to shift the chain to the bigger chainring too.

For most modern derailleurs you will want the cage to pass as close as possible to the biggest chainring without touching it, like this setup:

Unfortunately on older derailleurs like this one, this can cause the chain to jam between the inner cage and the teeth on the big gear as you try to shift. To fix this, position the derailleur a few millimeters higher than normal, like this:

If you do reposition the derailleur, then recheck the limit screw adjustments and tighten down the clamp bolt once you're satisfied with it. These adjustments take some practice, and some trial and error. Be patient, adjust in small increments, and eventually you'll find a nice setup.

Note: If you have an indexed double chainring setup, the low limit screw determines the resting position of the front derailleur on the small gear. The shifter determines the position on the bigger gear, but the high limit screw still needs to be adjusted to prevent the chain from overshooting the big ring. You will use the barrel adjuster that's connected to the front derailleur's cable to set the position of the front derailleur in the big gear.
If you have an indexed triple chainring, then the limit screws will control the smallest and biggest chainrings. The shifter will control the middle position, so use the barrel adjuster to position the cage where it doesn't rub the chain while it's on the middle chainring.