Front derailleurs are the most unnecessary component on a bike (we aren't talking about accessories). There is little performance difference between cheap ones and expensive ones; and if something happens and you have to ride your bike without one, you can still shift between the chainrings using your finger or a stick. We aren't suggesting you do this, just pointing out that if someone is holding a gun to your head and making you decide in three seconds which of the "key" components you have to give up, save your life and say, "The front derailleur."
Attaching them - front derailleurs come in two styles.
- Clamp-on. This is the traditional way, and is still the most common. A clamp goes around the seat tube, and is secured with a bolt. Different sized seat tubes require different sized clamps. The most common by far is 28.6mm, or 1 1/8-inches. Next is 31.8mm, or 1 1/4 inches. Those two sizes cover probably 98 percent of the clamp-on front derailleurs.
- Brazed-on. Probably a better name is "direct attachment" or "bolt-on," because brazing refers to a metal-joining method that doesn't apply to non-steel frames. But when the type of front derailleur in question was first developed, it was for steel frames with brazed-on mounts, and so...technically accurate or not, the name has stuck and everyone knows what you're talking about when you say "brazed-on front derailleur."
Of the two types, there is the widest selection among clamp-ons. Brazed-on styles tend to be available only in the upper price ranges. Not a huge deal, but a fact nonetheless.
There is one marked advantage to a clamp-on front derailleur, though. Maybe two.
One is that you can move the front derailleur up and down over a wider range than is possible with a brazed-on style. Most brazed-on fronts are positioned for 52/53 tooth chainrings (on road bikes), and if you want to run a 46t big ring, you're stuck. Granted, most bikes come with the bigger rings, but an increasing number of riders are figuring out that those rings are too big, and life is a lot better with a smaller big chainring. So, that's the main thing.
The second issue with brazed-on fronts is that once in a rare while, a seat tube fails at the brazed-on mount. A tube is stronger if it remains round and unheated. Clamp-on front derailleurs have never been known to cause or in any way be associated with broken seat tubes. Still, the main reason for going with a clamp, is the increased vertical range, which doesn't limit your choice of rings. And, if you know you'll always ride rings from 50t to 54t, then you can safely go with a brazed-on/direct mount style.
One way to differentiate front derailleurs is by intended purpose: Road double, Road Triple, and Mountain.
Road double front derailleurs are designed for two front chainrings with differences of 14t or less. For instance, 53x39, 52x42, 53x40, and so forth.
They have short cages, and the inner and outer cages are at nearly the same height. In other words, when you're looking right at one broadside, the outer cage pretty much blocks the inner one.
Road triple front derailleurs look a lot like road doubles, but the cages are longer (they drop down more), and the inner cage is lower than the outer one. Shimano road triples look like mountain fronts in this way, with a huge offset. The Campy road triple has less offset, and looks a lot like its road double model.
Mountain front derailleurs are designed for big tooth differences between chainrings-46x36x24, 42x32x20, and so on. Downshifts, from big to smaller rings, are easy. Upshifts are harder, and are aided by lower inner cages with strategically placed contours which coerce the chain up and outward to the larger ring.