avhed

I know the shifters have stops on them, but what about the derailleurs? Especially the front one for a double chainring since the limit screws already create "stops".

bt

come on bro...you can do better than this right?

e_guevara

The derailleurs, both front and back, do not have detents (stops) in them. The FD's cage and the RD's parallelogram move freely along their hinges' range of motion, limited only by the high and low limit screws.

As you may have already guessed, the "step" in the derailleurs are controlled by the shifters.

e_guevara

Forgive those who are not mechanically inclined, or those who haven't had the chance (or don't want to) work on their own bikes.

bt

i forgive you

Fiery

Requirements for indexed shifting, other than the actual shifters, are compresionless housing and sufficiently precise derailleurs (this means slanted parallelogram design for the rear derailleur, and low tolerances in general). A top pulley with some designed lateral play also helps a lot for rear shifting. Lastly, cable pull of the derailleurs needs to match the cable pull of the shifters.

All of the requrements except for the last one come by default with all derailleurs made in the last couple of decades at least. The only thing you need to worry about is properly matching the shifters to the derailleurs and the cassette. Notice that different nuber of gears doesn't have to mean different cable pull; with Shimano, for example, you can use the same rear derailleur with 8, 9 or 10 speed shifters without any problems.

avhed

Noticed all these requirements are needed in the rear derailleur. So I still am not clear about a double chainring front setup.

e_guevara

The limit screws limit the movement of the FD cage to "stop" the chain from going over the big ring (H screw) or preventing chain suck (L screw).

The mechanism that enables the "step" for either shifter (left-front or right-rear) is the ratchet mechanism inside the lever. When you move the lever, it rotates the gear a certain distance where the cable is connected and catches a pawl. This movement pulls on the cable that pulls on the derailleur cage (front or back) by a given distance. The distance the cage moves is limited by distance between the teeth of the gear inside the shifter. This is where the "step movement" of the derailleurs come from.

Clearer now?

joejack951

As already noted, the derailleur's stops limit how far out and in the shifter can move the front derailleur. But for most modern doubles and any triple, there are also intermediate positions (middle chainring for triple and trim positions for both double and triple) that are determined by the ratchet mechanism in the shifter (just like the rear derailleur and shifter work together).

Homebrew01

Campagnolo double drivetrains are not "indexed", you push the shifter, and the limit screws prevent over shifting. The front shifter has 5 or 6 intermediate clicks so that you can trim the front derailleur to avoid chain rub.

Fiery

The base requirements are for back and front indexed shifting alike. There are extra details for rear shifting simply because front shifting is a lot less sensitive in general, especially on a double. In particular, there's no difference in the function of the limit screws on a back and front derailleur; they limit the extremes of derailleur movement and that's it. They do the same no matter the number of gears, or if shifting is indexed at all.