Originally Posted by Kapusta

That simple? No its not.

You need a way to move the lever with the jockey wheel. But it is not a simple lever...

You also need a way to take up chain slack as you go between different cogs. So that jockey wheel shown is actually just the upper jockey in a tension arm (cage) like you see on a rear derailleur. So you are moving the whole cage.

What I just described is basically a rear derailleur in terms of complexity. The only real difference is that the linkage moves the jockey wheel vertically instead of at an angle. ..