Originally Posted by
Darth Lefty
It still needs to do three of the four functions of the rear derailleur, just not the pantograph.
And there still needs to be something like the pantograph or maybe a helical thing hooked up to a shifter to deploy the telescoping cassette.
Yeah, I guess it would not need to be the slant-parallelogram. That part could be a single spring-tensioned pivot (similar to what the chain tension cage uses). But the rest (cable activation, chain tension cage, b-adjust screw, need for a clutch for some applications) is pretty much the same design as a current RD.