Shimano Altus Rear Derailleur
The best-designed cheap rear derailer in existence. Most derailers have 11t pulleys, and that works well. The ALTUS has a 13t top pulley and a 15t bottom one, and what's the big deal there.
The bigger pulleys "use up" more chain, so the derailer cage that houses them don't need to be as long and low-hangy. Plus, the bigger pulleys turn more freely. There is no reason on godsgoodearth that all derailers shouldn't have big pulleys, and maybe someday they will. But for now it's the ALTUS, a super-shifting Shimano.
This is my/Grant's personal favorite rear derailer, but that doesn't mean it should be yours. I like it because the pulleys are huge (13t for the top one, 15t for the bottom one, compared to 11/11 on a normal rear derailer). This means they take up more chainslack and don't need as long a cage. The bigger pulleys turn easier, too. I don't care about that, but I find it interesting that Campagnolo retrofits giant pulleys on its derailers for pro riders sometimes, to reduce turning friction.
The thing I really like is that Shimano introduced this big-pulley design near the low-end of its range, probably because they felt it was too risky to introduce on their elite models, figuring the low-brows wouldn't be as tradition-bound. So there it is: Shimano's best design in a cheap derailer. There are now becoming some other Altus variants. The groovy one is the M310.
Indexes 7 or 8 speed, frictions up to 9.
Wraps 43 teeth*.
Max rear cog, 34t
* What's "wrap"?
It’s the difference in big and small chainring teeth PLUS the diff btw big and small cassette teeth. For instance:
Big chainring 46. Small 24. That diff is 22.
Big cassette tooth 32, small 11, that’s 21t