Derailer - Rear - Shimano Altus M310, silver
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
22t + 21t = 43t. That's what the Altus can handle.
Let's say you're bumming out because your big and small rings are 46 and 24, but your cassette is 11 x 36. You technically need 22 + 25...47t. And on top of all that, the Altus is rated to 34t maximum, so you're double-screwed out of it.
But wait! In friction mode, any Shimano rear derailer will go at least 2t more than the rating, so the 36 should be (and in our experience is) no problem.
And you aren't supposed to ride the small chainring x small rear cog combo, anyway. If you can limit your small chainring to a 15t or larger rear cog (36 - 15 +21), then you're back in clover.
There's no blatant danger by riding a smaller rear cog than that. The chain sags, but nothing breaks. We say DON'T DO it, but--why would you, anyway? You need the small front ring ONLY for low gears, and the low gears are really the biggest three or four cogs in back--probably starting at at least 20t.
Being a bike rider means being able to think, to sort things out, to make sense of logical stuff and simple math, to not be intimidated by a slightly different vocabulary than what you encounter in your daily life...and especially, by not being afraid to ask questions to clarify what is unclear to you. We are here to get you through anything. If this is unclear, call us, and ask to speak with, "somebody who can explain derailer wrap to me." Likely, the person answering the phone can, but you might get passed on to somebody else, anyway.