Shimano ALTUS rear derailer (RD-M310) - 17167d17
The best-designed rear derailer in existence, and only $28. Most derailer 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 for only $28.
Shimano Altus M310 rear derailer.
Indexes 7 or 8 speed, frictions up to 9.
Wraps 43 teeth.
Max rear cog, 34t
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
The 44t diff exceeds by a tooth, but two things here: One, I bet Shimano gives you a tooth to work with, and the bigger TWO, the wrap capacity assumes you’ll be riding on the 24t x 11t combination, which of course you shouldn’t. That’s the one that requires 44t of wrap. You should use the small chainring only with the biggest four or five rear cogs, in which case the 43t wrapacity is way more than enough.
The max rear cog of 34t assumes predetermined derailer hanger design and chain length AND indexing. It’s still generous at 34t, but you can eke out another 2t if you use friction and remove a link of chain.