Shimano XT RD-M771 Rear Derailleur help?

Question

Would the Shimano XT RD-M771 Rear Derailleur work with an 8-speed bike? And what do terms like "mid" and "long" cage, and SGS and GS (top normal?) mean?

Answer 1

I do not see any reason why the rear derailleur would not work with an 8 speed cassette as the indexing is applied at the levers.  The cage is the part on the rear gear that holds the two jockey wheels and applies the tension to the chain.  The distance between the centres of the Jockey wheels is what is meant by "mid" and "long" as you also get a "short" but that is used on racing bikes.

I do not know what the letters SGS and SG mean but the top normal I understand.  Top normal is if you disconnect the gear cable and rotate the gears in the normal way the chain will go on to the top gear (smallest cog).  Some derailleurs are made to work the opposite way, with no cable attached the gear will go on to the largest cog (lowest gear).  Hope that helps.

Answer 2

Yes, it will work. Your rear derailleur has no idea how many cogs there are in a cassette.  This relates solely to your shifters, assuming they are indexed.  So, "10 speed" derailleurs work fine on 8 speed bikes (or 5 speeds, for that matter).  There are two limits to set, high and low gear, and virtually all derailleurs have the same range capability.  See the link I've posted below.

Cage length affects the overall gear range you can accommodate, and long cages are typically used on mountain bikes and road bikes with triple cranks in the front.  The purpose of the longer cage is to to take up more chain slack.  A long cage will work just fine on a double setup with a narrow gear ratio; it just won't shift quite as crisply, and is not necessary.

Most rear derailleurs are high normal.  That means that the derailleur spring, unimpeded, will bring  the chain to rest on your high gear (smallest cog) in the rear.  A few years ago Shimano introduced "Rapid Rise" in its XTR mountain bike line, which is a low normal setup.  This reverses the shifting, and the spring is trying to push the chain onto the lowest (bottom) cog -- the largest one, closest to the hub.  I tried it and didn't like simply because it required shifting in the opposite direction to that which I had become accustomed.

SGS and GS are Shimano terms.  GS is mid cage length and SGS is long.