I think a (complete?) list of 9s XT rear mechs (in order of introduction) runs thusly;
RD-M750 (H,T)
RD-M760 (L, T)
RD-M751 (H,T)
RD-M761 (H,T)
RD-M770 (L, T, nb)
RD-M771 (H,T, nb)
RD-M772 (H,S, nb, 36T)
where
H = high normal
L = low normal
T = two sprung pivots
S = single sprung pivot ('shadow' type)
nb = no barrel adjuster on mech
36T = 36T max sprocket size (the others are nominally 34T max)
RD-M773 onwards are 10s mechs which have a different pull ratio.
RD-M751 (which shimano 'launched' in a hurry once they realised that the world wasn't wildly enthusiastic about low normal mechs like the RD-M760) is almost identical to RD-M750 but it lacks some of the rubber boots on RD-M750, and uses cage plates that are 'styled' (yuk) like the RD-M760 ones. It also has an optional rubber buffer on the lower knuckle that means that it doesn't clonk annoyingly when fitted on some full suspension designs (eg Giant NRS, ask me how I know....
)
All models are available in several different cage lengths (SGS is the longest one) and weigh about the same in any given variant. I have replaced the (steel) inner cage plate with an aluminium one in some cases which makes a significant weight saving (down to ~225g IIRC). The replacement inner plate can be shaped so that it won't so easily snag in the spokes too....
All models work reasonably well but the styling on some is a bit, er, 'more challenging' than on others. In theory RD-M772 ought to last longer because there is only one sprung pivot that is meant to move when the mech is in service, but the jury is out on that one. They are all good mechs but I personally can't abide the low normal approach on RD-M760 and RD-M770.
Why a XT 9s rear mech? They are very versatile mechs; they will work OK on all shimano 6s, 7s and 8s index systems (apart from with 6/7/8s dura-ace shifters) will (obviously) work with all road/MTB 9s systems , as well as with all 'road' 10s STIs (apart from ST-4700). They seem to have more durable pivot bushings, and more durable parallelogram bushings than the basic Deore 9s models or earlier 8s mechs. They are (like ultegra mechs) also fitted with much more durable pulleys; the upper pulley is sealed and has ceramic bushings, the lower pulley has a 2RS type cartridge bearing with additional dust shields. If you keep the mech adjusted correctly, and repack
both pulley bearings with a decent grease (eg finish line Teflon) at suitable intervals the pulleys will outlast the rest of the mech.
So yeah, if you have RD-M771, use it. The only reason for buying another mech like RD-M772 is that it has a slightly larger capacity, and perhaps if you don't buy one now, you many not be able to in a year or two's time (the oldest was launched nineteen years ago, and the youngest about eight years ago....
). I have a small stash of XT 9s rear mechs squirrelled away; having ones available that I can throw onto almost any dedangler-bike I own as needed is a bit of a plus point.
cheers
