If you've followed the blog for enough years (which you might not want to admit to), you'll remember me waxing rhapsodic about the M571 front derailleur (and pirating a funny story of Feldman's in the process). In the intervening years (I believe the 570 series derailleurs came out in the early 'aughts) Shimano has done everything possible to make front derailleurs suck - consider if you will:
-They got heavier - an XT front derailleur porked up to over 180 grams at one point.
-They didn't shift any better. If anything, they got worse, IMO.
-Their ridiculous cable clamping systems and dual-pull contraptions took up so much space that many short-chainstay bikes had problems with tire/derailleur clearance.
-They stopped making 28.6mm clamp derailleurs entirely, so you had to fiddle with annoying shims all the time.
Well, maybe they've finally done something positive. I just ordered (the day it came into stock) a M771 direct-mount derailleur from BTI. It's going to go on Sarah's new FS bike, I think, since my strategy is to always test things on my wife (FWIW, that's an inside joke for folks who remember the old frameforum and a certain thread about head tubes). Also, I can't test it myself, since I don't have any mountain bikes with front derailleurs.
There are several things I like about the idea, at least in theory:
-Should be lighter. Probably back down to where we were with the 570/750 series stuff. I'm guessing the mounting bracket is about the same weight as a clamp, so it probably won't be outright lighter, but you never know.
-The mounting hardware is simple and the bracket should be pretty easy to make (I'm guessing Shimano does not include one with the derailleur, at least not in steel!)
-Shifting should be excellent - the derailleur is attached to the frame in a pretty darn rigid manner. No aluminum shims or hinged clamps. In theory, this should mean better shifting under load.
-There's a built-in cable stop, so no need to have a stop on the frame if you don't want one (and you could easily hack off the stop on the derailleur if you don't want it). I'm guessing it would be better to have one anyway, though, since for top-pull applications that's going to be a lot of extra housing running down the seat tube, so much so that you'd probably want a ziptie type guide for it anyway.
And here's the big one: you no longer need the seat tube to meet the BB shell to make the frame work with a front derailleur. Yes, E-type derailleurs already accomplish this - but they also suck unbelieveably. This means you can have super-short chainstays on your 29er and still run a front derailleur - because the seat tube can be offset 20+mm forward to keep it out of the way, and the front derailleur can still be installed in the right spot. Kickass.
On the minus side, though, there's no up/down adjustability, so you're stuck with the standard 22/32/44 drivetrain with this setup. It's possible that you could move the mount up or down for different drivetrain styles, but regardless, you're stuck with one front derailleur position forever. For some people, that'll be a deal killer. For others, it's probably not a big deal.
Anyway, more info (and some pictures of how it mounts up) in a week or two when I get it installed.