Bicycle Mechanics - Shimano Deore XT M771 Front Derailleur vs. M773

I'm rebuilding a 1998 Diamondback Apex as my trekking bike for backroad and singletrack touring here in Oregon. I'm trying to figure out which front derailleur to purchase for a 48 tooth front chainwheel. Can anyone tell me the difference between the Shimano Deore XT M771 Front Derailleur and the M773? Shimano lists the M773 for "trekking." Can anyone tell me what the difference between the two? The M773 seems to weigh about 20 grams more. Is it just a more durable model?

Edit: Oops, I meant the M770 and the M773. Both are rated for 48tooth.

Most front derailleurs have three specifications:

1) Clamp diameter: 28.6, 31.8, or 34.9mm (unless it's an E-type that attaches to the bottom bracket)
2) Pull direction: top-pull (cable comes from the top of the frame usually along the top tube) vs bottom-pull (cable comes from the bottom usually under the bottom bracket)
3) Swing direction: Top-swing (the M770) vs bottom-swing (the M773)

Most newer Shimano FDs are multi-clamp (34.9mm with shims for smaller seat tubes) and multi-pull (cable can be routed from the top or bottom).

To some extent, the swing direction is more style than function. However, if the frame was designed for a particular type, then there may be interference with the frame when switching styles.

Traditionally, top-swing went with top-pull and bottom-swing went with bottom-pull. [edit: had this backwards]

Do you know what the frame's original FD's swing-direction was?

The cages of some fd's are shaped differenty to match the size of the largest chainring -- so that is an additional dimension to take into consideration. However, you have been misinformed that both the 770 and 773 are designed for a 48t big ring. The 770 is only rated for a 44 tooth, so you should choose the 773. There can be a little bit of a fudge factor, but generally you're better off getting the one matched to the chainring size. The other difference is the 770 is top-swing (clamp is situated below the cage) and the 773 is bottom swing (clamp is above the cage). I would choose a bottom-swing every time -- they last longer. Get the 773.

The cages of some fd's are shaped differenty to match the size of the largest chainring -- so that is an additional dimension to take into consideration. However, you have been misinformed that both the 770 and 773 are designed for a 48t big ring. The 770 is only rated for a 44 tooth, so you should choose the 773. There can be a little bit of a fudge factor, but generally you're better off getting the one matched to the chainring size. The other difference is the 770 is top-swing (clamp is situated below the cage) and the 773 is bottom swing (clamp is above the cage). I would choose a bottom-swing every time -- they last longer. Get the 773.

Thanks for the info. I didn't know that bottom-swings are more durable. I'll go with the773.