Cyclocross - cyclocross rear derailleur

Sorry to be a pest all.

I have a question regarding rear derailleurs. I see some people going for mtn bike sets while others use road sets.

It would seem to me running an XT rear would be better than an Ultegra set up.

Is there a reason NOT to go with the mountain rear derailleur?

An XT long cage would seem to be a good set up for a 12-25, with a 44/34 up frant no?

I would prefer a long cage RD in case I wanted to throw on a larger cassette for flexibility sake and nothing else. If you weren't going to use a larger cassette and knew that a road style RD was fine for your drivetrain, then shifting will typically be more accurate and faster with the short cage.

Also, I would think the XT is perhaps a little more robust than an equivalent road ( Ultegra ) one but that's probably just opinion as I've never had the best longevity with my XT RD's, then again they get abused more than my road Ultegra's!

haha.. i raised a similar question the other day. A short cage can accomodate your 25T in the rear and your double set up. I think the purpose of a long cage is really to take up the slack from the combination of a bigger cog in the rear and jumping around from the smallest chainring to the biggest one..

I'm new to this CX thing so I might be full of crap.

so maybe a Shimano RD-M772 XT Rear Deraillier in the midcage would be a good bet.

will it accommodate a 12-27 if I needed it to?

The short cage RD will support the 12-27. I've read where people have used 12-34 with a RD before. Just don't cross chain!

I have used ultegra short cage rd with a 12-27 before no problems at all.

It would seem to me running an XT rear would be better than an Ultegra set up.Why?

I would just guess that the XT and other mountain stuff would be a bit more willing to take a beating.

My Campy, Record rear works with an 11x30 setup.

Is there a reason NOT to go with the mountain rear derailleur?

An Ultegra double RD is lighter and will shift a 12-25 or 12-27 cassette better than an XT mid cage. With a 44-34 crank you should be fine on all but the steepest climbs but you could also spin out above 30 mph. There would be enough chain wrap left over to consider a 46-34 or 48-34 crank though. I wouldn't use it with a mountain bike cassette.


so maybe a Shimano RD-M772 XT Rear Deraillier in the midcage would be a good bet.

will it accommodate a 12-27 if I needed it to?

I use and prefer an XT mid cage myself. I run a 38-48 crank with 9 speed cassettes and it lets me commute through the summer with an 11-28, race cross in the fall with a 12-27, and ride the odd trail with an 11-32. However, it limits you to a 10-12 tooth difference in the front if you plan on running a mountain cassette.