Shimano XT Rear Derailleur Capacity Question

[yellowlebron]

Hey guys,

I'm interested in getting a MTB Derailleur (Shimano XT M771) for my road bike (Polygon Helios 400) so I can access lower gears. The current maximum low sprocket supported by my rear derailleur is 27T. Currently my bike has a 2x9 setup (53-39), and the shifters are 9 speed.

For the Shimano XT M771 9 Speed Medium Cage Rear Derailleur, the total capacity is 33T. After some research, apparently the formula for determining the capacity needed by your rear derailleur is worked out by adding the difference between big and small chainring to the difference between big and small cassette sprocket. So for me, that would be (53-39)+(32-11) which works out to 35T, which is only 2T more than the total capacity of my rear derailleur.

I've read that there possibly may be some leeway with Shimano capacities, but I'd just like to confirm that this will be alright before making the online order for the parts. I would be riding pretty frequently in the low-low combination, as there are some ludicrously gnarly hills on the way to work and I am riding with a 15kg backpack, so I'm only really interested if I am able to access all the gears with my proposed setup.

Cheers everyone

[m@]

I'm running a medium-cage 10-speed XT shadow+ RD two teeth past its theoretical capacity on my 29er; Google research suggested that this is fine and I've had no problems.

Just make sure you avoid big-big cross-chaining...

[toolonglegs]

Make sure you order an inline cable adjuster as well... unless you have some on your frame downtube that is.
I run that derailleur on my cx bike ( 11-32 38/46 ).

[Blakeylonger]

Shimano still make a "trekking" derailleur with a barrel on it.

But why not just get a compact crankset? 34:28 is about the same as 39:32. Then if that's *still* not low enough you can go to a mech that will accept a 32t to get super low gearing.

Or... Carry less stuff to work?

[Baalzamon]

Having done my own research and just purchased the XT 771 RD myself. Be aware that your shifters must be of the shadow range. If not do some research to find out if they will work.

[Duck!]

Having done my own research and just purchased the XT 771 RD myself. Be aware that your shifters must be of the shadow range. If not do some research to find out if they will work.

My bold. No such thing exists. "Shadow" is a name given to a particular design of Shimano rear derailleurs; it has nothing to do with the shifters. Compatibility issues arise with MTB 10-sp. componentry; this family of shifters & derailleurs uses a unique leverage ratio, so will not work with anything else. 7/8/9-sp. MTB, and 7/8/9/10-sp. road gear all use the same leverage ratio, so you can mix & match those components successfully. M770, M771 & M772 (XT 9-sp, with M772 being the Shadow type) will work perfectly fine with any road shifters up to 10-sp. (I haven't yet played around with 11-sp. bits with a view to see if there's any cross-compatibility, but I suspect there isn't). M773 & anything in the M780 series are 10-sp, therefore won't work outside their own family.

[bychosis]

Shadow is available in 9spd and 10spd (both cable pull ratios), clarifying the thorough description above. Shadow is the name given to the design that doesn't stick out as much. It is better for MTB so you don't catch it on stuff and break it.

I'd be reasonably happy to run the med cage but as somewhere above be careful not to big big cross chain. I ran a med cage derailleur on a triple ring MTB setup after ordering the wrong derailleur, should have got a long, and it was OK except when in extremes of gear selection - something that is easier to monitor with shift indicators. On that setup my chain was a bit long so I couldn't over stretch the derailleur in big big combos but the chain did drop off more than usual in small small combos