I'm (still) looking at getting a 9 speed mountain rear derailleur to use on my 10 speed road setup (see here). The reason being that I want a wider range cassette, maybe an 11-32 or 11-34. Since I'm using this with a compact double I naturally thought that it would make sense to get the medium cage version (GS) instead of the long cage (SGS). I've read the long cage is for triples and the medium cage is for doubles. However, I noticed that the total capacity is:

• GS: 33T
• SGS: 45T

Now, for a compact crankset the difference is 50-34=16. Then if we add that to the differences in the cassettes listed above we get:

• 11-34 = 16 + (34-11) = 39T
• 11-32 = 16 + (32-11) = 37T

Both of these would seem to indicate I should get the long cage. But I seem to recall some saying the total capacity doesn't matter much. Maybe it means I shouldn't cross chain, which isn't a good idea anyway, but sometimes happens on accident and wouldn't want it destroying things. It also surprises me that this is the case with the medium cage mtn derailleur, is that because in those cases the difference on the front chainrings isn't usually as large as with a compact crankset?

Just get the long cage derailleur and spare yourself the concern. Then you will have some leeway as regards chain length and you will not be up against any limits. This would be my choice.

Thanks, I think that's what I'll do. I did happen to notice one other issue. The website I'm looking at buying this from says that the RD-M771 only handles a maximum size for front chainrings of 44T. Is that really true? I've read I should be able to use this on a road bike, which typically has at least a 50T front chainring. Is that spec overly zealous?

Rear derailleur specs: https://productinfo.shimano.com/#/spe...r%20Derailleur

I've got a hunch the site accidentally copied & pasted from front derailleur specs. The size of your chainrings doesn't matter at all to your rear derailleur. The difference between chainring sizes matters when calculating wrap capacity, but the particular size of each ring doesn't matter.

That makes sense. Thank you.

I agree with the advice you've gotten so far. Specifically, get the long cage.

Personally, I'm using a RD-M772 SGS (same RD but with the Shadow cable routing design) with two different triple cranks and wide range cassettes.
One bike has a 48/36/26 Shimano XT FC-M730 triple and 11-34 10 speed cassette.
The other has a 52/42/30 Campagnolo Racing T triple and 11-34 10 speed cassette.

They each have 55t total wrap, and work fine. NOTE, I'm careful never to shift into big/big, but my chain is long enough to survive it.

I use 10 speed BS78 bar-end shift levers with each.

Not being able to cross-chain on a compact is a serious annoyance. A compact only has overlap on the last 2-3 gears, so if you can't use the small-small combinations, you're going to need to front shift significantly more often. Get the long cage and avoid the hassle.

Yeah, good point. Thanks for the feedback. That's what I've done, I purchased the RD-M771 SGS (long cage). Now I'm trying to decide if I want to go to 11-32 or 11-34. I guess it's not that big of a difference in gear ratios (1.06 vs 1.0).

Whichever you decide make certain that the chain is long enough to go into the big-big combination without difficulty.

Thanks everyone. I know it's a little outside the scope of the original question, but does it matter what type of 10 speed cassette I get? I can go the mtn route and get an XT 11-32 or 34 cassette, but I also notice there are Tiagra 10 speed cassettes that go to 11-32 (or SRAM road cassettes). I've seen some threads say it doesn't matter and others say that it does matter, that I need to get a road cassette.

You may want to consider an 11-36; those extra teeth will be very welcomed on the steep grades.

Thanks. I would say it all depends. A few years ago, before my knees and body started falling apart, I did lots of difficult climbs in Colorado with a compact on front and 11-28 cassette just fine. But, three knee surgeries later ... we'll see. I think I'll start a bit lower, with 32T, but having this derailleur will allow me to expand that if I need to. Perhaps I should leave extra slack in the new chain to go bigger in the future if I need to?

It doesn't matter for 10 speed, they'll both fit on a standard 8/9/10 freehub body.

The big differences are in materials, and therefore durability and weight. The cheaper cassettes have more steel cogs, and more separate cogs. The more expensive cassettes use more aluminum cogs, often mounted together on spiders. Aluminum cogs wear down more quickly than steel ones, but weigh less. Your choice.

Personally, I use XT CS-M771 cassettes. The cogs are mounted to a couple aluminum spiders.
They're ~$45 online, but can be found cheaper from the german bike sites (like bike-discount.de). Obviously, shipping is more expensive from zee germans (~$23 for bike-discount.de), so I pool purchases together and make one big order from them.