XTR M952 Short Cage - Maximum Rear tooth capacity

[Retrorockit]

Another option would have been an 11-28 cassette.

 

[bold stelvis]

Again, just to avoid confusion

Max sprocket size != capacity
they are two different things. Cage length mostly affects max capacity only (ie the ability to manage chain tension over the entire range)
max sprocket size is mainly down to how far out you can manage to get the derailleur guide jockey wheel to sit so it doesn't collide with the largest sprocket - cage size doesn't really matter.

 

[Retrorockit]

Again, just to avoid confusion

Max sprocket size != capacity
they are two different things. Cage length mostly affects max capacity only (ie the ability to manage chain tension over the entire range)
max sprocket size is mainly down to how far out you can manage to get the derailleur guide jockey wheel to sit so it doesn't collide with the largest sprocket - cage size doesn't really matter.

Cage size usually doesn't matter. But with some newer derailleurs being able to run up to 11-51t cassettes (40t range) some short cage SS can't handle that any more. 11-40t =29t range and that's about the limit for SS cage derailleurs. The common MTB GS mid cage is probably OK for 1x drivelines. I avoid SGS cage derailleurs if possible for 26" bikes due to it being very close to ground level in the mid gears.
There are derailleur hanger extensions that allow older derailleurs to run with larger cassettes. Then cage length (total capacity) becomes the determining factor.
Derailleur factoid. ( factoid= I haven't tried all possible combination myself) The 10/11 speed Shimano SLX clutch type derailleur landed almost on top of the SRAM 1:1 cable ratio (SRAM1:1.11, vs 1:1.13 Shimano). I have tested this by using a SRAM NX gripshifter with SLX 11 speed on 1x11. The 1x conversion lets me run a Front Freewheel setup ( seriously retro concept) on my vintage XC bike. The retro factiod is that this derailleur should work with SRAM 8/9 speed gripshifters including the very retro 1/2 pipe versions. The Zee M640 is an SS cage version of the SLX clutch derailleur for DH (28t), and FR (36t). The hanger is the only different part# according to Shimano's exploded drawing. No plastic bits to break off. The FR version hanger is also the same part# # as the SLX hanger. Just be aware that my testing was done with Jagwire Elite Link solid metal cable housings for maximum shift accuracy. These are retro, and great for bringing mechanical disc brakes up to modern standards, but it's a very pricey cable kit. If you have slack cable housings flapping around inside the frame or crappy old cables this may not work.

 

[integerspin]

Put a photo from the side of the mech with the mech over the big sprocket.

 

[Retrorockit]

I'll get a photo up tomorrow.
Here's a page on the XTR RD-M950

Shimano Rear Derailleur Specifications - the Buyer's Guide, 2015

Bicycle rear derailleurs Shimano features and comparison, bikepro.com

www.bikepro.com

You'll notice that the GS has 33t total capacity 22t front difference, and 11-32t cassette so in 2x MTB it would be 12 t difference in front with 21t in the rear
In a 1x situation an 11-40t cassette is within spec as far as total capacity goes.
The SGS has the same 22t front and rear 21t limits listed, but 38t total capacity.
My XTR is an M960 GS, It shows the same 33t GS capacity, but 43t for the SGS long cage. It is rated by Shimano for 34t max. cassette. IDK if it's actually any different or they just released a bigger cassette than they offered before.