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Hi wondering if someone could help me. I've always struggled understanding this and have finally got the time and guts to ask! Online provides answers but none are in a simple explanation for a non mechanical minded cyclist.
The Shimano XTR RD-M972 Shadow rear derailleur was offered in two sizes: GS (mid-cage) and SGS (long-cage). In all cases, the max cogset that can be used is an 11-34. The difference is that the mid cage size has a 33 tooth total capacity while the long cage takes 45 tooth total. I actually remember Shimano calling the SGS (Super Long Cage and the GS "Long Cage". The SGS is indeed very long!
So they both fit the largest cassette available at the time I get that. What I don't understand is the term "total tooth capacity". Is that referring to the front chainring? I would be using a triple with a 44t largest cog.
SO, am I right in thinking it's not as simple with the M972 rear mechs as saying if going old skool route with a triple setup up front I need the SGS and if I'm dropping the front mech and going for a single chainring setup (32t) upfront I would go GS? Or would the GS medium/long cage be just fine with a 11:32 cassette and triple ring setup max 44t or is the total capacity at 33t going to cause issues using a 44t upfront?
Okay so confusing, any help much appreciated. Long live 9 speed...
The Wikipedia entry for "Derailleur gears" covers this under the section headed "Cage length". There is also a thread on here from 9 years ago when the trendy people were using front mechs https://singletrackmag.com/forum/topic/rear-derailleur-total-capacity/ . The posts are not in the correct order now but Mister P said
"Total capacity is worked out by adding the difference between big and small chainring to the difference between big and small cassette sprocket.
(44-22)+(32-11)=43"
I believe you will need the SGS if you are running a front mech.
I've had a coupleof those mechs, still got one.
Both bikes were 24/32/44 at the front, one was 11-32 at the back and the other was 11-27. One had GS, the other SGS. I could swap cassettes between them without any trouble. So 41 and 36 tooth differnece is fine on either in the real world.