Medium cage derailleur, 11-51t cassette

I’m currently running an 11 speed shimano set up with medium cage rear derailleur – 30t chainring, 11-46t cassette. I’m in the process of replacing the whole drivetrain. If I get an 11-51t cassette will that work (keeping 30t chainring)?
Officially it looks like it’s a 39t capacity mech, this would be 40t.
I don’t want to buy a cassette that won’t work but suspect it might all be ok. Or will it guarantee flaming death?
Anyone running similar?

Anyone?

Sunrace do a mech extender that will take a medium cage to large cage mech, worked on the other halves bike, about £25 from Merlin cycles.

I’ve been using 50 and 51t cassettes with a medium cage 11sp SLX mech on my hardtail for a couple of years with no issues. Didn’t even need an extender thingy. Fitting the extender gives better chain wrap on the smallest cog but works fine without. There’s plenty of clearance with the b screw wound in.

My ebike came with 51t cassette and a medium cage SLX mech. No issues. Just swapped to a medium cage XT and again, no issues. This is 12sp though.

I reckon you’ll be fine although you might need a new chain as it might be too short.

But then a new cassette might need a new chain anyway.

Those chinese hanger extenders are a few quid on ebay , I’ve had no issues with them and they are a help in dropping the mech down from the biggest sprocket . Once had to use the Dremmel on one which hadn’t quite room in the cut out when I altered a vintage Campag set up. But it sounds like you should be ok as Shimano often understate the capacity of their mechs.

Wait, what ?, 12sp medium cage works on 51t cassette? Seriously, why the hell am I using a long cage that’s periously closer to the ground than I’d like, anyone else running a medium on 51t. 45t cog to 51t seems a big ask for a medium cage.

yup. 11 speed xt m8000? on the deore 11-51 no isses

I’m using a GRX rear mech with a 11-51 with no extender and that’s only rated to 42 so I can’t see why an MTB mech wouldn’t work

Brilliant – thanks all

Wait, what ?, 12sp medium cage works on 51t cassette?

Sort of not really.

First Shimano don’t make a med cage SLX mech so not sure what @bens had on there until now. A direct replacement for the 12spd SLX will be the long cage SGS XT. I have a med cage GS XT on my bike (10-45T) and I put a 51 on there to test, and decided it ran too close to shift nice, so am sticking with 45T (which to be fair is adequate anyway and lighter). Hoping that someone like Garbaruk comes out with a 10-48 which would be perfect if we got +1T on #2 and +2T on #1.

EDIT – It does work BTW. Just not as well as I would like.

Am confused by the use of “medium cage” too so will ignore that. Aren’t they all long cage now?
But can confirm that my slx m7000 mech works fine with 11-51 cassette. So much better spread and range than with the previous 11-46. Had to give it a few turns on the b screw I think. This is on a hardtail and obviously not all frames are the same. It’s essentially the same in use as the m6100 12s deore on the same cassette on the fs. Listed maximum big cog is 46t but copes with 51.
Costs nothing to try. If it doesn’t work buy a deore 5100

Am confused by the use of “medium cage” too so will ignore that.

You can get GS (Med) caged XT and XTR 12 speed mechs. The XT one has only just come out under a veil of secrecy it would appear so most people probably dont know about them. Looks like a 10speed mech its so short 🙂 I have one on my bike… It shifts brilliantly and is miles from the ground relative to an SGS… As I said it works with 10-51 but I couldn’t hand-on-heart recommend it.