Looking at specs on the Shimano site, my M8000 GS medium cage rear mech a) is fine at 2×11 with a 42t max rear sprocket, and b) has a capacity of 39t.

Since capacity is rear max-min plus front max-min, the rear cassette is (42-11)=31t so that leaves 8t difference for the front chainrings.

However, all of Shimano’s current XT 2×11 chain sets have 10t difference though e.g. 38-28, 36-26 and 34-24, same as the 38-28 setup I was planning to use, which makes 41t total when used 2×11 with the 11-42 cassette.

What am I missing?! And will I die using a medium cage mech 2t over capacity?

If you’re careful not to go big big you’ll be fine, but I’d be inclined to go SGS for double with a 42T. GS should be good for 11-40with double assuming your bike (FS?) doesn’t have huge chain growth.

Unless they have changed the spec, 2×11 setup could only be run using the medium cage derailleur with thr 11-40 cassette, as the mech has a 39 tooth capacity.

Technically you should use a SGS rear mech, but the GS would be ok. If you set the chain the right length it’ll be baggy in the 28/11, when you shift to the 13 it’s back in spec and you wouldn’t be at that end of the cassette in the granny ring anyway.

Akers +1 – I thought if running 2×11 then rear cassette was 11-40 not 11-42.

Should have said, hardtail so less issues there. And already have new mech, cassette, chainrings so didn’t fancy buying either another cassette (40t) or another mech (SGS), and the chainrings are already pretty close to each other…

Akers – spec here:

https://bike.shimano.com/en-EU/product/component/deorext-m8000/RD-M8000-GS.html

specifically says:

Low sprocket_Max. 42T (2×11-speed) 46T (1×11-speed)

but also

Total capacity 39T

which is what is throwing me, as it doesn’t then fit their 2x chain sets! Anyone know a bike speccing full XT 2×11 with the 42t and GS medium mech?

Good point jabbi, hadn’t thought of it that way round, was thinking is it going to explode the mech etc if too tight but if it’s setup to be only too loose in 28/11 that’s not really a problem! Maybe that is their way of seeing it too, you shouldn’t cross chain that much especially from small front to small rear, but still that’s not how it’s usually worked out as far as I know, and not what their specs say!

Guess I will give it a go and keep fingers crossed, thanks all for at least confirming it should be doable!

Sweaman – that’s how the numbers all add up, apart from the line on Low sprocket max capacity – you can’t actually run the 42t with a 2×11 using their M8000 2×11 chainsets, and I thought Shimano were usually quite good at considering the other components when setting the limits.

Might be a fitted once GS mech for sale/swap in the near future…

I run 105 with a 27 limit but with a 30 rear max sprocket, without any issue.

Sometimes shimano are conservative in the limits they set.

I’d give it a try before thinking about upgrading and then I’d probably look at trying to get new plates for the mech to make it SGS rather than a complete new mech. Is that possible for your mech?