Max derailleur capacity vs max low cog capacity?

There's a few things going on here, such as the total capacity, which is the last line in the spec, and is effectively a function of the cage length because it's determined by how much chain slack can be taken up. This is 41t for the mech you've linked. This is the maximum value for the difference between the big ring and small ring plus the difference in big cassette cog and small cassette cog.

For a 2x11 with 22t and 32t chainrings, this is:
(32-22)+(42-11) = (10)+(31) = 41. This is the same as the total capacity, so you can use an 11-42 cassette with a 10t difference up front.

For a 1x11, there is no difference in chainrings, so you use the 35t difference between the 46t cog and the 11t, which is well within the capacity.

The other thing which comes in is the geometry of the mech, which largely determines how big the biggest cassette cog can be. The geometry of this mech makes the biggest cog it can handle a 46, so even though an 11-50 would only technically require a 39t capacity which is within the 41t capacity spec, you still can't use one because the mech can't clear the 50t cog. This is how things like Goatlinks work, by changing the geometry a bit to make the mech clear a bigger cog.

In reality, manufacturers are often a bit conservative on these things, and they can be pushed a bit, especially if you are willing to accept a compromise, such as the chain being a bit loose on the small-small combo on a 2x setup. The Goatlink is also a compromise, because they move the mech lower for the whole range, giving increased range at the cost of worse shifting on the small cogs.

reaperactual said:

JBA said:

I assume it is because with a 1x system you will usually have a smaller chainring (28T - 36T). However, with a 2x system the large chainring could be up to 46T.

On my old 3x9 set up when fitting a new chain I would check length was correct with an extreme big/big and small/small test on a works stand.

The big/big was only 40:36 but with what your saying it makes perfect sense. I imagine no derailleur could handle a big/big (e.g. 46:42!) combo that you describe.

Shimano state these figures because this extreme combo, although rare they can't rely on end user not to fit something like this, shift into that extreme gear and rip off their derailleur! Thanks for the response JBA. 😎👍

Interestingly, the actual size of the chainrings makes no difference, only the difference between them. As long as the max difference is only 10t, you could use whatever chainrings you want. You could build a road bike with a huge gear range using an 11-42 cassette with 52t and 42t chainrings, and as long as you used a shifter which could operate the right mechs (another minefield of complexity there), it would all work perfectly.