I recently got an long cage rear mech as part of a bundle of second hand stuff.
I'm building up 1x11 (11-42) and know that i only need a medium cage but, is there any harm to running a long cage mech on a 1x setup ?
No.
But you may find with a 1x set up you need a NW front ring and/or a clutch mech.
No harm, but it may swing about more than a medium would.
As above you will want to be running a NW chainring and clutch mech, or a chain device if you're building a 1x setup.
Thinking of going 1x11 myself.
Does cage length matter when considering cassette size? Id be looking at 11-46.
Or is it as simple as 1x = medium, 2x = long?
It is a clutch mech (m9000) and i have a 'proper' shimano ring to put on up front (though if that doesnt work out i'll look at NW).
It'll be a few grams heavier, give a few mm less ground clearance and I'm assuming that the clutch would essentially take care of any extra swing but shifting performance should be the same?
A clutch mech may feel stiffer to shift, but not much. You can adjust them, but if you adjust it too much its no longer doing its job.
You can forget the grams and mm's of clearance in the real world. Dropping a chain or not will be your issues.
Get a NW ring, they are cheap and dont worry about the long mech.
Thinking of going 1x11 myself.
Does cage length matter when considering cassette size? Id be looking at 11-46.
Or is it as simple as 1x = medium, 2x = long?
The proper calculation to work out cage size is:
(biggest chainring - smallest chainring) + (biggest cassette - smallest cassette) = derailleur capacity
On my 1x 11-36t a short cage mech is recommended. However if I'd have gone for a 1 x 11-42t, I'd have needed a medium.
In the event you only have a single chainring, you count that as 0 and just do the cassette calculation to get your chain capacity.
Each rear derailleur has a given chain capacity (how many extra links is can 'absorb' when shifting from big to small gears.
So just do the calculation on your chosen gearing, then get the shortest mech you can while keeping within it's specific capacity.