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I am 'updating' my old bike for my son. It is currently 3x10 but i picked up some 2x10 cranks and shifters for it. Will the 3x10 front deraileur i have work fine on the 2x10 set up or should i order a 2x10 specific one?
The 3x10 will work. Just use the limit screws to stop it going in to the 3rd position.
Great. Saves me a few quid.
Correct. Will work.
Usually.
If the derailleur, crank and shifter are all 3x then the limit screws will let you lock it down to 2x travel.
If the crank is a true double and shifter 3x (and you're exceptionally picky) you may find it doesn't quite shift properly - a bit like tension never being quite right. Offsets and pull are the problem. The 2x/3x switch you used to get on Shimano left pods fixes it if available.
Usually it's not a problem but there are combinations that can be annoying.