would you position a 3x mech as though the big double chain ring was the equivalent of the old big triple ring, ie drop the mech down the seat tube a little, or leave the mech in the same place and treat the new 2x rings as the granny and middle tripples?
Not sure that makes sense, very badly phrased, sorry!
OK - going from 3x to 2x, if I keep my old mech, should I be repositioning it on the seat tube?
thanks!
If it's currently 'correct' and working, just take of the big and fit the bash.
Meant to explain that I was going from 3x10 to genuine 2x10, rather than just removing the big ring, so no bash option. New cassette, rear mech and chain, but reusing an XT front mech.
Given the rings on a 2x will be different sized to the rings on your 3x chainset, it should need moving to be positioned correctly.
Good question, cos most people assume they need to lower the FD when they take the big ring off, then wonder why they get cr@p shifting.
Fact is, todays FDs have a quite sophisticated shape on the inside of the cage, with wider bits where the chain runs on each chainwheel so you can get the use of the whole cassette at the back, and narrower bits inbetween to aid with fast shifts between chainwheels.
If you then lower it, your remaining chainwheels are out of sync with the narrow/wide bits in the mech cage, and shifting suffers.
If you could lower it enough so that the middle is running where the big was and the granny where the middle was, then OK, but before it gets that low the cage would interfere with the chainstays.
So, sorry for the long answer, but the quick version is to leave it exactly where it was when you had three chainwheels, or if your new double 'big' ring is larger than your old middle, then you should actually [i]raise[/i] it a bit for best shifting performance.
Does it look good sticking up there - well no, but shifting is best. If you're worried about looks, then get a double specific mech...
Thanks, that makes sense and was sort of the reason for the question.