Originally Posted by
FatBottomedGirl
My actual question is: how important and/or critical is that?
Will this affect my shifting speed/quality?
The B-screw sets the position of the upper pulley relative to the cassette. The closer the pulley is to the cassette, the better the chain can catch on the teeth and ramps on the cassette which will improve shift quality (faster engagement). If the pulley is too close, it will interfere with the movement of the derailleur when you shift to the bigger cogs.
How should I set it? have the upper jockey as close as possible to the bigger cog when on the granny in front without it being unsafe?
I read that ~6mm was a good number... is that correct?
Set the B-screw while on the largest cog just enough that the upper pulley leaves the cog at an angle (not parallel to the ground).
I tend to ride small cassettes (I mean with small large sprockets: 12-23 being my favorite) which means an optimal B-screw setting would lead to it being pretty extreme a setting...
Yes. But most of the time, the factory setting of the B-screw is good enough for a lot of people.
Is it worth it? or should I leave a bit of an extra slack to allow switching to a little larger cassettes for when I am climbing (12-25 is my mountain cassette)?
As I've said, the default setting is good enough. I wouldn't adjust it if you're not having problems shifting to larger cogs. It's not that difficult to do though. You can readjust if and when you change cassettes.
Or you can adjust it for the 12-25. The change is small enough that it will not be that far from the 12-23 - unless you're OC about your shifting then do the former.