mathof wrote:In fact, darker roasts do require coarser grind settings for similar timings. I've noticed that, but I've never been able to figure out why it should be so.
I think there's a general rule that the more brittle the bean, the more fines you get grinding, and the coarser you need the grind to be to get the same flow (i.e. the amount of fines controls the flow rate). I also think beans are at their most brittle at the start of the second crack, a medium dark roast. When you get really dark, the oil (I'm guessing) softens them. And for some reason I don't know, Sumatra coffees produce fewer fines and meed finer grind settings.
Fines get a bad rap when people talk about grinding; but without them, there is no espresso.