We all know not to ride big ring-big sprocket or small ring-small sprocket because the chain is at too sharp an angle. Richard Ballantine in Richard's 21st Century bicycle book says that triple chain rings on 7,8, or 9 speed blocks you should avoid 2 cogs on either side of the extreme, and that the middle ring shouldn't be used with the extreme cogs either. Is there a rational way of figuring out which angles are too extreme? Does the chain width effect the acceptable angles? Does anybody really know these things or is it all trial & error? If so, why don't production bikes come with charts of the "right" gear combos and the no-nos?