Join Date: Feb 2009
Location: Columbus, OH
Bikes: A beautiful columbus steel frame, 1986 Schwinn Voyageur touring bike, Currently Undergoing Overhaul
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Whoa, hold up fella, how did you break the crank? Anyway, you probably could go to a triple and still use the shifters and fd, but your chainline might be a bit off depending on your preferred chainring. I would look at getting just the new crankarm . . . probably the cheapest way to go anyway.
As for the external BB, yes, probably better (but I'm not sure you would notice it that much).
1) Stiffer because the bearings are closer to the point at which you are applying pedal force
2) Larger bearings (I think) and they can probably handle more weight
3) Serviceable, much less complicated to service and adjust than the sqare taper (like mine).
Regardless, my wife's bike has an external BB, and I can't tell anything different. Again, I would just replace the crankarm(s). Keep your chainline (you would probably get more annoyed with a crooked chainline that would cancel any improvements with an external BB).
Seriously though, how did you break your crank?