Join Date: Aug 2005
Location: Madison, WI
Bikes: Gary Fisher Aquila (retired), Specialized Allez Sport (in parts), Cannondale R500, HP Velotechnic Street Machine, Dented Blue Fixed Gear (retired), Seven Tsunami SSFG, Specialized Stumpjumper Comp Hardtail (alloy version)
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IMO external bearing BBs are, on the whole, a better bottom bracket. I do understand the argument that your average rider won't benefit as much as a pro rider from the increased stiffness. However, the bearings can be larger, less prone to wear and much easier to replace. As well, square taper BBs and cranks have always been subject to greater metal fatigue stresses than the equivalent external bearing.
External BBs are easier to install, service and replace (if necessary). In particular Shimano receives high marks from me for including bearing pre-load in their external BBs. SRAM does a good job with their external BBs not needing preload if the BB shell is properly milled. Something you don't have to worry about with modern carbon fiber frames, or high end alloy frames (both steel alloys and aluminum). Campy is trick if you don't mind mucking about with wave washers.
and as to Tom Boonen winning the world champ on a square taper bb... that was several years ago (2005 to be exact). Since then he has been riding on (and winning on) either external BBs or BB30 spec frames. Up until next year when the entire Quick Step team will be on Mercx frames, probably with external BBs, more likely than not the bikes will be spec'd with Campy Record, which dropped square taper not too long ago.