external vs sealed bottom bracket?
 

Is it merely coincidence that most of the more $$ bikes that i have looked at have 'external' bottom brackets vs sealed ones with the square 'peg' for the crank arm?

is one more servicable or better than another? I have been trying to search, but may have been using poor search terms, and the threads i did find were old from when these things were new apparently.

thanks all

Most modern square-taper bottom brackets are sealed-cartridge units. Usually quite durable, but need to be replaced when the bearings start to go.
I believe most external-bearing units are sealed-cartridge-bearing as well, although some parts of the unit may be replaced without having to replace everything.

The reason for external-bearing units becoming popular is stiffness of the crank (and the "it's the new thing" fuss, as most riders don't stand to benefit much from the increased stiffness, and remember that Tom Boonen won the world championship on a Campy Square-Taper BB while Shimano had already upgraded its high-end systems to external bearings.

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.

ALSO

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.

Cartridge bottom bracket systems like shimano's un72 are not servicable but run for a looooooong time, very good seals.

The new external BB are indeed much stiffer.. there is noticeably an increase in rigidty, but they don't roll as smooth as the un72 and increase the Q-factor of your pedals(how far apart your feet will end up, cowboy) and may mess up your chainline . I know shimano's hollowtech II external BB is servicable. One can disassemble the seals and clean out the contaminated grease, but they'd need a special bearing press to remove and pop in a brand new cartridge bearing.

I agree that external-bearing BB's are technically better. I just don't think they're much better, in ways that end up being meaningful. For example:
- it's true that square taper BBs and cranks are subject to greater metal fatigue stresses. but this didn't mean that they were likely to break. square-taper is a very mature technology in terms of both taper interface and bearing durability.
- it's true that external bearing BBs are x% stiffer than comparable square-taper system. what I haven't seen is blind-test power-meter data (where the rider doesn't know what kind of BB he's using) that quantifies actual output difference this means to a cyclist.

that all said, I don't race, and I'm probably judging external BB systems more harshly on cost than I should. I just haven't seen any reason to upgrade any of my bikes.

No modern shimano 9/10 speed nor campy 10/11 use a square taper interface. Anything older is not going to be appearing on a new bike, that would be 8s < lower. Road bikes anyways. Square taper for all intents and purposes is deprecated on modern offerings.

#1 problem with square tapers, the interface, "ovalized" crank arms(left ones take the brunt) and flattened crank-interface surfaces. Not saying that external bb are idiot proof, but they're pretty straight forward.

I'm aware of the "increased stiffness and strength" argument but have recently replaced my FSA triple external BB chainset with a Campag Chorus triple chain-set, paired with an old Record BB. The difference in the amount of resistance is amazing.