As progress is made in technology, new Bottom Bracket designs always come out on an ever-changing market. Two recent examples of this are the BB30 and BB90 units.
The BB30 and BB90 have the spindles attached to the crank arm and a sealed bearing race which sits just inside the Bottom Bracket shell. The design essentially enlarges the frame’s Bottom Bracket shell both in diameter and width to 87mm (BB30) and 90mm (BB90). The large bearing units are then pressed into the Bottom Bracket shell and the crankset is attached in the same way as an external/outboard Bottom Bracket system.
These modern types of Bottom Bracket unit have the inherent advantages seen in any outboard Bottom Bracket (such as lightness and stiffness.) However, as the Bottom Bracket shell is much wider and of larger circumference, even more stiffness is created – making it a desirable choice for top-end road bikes.
This BB30 system was invented by Cannondale in 1999 and is now becoming a standard system within top-end road bikes with many frames and component manufacturers adhering to its design spec. Special techniques and tools are needed for the fitting and removal of these systems.
The BB90 is high-end technology manufactured by TREK. It also has the advantage of being compatible with any chainset, giving you a great choice.
In any Bottom Bracket unit, considerable force is at play within it. With sealed bearing units that sit against the frame such as with the BB30 or BB90, if anything is awry, then the bearings can damage your (very expensive) frame and make the frame a write off.
Also, with BB30s you must use the compatible chainset, limiting your choice (although more are being developed as we speak.)