If the History of Bottom Brackets 101 wasn't taught in your school, you probably don't know that Shimano was one of the first to adopt the external bottom bracket. Its Dura Ace SM BB9000 Bottom Bracket sits at the top of the line in terms of being serviceable, reliable, and exceptionally durable. The SM BB9000 mates with the new Dura Ace groupset and uses smaller bearings than the previous 7900 version. This results in a decrease in friction, making them roll even smoother. The new seals that have been incorporated result in a 50% reduction in drag without decreasing the protection levels of the bearings.