Bottom Bracket Cup Set BASIC

A traditional bottom bracket has 2 bearing cups that screw into the frame, a right hand, or fixed cup and a left side cup. The fixed cup is the one that goes on the chainring side. It usually has no adjustability. It is installed first and it tightened until the flats on the cup are snug against the bottom bracket shell.

The second cup is the left hand, or adjustable cup. Because component manufacturers have to make bottom brackets that will fit a wide variety of bicycles, built with varying degrees of precision, the left cup is installed last. After making sure that it is optimally tight so that the bearings run freely, without binding, it is in some way locked down.