The Chain Keeper BB attaches to frames with outboard bottom brackets. This allows the Chain Keeper BB to be used with carbon fiber frames and frames with non-standard tubing. Designed with adjustability in mind the Chain Keeper BB can accommodate a range of chain-line measurements and chainrings from 28 to 39 teeth. It weighs less than the lightest road derailleurs on the market. Plus you lose the weight of the shifter, cable and housing, which can be substantial. The following table lists the spacer arrangement for most outboard bottom brackets when installed in either 68mm or 73mm shells. Hollowtech II, MegaExo and Giga-X-Pipe bottom brackets are designed to have their bearing cups spaced 75.5mm apart. The bottom bracket shell, 2.5mm spacers, and Chain Keeper BB mounting plate are counted towards this total width.