The Chain Keeper BB is designed for use with external (outboard) bottom brackets. This allows the Chain Keeper BB to be used with full suspension frames, carbon fiber frames and frames with non-standard tubing. It is attached by a plate that replaces the 2.5mm spacer found on many external bottom brackets. The Chain Keeper BB can accommodate a range of chainline measurements and chainrings from 28 to 39 teeth in size. Weighing less than the lightest road derailleurs on the market, a Chain Keeper can help you cut weight from your bike. Plus, you lose the weight of the shifter, cable and housing, which can be substantial. The Chain Keeper BB is compatible with frames with 68mm or 73mm shells, and will work with Hollowtech II, MegaExo, Giga-X-Pipe and other outboard bottom brackets. Specifications: Material: 6061 Aluminum Weight: 69g Chainrings Size: 28T 39T Black only.