Features: The hollow steel shaft and external bearing design provide strong stepping rigidity and direct force transmission, reducing force loss and making it more efficient than traditional square hole/spline shafts. Adopting sealed bearings, dustproof and waterproof, suitable for complex road conditions. All of it are modularization designs, and the bearings can be replaced separately, with low maintenance costs and extended overall service life. Threaded Connection design enhances lateral stability to avoid creaking sound. BB51/BB70 are threads (34.8x1.37x24TPI), suitable for 68mm (highway)/73mm (mountain) tees; BB91 is suitable for 86.5mm (highway)/91.5mm (mountain) press in type five joints, with strong versatility. Specifications: Component: Aluminum alloy Size: about 4x3cm/1.57x1.18inch Package Includes: 1pc Bottom Brackets Adapter note: Please allow 1-2cm errors due to manual measurement, make sure that you do not mind before you order. Due to the difference between different monitors, the picture may not reflect the actual color of the item.