The carbon form widely used for lightweight, high-performance bicycle frames is carbon fiber.
A bicycle frame is not usually made from loose carbon fibers alone. The fibers are embedded in a polymer resin, producing a carbon-fiber-reinforced polymer composite. Manufacturers can arrange the fibers in different directions and vary the layers through the frame, tuning stiffness, strength, and compliance for particular riding forces.
This directional control is a major advantage over conventional metal tubing. Carbon composites can be molded into aerodynamic or highly integrated shapes, while a frame designer can reinforce areas such as the bottom bracket and head tube without making every part equally heavy. Modern carbon frames may use joined tubes, bonded lugs, or one-piece monocoque construction.
Carbon fiber is often confused with graphite because both are carbon-based materials, but graphite powder is not the structural reinforcement used in a typical bicycle frame. Fullerene is another carbon form with very different molecular structures. Carbon frames can be exceptionally light and strong, though they may be vulnerable to hidden impact damage and are costly to manufacture or repair.