Your bike wheels are the first component of your bike, road or mountain, to encounter shock or vibrations. Their rolling mass is critical to momentum and weight. Below we’ve created a wheel anatomy guide for you to be able to identify the components of your wheels before repairing or upgrading.
The rim is the hoop-shaped metal or carbon fiber component that makes up the perimeter of the wheel. The exterior consists of two steep, hook-shaped walls that allow the bead of the tire to form a tight seal. Inside is a smooth, concave surface on which a tight layer of plastic or tape is placed to prevent punctures and leaks. Throughout the rim’s interior are evenly spaced holes, called eyelets. The eyelets are flanged, allowing them to house small cone-shaped nuts, or nipples, which are tightened to the threads of the spokes.
Spokes are thin, cylindrical lengths of steel or aluminum, or, in some cases, more exotic materials like carbon or ceramics, that connect the hub at the center of the wheel to the rim at the exterior. They are tensioned evenly to increase the integrity of the wheel.
The hub is the rotating metal mechanism found at the center of the wheel. It features: an axle, which is fixed to the dropouts of the bike; a bearing system, which allows it to spin; and a flanged shell, which provides circular sets of eyelets for the spokes to be fixed to. Hubs vary by width and axle diameter, with wider, bulkier axles belonging to more aggressive bikes. The number of eyelets can also vary, as different wheels have different spoke counts. Hubs made for bikes with disc brakes will feature either a bolt pattern or a splined edge for the rotor to be fixed to. While front hubs can spin freely in either direction, rear hubs are more technologically advanced, and boast an integrated drive mechanism.
The freehub body is the splined drive mechanism that is fixed to the drive side of the rear hub. On the exterior, the freehub body features a unique pattern of splines, which allows the cassette to be fixed securely to it. On the interior of the freehub body is a set of spring loaded pawls. The pawls engage against complementary splines on the inside of the hub shell. While the hub is freely spinning, the pawls ratchet freely against the splines, but when the rider begins to pedal, the pawls engage against the splines, allowing the drivetrain to rotate the wheel and propel the bike forward.