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A bike axle is a component on a bicycle that allows the wheel's hub to rotate. It is generally a solid piece of metal which is round and threaded. The threads allow axle bolts and bearing crown races to be secured in place. Sometimes the axle is hollow to allow for use of a quick-release system, though traditionally solid metal axles have been used for strength and rigidity. A bike is likely to have two of these components, one for the front wheel and one for the rear.
The hub is the set of bearings inside the shell at the center of the wheel. This component is primarily responsible for allowing the wheel to rotate. A bike axle is a component within the hub, and it is the center point around which the hub bearings rotate. To allow for freer rotation, bearing cones are screwed onto the threaded bike axle on either side of the hub. The bearings themselves are seated in a bearing race within the hub, and the cones will press against the other side of the bearings to secure them in place. Locknuts will then be screwed onto the bike axle to help lock the cones in position.
Two primary types of bike axle exist: solid axles and hollow axles. Hollow versions are used to accommodate quick-release skewers, which are metal rods that slide through the center of the axle. A lever is attached to one end of the skewer, and a specially designed bolt on the other. When the lever is flipped upward, the skewer will tighten inward, securing the wheel into the dropouts of the bike frame or fork. This ensures the wheel does not move or otherwise dislodge during riding. A solid bike axle, conversely, does not have a hollow center. The wheel is secured into the frame dropouts or fork by using bolts screwed onto either end of the axle.
Sometimes the hub will be specially designed to accept sealed bearings rather than loose ball bearings. If this is the case, a specially designed axle will be used instead of the more traditional threaded versions. Sealed bearings are often press-fitted into the hub shell, and the threadless axle unit will slide into place within the bearing itself. It is likely that the metal axle will be slotted or otherwise cut to ensure it presses on the bearings properly.