Every wheel needs one, and bicycles are no exception. A bicycle hub contains an axle and bearings on which the wheel spins. The hub also serves as an anchor for the spokes, which attach at the hub flanges and radiate out toward the rim. Hubs need to be serviced from time to time because the bearings inside can become worn with use. Most hubs feature sealed bearings, which come in a cartridge "sealed" by rubber. The bearings are installed in each side of the hub.
An average wheel will contain at least 28 spokes, and if they aren't properly adjusted, everything from the strength of your wheel to its very shape will be affected. Spokes are anchored at one end to the hub and attached at the other to the rim. When tensioned properly, spokes pull the rim evenly inward and give the wheel strength to withstand jolts from the road.
The tire protects the wheel from the road, and a good one keeps the bicycle rolling smoothly. The tire is a blessing to the cyclist, who has it to thank for absorbing so much of the shock from the road. Bicycle tires range in size according to the dimensions of the wheel. Tread patterns on tires vary depending on the type of road surface for which the tire is intended. Generally, off-road tires feature deep tread for increased traction, while road tires are smoother to reduce rolling resistance. The average bicycle tire is composed of fabric cloth coated with rubber. Kevlar may be added to the rubber to increase puncture resistance.
The rim defines the overall shape of the wheel more than any other component. The rim is a hoop, composed usually of aluminum. Similar to tires, the thickness and weight of a rim will be determined by its intended use. A mountain bike will require a more heavy-duty set of rims than a road bike. The rim serves multiple functions. Its surface provides a gripping point for brakes; its outer diameter holds both the inner tube and tire in place; and it acts as an attachment point for the spokes.
Unique to the back wheel, a cassette is a cluster of sprockets that serves as a portion of the drive train. Each sprocket is a different size, providing a different gear ratio from which the cyclist can choose. The cassette attaches to the rear wheel by virtue of a freehub body, which in turn attaches to the hub. Over the years, the number of sprockets a cassette can contain has continued to increase, offering a greater range of gear ratios. Some cassettes contain as many as 11 sprockets.