Although there are many special wheels designed to reduce weight and wind resistance, most bicycle wheels consist of a circular rim attached to both sides of a central hub by wire spokes under tension. The spokes usually hook into the edge of the hub at an angle rather than perpendicularly in order to convey the tangential force from pedaling most efficiently from hub to rim. Spokes are also usually laced over and under one another to maintain their tension and to reduce their tendency to loosen. Keeping the spokes under tension allows the bicycle wheel to be exceptionally strong while also exceptionally lightweight. This combination of benefits lasts only as long as the rim is both radially true (perfectly circular) and laterally true (in one plane), however. If a spoke loosens or breaks, the balance of tensions among the remaining spokes is thrown off, and further riding will rapidly cause the rim to become distorted or even irreparably bent into a characteristic potato-chip shape.
A circular rubber tire lined with an airtight inner tube (commonly made of latex or butyl) surrounds the rim. The inner tube is filled with air at a designated pressure. Most bicycle wheels fall into two general categories: those designed for clincher tires and those designed for tubular (sew-up) tires.
Clincher tires are by far the most widely used. Utility and mountain bicycles usually use clincher tires wider than 5 cm (2 in), sometimes with deep tread. Touring and hybrid bicycles commonly have clincher tires that are 2.9 to 3.8 cm (1.125 to 1.5 in) wide, and road- and track-racing bicycles usually have clincher tires less than 2.5 cm (1 in) wide. Air pressures for clincher tires range from as low as 35 pounds per square inch (psi) for the fattest mountain-bike tires to more than 150 psi for the narrowest racing tires. Rims designed to accept clincher tires are usually made of steel, a steel alloy, or aluminum. The rims have a roughly U-shaped cross section with the two open sides facing out toward the tire. Clincher tires are also U-shaped in cross section, but their two open sides point in toward the hub. When the inner tube is inflated, the air pressure keeps the tire firmly in place against the rim.
Tubular tires are very narrow (as narrow as 19 mm, or 0.75 in) and highly pressurized (120 to 170 psi) to give racing bicycles the least friction with the ground and the most nimble handling. In cross section, tubular tires are indeed shaped like a tube, completely enclosing the inner tube. The edges of tubular tires are hand-sewn together with a special needle and thread; the sewn edges are then glued onto a smooth rim. The rim has no projecting sides to hold the tire in place. The high air pressure within the tire pushing inward against the rim secures the tire. The rim, which may be made of aluminum alloy, carbon-fiber composite, or even wood, may be a hollow structure in cross section to maximize strength while minimizing weight.
Specialized wheels without wire spokes have been designed for high-performance racing to minimize the wind resistance and turbulence normally created by spokes slicing through air at high speed. One design is a monocoque (single-unit) or injection-molded solid structure consisting of three to five thin, flat airfoil blades (sometimes still called spokes) connecting the rim to the hub. The airfoils actually generate a bit of lift. Another design is a disk wheel, in which the rim is attached to each side of the hub by a thin plane of fiberglass or stringed Kevlar under tension. Disk wheels are generally used only for the rear wheel, because their unperforated surface can make the bicycle susceptible to crosswinds. There are many other specialized wheel designs that have been marketed in recent years to reduce weight and wind resistance.
Mountain-bicycle wheels are usually 65 cm (26 in) in diameter and are clinchers. Road-bicycle wheels, which may be clinchers or tubular, used to be either 70 cm (28 in) in diameter or the slightly smaller size known as 700 C, which allegedly stands for 700 mm but in reality ranges from 66.6 to 68.6 cm (26.2 to 27.0 in) in diameter. (The actual sizes of bicycle tires vary and are only partially standardized.) By the late 1980s the 700 C size was featured on most new road bicycles. BMX bicycles usually have wheels 50 cm (20 in) in diameter, and some folding bicycles have even smaller wheels.