Typically, people don't spend a lot of time thinking about their tires, after all, why should you? They just work. But a tire is quite an amazing piece of engineering once you get inside it. A tire has to hold up tons of weight on a cushion of air, stay in good contact with road surfaces, give excellent grip and flex when those tons of weight go around a corner and spring back exactly to its original shape. And it has to do this over and over for literally millions of high-frequency cycles.
Plies: The body plies constitute the tire’s basic skeletal structure. Plies are usually composed of polyester or other fiber cords wound together and sandwiched in rubber as well. Radial plies all run perpendicular to the direction of the tire’s spin, and it is this pattern that gives a “radial” tire its name, as opposed to “bias-ply” tires in which the plies are placed at overlapping angles. Fiber cords are used because they are quite flexible, but inelastic, that is, they do not stretch. Thus they allow the tire to flex but keep it from deforming or losing shape under pressure. Plies can be damaged or cut, usually by a sharp impact. When that happens, the rubber becomes unable to resist high air pressures and begins to “bubble out.”
Steel Belts: The steel belts run longitudinally around the circle of the tire. Steel belts are made up of thin steel wires that are woven together into thicker cords, then woven again to form large sheets of braided steel. The sheets are then sandwiched between two layers of rubber. Most passenger tires contain two or three steel belts. Some manufacturers will now also wind Kevlar cord or other materials around the belts to improve rigidity and other running characteristics.
Cap Plies: Above the steel belts and towards the tread are the cap plies, which are much like the steel belts, except that the sheets are composed of woven fibers, again usually nylon, Kevlar or other fabrics. These inelastic plies help to hold the tire’s shape and keep it stable at high speeds, so usually only tires with a speed rating of H or higher will contain one or more cap plies. The number and composition of belts and plies can be found imprinted on the tire sidewall.
Many tires are now made with “jointless” steel belts and cap plies. Instead of simply clamping the ends of the belts or plies together, which creates a slight roundness irregularity in the tire, the ends are woven or otherwise seamlessly connected. This tends to result in a smoother-running tire.
Bead and Chaffer: The area where the tire seats against the edges of the wheel, creating the seal that holds air in the tire is called the bead on both the wheel and the tire. In tires, the beads are composed of two braided steel cords encased in a very thick rigid plug of rubber called the chaffer. The chaffer protects the body plies against abrasion from the steel bead wires and helps to stiffen the bead area of the tire.