Let’s take a look inside your tires with both a cutaway view and a cross-section.
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.
Liner: Covering the inside of the tire is the thin rubber liner. The rubber of the liner is made as gas-impermeable as possible, but air will still slowly leak out of the tires via osmosis.
Sidewall: In construction terms the tire sidewall is the outer layer of rubber in the sandwich of materials that runs vertically from the bead to the tread. The sidewall layer is extra thick, both for strength and so that the tire’s identifying information may be embossed on it.
In more general terms, "sidewall" is used to denote the entire side construction of the tire, from the outer wall to the inner liner.
Tread Area: Above one or more layers of cushioning gum, which help to give a softer ride, lies the business end of the tire – the tread. Tread rubber compositions can and will occupy an article in themselves, but suffice it to say that it is here that most of the real compromises involved in tire making must be made.
In general, a hard tread composition will wear extremely well, but not provide a lot of grip. Soft tread rubber will grip well but wear much faster.
Grooves and Sipes: The tread area is separated into independent tread blocks by the deep channels known as grooves, which both define the tread blocks and help to channel water out from underneath them. Sipes are the small cuts made in the tread blocks themselves. Siping patterns in the tread blocks tend to suck up water and allow the tread blocks to flex, providing better grip on wet or snowy roads.
Rib: Many tires feature an unsiped central rib. By reinforcing the natural weak point at the center of the tread, the rib increases the rigidity of the tire in several dimensions.
Shoulder: The beveled or rounded area where the tread turns into the sidewall. How the shoulder is formed and siped affects how the tire corners. The shoulder flexes more than just about any other part of the tire. Nail punctures or other kinds of damage to the shoulder should not be plugged or patched, as the shoulder flex will eventually work the repair loose.
Once all of the various components that make up the tire are assembled, the “green” tire is placed into a heated press which forms the tread, melts the sandwiched layers together and vulcanizes the rubber. This creates the long elastic polymer chains that allow the tire to flex well and still return to its original shape. At that point, you pretty much have yourself a tire!