In the early days, tires were simple things, essentially just wooden or metal hoops that "tied" the spokes of a wheel together. In 1887, Scottish veterinarian and inventor John Dunlop revolutionized the tire when he created an air-filled rubber tube for his headache-prone son's tricycle. When Charles Goodyear came along with his vulcanization process, the pneumatic rubber tire came one step closer to readiness for the high-speed, high-performance world of today. But that wasn't the end of the tire's structural evolution -- not by a long shot.
At the heart of almost every modern tire is something like Dunlop's original air-filled tricycle tube. Older tires were much like bicycle tires are today, with a round, air-tight tube inside of a thick, rubber casing. The problem with inner-tube tires is that, while reliable, they're also prone to failing at high speeds and under prolonged use because friction between the tube and outer casing overheats the inner tube's rubber. In 1944, a subsidiary of Goodyear called Wingfoot patented a tubeless tire in South Africa, and by 1955 all new tires in the U.S. were tubeless. The tubeless tire still uses an impermeable air tube that is "U" shaped in cross-section instead of "O" shaped. The wheel rim covers the open end, sealing it shut. The inner edge of the tire is reinforce by a bundle of thick, stiff wires wrapped in fabric; this is the tire "bead," which is what seals against the flanges on the wheel rim when air pressure pushed the sides of the tire outward. Another layer of wire and a thin layer of "chafer fabric" finishes off the bead.
The body of the tire is somewhat morbidly known as the "carcass." The carcass consists of the bead, sidewall, ply, "shoulder," and tread. Around the inner "Dunlop tube" are a series of "plies," or reinforcing strands of steel or fabric. Older tires used nothing but strands of steel and polyester, but many newer tires use light-weight, super-strong synthetics like Aramid, Twaron and Kevlar -- the same used for bullet-proof vests. Modern tires use "radial" construction, meaning that the primary supporting fibers run sideways around the tire, wrapping around the bead on one side, going over the inner Dunlop tube and over to the other bead. A chunk of rubber -- known as the "shoulder" and an "edge cover" over that -- goes over the top-sides of the inner tube, squaring it off on the sides and top. Another set of reinforcing "belts" go around the circumference of the tire, and these serve to keep the outside of the tire round. These are often steel, but can be other materials as well. A fabric-like "cap ply," often made of synthetic fiber, goes on over that; this and the steel belts are what you see when you melt all the tread off your tires with a massive burnout.
The outer layer that you see on top of all of this complex under-structure is the sidewall casing and the tire tread. The sidewall is actually the whole side of the tire from the bead to the tread, including the structure and plies underneath. In a radial tire, the fibers that wrap from one bead upward and over the tire act like springs of a sort, allowing the sidewall to flex and bounce somewhat for a softer ride; the nearly parallel ply design reduces the tire's rolling resistance, and combined with the reinforcing belts on top, make for a tire that rolls forward very efficiently. A taller sidewall -- aka "higher profile," or "higher aspect ratio" tire -- flexes more, giving a more comfortable ride. A shorter sidewall flexes less, making for a harder ride, but also keeping the tread centered over the rim under cornering for enhanced steering response. The tire's tread is usually a mix of many different ingredients, proportioned to optimized the tire for a particular type of performance.
The tire's tread relies heavily on Charles Goodyear's vulcanized rubber, and a number of other ingredients. Coal-like "carbon black" is a filler that might make up anywhere from 20 to 80 percent of a tire. Silica -- aka "glass powder or sand" -- hardens the tire, decreasing rolling resistance for better fuel economy. Other additive like silanol, silicone, and natural or synthetic rubber can be used to harden or soften the tire's tread for more grip, or for longer life and better fuel economy. Different parts of the tire's tread can be made of different rubber compounds as well; some tires use a harder compound in the center for better fuel economy and wear, and softer compounds near the edges for better handling. Tire tread compounds are kind of like the secret formula for your favorite soda, or the Colonel's herbs and spices -- they're almost always secret and proprietary.
Tread compound might be a secretive thing, but tread design can tell you a lot at a glance. The tread is divided into "ribs" that circle the tire, and "blocks" of rubber in the ribs. The grooves that go around the tire between the ribs are known as "voids," and they're used as constant reliefs in the tread to channel water from in front of the tire to the back. "Sipes" are the sideways or angled grooves going through the ribs, and they squeegee water from the ribs into the groove voids so it can escape. The best performance tires have sipes angled backward to efficiently get rid of water; this makes for a "directional" tire that can only roll in one direction, but which is far superior in the rain or snow to non-directional tires. Larger, unbroken blocks of tread are more stable at high speed than lots of little tread blocks, which is why you'll find that the best performance tires will have very flat tread blocks with only a few, very large voids and sipes. By comparison, lower-speed and off-road tires are "knobbier," with lots of smaller voids that make room for water to go through, and put more weight on each individual tread block for better traction off-road, and on ice and snow.
