Tire Construction
While Charles Goodyear is generally credited for the vulcanizing process (which makes rubber more flexible), England's Robert Thomson developed the pneumatic-tire concept. In elementary terms, a tire is no more than a group of materials that contain an air cushion around a wheel. These materials may include rubber, synthetic fabrics (rayon, nylon, polyester), and steel, and are wrapped in layers according to different designs.

Such designs have included belted, bias-belted, bias ply, diagonal ply, and radial. Since radials provide better ride characteristics and lower rolling resistance, they're the design of choice for standard equipment today. Michelin debuted a steel-belted radial in 1948, but it didn't become popular until the mid-'70s. No coincidence there, since that era marked the first gas crunch and softest-riding barges.

If you took a hacksaw to your tire, the cutaway would show the tread, belts, and beads. The belts stabilize the tire carcass and maintain the integrity of the air cushion by keeping out debris and sharp objects. Each of these belts, or plies, does a specific job, which is why the number and type vary by tire and tread or sidewall. The way these belts are wrapped around the tire defines radial (or belted, bias-ply, etc.).

The tire is held on the rim by the beads, the round cross-section inside edge of the tire. Inside the bead is the anchor cable, which acts much like the main cable on a suspension bridge and holds all the tire plies together. This cable is protected by a hard rubber cover that helps keep the sidewall stiff next to the rim and often by a layer of fabric to save the tire from rim abrasion or scuffing during installation. Wheel-mounted bead locks physically lock the tire on the wheel instead of relying on air pressure. Bead locks are not used on production vehicles, but in situations where the tire may pop off the bead because of low air pressure (sand, off-road racing) or where the wheel may spin inside the tire because of low pressure and/or excessive power (dragsters, sand buggies).

The tire's top layer is the tread, and with the exception of curb guards--thick ridges at the widest point of the sidewall--the only part of the tire designed to contact the ground. The tread does wrap around the side of the tire slightly for cornering grip and better water evacuation, but good off-road tires have extra tread or sidewall protection.

Tread Options
Apart from a data book or the sidewall, the tread pattern will tell you more about a tire than anything else. The contact patch (the area where tire meets ground) consists of a series of grooves (channels) that serve as primary water passages, ribs that run around the circumference of the tread, and individual blocks of tread rubber that often become much larger blocks after the tire has worn down. Sipes are the small edges in the tread blocks and ribs.

The basis for tread material is rubber, but there could be 30 or more different materials used in the compound. If a compound is called "hydrophilic" it means it's attracted to water, which makes it an excellent material for dedicated snow or rain tires. However, hydrophilic compounds tend to wear faster, just like sticky tire compounds. Recently, Goodyear introduced a compound derived from cornstarch and mixed with synthetic rubber, called BioTred. This compound is said to save fuel (and therefore lower emissions) and give better wet-braking performance, but sizes are limited because it's a new technology.