“A cigar is just a cigar,” goes the old saying, and “An automobile tire is a tire is a tire.” Well, neither description is that simple. Tires aren’t just an undifferentiated mass of rubber, and they aren’t all made the same.Automobile tires are made up of different materials forming distinct layers in specific kinds of designs. A series of plies reinforces the basic tire, giving the tire its strength and its shape. A bias-ply tire has an under-layer of plies, constructed wit a nylon weave. These plies run diagonally down the sidewalls to the bead, where the tire fits on the wheel rim. The plies converge in the centerline of the tire face. Although it’s strong, nylon ply tends to compress and set when under certain loads, and can result in “flat areas” on the tire. Because of the direction of the plies on these tires, the sidewalls don’t bulge even if the tire pressure is low.
A radial tire is made differently, constructed of polyester cords that run perpendicular from bead to bead, upwards–over the face of the tire, and downwards–to each sidewall. So a radial tire is “wrapped” at a right angle to the direction of the tread.
A radial design keeps the plies from rubbing against each other as the tire flexes, which means less rolling friction and thus better fuel economy than with bias-ply tires. A belt almost as wide as the tire itself runs below the tread, over the polyester, on the tire’s face. It helps stabilize the tread and reduce tire wear. Once made of rubber-coated fibers, today’s belts are now normally made from steel fibers, hence the name, “steel-belted” radial. This belt helps stabilize the tread, reducing wear.
Because of these performance advantages, in 1968 Consumer Reports acknowledged the superiority of the radial tire design, noting its longer tread life, better steering characteristics, and less rolling resistance. And two years later Ford debuted the first American-made vehicle with radial tires as standard equipment. That year the Lincoln Continental Mark III sat on Michelin radials. Michelin, which owned France’s leading automaker, Citroen, designed, developed, patented, and commercialized the radial tire (the first being the Michelin X car radial in 1946), following pioneering work by Californian Arthur W. Savage earlier in the century.
Italian design made its mark in 1974 when Leopoldo Pirelli (whose grandfather founded the global manufacturer–and exclusive tire supplier for the Formula One Championship) created a wide radial with a stance strong enough for the Lancia racing team’s powerful Lancia Stratos (the Stratos won the World Rally Championship in 1974, 1975 and 1976).
The design and technological performance specs continued to spread the popularity of radials throughout the automotive world across the decades until today, when they are the standard for virtually all autos and enjoy a 100% market share