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But the other five to seven pounds—the cords used to reinforce the plies and belts—may get some ink from the publicists. It's impossible to say categorically that one material is superior to another for making either ply cords (the crosswise components) or belt cords (the circumferential components). Nonetheless, each of the popular combinations has certain advantages and disadvantages worth reviewing.

Rayon is the most prevalent reinforcing material for plies these days. It has adequate heat resistance and adheres well to rubber, but it will develop a set, resulting in the characteristic thump of flat spots the first few miles in the morning. It's also not uncommon to find it in the multi-ply belt of a radial.

Despite its strength, nylon is becoming less popular, largely because it stretches, shrinks and develops flat spots. You'll mainly find it wrapped in the belts of high-performance tires, where shrinkage is an advantage because it holds other belts in place, and flat spotting is tolerable.

Polyester is almost as common as rayon on passenger car tires. It may be somewhat more resistant to the elements than rayon, and its only real disadvantage is that it shrinks if overheated. It's used in both plies and belts.

Fiberglass is quite strong and very resistant to heat but is intolerant of flexing and is easily cut. It can be found in the belts of a few specialty tires.

Aramids, such as Kevlar, are light, strong and durable, making them excellent for belts. Their only real disadvantages are bulk and cost.

Steel is rigid and strong, making it the allaround favorite for belts. Its few disadvantages include weight and a lack of adhesion to rubber.

Treads

Tire engineers laboring away at their computers may take umbrage at this statement, but those of us whose driving aspirations are somewhat lower than Mario Andretti's don't need to worry much about tread design. All we have to do is choose a tread type that's suitable to our climate.

If you live in an area without snow, you're better off sticking with a conventional highway tread. It will have about 35% voids (spaces between the tread blocks) to usher rain out of the way. And it will give excellent dry pavement traction and long wear. Some designs may do a slightly better job of squeegeeing water to the sides than others, but major differences are found only among the latest generation of directional (mounted facing one way) high-performance tires.

For all but the balmiest and snowiest portions of the country, choose an "all-season" tire. (The initials M-S or M&S will be molded somewhere on the sidewall.) On packed snow or ice, the M&S tire is superior to either a street or a snow tire, and it will wear nearly as well as the highway tread. Many new cars now come fitted with all-season tires, and they're among the most popular replacements. There are three main differences between M&S and highway tires: The M&S has a slightly higher void area, it has grooves through the shoulder to help it self-clean of mud and snow, and it has a higher percentage of polybutylene rubber to keep it flexible at low temperatures. For the best handling, they should be run on all four wheels.

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