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Elizabeth McCully, age 12, of Tonawanda, N.Y., for her question:

HOW MANY TYPES OF AUTO TIRES ARE THERE?

There are three basic kinds of automobile tires: bias, belted bias and radial or radial ply.

Bias tires are built with the cord fabric running diagonally, or on the bias, from one rim to the other. Each ply is added so that its cords run at an angle opposite to the angle of the cords below it.

As an automobile moves, the plies of the bias tires rub against each other and against the tread in an action called flexing and squirming. This action produces inner heat, one of the major causes of tire wear. Extreme heat can separate the tread or split the piles.

Belted bias tires are used on most new cars. They are made in the same way as bias tires, but a belt of fabric is placed between the plies and the tread. The belt is made of fiberglass, rayon, steel or some other material that does not stretch. It helps prevent punctures and reduces inner heat.

Radial tires are built with the cord fabric running straight across the tire from one rim to the other. All radial tires are belted.

The combination of radial ply and belting produces a tire that has longer tread life than either bias or belted bias tires. Radial tires give longer wear because they have less flex and squirm than bias ply tires. But radial tires do not provide so comfortable a ride, and they also cost more.

Low profile tires are used on some sports cars. They are available as bias, belted bias or radial tires. Low profile tires look pudgier than regular tires. They are wider from side wall to side wall than they are high, from tread to wheel rim.

Low profile tires put more tread into contact with the road than do regular bias, belted bias or radial tires. This additional tread creates a wider track or footprint.

The wider footprint of a low profile tire also gives greater traction. This added traction gives a driver more control of his car at high speeds and around curves than he would have with any regular type of tire.

Tire tread patterns are designed for a variety of special purposes. Snow tires, for example, have a tread with extra deep grooves. This tread bites into snow and mud and it provides exceptional traction.

After the original tread pattern has worn down, if the tire is in good condition it can sometimes be retreaded or recapped. First, a special machine is used that rubs away the old tread. Then a workman applies new tread rubber and puts the tire into a mold. The new tread and tread pattern are then vulcanized to the old tire.

A Scottish engineer named Robert Thomson invented the pneumatic tire in 1845. At that time, most vehicles had wooden wheels and steel tires. The steel tires preserved the wood and wore well. Thompson's tires gave a smoother ride.

 

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