Construction fundamentals

A normal tire has four parts;the tread, the sidewall, the bead which holds the tire securely to the metal wheel rim, and fabric plies. The character of the fabric plies, which give the tire its skeletal structure, determines the tire's strength, stability, and resistance to bruises, fatigue, and heat. The plies vary in number and material. Without them, a rubber tire would be little more than a soft rubber balloon. The latest developments in tire de sign have been in the areas of tread width and ply construction.

Bias-ply tires were the most common type until a few years ago. They are still used frequently, mainly because of their low cost. But, as tire fabricating has advanced technologically, other construction methods have become more popular because they offer better performance.

In bias-ply construction, the fabric cords or plies are molded to the carcass of the tire in a crisscrossed pattern much like a herringbone tweed. The angle these cords make with each other determines certain characteristics of the tire;high speed stability, ride harshness, and handling. Generally speaking, the lower the cord angle, the better the high speed stability, but also the harsher the ride. Usually, cord angles are about 35 degrees from the centerline of the tire, giving strength to both sidewall and tread. Alternate plies extend in opposite directions. These layers of cord can be made of one of several materials;rayon, nylon, or polyester.

Bias-ply tires come in either 2-ply, 4-ply, 6-ply, or 8-ply. Most 2-ply tires now use very strong, heavy plies and have the equivalent strength of four normal plies. These tires are designated as 2-ply/4ply rating. The same up-grading can hold true for a 4-ply tire; it may carry an 8-ply rating. This construction provides flexibility and two-directional strength in both tread and sidewall areas. It is a very serviceable construction and one that has been a standard in the tire-making industry for many years.

The bias-belted tire has gained wide acceptance both as original equipment and as a replacement tire. It offers the car owner high mileage and great resistance to road hazards, plus excellent traction. Basically, the bias-belted is an improvement upon the bias-ply. It is constructed with two or four plies of a strong inner fabric to which rubber sidewalls and tread are bonded. However, before the tread is applied, belts which encircle the circumference of the tire are added.
A tire with bias-belted construction keeps its tread firmly on the road, greatly reducing tread squirm, a major cause of tire wear. The belts make the tire much more resistant to punctures, cuts, and bruises. A bias-belted tire can be made of several combinations of materials. Plies and belts of rayon, nylon, polyester, fiberglass, and steel are common But it is the construction principle, the combination of body plies and belts, more than the materials, which gives the bias-belted tire its many advantages.

Though more expensive than the con-ventional tire, the bias-belted provides greater value for the motorist. For an investment of approximately 25 percent more, he can expect to gain up to 40 percent more mileage compared with a regular bias tire.

When do you need new tires?
You should carefully inspect your tires at least once a month. Certainly you should never go more than 5,000 miles without checking them.
If a tire is smooth, get a new one. If the tire is worn unevenly, it indicates a mechanical malfunction somewhere. The tire probably should be replaced and the condition that caused the damage should be corrected. Uneven wear can indicate several things wrong;misalignment, the most common reason for fast wear; worn shock absorbers; a worn, loose front end; wheels out-of-balance; or a combination of these.