The main parts of a modern pneumatic tire are its body, tread and sidewalls, and beads. The body is made of layers of rubberized fabric, called plies, that give the tire strength and flexibility. The fabric is made of rayon, nylon, or polyester cord. Covering the plies are sidewalls and tread of chemically treated rubber. The sidewalls form the outer walls of the tire. The tread is a thick hoop of rubber that comes into direct contact with road surfaces. To improve its traction, the tread has patterns of deep and shallow grooves and channels, depending on the intended use, and also may have protruding metal studs for icy or snowy conditions. High-performance tires have treads optimized for warm weather, and winter (or snow) tires are optimized for cold and snow; all-season tires are general-purpose tires. Imbedded in the two inner edges of the tire are steel hoops, called beads, that hold the tire to the wheel rim.
In the older type of pneumatic tire, air is sealed in an inner tube of butyl rubber beneath the body. In a tubeless tire the seal between the beads and the wheel rim is airtight and the underside of the tire body is coated with butyl rubber to keep the air from escaping. A puncture in a tire leads to loss of air and a so-called flat tire. Self-sealing tires are lined with a rubber or rubberlike compound that, when the tire is punctured by a slim object, such as a nail, coats the object and seals the hole to prevent air from escaping. A recent innovation is the run-flat tire. In the most common version, the sidewall is reinforced so that, in case of a large puncture and a total loss of air pressure, the tire is self-supporting; the vehicle can continue operating as if there were no tire problem for up to 125 mi (200 km). An innovative bead design keeps the tire securely on the rim. Such tires are often linked to a pressure monitoring system that alerts the vehicle operator to the puncture.
The most important feature of tire design is the arrangement of the cord, or ply. The three main types are bias ply, radial-ply belted, and bias-ply belted. In a bias-ply tire the cords in a single ply run diagonally from the beads on one inner rim to the beads on the other. However, the orientation of the cords is reversed from ply to ply so that the cords crisscross each other. In a radial-ply (also called radial-ply belted) tire the cords in every ply run perpendicularly from the beads on one inner rim to the beads on the other, and there is a rigid belt, usually of fine steel wire, between the tread and the plies. This construction provides longer tread wear but a rougher ride. In a bias-ply belted tire the cords in the plies are aligned as in a bias-ply tire, but a rigid belt, usually of synthetic fabric, is added. This tire has longer tread life than a bias-ply tire and provides a more comfortable ride than does a radial-ply tire.
Pneumatic tires are made in a variety of sizes to accommodate a variety of vehicles. The size is usually expressed by a standardized code of the form Axxx/yyBzz, where A designates the type of vehicle the tire is made for, such as P (passenger) or LT (light truck); xxx denotes the tire width in millimeters; yy denotes the aspect ratio (the ratio of the tire's height to its width); B is an R if the tire is of radial-ply construction; and zz is the wheel-rim diameter in inches. In addition, an alphabetic speed rating and a numeric tread-wear rating are embossed on the outer wall of the tire. Most tires are of the balloon type, with a large cross section and thin sidewalls. The large size permits a low inflation pressure, and the increased tread area gives better traction and braking qualities. Excessive tire wear is caused by incorrect inflation, wheel misalignment, sudden braking, and high speeds.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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