Figure 8-34.- Uniform Tire Quality Grade System ratings on the sidewall of a tire.

vehicle. Though it is strong enough to hold only a few pounds of air when not confined, the tube bears extremely high pressures when enclosed in a tire and wheel assembly. Because the tube is made of comparatively soft rubber to fulfill its function, it is easily chafed, pinched, punctured, or otherwise damaged. Tubes generally are made of a synthetic rubber that has air-retention properties superior to natural rubber. There are two types of synthetic rubber tubes- butyl and GR-S. A butyl type tube is identified by a blue stripe, and GR-S has a red stripe. Other than the standard tube, there are three special typesoftubes- radial tire, puncture sealing, and safety.

RADIAL-TIRE TUBE- The construction of an inner tube for use in a radial tire differs from the tube used in a bias tire. A radial tire flexes in such a manner that it concentrates the flex action in one area and at the edge of the belts in the shoulder of the tire. This concentration of stress will damage a standard tube causing it to fail. To overcome this problem, the radial tube is made of a special rubber compound that is designed to overcome this concentrated stress; Therefore, standard tubes must NEVER be used in radial tires.

PUNCTURE-SEALING TUBE- This type of tube has a coating of plastic material in the inner surface. When the tube is punctured, this plastic material is forced into the puncture by the internal air pressure. The plastic material then hardens, sealing the puncture.

SAFETY TUBE- The safety tube is really two tubes in one, one smaller than the other, and joined at the rim edge. When the tube is filled with air, the air flows first into the inside tube. From there the air passes through an, equalizing passage into space between the two tubes. Therefore, both tubes are filled with air. If a puncture occurs, air is lost from between the tubes. However, the inside tube, which has not been damaged, retains its air pressure. It is sufficiently strong enough to support the weight of the vehicle until the vehicle can be slowed and stopped. Usually, the inside tube is reinforced with nylon fabric. The nylon fabric takes the suddenly imposed weight of the vehicle, without giving way, when a blowout occurs.