establishes ratings for treadwear, traction. and temperature resistance (fig. 8-34). These tests are conducted independently by each manufacturer following government guidelines to assign values that represent a comparison between the tested tire and a control tire. While traction and temperature resistance ratings are specific performance levels, the treadwear ratings are assigned by the manufacturers following field-testing and are most accurate when comparing tires of the same brand. Tire grades are as follows:

TREADWEAR - Treadwear receives a com- parative rating based on wear rate of the tire in field-testing following a government-specified course. Treadwear is given as a number: 100, 120, or 130, for instance. The higher the number, the more resistant the tire is to wear. For example, a tire grade of 150 wears 1.5 times longer than a tire graded 100. Actual performance of the tire will vary significantly depending on conditions, driving habits, care, road characteristics, and climate.

TRACTION - Straight-a-head wet braking traction has been represented by a grade of A, B, or C with A being the highest. In 1997 a new top rating of "AA" was introduced to indicate even greater wet braking traction. Traction grades do NOT indicate wet cornering ability.

TEMPERATURE - Temperature resistance is grades A, B. or C. This represents the resistance of the tire to heat generated by running at high speed. Grade C is the minimum level of performance for all passenger vehicle tires as set under Federal Motor Vehicle Safety Standards. This grade is established for a tire that is properly inflated and not overloaded.

NOTE

Uniform Tire Quality Grade ratings are NOT required on winter, light truck, and commercial tires.

TUBES

Tubes (inner tubes) are circular rubber containers that fit inside the tire and hold the air that supports the

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.

WHEELS

Wheels must have enough strength to carry the weight of the vehicle and withstand a wide range of speed and road conditions. Automobiles and light trucks are equipped with a single piece wheel. Larger vehicles have a lock ring (side ring) that allows for the