Every passenger tire sold in the United States possesses a DOT number (serial number) molded or branded onto its sidewall. The DOT numbers certifies that the passenger tire meets or exceeds Department of Transportation Standard 109 (49 CFR 571.109 – Standard No. 109; New pneumatic and certain specialty tires). This standard specifies tire dimensions and laboratory test requirements for the testing of tires.
Tire companies do not test every tire. Instead, a tire manufacturer takes a sample tire from a group of tires with the same green tire specification. The complete design of a tire largely does not change, but minor changes are constantly being made to a particular tire specification. Within each group with the exact same design, tire manufacturers will use sample tires to perform Standard No. 109 testing, which include tests for bead unseating resistance, strength, endurance, and high –speed performance.
The bead-unseat tests measures the amount of force required to unseat the tire from its rim. A tire manufacturer will inflate its sample tire to the recommended air pressure in pounds per square inch. The test requires the increased pushing of a bead unseat fixture into the tire’s sidewall. The object is applied with increasing force to the sidewall, until the bead unseats from the rim. The amount of force necessary to unseat the bead on a tire with a designated section width of 6 inches shall not be less than 1,500 pounds. A tire with a section width of more than 6 inches but less than 8 inches must require more than 2,000 of force to unseat its bead. Finally, a tire with a section width of 8 inches or more requires a test result of more than 2,500 pounds of force to unseat the bead. In addition to section width, the sample tires are tested according to their load range. A tire with a load range of 880 pounds or less requires force of more than 1,500 pounds to unseat the bead. More than 2,000 pounds of force is necessary to unseat the bead of a tire with a load range of more than 880 pounds but less than 1,400 pounds. Finally, a tire with a load range of more than 1,400 requires more than 2,500 pounds of force to unseat the bead.
Tire companies use a Plunger Test to measure tire strength. A sample tire is inflated to its maximum air pressure, and a plunger with a hemispherical end is applied perpendicularly to the center of the tire’s tread. An increasing force is used until the tire punctures. The energy in inches per pound is measured, with a passing score indicated in the Standard.
Endurance and high-speed tests are conducted with an inflated tire spinning against a drum to simulate road operation. The endurance test spins the tire at 50 mph in an ambient temperature of 100 degrees Fahrenheit. A passing tire maintains its integrity for specified amounts of time under three separate load capacities: 85%, 90% and 100%. High-speed tests place a fully inflated tire into a drum. Under 88% of maximum load, the tester spins the tire at 75mph, 80 mph, and 85 mph. A passing tire maintains its integrity for 30 minutes under each condition.
Before a tire is placed into the stream of commerce, sample tires in its specification group must satisfy each of these four tests. A failing test results in changes to the specification.
Written by:
Guy L. Watts, II
Watts Guerra LLP
4 Dominion Drive, Bldg 3, Suite 100
San Antonio, Texas 78527
© Watts Guerra LLP 2015
