Laboratory Tire Bead Unseating – Evaluation of New Equipment, Pressures and “A” Dimension From ASTM F-2663-07as

Under the Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act, the National Highway Traffic Safety Administration (NHTSA) was assigned the task of revising and updating the Federal Motor Vehicle Safety Standards (FMVSS) for tires. Part of this effort included research to support a possible update or replacement of the FMVSS No. 109 Tubeless Tire Bead Unseating Resistance test. In the current FMVSS test, a profiled metal “block” is located perpendicular to the sidewall of a mounted and inflated tire, then force is applied until the bead component (region where the tire meets the rim) unseats with the resulting air loss or the specified minimum force value is achieved. The objective of the test is to evaluate the ability of the tubeless tire’s bead to remain seated on the rim and retain tire inflation pressure when the tire is subjected to high lateral forces such as during severe turning maneuvers, curb scrubbing, or negotiating deep ruts in the roadway. Since introduction of the FMVSS No. 109 in 1967, there has been a steady introduction of larger wheel diameters and smaller tire sidewall aspect ratios. For tires with rim codes above 18 (e.g., P345/30R18), the bead unseating blocks in the current Federal standard can contact the rim or slide down the tread during tests (especially for low aspect ratio tires). To account for these issues, an American Society for Testing and Materials (ASTM) task group working to develop a new standard test method for bead unseat testing (ASTM F2663-07a) proposed using the original FMVSS No. 109 equipment and two larger-radius bead-unseating blocks with a new method for positioning the block on the tire sidewall. The agency conducted an independent evaluation of the new ASTM standard using nineteen models of passenger and light truck tires that ranged in nominal widths from 155 to 345 millimeters, aspect ratios from 30 to 80, and rim codes from 12 to 28. For some tires, tests were conducted at two inflation pressures to evaluate the effects of pressure on unseat force. The three blocks used in the ASTM standard (the original FMVSS No. 109 block and two new designs) allowed testing of all tires to the minimum FMVSS No. 109 bead unseat force requirements without contacting the rims. The new ASTM method for positioning the block on the sidewall generally produced lower unseat forces than the method in the current Federal standard, meaning that the ASTM method is more severe. Furthermore, occurrences of the test block sliding across the tread when testing with the FMVSS No. 109 positioning method did not occur when using the new ASTM method. As expected, the results of tests at two tire inflation pressures indicated that tire bead unseating force was sensitive to inflation pressure, with lower unseating forces recorded at lower pressures.