Experimental identification of the split in the fundamental air-cavity mode of loaded tires in the absence of rotation

A tire’s fundamental air-cavity mode, typically occurring near 200 Hz for current passenger car tires, can split into two adjacent modes when a tire is loaded, thus creating a fore-aft mode at a lower frequency and a vertical mode at a higher frequency. These adjacent modes are important since they create dynamic force inputs to the suspension system, and thus, they can contribute to vehicle interior noise in the vicinity of 200 Hz. This work was focused on experimental observations of a set of 24 commercially available tires. A specialized test rig was built to load the tires, and the tires’ mobilities were measured using a Doppler laser-scanning system. It was found that the frequency splits ranged from 2.0 to 11.2 Hz at rated load from the mobility and dispersion diagram converted by wavenumber decomposition, which was also observed in a roving test around the tire’s circumference using an impact hammer. From these results, the regression model on the frequency split over input parameters such as rim size, stiffness, inflation, and applied load was developed, and it agreed with actual test values. Finally, the relation between the magnitude of the frequency split and the applied load showed a nearly fourth-order dependence, while Thompson’s analytical estimation presented a second-order dependence. The resultant dispersion curves obtained from the wavenumber decomposition of the spatial mobility data made it possible to identify the strength of the interaction between the two acoustic modes and circumferential treadband structural resonances near 200 Hz.