Aerodynamic Sensitivity Analysis of Tire Shape Factors

It is well known that the wheels and tires account for approximately 25% of the overall aerodynamic drag of a vehicle. This is because the contribution of the tires to aerodynamic drag stems from not only aerodynamic drag itself directly caused by exposure to the main flow (tire C_D), but also from aerodynamic drag indirectly caused by the interference between tire wakes and the upper body flow (body C_D). In the literature, as far as the authors are aware, there have been no reports that have included the following all four aspects at once: (1) C_D sensitivity to detailed tire shape factors; (2) C_D sensitivity differences due to different vehicle body types; (3) C_D sensitivity for each aerodynamic drag component, i.e., tire C_D and body C_D; (4) Flow structure and mechanism contributing to each aerodynamic drag component. The purpose of this study was to clarify C_D sensitivity to tire shape factors for tire C_D and body C_D considering two different vehicle body types, sedan and SUV. In wind tunnel testing, many tire configurations were prepared for a parametric study. Detailed shape factors of each tire were three-dimensionally measured by laser beam scanning in the test section taking into account tire deformation. For decomposing the aerodynamic drag components, measurements were made of C_D and also the pressure at the vehicle base in the tests. For analyzing the mechanism causing C_D differences, flow fields were obtained by measuring the surface pressure on the test vehicles and also the spatial pressure and velocity were measured at cross sections of the body side. Multiple regression analyses clarified C_D sensitivity to tire shape factors for tire C_D and body C_D with respect to the sedan and SUV body types. This study revealed high C_D sensitivity factors that were independent of exterior styling. Furthermore, analyses of computational fluid dynamics (CFD) simulation results identified the flow fields contributing to each aerodynamic drag component, tire C_D and body C_D.