Study On Self-excited Shimmy Hopf Bifurcation Characteristic And Lateral Stability Of4WS Vehicle

Both of vehicle handling and stability characteristics and shimmy belong to automobile stability. However, modern car pursues higher speed, better comfort, stability and security, while the car shimmy problem and lateral stability problem becomes more prominent. Four-wheel steering vehicle can effectively improve the car’s mobility, while four-wheel steering vehicle yaw rate and lateral slip transient response can be significantly improved to enhance handling and stability, but there are limits for four-wheel steering vehicle to enhance the performance. This paper focuses on the problem of four-wheel steering shimmy induced by self-excited vibration and four-wheel steering vehicle lateral stability issues under lateral aerodynamic force. The researches can provide references for enhancing driving performance by obtaining the mechanism of shimmy and factors of lateral stability.In order to obtain the shimmy and lateral stability characteristic of four-wheel steering vehicle, three models are established taking a domestic sedan as the prototype:(1) Front wheel shimmy model considering lagged dry friction of the kingpin.(2) Four-wheel steering vehicle shimmy model based on front shimmy model.(3) Lateral stability model of four-wheel steering vehicle considering lateral aerodynamic. The above-mentioned models are simulated, then the front wheel multiple limit cycles shimmy of four-wheel steering vehicle, steering wheel shimmy of four-wheel steering vehicle and lateral stability of four-wheel steering vehicle are analyzed. The results show that:(1) Front wheels tends to generate shimmy more easily than rear wheels and rear wheels shimmy can be controlled more easily. When shimmy occurs on rear wheels, shimmy must occur on front wheels. Both the range of bifurcation speed and the amplitude of front wheel shimmy are larger than that of the rear.(2) Lateral aerodynamic acted after mass center can enlarge stability area than that before mass center. As vehicle speed increases, the tendency for stability area decreases obviously. When vehicle drives on low friction coefficient road, lateral aerodynamic make the stability area smaller, or even disappear.