Dyniamic Analysis Of Vehicle Shimmy System With Multiple Movement Pair Clearances Of Steering Mechanism

Generally it is believed that front wheel shimmy is relative to horizontal movement of front axis,rotational movement around longitudinal axis of front axis and rotational movement around king-pin of front wheel.It is a very common and harmful vibration.To a certain extent it performs negative influence on power performance,fuel economy performance, vehicle handling stability, ride comfort and driving safety.Steering wheel shimmy of the vehicle is a problem which many factors contribute to , including clearances of movement pair of steering trapezoid mechanism which is the important part of steering mechanism. Previous studies show that single clearance of movement pair of steering mechanism can make great influence on nonlinear dynamic response of vehicle shimmy system .In fact there are multiple clearances in every movement pair of steering trapezoid mechanism. Therefore, it is important to analyze the influence of multiple clearances of movement pair of steering mechanism on vehicle shimmy system.The steering trapezoid mechanism with two clearances is taken as the research object in the dissertation.The steering trapezoid mechanism with two clearances is simplified as a four-bar mechanism with two clearances.For this reason ,based on previous research on the influence of single clearance of movement pair on the four-bar mechanism and vehicle shimmy system, this dissertation has focused on the influence of multiple clearances on the four-bar mechanism and vehicle shimmy system,and compares with the influence of single clearance on the four-bar mechanism and vehicle shimmy system though examples . The influences of other factors on shimmy are also analyzed, and several effective measurements for shimmy attenuation are discussed. The research work are mainly as follows:1. Based on nonlinear dynamics theories,the nonlinear dynamic model of a four-bar mechanism with multiple clearances is established, the influences of multiple clearances on the response of the system is analyzed. The analysis show that the system experience the variation from periodic motion, to quasi-periodic motion, and further to chaos with the increment of the clearance. Compared with dynamic behavior of the four-bar mechanism with single clearance through examples, the mechanism with multiple clearances is more prone to induce the chaotic motion. 2. Based on the nonlinear dynamic model of the four–bar linkage mechanism, a 8-DOF nonlinear dynamic model of vehicle shimmy system with consideration of multiple clearances of movement pair of steering mechanism is established with Lagrange equations. With this model, the simulation model of vehicle shimmy is established with MATLAB&Simulink.3. Based on the simulation model of vehicle shimmy, examples with different clearances of the movement pair are analyzed with 4-order Runge-Kutta method in order to evaluate the influences of the clearance on the dynamic behavior of the vehicle shimmy ststem. Simlation results show the clearance is an important factor leading to chaos of the vehicle shimmy system .Compared with the influence of single clearance of movement pair of steering mechanism on nonlinear dynamic response of vehicle shimmy system through examples, multiple clearances of movement pair of steering mechanism are more prone to induce the chaotic motion. Moreover, steering gear stiffness, the speed of vehicle, steering gear damping and related parameters of tire are critical parameters for shimmy. And appropriate ranges for the value of these parameters are presented, which provide an effective method for attenuation of the shimmy.