Dynamic Analysis And Intervention Strategies Of Vehicle Shimmy Under Braking Conditions With Consideration Of Clearance Joints

Vehicle shimmy is a kind of quality defects which would affect driving status.It is related to the design,matching and assembly quality of different components in chassis system.Particularly,it is easy to cause performance degradation of steering and handling and lateral dynamic response when shimmy occurs under braking conditions.It may even lead to losing control of vehicles and bad accidents with severe shimmy oscillation.Continuous changes of lateral force and longitudinal slip,load transfer while braking and coupling in joints with clearance make influence on the dynamic response of vehicle shimmy system.Comprehensive research on these factors contributes to suppression of shimmy oscillation and improving handling stability performance.In this thesis,a seven-DOF dynamic model of vehicle shimmy system with consideration of clearances in two movement pairs between the steering tie rod and the tie rod arms of both sides,lateral force and longitudinal force under combined conditions,load transfer while braking and vehicle yaw response is established.The derivation process is based on Lagrange equation method.An improved Rosenbrock algorithm is applied to solve the system equations with MATLAB.The time-varying characteristics of parameters and the data transfer are included in the iterative process.The steady state response of the vehicle shimmy system is investigated with time history,power spectrum,Poincaré mapping,clearances joints trajectories,bifurcation characteristics,stochastic assembling clearance and global analysis.It can be found that the response of the shimmy system considering dynamic coupling in multi-clearance joints is more complex and the relevant vibration amplitude increases a lot.The proportion of quasi-periodic motions or chaos is more than that with consideration of only one clearance joint.In addition,multi-clearance joints can reduce the critical clearance value and vehicle speed above which quasi-periodic motions or chaos may occur.Therefore,the system stability can be deteriorated in consideration of multi-clearance joints.Moreover,the Naimark-Sacker bifurcation with amplitude jump may occur as vehicle speed increases.Furthermore,the transient response characteristics of vehicle shimmy system under braking conditions are analyzed.Then the influences of vehicle speed,load transfer characteristics,tire weight,clearance size,clearance contact stiffness,front axle length and alignment parameters on dynamic behavior are investigated in view of the time and peak value of transient response.The key parameters that affect the transient behavior of the shimmy system are clarified,and the measures to suppress steering wheel shimmy and improve vehicle handling stability are put forward.A further investigation into the dynamic mechanism of steering wheel shimmy with analytic method is presented.There is phase difference between aligning torque and shimmy angle.It can be concluded that shimmy is a self-excited vibration phenomenon which is caused by the feedback with time delay.Furthermore,Numerical examples are employed to verify the related conclusions.The response mechanism for vehicle shimmy is investigated in view of energy input.The transfer path of vibration energy and response mechanism of clearance contact force is clarified.Moreover,the increase of the shimmy oscillation under braking conditions is explained based on the trend of system energy input.In the end,the shimmy experiments of a test vehicle is designed and carried out.The experimental method and data are analyzed,which can verify the accuracy of the dynamic model.Some measures for attenuation of steering wheel shimmy are presented based on the optimum damping matching characteristics of the steering damper.