The Effects Of Tyre Dynamics On Vehicle Shimmy

Shimmy is a self-excited vibration which can occur in various wheeled mechanisms,such as aircraft nose landing gears,cars,motorcycles,and supermarket trolleys,etc.Shimmy for cars,also known as "vehicle shimmy",or "death wobble",can increase tyre wear,lead to vehicle handling problem,and even cause danger.The elasticity of pneumatic tyre is considered to be one of the crucial factors that leads to shimmy,thus,the selection of a proper tyre model is of great importance.Two tyre models: Pacejka's Magic Formula and the Delayed Tyre Model are compared in this study of the vehicle shimmy problems,combined with brush-type tyre and stretched string-type tyre assumptions.To start with,a 3 degree-of-freedom(DoF)model considering the front wheels,the steering system and the dependent suspension is established.Linear analyses including the stability charts and the vibration modes are carried out to investigate shimmy,and the results are verified by the numerical simulations.In the previous studies,the Delayed Tyre Model shows the advantage of predicting some extra instability at low speed for the 1 DoF single-wheel shimmy models.Here,with reduced damping and stiffness values representing a worn structure,the Delayed Tyre Model also predicts shimmy in the high speed range,when the front wheels vibrate in opposite phases.For an electric vehicle with independent suspensions,the 3 DoF model is not feasible any more.Thus,a more intricate 5 DoF model is built taking into account the vibrations of the independent suspensions and the pitman arm in the steering system.With the more realistic stretched string-type tyre assumptions,the two tyre models predict very similar stabilities.Apart from the stability analysis,the influence of different parameters on shimmy is studied.Last but not the least,the kinetic energy distributed on each generalized coordinate is calculated and compared between different system models.Suggestions for attenuation of shimmy are given accordingly.