Research Of Coach High Speed Waving

Coach will get some lift and drag forces from the air flow when it runs at a high speed.These forces will decrease side slip stiffness of the wheel and reduce the capability to hold the ground.So when the steering wheel get a force input from the driver,yaw rate of the coach is difficult to get to be stable and the vehicle will not responses to the action of the driver.The driver will have a feeling that the coach is waving.This phenomena is what we called high speed waving.The main contents of this article are as following:(1) The moving characteristic of the coach is briefly analysed based on the two free degrees model.The influence to high speed waving by the windage of the steering system,low stiffness area,wheelbase,side slip stiffness and velocity is researched.(2) The moving characteristic of the coach is briefly analysed based on the three free degrees model.The influence to high speed waving by the ratio of side slip stiffness of different axles and the dimentionless velocity is researched.(3) A three dimention physical model of a coach is established.The air lift force and drag force coefficient are calculated at different velocity.Then the air lift force and drag force are calculeated through some formulas.(4) Coach dynamic formulas of ten free degrees is established.The ten free degrees are longitudinal, lateral,yaw,pitch,roll,front wheel rotation to master pin and the rotation of four wheels.The side slip stiffness of the wheels are calculated by some formulas and they will be input to the coach dynamic formulas.The response of the coach to the force input is analysed at different velocity.The phenomena of high speed waving is researched when the coach gets the air lift force and drag force and reduces the capability to hold the ground.Research results show that air lift force and drag force maybe will make the coach get high speed waving when it runs at a high speed.Increasing wheel base and side slip stiffness of rear wheels can help to reduce this phenomena.