Virtual Tests And Improvement Of Automobile Chassis's Handling

The car industry of China is experiencing a phase of high-speed development; meanwhile, the ability of independent design is comparably weak. As a key technology, the skill of chassis design needs to be developed. Nowadays, thanks to the virtual test method, the handling of cars'dynamic models can be analysis before the real vehicles are built, and the correlative performance can be assessed by setting up the evaluation system, so that the improvement of chassis parameters can be made quickly. This new research method can shorten the period of chassis design, bring down the expense and reduce the risk, and it's of great significance for the development of design ability of Chinese car industry.This thesis emphasizes the process of using virtual test method to improve the handling of cars. Focusing on the topic, the following work is done.1. The building process of parameter-based vehicle dynamic model. Here the software Carsim is used to build parameter-based vehicle dynamic model. In order to get the parameters needed by the model, there are two kinds of preparations. On the one hand, a lot of real tests are done, including steering stiffness test, steering gear ratio test, power steering test, steering dry friction test, parallel wheel travel test, opposite wheel travel test, longitudinal tire force test, lateral tire force test and aligning torque test. On the other hand, ADAMS model and correlative TestRig simulation is used to get useful parameters. The two ways of getting the vehicle parameters can proof each other.In order to meet the needs of all the handling tests, a Matlab-Simulink model is connected to the Carsim system by means of certain interface in order to improve the original Steering model in Carsim: Steering power, dry friction and inertia factor are introduced into the original system to make the model more real and able to perform simulation such as on-center handling test; Thanks to the additive Simulink model, the new steering system have both angle input and torque input, which is essential to steering returnability test.Slalom, steady-state steering and on-center handling test are used to proof the correctness of the vehicle dynamic model. The contrast between the virtual results and real car test results shows that the Carsim model with the cooperation of Simulink is able to reflect the dynamic characteristic of the real vehicle and it can be used to evaluate the real vehicle's handling.2. Study on the objective evaluation method of cars'handling. Of all performances correlative with vehicle handling, this thesis stresses six important points according to the research project. They are: steady-state steering performance, transient-state steering performance, on-center handling, roll, pitch and steering returnability. The phenomena of every point are explained, and the evaluation indices of them are given. The typical value of each index is found out in correlative papers, which is useful for the analysis of a vehicle's handling.3. Virtual evaluation of a certain car's handling and its improvement method.According to the virtual handling tests and objective evaluation, some defects are discovered in the vehicle's transient-state steering performance, pitch, steering returnability and on-center handling. The problem is that the vehicle has weak steering returnability, severe steering lag and pitch gradient.In order to improve the three shortcomings, this thesis tries to find improvement method in the parameters of the steering system, suspension kinematics and compliance, and then comes to the following conclusions:1) The dry friction of steering system can affect steering returnability greatly. It's proved that the steering returnability can be controlled within the ideal range by reduce the dry friction to 90 percent of the original vehicle.2) Steering lag is determined by the front and rear equivalent side-slip stiffness. Increasing the front equivalent side-slip stiffness and meanwhile keeping the stability factor unchangeable can reduce steering lag. It can be discovered through tests that the suspension compliance coefficient Steer/Fx can affect equivalent side-slip stiffness. So increasing this coefficient is a good way to improve the transient response speed.3) According to correlative kinematics analysis, the longitudinal movement of the original vehicle's front wheel can not resist vehicle pitch. So the kinematics characteristic is changed to: front wheel moves forward when suspension is compressed. And the correlative virtual test proves that the change in kinematics can reduce vehicle pitch effectively.Integrating the analysis above, a holistic improvement scheme comes out and modifications are made according to it. At last, the contrast between the handling evaluation of the original and modified vehicle model shows that the scheme can improve the vehicle's handling effectively.