Study On The Suspension K&C Characteristics Of A Mini-car And Its Influence On The Vehicle Handling Stability

As a crucial feature of the system assembly, the kinematics and compliance of the suspension system in a vehicle have direct influence on the vehicle handling stability and its ride performance. They are hot focuses in the field of traditional dynamic study in China and oversea. The so-called suspension kinematics (K characteristic in short in the following paper) describes the movement of the wheels due to the springs transform and the wheels turn. And the so-called suspension compliance (C characteristic in short) describes the wheel location parameters changes caused by the forces and moments occurred between the wheels and the road. These two together are called Kinematics and Compliance (K&C characteristics in short). The Study and analysis of suspension K&C characteristics can provide an important theoretical and experimental foundation and a guide for designing the suspension reasonably, tuning and optimizing of vehicle performance, balancing the controversy between vehicle NVH requirements and handling stability target, as well as improving the vehicle performance quality.Associated with the project of a mini-car chassis platform development in a certain domestic OEM, this dissertation aims to carry out a study on the suspension K&C characteristics and its influence on the vehicle handling stability. Applying reverse engineering mainly, the newly-developed prototype car is designed in reference with a benchmark car. Choosing a same suspension structure type as the benchmark car, the prototype car has a McPherson independent front suspension and a five-pole spiral spring dependent rear suspension.Firstly, the structure-oriented model of the front and rear suspensions of the benchmark car is set up by using ADAMS/Car. And the experimental study of the K&C characteristics of the front and rear suspensions of the benchmark car is carried out by applying the vehicle uni-axial suspension K&C characteristics test bench developed independently by ADSL. Theoretical explained, the suspension K&C characteristics analysis is a quasi static analysis. So the suspension K characteristic analysis should include the bush influence. During the reverse engineering, the bush original transformation influence should be considered when the vehicle model is validated. A mounts of test data of suspension K&C characteristics are got in the experimental work, which provide real and reliable basis for the study of suspension K&C characteristics and its influence on the vehicle handling stability.Secondly, the given aim of suspension K&C characteristics is achieved by studying the disposal of the hard point of the prototype car and the selection of the bush rates. This lays the basis for the design improvement and adjustment of the suspension. On the basis of introducing the control mechanism of the suspension K&C characteristics, the differences between the suspension K&C characteristics of the prototype car and the target characteristics (namely, the testing data of suspension K&C characteristics of the benchmark car) are compared, and studied. Sensibility analysis is carried out to determine the influence degree of design factors such as the hard point coordinates and the bush rates on the suspension K&C characteristics responses. And then, key design parameters can be selected for improvement. The regression model of hard points V.S. K&C characteristics of the front suspension of the prototype car is set up by applying testing optimizing method. According to this, an improvement plan is put forward and parts of the hard points are adjusted to make the suspension K&C characteristics of the front suspension of the prototype car match the target values. Using DOE method, the influence of the bush rate changes on the suspension K&C characteristics of the rear suspension of the prototype car is analyzed in the case of that hard points are given.Then, by using ADAMS/Car, a structure-oriented dynamic model of the benchmark car is set up. There are 93 degrees of freedom in the system. The comparisons of the simulation and testing data in 6 items of international handling stability working conditions show satisfied matching results. The steering system model considers the steer inertia, hard rates, transmission ratios, impetus and friction. But steering damping and gap are excluded. The UniTire model is adopted as the tire model, which has good extensibility, and the parameters are achieved through tire dynamic characteristic testing data identification.Finally, the conceptual suspension module in ADAMS/Car is introduced, which provides a functional suspension modeling method based on the assembly characteristics and in no need of detailed information of hard points and bush. By using the conceptual suspension module, a vehicle dynamic model is set up, which has only 14 degrees of freedom while still a multi-body system by nature. The differences in vehicle handling stability performance between prototype car and benchmark car caused by the suspension K&C characteristics are compared, and the influence of K&C characteristics of the front suspension on the vehicle handling stability is studied by applying the conceptual suspension module. The influence of the rear suspension hard points and bush on the vehicle handling stability is analyzed by using structure-oriented suspension model.Major Innovations of the Dissertation:(1) The relation between hard points and K characteristic of the front suspension of the prototype car is analyzed by using DOE method, and the improvement plan is put forward to make the K characteristic of the prototype car match the target value.(2) The relation between bush rates and C characteristic of the rear suspension of the prototype car is analyzed by applying the DOE method in case of given the hard points in order to lay the base for the improvement of the suspension C characteristic and the vehicle adjustment. The system statistic regression model got from the calculation in the DOE method is convenient in analysis and foretelling the influence of simultaneous changes of many design parameters on the system response and also can determine the optimised value of the design parameters under the given target response. This method helps engineers to get much more direct knowledge of suspension and have more choices. It has advantages in checking and when necessary adjusting the deviations of hard point coordinates and bush rates in reverse engineering. Of cause, it may also be applied in forward design of new suspension system.(3) The influence of the hard points and bush rates of the rear suspension on the vehicle handling stability is analyzed by using the established structure-oriented vehicle dynamic model which has 93 degrees of freedom.(4) The influence of K&C characteristics of the front suspension on the vehicle handling stability is analyzed by applying the 14 degrees of freedom vehicle dynamic model on the basis of assembly characteristics. This method helps to conveniently investigate the influence of a single factor on the vehicle handling stability.