The improvement of vehicle ride comfort and acoustic performance not only needs to spend energy on the detailed analysis and design of individual parts of the system, but also needs to accurately study the interaction between the whole suspension system and the components of the system. Usually, due to the underestimation of the impact of a single part on the system performance, some companies spend a lot of manpower and material resources on the independent research and development of suspension or damping system components, but the system performance is not satisfactory.
As a leader in vibration and acoustic control technology in the field of global automotive industry, wibuck has accumulated rich experience in many years of product research and development practice, and formed its own unique and effective product development system. In weibak company, whether developing new automobile chassis parts or optimizing the performance of existing chassis parts, it always studies the vibration performance of automobile subsystem as a whole, identifies the existing performance improvement space, and then optimizes the performance of a single product through part design technology, Finally, the success of the developed parts is verified by multiple stages of testing.
Part development and test verification technology
The detailed system characteristic information obtained from bench test can be used to establish computer simulation model. The model includes all suspension elements, control arms or connecting rods, and sometimes subframes with ideal rigidity, which are connected with each other through coupling elements or force units (see Figure 1). The model is described by the part characteristic parameters that affect the comfort. The switching variable is used to control the on or off of the simulated parts, so as to compare the test results with the calculation results, analyze the impact of the parts on the system performance and estimate the improvement space of the solution on the system performance. Each part in the system can be replaced by components with ideal characteristics for performance contribution analysis to evaluate their impact on system performance. The performance improvement space of the optimized parts can be estimated through the performance comparison between the initial state and the ideal state. The combination of contribution analysis and performance improvement potential analysis technology can greatly save the design cost. After the contribution analysis is completed, the main optimized parts can be determined. For example, the friction of the piston connecting rod of the liquid filled shock absorber is the main source of the increase of parasitic stiffness under small excitation, but it is not absolutely necessary to replace it with an expensive low friction shock absorber. Usually, the required comfort effect can be achieved by matching an improved top support for the existing shock absorber.
Most suspension parts show strong nonlinear characteristics, which means that their dynamic characteristics depend on factors such as vibration amplitude, prestress and temperature. When measuring nonlinear parts, it is very important to select test parameters and test conditions consistent with the actual service conditions. Wibuck company has developed a set of standard tests for testing automotive vibration control parts, including quasi-static test of part stiffness characteristics, frequency test to identify the dependence of dynamic stiffness on test frequency, sweep measurement of dependence between dynamic stiffness and excitation amplitude, preload test to determine the influence of geometric nonlinearity on dynamic stiffness, etc. Firstly, the prototype will be evaluated according to these standard methods, and the test results will be compared with the part design drawings. If the prototype meets all technical requirements, it will be installed on the axle for assembly bench test.
Fig. 2 axle test bench
Various types of axle test stands (see Fig. 2) have been independently developed and built by wibuck company for the test and analysis of automobile axle assembly. In all tests, the axle is connected to the rigid frame structure of the test bench through 3 axial load measuring units. These load measurement units are located at the interaction point between the axle and the body to detect the internal force between the axle and the body. Compared with the bench test of parts, the main challenge in axle analysis is that it is a single input / multi output system. If the hub is excited in one direction, a response force will be generated at all interfaces interacting with the vehicle chassis.
Figure 3 transmission path analysis
The input stiffness and transfer stiffness can be obtained by testing the axle without wheels. For the same input displacement, large transfer stiffness means that large forces will be generated at the output end. Accordingly, these dynamic forces are mainly responsible for the vibration and acoustic comfort of the vehicle. Figure 3 shows the relationship between transmission stiffness and frequency. In the figure, each line represents a transfer path, and color is a measure of stiffness distribution. The figure also shows the characteristics that the transmission path changes with the change of frequency as a function of frequency. With the change of frequency, the maximum force will appear in different positions of the body. This can provide the vehicle manufacturer with information about the possible interaction between the axle and the body. The key point in the whole vehicle design is to ensure that the resonance frequency of the body is staggered with that of the axle.
After analyzing the input and transmission stiffness of suspension, it is necessary to study the axle with wheels. The test analysis of the axle is realized by sinusoidal and random excitation of the hub by the hydraulic exciter. In addition to the force measurement technology mentioned above, the test can also be detected by a high-speed optical measurement system (see Figure 4). The data obtained from the test of the optical measurement system reveal the motion curve of the wheel runout in the vertical direction of the suspension under transient excitation, and the evaluation of the damping behavior of the axle, which is carried out in the vertical direction and the front and rear direction of the vehicle respectively. The part that has a decisive impact on the damping performance in the vertical direction is the shock absorber. In addition, the damping of the front and rear subframe suspension will also affect the handling performance of the vehicle.
Fig. 4 installation of high speed optical measurement system
In addition to the required stiffness characteristics and curves, the service life of parts needs to be considered in part design. Weibak company has successfully used the finite element technology to predict the service life of parts many years ago. The goal is how to meet the service life requirements of parts on the first sample vehicle. Firstly, the load / time curve is transformed into stress or strain / time curve by finite element tool in order to make durability prediction. Counting techniques, such as rain flow counting, are used to grade the results, which will be linked to material properties in the next step. The fatigue characteristics of the material are expressed in the form of woehler curve or Haigh diagram. The calculated damage factor provides engineers with the index of service life of parts under actual load conditions.
If the bench test results of the axle assembly are consistent with the calculated results, the parts will be installed on the prototype vehicle, and the success of the final design will be verified by the subjective and objective evaluation of the whole vehicle.
In order to improve the ride comfort and internal acoustic characteristics of the vehicle, not only the single part needs to be improved, but also the vibration influence of the whole suspension system needs to be analyzed.
This paper describes the relevant methods used by wibuck company in the development of chassis system vibration control parts. The combination of test and simulation makes the development process of the new generation axle more efficient in time and cost.
Responsible editor: GT