Optimization, Tuning And Experiment Research Of A Light Bus With Variable Stiffness Suspension

With developing of automobile technology, requirements for ride comfort and handlingof vehicles is also increasing, especially for commercial bus. Ride comfort is related topassengers’ physical and mental health, and handling stability directly affects the lives andsafety of vehicle occupants. Due to vehicle suspension system is the major system that has adirect impact on ride and handling stability, so how to make vehicle body keep right postureand small vibration in all driving conditions has been a constant pursuit goal for all chassisand suspension engineers. For commercial vehicles, variable stiffness suspensions areusually used to ensure the ride comfort and handling stability under different loads. Becauseleaf spring suspension has many advantages, such as simple structure, mature technologyand low cost, etc., so it is still the mainstream suspension of such vehicles. At the same time,with the development of automotive lightweight technology and improvement of materialsand manufacturing technology, variable stiffness leaf spring eventually develop into atwo-leaf spring form. In this paper, the mechanical properties of this leaf spring was studiedtheoretically. The calculation formula of its complex stiffness has been obtained, and anefficient and realistic calculating method of contact load has also proposed.The subjective chassis tuning of the optimized vehicle is necessary for the suspensionsystem optimization, in addition to the matching front and rear suspension systems tocalculate the basic performance and K&C characteristics analysis and vehicle simulationand optimization, then the chassis performance of the vehicle is further optimized, so thefinal match suspension tuning parameters will be obtained. At present, the chassis tuning hasjust start in domestic manufacturers. Lack of experience, subjective evaluation rarely, lack oftuning methods and other issues seriously restrict the manufacturer’s chassis developmentcapabilities. Therefore in this paper, a set of actual chassis tuning methods and processes wasproposed by means of a variable stiffness suspension optimization project, which is in linewith domestic manufacturers. As auto industry increasingly fierce competition, virtualization technology has beengradually adopted throughout car development process to shorten development cycle andreduce development cost. In the last part of the thesis, the road reliability of suspension wassimulated to reduce test time and test cost. Various kinds of virtual road were respectivelybuilt in Adams, and the suspension load spectrum was respectively obtained by virtual roadtest and was used for suspension fatigue reliability analysis. Therefore the productdevelopment are all carried out virtually, which will greatly reduce the cycle of a new carand development costs to a minimum. In the this paper, suspension optimization, chassistuning and reliability simulation of processes and methods would have guidance andreference value for the development of domestic automobile chassis and suspension.The main research work and contents of the paper include the following sections:1. Because the theoretical matching of suspension is an important work in the earlierstage of the car development project. It is the basis of the suspension K&C characteristicanalysis, simulation and optimization of virtual infrastructure and vehicle chassis tuning. Thetraditional theory of match suspension system was firstly arranged, and divided into fivematching aspects by the suspension structure, which are the suspension stiffness matching,suspension damping matching, suspension roll angle stiffness matching, rubber bushingstiffness matching, buffer stopper stiffness matching. The corresponding matching processesand methods were put forward, and the suspension parameters of the variable stiffnesssuspension were matched and analysised respectively.2. The total stiffness and contact load of the variable stiffness leaf springs werecalculated and analyzed in theory. Because the leaf spring suspension of this study is a newleaf spring and also variable stiffness final form, there are few studies on its mechanicalproperties. A simplified model of the two-stage variable stiffness parabolic leaf spring wasbuilded based on the characteristics of the leaf springs. The stiffness calculation formulas ofparabolic leaf spring were respectively derived under end and central load. According to thematerial mechanics theory and equal deflection deformation theory, the total stiffnesscalculation formula of two unequal length parabolic leaf springs was deduced and the total stiffness calculation formula of three parabolic leaf springs was obatined by thesuperposition principle. At the same time, the total stiffness calculation formula of unequalfront and rear arm leaf springs was also given correspondingly. This provides the necessarytheoretical guidance for these final leaf spring design calculation. While another importantfeature(contact load) of the two variable stiffness spring are studied theoretically. From theperspective of the unsprung mass dynamic load, using of probability theory, contact loadcalculation method has been more realistic, combined with practical experience inengineering. The obatined formulas and methods will provide an effective theoreticalguidance for leaf spring suspension design and application.3. K&C characteristics of the front and rear suspension system were analyzed in detail,and the parameters effect on K&C characteristics were also researched. K&Ccharacteristics are the main performance of the suspension, which affect the precise of thewheel alignment parameters, and the handling stability and comfort of the vehicle. K&Ccharacteristics of the vehicle front and rear suspension systems were all analyzed, andchanging trends of K&C characteristics of the vehicle front and rear suspensions wereresearched under parallel wheel track, opposite wheel track, longitudinal load, lateral loadand aligning torque simulation. The effect of changes in the characteristics of each vehiclehandling and stability performance or ride comfort were also analyzed. K&C characteristicsof variable stiffness asymmetric leaf spring of independent suspension were analyzedcomprehensively. How the main spring stiffness, deputy spring stiffness, contact load and thearm length difference have an effect on the suspension K&C characteristics wererespectively analyzed. This would provide the right guidance for subsequent optimizationand the leaf spring design.4. The suspension parameters were optimized based on simulation of vehicle dynamicsmodel. To further optimize the parameters of front and rear suspension system, you need touse a virtual model of vehicle to test ride and handling stability. An accurate multibodymodel is the basis of virtual optimization. First, the mechanical properties of the vehicle tirewere tested. The magic formula tire model parameters and the corresponding properties file were obtained through parameter identification. Then the inertia parameters of the vehiclemass were tested. The dynamics models of each subsystem and the whole vehicle wereestablished respectively using the parameters obtained by test. Then the model was verifiedby the test of ride and handling stability. The results show that the vehicle model can be usedto optimize front and rear suspension parameters. Finally, the parameters of the front andrear suspension system were optimized by the simulation and optimization, usingmulti-objective genetic optimization algorithm. The optimized result of the front and rearsuspension were got, and comparative results show that ride and handling stabilityperformance of the vehicle has been significantly improved.5. The chassis tuning of a light bus was studied. Due to the simplicity in the modelingprocess and the nonlinearity of vehicle components, the virtual optimization results are notnecessarily the best, so it has to tune the real vehicle chassis to achieve optimal performanceof the vehicle. The chassis tuning mainly rely on subjective feelings and engineeringexperience of reviewers and it evaluates the coordination between some parameters of frontand rear suspensions. Based on the feature of the studied variable stiffness suspension,chassis tuning mainly includes four aspects.1. Based on the relationship between front andrear suspension roll angle stiffness, it is to adjust handling and stability, and thus todetermine the multiple sets of different sizes front and rear stabilizer bars.2. The differentleaf spring contact load was given considering the variable stiffness leaf spring contact load.3. In order to optimize vehicle ride under the high-frequency excitation, different leaf springbushing stiffness was considered.4. The shock absorber damping force curve was adjustedand different weight coefficients was given according to the main spring stiffness and totalstiffness to match the different types of damping curves. At last, according to the principlesof orthogonal experiment, several sets of project were proposed by DOE test. There are twokinds of load states(unload and full load). Many chassis subjective evaluationer give theirscore at21items of handling stability and9items of ride comfort performance. The besttuning program was obtained by statistical evaluation. Based on the actual situation of thebus, a set chassis tuning flowchart and tuning methods were respectively proposed，which is in line with domestic manufacturers. This will provide some guidance and reference modelfor the domestic automobile chassis tuning.6. The road test reliability of the leaf spring suspension was simulated, and the fatiguelife of the leaf spring suspension was analyzed. In accordance with the reliability testprocedures of the manufacturer, Adams_Car road modeler was used to generate the virtualdurability test pavements, combined distribution and probability theory and Matlabprogramming processing methods. They are verifyed by a real road test. Then the virtualreliability test of the vehicle multibody model were carried out under the same conditions, sothe virtual load spectrum of the leaf spring root were all obtained at six directions. The finiteelement model of the leaf spring was builded and validated. Then the finite elementsimulation results were entered into Patran software Fatigue Module and the leaf springfatigue life was simulated. The actual vehicle road test results were compared with thesimulation results, and the comparison showed that the leaf spring fatigue failure were inagreement basically. This shows that the analysis methods and processes of reliabilitysimulation used in this chapter is the correct and effective. The analytical methods andprocesses virtual reliability test vehicle will have a certain reference value in the future.