| The bushings and control arms are key components of a suspension system. They areusually subjected to large static and fluctuating loads, and often fail due to the nucleation andgrowth of defects or cracks. The fatigue of the bushings and control arms will lead disastrousaccident. For this reason, both the automotive manufacturers and their customers would likeautomotive designers to perform a series of strict fatigue test and fatigue analysis on thebushings and control arms. During the fatigue analysis and experiment for the bushings andcontrol arms, the loads from joint forces and moments in suspension systems are importantinputs, and must be determined firstly.As the control arm of a front suspension connects to car body using bushings andconnects to wheel knuckle using ball joint, while the control arm of a rear suspensionconnects to car body and wheel carrier both using bushings, different models and calculationmethods should be developed for front suspension and rear suspension respectively.Taking a front double wishbone suspension and a rear multi-link suspension as thestudying example, a modeling and calculation method for obtaining the forces applied to theball joints and the bushings, and moments applied to the bushings is proposed and developedin this paper. In modeling of a suspension, a bushing is modeled with three translationalstiffness and three torsional stiffness along and around three orthogonal axes. The force(moment) versus displacement (angle) along (around) one axis is nonlinear and is simplifiedas piecewise linear at five section.The joint forces and moments for ball joints and bushings of a front double wishbonesuspension system are calculated with the proposed method in this paper, and compared withthat obtained using two conventional methods (method I and method II) when a vehicle runsin some typical and extreme loads. The joint forces and moments for ball joints and bushingsof a rear multi-link suspension system are calculated with the proposed method in this paper,and compared with that obtained using conventional method I when a vehicle runs in some typical and extreme loads. Conventional method I simplifies the connections between thecontrol arm and body as rigid link. Conventional method II assumes that the control arm linkswith the body using bushings, in which the bushings are modeled as three linear stiffnessalong three orthogonal axes. The calculation and comparison demonstrate that when anextreme load is applied to the suspension, the joint forces are quite different using differentmodels. The nonlinear elasticity of a bushing has great effects on the calculated results of jointforces. Also, the calculated joint moments are large and should not be ignored. So it isconcluded that if one want to get the joint force and moment accurately, the nonlinearelasticity of a bushing must be considered, and the proposed method in this paper should beused under extreme load. The joint forces and moments obtained from the proposed modeland method in this paper can be used as input loads for the design and fatigue test of bushings,control arms, trailing bars and steering knuckles.Also a front double wishbone suspension and a rear multi-link suspension are built inADAMS/View. The calculated forces and moments using the proposed models and ADAMSmodels are approximately equal under the two typical loads and two extreme loads, whichdemonstrates that the modeling and calculation method proposed and developed in this paperis a very efficient and accurate method. A finite element model of a control arm is set up. Thejoint forces obtained from the proposed model and method in this paper are used as inputloads for the strength analysis of the control arm. The joint force calculation programs are allwrited in MATLAB, and the software for calculating joint force is developed using GUI tointegrate all the programs. |
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