Effect Of Suspension K&C Characteristics On Handling Stability And Its Objective Evaluation For Commercial Vehicle

In the chassis development stage of commercial vehicle,the effect of suspension Kinematics and Compliance(K&C)characteristics on vehicle handling stability is significant.The suspension K&C characteristics need to be optimized according to the whole vehicle performance target based on analyzing their influence on anti-roll,aligning torque varying and understeer characteristics.The ability of suspension anti-roll is determined by the height of roll force center and roll stiffness.Increasing the height of roll force center is useful to improve the anti-roll performance.However,the tire lateral jumping motion is also enlarged,which will deteriorate the tire adhesion performance and straight line stability.What's more,this research is not particularly adequate.The additional side-slip angle caused by suspension K&C characteristics will generate a relative force feedback,which changes the understeer characteristics,but this research is still not comprehensive enough.In the progress of wheel steering,the movement of imaginary kingpin determined by control arms of multi-link suspension leads to change the suspension kingpin alignment parameters,the effect on aligning torque need to be analyzed.At the same time,in order to evaluate the handling stability of commercial vehicles,developing the evaluating index is an important content.And it is great significant to reduce the cost on suspension development.Based on the problems mentioned above and the previous theory of suspension K&C characteristics analyzed,this thesis will focus on the influence of suspension K&C characteristics on commercial vehicle handling dynamics and evaluating method.The main contents are listed as follows.(1)Research on the theory of suspension K&C characteristics,which is used in thedevelopment stage of commercial vehicle suspension.The suspension K&C characteristics were analyzed through the quasi-static analysis theory.The ability of suspension anti-roll was analyzed by the concept of roll force center introduced by Prof.Konghui Guo.The roll force center height was calculated and validated by the experimental result.By using the virtual work principle,it was proved that the tangential velocity direction of the moving locus of the suspension wheel is perpendicular to the constraint reaction force of the suspension guide rod.Increasing the height of roll force center is useful to improve the performance.However,the tire lateral jumping motion is also enlarged,which will deteriorate the tire adhesion performances and straight line stability.The effect of suspension additional side slip angle and kingpin alignment parameters on vehicle handling performance was elaborated.(2)The theoretical model of vehicle handling was established considering the suspension roll steer and roll camber.The general transfer function of yaw rate was derived,and the steady-state response,frequency characteristics,and root locus were analyzed.By the derived equivalent steady factor considering the suspension roll steer and roll camber,the understeer characteristics was analyzed.Positive roll steer at the rear wheel and positive camber change at the front wheel,or negative roll steer at the front wheel and negative camber change at the rear wheel,could lead to change the vehicle steer characteristics toward understeer tendency.Based on the frequency response function,the effect of suspension roll steer on amplitude-frequency and phase-frequency characteristics was analyzed.In the range of low frequency,the effect of roll dynamics reduces yaw rate gain,so enhancing the vehicle understeer characteristics.The effect of roll dynamics reduces phase delay in the range of high frequency,which reduces response time.By establishing the characteristic equation,the root locus diagram with the introduction of wheel roll steer was analyzed.Vehicle roll effect reduces the stability margin with negative roll steer at the rear wheel and enhances the stability margin with positive roll steer at the rear wheel.(3)The identification of suspension kingpin alignment parameters and influence on aligning torque were studied.Based on the theorem of screw axis,the identification modelwas established to evaluate the suspension kingpin alignmen parameters validated by the experiment result.By establishing the low-speed turning model,the aligning torque was calculated and validated by the experiment result.The effect of suspension kingpin alignmen parameters on aligning torque was analyzed.(4)The lateral dynamics characteristics of suspension were studied.With the random road excitation input,the tire lateral motion is generated as the suspension jumping,which leads to the lateral force.For the tire elasticity,relative to the input of tire lateral motion,the tire force performance non-steady state characteristics.The coupled suspension dynamics model was established.And the calculation of wheel camber and wheel center lateral displacement as wheel jumping was validated by the experiment result.According to the amount of tire lateral movement,the suspension lateral force was calculated.And the influence of structural parameters on the lateral force was analyzed.The simulation of whole vehicle lateral acceleration was validated by the test result.(5)The evaluation system of handling stability considering suspension K&C characteristics for commercial vehicles was established.Based on elaborating the theory of total variance calculation according to the transfer function,the total variance of vehicle angle input was calculated to evaluate the transient-state response.According to the model of aligning torque calculated,the vehicle slow-speed turning was evaluated by using the evaluation indexes of low-speed turning.According to the coupled suspension lateral dynamics model,the RSM of yaw rate was introduced to evaluate the vehicle straight line stability.Based on the driver-vehicle-road closed-loop driving model,the various individual evaluation indexes and comprehensive evaluation index were calculated to realize the closed-loop evaluation.The main innovations of this dissertation include:(1)Equivalent theoretical model of vehicle handling was established considering the suspension roll steer and roll camber.The general transfer function of yaw rate was derived,and the steady-state response,frequency characteristics,and root locus were analyzed.(2)The coupled suspension lateral dynamics model was established.And thecalculation of wheel camber and wheel center lateral displacement as wheel jumping was validated by the experiment result.According to the lateral movement of wheel,the suspension lateral force was calculated.And the influence of structural parameters on the lateral force was analyzed.The simulation of the whole vehicle lateral acceleration was validated by the experiment result.(3)The evaluation system of handling stability considering suspension K&C characteristics for commercial vehicle was established.The derived evaluation indexes were total variance of vehicle angle input,equivalent steady factor,RSM of yaw rate,aligning torque evaluation indexes,evaluation index of closed-loop system.