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The influence of self- and forced-steering axles on the directional dynamics of a tractor-semitrailer is investigated through computer simulations. The dynamic characteristics of a self-steering axle are analytically modeled and integrated to a three-dimensionalnonlinear directional dynamic model of the vehicle. Two forced-sleering algorithms relating !he vehicle speed and response quantities to the angle of the wheels of the steerable axle are formulated. and integrated to the nonlinear directional dynamic model of the vehicle. Computer simulations are performed to determine the directionalresponse characteristics of a tractor-semitrailer with self- and forced-steering axles for low as well as high speed maneuvers. The directional response characteristics of the vehicles with self- and forced-steering axles are discussed in view of the self-steering parameters and forced-steering gains, and compared to those of the vehicle with conventional axles.

The increased number of heavy trucks on today's highways, along with the extended driving hours, resulted in increased demand for improved driving conditions and prompted concern about the dynamic pavement loads. The dynamic pavement loads are one of the major causes of pavement deterioration. Passive suspensions, while being very reliable and easily implementable, fall short of satisfying the various conflicting design requirements. The overwhelming improvement of ride quality resulting from the use of active suspensions seems to have overshadowed their effect on tire generated pavement damage. An in-plane tractor-semitrailer model is used to evaluate the relative performance of fail-safe active and passive suspensions. Both full state feedback and limited state feedback are used in the design of the active suspension.

A covariance analysis technique is proposed to derive the optimal suspension parameters of an articulated freight vehicle. A performance criteria comprising vehicle ride response, suspension deflections and tire deflections related to dynamic wheel loads, is formulated for the 9 degrees-of-freedom (DOF) in-plane model of the vehicle. The range of suspension parameters to achieve four different design requirements is identified and a parametric study is performed to make initial parameter selection using the covariance analysis. The optimal suspension parameters are then identified from the results of the study. The study concludes that the proposed technique can yield the optimal solution in a convenient and highly efficient manner.