| This thesis develops parameter variation techniques for calculating the set of vehicle parameters that result in the best ride comfort for the driver. The model is a fifteen degree-of-freedom (15 DOF) tractor semi-trailer vertical dynamic ride model. The modeling and simulation techniques used in this thesis are extensions of the research performed by Trangsrud [1] and Vaduri [3]. Features of the model include suspension characteristics for (a) each of the five axles (tractor steer axle, two tractor drive axles, and two trailer axles), (b) tires, (c) a flexible engine mount, (d) the tractor cab, (e) the driver's seat, and (f) a fifth wheel suspension system. Also taken into consideration are the beaming effects of the tractor and trailer frame. The simulation of the model is conducted using MATLAB. The input to the system is a user-defined power spectral density (PSD) function of the vertical road irregularities. Other user inputs include the beaming frequencies of the tractor and trailer frame, tire types, cab suspension configurations, seat suspension configurations, and fifth wheel suspension configurations. Outputs from the simulation include root mean square (RMS) accelerations experienced at the driver's seat and at the center of gravity (CG) of the trailer, static axle loads and deflections, various transfer functions of response variables, and surface plots of the RMS combined weighted acceleration at the driver's seat and the RMS vertical weighted acceleration at the trailer CG as different parameters of the vehicle are varied. In addition, the RMS acceleration spectra of the driver are plotted together with the ISO 2631 [5,7] comfort curves. Results from the case studies explored in this thesis suggested lowering the stiffness values for the axle suspensions and tires and raising the corresponding damping values. Also, beaming frequencies of the tractor and trailer frames should be kept above 20 Hz to avoid large accelerations caused by coupling with other modes. Finally, the implementation of an idealized vertically-oriented fifth wheel suspension system did not lower accelerations experienced at the driver's seat in the nominal vehicle, but was shown to have beneficial effects when coupled with a full cab suspension system. |
