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It is well known that the ride quality of trucks is much harsher than that of automobiles. Additionally, truck drivers typically drive trucks for much longer duration than automobile drivers. These two factors contribute to the fatigue that a truck driver typically experiences during long haul deliveries. Fatigue reduces driver alertness and increases reaction times, increasing the possibility of an accident. One may conclude that better ride quality contributes to safer operation. The secondary suspensions of a tractor have been an area of particular interest because of the considerable ride comfort improvements they provide. A gap exists in the current engineering domain of an easily configurable high fidelity low computational cost simulation tool to analyze the ride of a tractor semi-trailer. For a preliminary design study, a 15 d.o.f. model of the tractor semi-trailer was developed to simulate in the Matlab/Simulink environment.

A simulation of the vertical response of a nonlinear 1/4 car model consisting of a sprung and an unsprung mass was developed. It is being used for preliminary evaluation of various suspension configurations and control algorithms. Nonlinearities include hysteretic shock damping and switchable damping characteristics. Road inputs include discrete events such as bumps and potholes as well as randomly irregular roads having specified power spectral densities (PSDs). Fast Fourier transform data analysis procedures are used to process data from the simulation to obtain PSDs, rms values, and histograms of various response quantities. To aid in assessing ride comfort, the 1/3 octave band rms acceleration of the sprung mass is calculated and compared with specifications suggested by the International Standards Organization (ISO). Cross plots of the rms values of acceleration, suspension travel, and the force of the road on the tire are used to compare the performance of various suspensions.

Providing acceptable ride quality of tractor semi-trailers is essential to their viability in the freight transport business. This paper describes the development of a design tool that may be used to investigate the vertical dynamic response and ride comfort of these vehicles. A 12 degrees-of-freedom (DOF) model of the vertical dynamic response was developed and simulated in MATLAB [1]. The model is analyzed in the frequency domain. The input to the model is a user-specified power spectral density (PSD) of the vertical road irregularities. Outputs include modal frequencies, damping ratios and mode shapes, frequency response functions, PSDs and root mean square (rms) vertical and longitudinal accelerations in 1/3 octave bands. The rms values are compared with the specifications for ride comfort cited in ISO 2631 [2].