Assessing Road Load Coefficients of a Semi-Trailer Combination Using a Mechanical Simulation Software with Calibration Corrections

The study of road loads on trucks plays a major role in assessing the effect of heavy-vehicle design on fuel conservation measures. Coastdown testing with full-scale vehicles in the field offers a good avenue to extract drag components, provided that random instrumentation faults and biased environmental conditions do not introduce errors into the results. However, full-scale coastdown testing is expensive, and environmental biases which are ever-present are difficult to control in the results reduction. Procedures introduced to overcome the shortcomings of full-scale field testing, such as wind tunnels and computational fluid dynamics (CFD), though very reliable, mainly focus on estimating the effects of aerodynamic drag forces to the neglect of other road loads which should be considered. The development of mechanical simulation software in recent years suggests it is becoming much easier to incorporate the effects of other road load forces in drag analyses and fuel conservation measures. This study analyzed road loads obtained using a typical mechanical simulation software under calm and zero-wind conditions. The results were compared to drag coefficients derived from field tests of a fully instrumented truck and published drag coefficient values. The study found that the aerodynamic drag coefficients obtained from the simulation software were higher than those of the field and published data. A calibration curve was developed using the field test results and simulated data to correct the simulated drag forces. Average drag coefficients obtained from the corrected simulation forces were found to fall within the range of published data. However, due to the limitations of the simulation software in fully accounting for environmental conditions encountered in the field, some sensitivities of the derived coefficients in the field were observed. The results from using the mechanical simulation software suggest reasonable estimates of total road loads can be derived if corrections are applied. Better estimates of the aerodynamic drag forces may be derived from other approaches such as CFD and wind tunnel tests.