In Western Canada, log-transportation costs are often the highest phase cost of supplying wood to the mills. The industry has attempted to reduce costs by adding axles to trailers to increase the legal payloads. However, with increased payloads, traction has become a limiting factor for Class 8 tractors, especially in the off-highway portion of the log hauling cycle. In response, FERIC initiated a feasibility study in 1989 to investigate the potential of a tridem tractor addressing the industry's needs for improved tractive performance. A computer simulation indicated that a tridem tractor has more tractive ability than the conventional tandem drive tractors, and that log truck combinations with a single articulation point are more dynamically stable than those with two or more articulation points. The principal drawback predicted by the simulations was a reduced level of steering responsiveness for the tridem tractor configurations. The report recommended that a tridem tractor be evaluated while in revenue service to verify the simulation results, and that operational costs and productivities be determined (Amlin 1992).
Typically the log-hauling vehicles studies have a high centre of mass (centre of gravity) which contributes to a low rollover threshold. Rollover threshold is defined as the maximum severity of steady turn that a vehicle can tolerate without rolling over. Two vehicle parameters that present opportunities for improving this condition were identified durin the research.
Over the last decade, the log transportation sector of forest industry in Canada has improved productivity by adding more axles to the trailing system of additional 5-axle configurations, to the detriment of vehicle mobility. If payload capacity is increased by adding a drive axle to a tandem drive group, the resulting tridem drive system will provide both productivity and traction benefits. However, this changes the handling performance of the vehicle, specifically the steering response during low-speed, tight turns. To determine the tridem tractor's steering response, a testing program was undertaken to validate computer simulation models, and following validation these models were used to determine the appropriate vehicle parameters to ensure acceptable handling on both high- and low-friction surfaces. Recommendations for vehicle parameters are presented in terms of the respective weights and dimensions regulations for the provinces of British Columbia and Alberta