A study of the yaw stability of tractor-semitrailer combinations
Authors: R. D. Ervin, C. Mallikarjunarao
A simplified analysis of the steady-turning characteristics of the tractor-semitrailer combination shows that, in order for a "jack-knife" type of directional instability to occur in the absence of braking, the tractor must, itself, exhibit an oversteer behavior and must be operated above its critical speed. When considering nonlinear aspects of the behavior of such vehicles, it becomes convenient to examine the potential for instability using the so-called handling diagram. When the nonlinear tractor is outfitted with a tandem drive axle arrangement, there ceases to exist a unique handling curve representing all steady-state conditions. Rather, a family of handling diagrams exists, with a distinct curve representing each value of forward velocity. Numerical computations employing a rather complete model of the vehicle were conducted to examine the influence of various design and operating parameters upon the yaw stability of tractor-semitrailer combinations. Results show that the inherent oversteer tendency in the steady turning behavior of typical tractors at increasing levels of lateral acceleration can be eliminated by large changes in the front/rear distribution of roll stiffness. Other commonly-experienced parametric variations are seen to significantly aggravate the basic tendency toward oversteer behavior.