Ride and directional dynamic analysis of articulated frame steer vehicles

ABSTRACT Pazooki Alireza, Ph.D. Concordia University, 2012 Articulated frame steer vehicles (ASVs), widely employed in different off-road sectors, are generally unsuspended vehicles. Owning to their complex operating environment, high mass center, relatively soft and large diameter tires, wide load variations and load distribution, and kineto-dynamics of the frame steering mechanism, these vehicles transmit higher magnitudes of low frequency whole-body vibration (WBV) to the operators and also exhibit lower roll and directional stability limits. While the superior performance potentials of axle suspension in limiting the WBV exposure have been clearly demonstrated, the implementations in ASVs have been limited due to the complex design challenges associated with conflicting requirements posed by the ride and roll/directional stability requirements. Growing concerns on human driver comfort and safety, and increasing demands for higher speed ASVs such as articulated dump trucks, however, call for alternate suspension designs for realizing an improved compromise between the ride and stability performance. This dissertation research is aimed at analysis of a torsio-elastic axle suspension concept for achieving improve ride, while preserving the directional stability limits of the ASV. For this purpose a comprehensive three-dimensional model of the articulated frame steer vehicles is developed for design and analysis of the proposed axle suspension concept. The model is formulated considering a three-dimensional tire model, tire lag, coherent right- and left-terrain track roughness, and kinematics and dynamics of the steering struts. Field measurements were performed to characterize the ride properties of a conventional forestry skidder and that of a skidder retrofitted with the rear-axle torsio-elastic suspension under different load conditions. The measured data were analyzed to assess the ride performance potential of the suspension and to examine validity of the simulation model. Both the field measured and simulation results revealed that the proposed suspension could yield significant reductions in the magnitudes of vibration transmitted to the operator location, irrespective of the load and speed conditions. A simple yaw-plane model of the vehicle is also formulated to study the role of steering system design including the steering valve flows, kineto-dynamics of the steering struts and leakage flows on the snaking stability limits of the ASV. The results showed that the critical speeds are strongly dependent upon the kineto-dynamics of the articulated steering system. The comprehensive three-dimensional model subsequently used for analysis of integrated ride and roll/directional stability limits of the vehicle and the axle suspension designs. The stability performance measures are defined to assess the vehicle stability limits under steady as well as transient directional maneuvers. The results show that the proposed rear-axle suspension deteriorates the stability performance only slightly, irrespective of the load condition. It is concluded that the proposed suspension concept could yield a very good compromise in ride and stability performance. The proposed model could serve as an effective and efficient tool for integrated ride and handling analysis and to seek primary suspension designs for an improved compromise between the ride and stability performance of ASVs.