Abstract

The design of a vehicle suspension involves complex compromises due to conflicting ride comfort and handling requirements. High load capacity and high mass center commercial vehicles, especially, impose greater design challenges due to their relatively low rollover immunity. Road vehicles, invariably, employ auxiliary roll stiffeners such as antiroll bars to realize a better compromise among the roll dynamic and ride comfort performance. The anti-roll bars, however, add considerable weight, exhibit negligible damping and cause stronger coupling between the roll and vertical modes. Alternatively, roll-connected hydro-pneumatic suspensions offer superior anti-roll performance, while preserving the soft vertical ride characteristics. Reported studies have shown that such suspensions can provide anti-roll characteristics similar to an antiroll bar but with considerable roll damping and less weight. The feedback effects of the hydraulic couplings in such suspensions yield negative damping force in the vertical mode, which have not yet been explored.
This thesis research presents a systematic study of negative damping features of the roll-coupled hydro-pneumatic suspensions and its significance for realizing variable damping properties. Three different configurations of hydro-pneumatic struts were conceived for realizing hydraulic couplings in the roll plane. Analytical models of the roll-coupled suspensions were formulated considering ideal gas law, turbulent flows through orifices and damping valves, laminar flows through interconnections, floating piston dynamics and fluid compressibility. The analytical formulations were used to describe the negative damping feature attributed to the flow feedbacks. The vertical and roll mode damping and stiffness properties of the proposed configurations were derived via analytical relations, which showed that hydraulic couplings yield high roll stiffness and damping with only minimal effect on the vertical mode properties. The simulation results demonstrated two negative damping force components of a strut attributed to flows through the interconnecting pipes and flows through orifices in the connected strut. These negative damping force components, however, contributed to only positive roll damping moment. A methodology to enhance negative damping force of the connected struts was proposed for realizing variable damping properties similar to those of the conventional damping valves. Deployments of small size multiple interconnections or the flow-control valves across the struts resulted in comprehensive magnitudes of negative damping force components. Simulation results were obtained under lateral acceleration excitation idealizing the centrifugal force encountered during a steady-turn maneuver, a road bump, and in-phase and out-of-phase harmonic road excitations. The results were obtained for unconnected and connected struts with and without the damping valves and interconnection flow valves. Comparisons of the results revealed that interconnection valves can provide variable damping properties similar to the damping valves. The interconnection valves, however, offer greater design/tuning flexibility since these are mounted externally. The results suggested that further efforts in parameterization of the coupling flows will be worthy for realizing optimal damping properties of the roll-coupled hydro-pneumatic suspensions.