5 th Australasian Congress on Applied Mechanics, ACAM 2007 10-12 December 2007, Brisbane, Australia Determination of Modal Parameters of a Half-Car Fitted with a Hydraulically Interconnected Suspension from Simulated Free decay Responses Nong Zhang 1 , Wade Smith 1 , Jeku Jeyakumaran 1 and William Hu 1 1 Faculty of Engineering, University of Technology, Sydney Abstract: This paper presents an alternative approach for determining the vibra tion modal parameters, in terms of natural frequencies, damping ratios and modal shapes of a roll-plane half-car fitted with a general hydraulically interconnected suspension sys tem. The dynamic model of the system, which consists of the sprung mass and the HIS and the wheels, is formulated using the state space representation approach. The state variables describing rigid body motions of the sprung and unsprung masses are heavily coupled with those describing the dynamics of HIS fluid circuits. A numerical simulation scheme is developed to obtain the transient and free decay responses of the half car vehicle using specific initial conditions or road inputs. The obtained results are compared with those determined from the free vibration analysis of the system using the transfer matrix method. Discussions on the advantages and limitations of the presented method are also provided. Keywords: Hydraulically interconnected suspensions, multi-body systems, dynamics of fluid circuits 1 Introduction Fatal crashes due to vehicle rollovers have been frequently reported in recent years around the world. Four Wheel Drive vehicles (4WDs), typically having higher mass centres, are particularly vulnerable to this type of accident, with over one third of 4WD fatalities involving rollover [1]. The events leading to vehicle rollovers are complex with many factors influencing the vehicle motion. However, good suspensions can greatly reduce vehicular rollover propensity during extreme manoeuvres. Recently, research efforts have been focused on advanced suspension systems, with a view to overcoming the inevitable compromise between ride and handling performance encountered in conventional suspensions. These advanced suspensions generally include variable or adjustable stiffness or damping parameters, most often achieved through active or semi-active means [2, 3]. However, completely passive hydraulically interconnected suspension (HIS) is also becoming increasingly popular for passenger vehicles. In addition to having the normal functionality of a conventional suspension, a HIS has advantages in providing additional stiffness in roll, bounce, pitching, articulation, or their combinations, depending on the system configuration. A typical HIS often contains several fluid circuits that link double-acting cylinders, damper valves and accumulators through pipe, hose or curved fitting elements. Kinetic H2 suspension is one example [4]. A HIS behaves nonlinearly in relation to external disturbances. Therefore, unlike a conventional suspension, a lumped mass assumption does not apply to a vehicle with a HIS because the highly pressurised fluid within the circuits is a distributed mass system. The vibration modes of bounce, roll, pitching, articulation and wheel hopping of the vehicle system can not be easily determined using the conventional multiple lumped mass, s pring and damping approach. Although rich practical knowledge on HIS systems has been gained from experimental studies and applications in both racing and passenger vehicles [4-6], only recently a systematic theory and solution procedure for determining the dynamic characteristics of HIS vehicles has been presented by Smith, et al [7] and Zhang, et al [8]. This paper presents an alternative approach for determining the vibration modal parameters, in terms of natural frequencies, damping ratios and modal shapes of a roll-plane half-car fitted with a general hydraulically interconnected suspension system. The sprung mass, suspension springs, wheels and tires are modelled using the free body diagram method. The individual fluid elements, such as lines, valves and accumulators of the HIS fluid circuits are modelled using their own dynamic models. A state vector for the vehicle dynamic system is so defined that it contains all the independent displacements and velocities of the sprung mass and wheels, and the pressure and flow at all nodes (between which are various hydraulic elements) of the fluid circuits. The dynamic model of the system, which couples the sprung mass and the HIS and the wheels, is then formulated using the state space representation approach (see the details in reference [8]). The state variables describing rigid body motion are heavily co upled with those describing the dynamics of the HIS fluid circuits. The dynamic