AbstractSemi-active suspensions (as opposed to purely active or passive) have recently been successfully applied to the vibration isolation of ground transportation vehicles [1,3,4,5]. In the present work, a digital computer model of a vehicle incorporating passive and semi-active suspensions is developed using bond-graph modeling techniques. Special attention has been devoted to the modeling of a pneumatic semi-active actuator consisting of a piston-cylinder combination and a controller, composed of an electromagnetic torque motor and a valve. A two-port C-field was used to model actuator compliances caused by air compressibility and air flows, whereas a CR-field summarized the control valve action. From the developed bond graph model the set of corresponding nonlinear differential equations describing the semi-actively suspended vehicle was easily obtained.