Recently, the application of active suspension systems in commercial vehicles is recommended to reduce road damages due to the generated dynamic tyre loads and to improve ride quality, structure safety and both handling and traction stability. The purpose of this paper is to design suspension controllers for a tractor semi-trailer system considering chassis elasticity and the controller time delay. The modal parameters of the vehicle chassis as a separate free-free substructure (connected tractor and semitrailer frames) are calculated using the finite element method and then incorporated with the rigid parameters and motions of the whole vehicle components. The connection (articulation) between the tractor and semitrailer frames is modeled by a vertical spring with its ends attached to the tractor and semitrailer frames. The stiffness values of the vertical spring is adjusted so that the relative displacement between the tractor and semitrailer frames in the vertical direction is equal to zero by applying a very high stiffness spring element. In the design stage of active suspension systems a first order filter is used to represent the time delay of the controllers. The stochastic linear optimal control theory is applied to generate the feedback gains based on a full state feedback control strategy. The dynamic performance of the actively suspended tractor semitrailer system is examined with different vehicle speeds, road roughness conditions and actuator controller time delay constants. The results showed that the given controller design plays a major role in improving both vehicle ride quality and reducing the dynamic tyre loads which is the main reason of road damage. Finally, conclusions and suggestions for further work are given.