The majority of] vehicle manufacturers and researchers have focused their efforts to find more practical systems than active suspension systems. Other researchers have tried to study the effect of preview sensors or time delay on suspension system performance using uarter vehicle models. This paper is directed to investigate the effect of road and wheelbase preview sensors on the ride performance of the semi-active suspension system. A half car model is used to represent the heelbase travel time, with only a single input for the system, that to the front axle. The input is assumed to be measured at a certain distance in front of the half vehicle. Optimal control theory is used to establish control laws which minimise cost indices. This indices are evaluated for several values of vehicle speeds (preview time). The results are presented in terms of performance criteria for either front or rear wheel. It has been shown that a worthwhile improvements are obtained when the wheelbase and road preview are modeled.