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A serious safety problem with tractor/semi-trailer combinations is roll-over. The roll-over threshold of a combination could be increased by using active roll control to roll a trailer into a corner. The key components of such a system are a tiltable fifth-wheel coupling, actuators to apply a moment to the trailer axles, a source of hydraulic power, sensors and a controller. This active roll control system is entirely self-contained within the trailer, so is effective when the trailer is pulled by any tractor or used with a convertor within a train. An active roll control equipped trailer behaves exactly like the same trailer with a conventional suspension except for a reduction in the effective height of the center of gravity of the trailer. Increases in roll-over threshold of 20% to 30% are typical over a wide range of trailer weights and CG heights.

The full potential of active suspension has not yet been realized because of the limitations of available actuators. Current active suspension systems use hydraulic rams with the flow of hydraulic fluid to the ram controlled by a servo-valve to exert the desired force. The objective of this paper is to describe an alternate arrangement of hydraulic actuator which combines great compliance with short reaction times. The heart of the arrangement is a set of hydraulic rams which are connected between the body of the vehicle and one wheel assembly. Each of the rams is connected via a separate three-way, two-position valve to an accumulator, kept at a constant pressure, or a reservoir depending on the position of its valve. The force exerted by the actuator depends on the piston areas of those rams connected to the accumulator.