Semi-active suspension, as used on the BMW HP4 and Ducati Multistrada, controls the damping valves electronically to vary suspension action based on changing conditions. Active suspension, however, completely replaces the spring and damper unit with an electromagnetic or hydraulic ram that physically moves the wheel to match the road surface. A couple of interesting variants are the http://www.bose.com/controller?url=/automotive/bose_suspension/index.jsp”>Bose Suspension System and the Michelin Active Wheel.
A patent issued to Ducati almost a decade ago, with the inventors listed as Claudio Domenicali and Filippo Preziosi, describes a rear suspension system with a hydraulic actuator in the linkage that can change ride height automatically. The application and issued patent are entitled “Motorcycle With Active Rear Suspension Unit Providing Improved Braking.” While it’s not a fully active system by any means, the basic concept and first building block are there.
The system senses when the motorcycle is braking, either via a pressure sensor, fork travel sensor or if the throttle is closed abruptly, and lowers the rear of the motorcycle by collapsing the actuator. This lowers the overall centre of gravity and allows the rider to brake more aggressively without having the motorcycle tip over. Once braking is completed, the actuator is extended, returning ride height to its normal position.
Operating only under braking, the ram would not have to alter ride height quickly; a fully active system must be able to move the wheel in milliseconds, and this is the essential difference between what is described in the Ducati patent and a suspension system that is truly active. The technology for active suspension is available and some automobiles are equipped as such, but for motorcycles the major stumbling block is the actuator. Electromechanical units as used in the Active Wheel or Bose system are bulky and require a considerable amount of electrical power; hydraulic rams may be smaller, but require pumps and power to operate them. One last consideration: While a ram of some form could easily replace a shock absorber in a single-shock rear suspension, the traditional front fork presents more of an obstacle as the ram must be a structural component as well.
Currently active suspension for motorcycles is prohibitively expensive and heavy, but given the progress of semi-active systems in the recent past, that may change in the not-too-distant future. The first applications would most likely be on touring machines; comfort and performance would be improved, but the added weight and cost would not be as much of a penalty as they would on a sportbike.