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An Active Camber Concept for Extreme Maneuverability: Mechatronic Suspension Design, Tire Modeling, and Prototype Development

March, 2010

Abstract

Since the beginning of the automobile, engineers have worked continuously to increase its maneuverability. As maneuverable as today’s cars have become, they have yet to reach their full potential. The goal of the active camber concept is to take the next step: to generate a vehicle with extreme maneuverability. The maneuverability of an automobile is limited by tire forces. While tire/road friction does limit the maximum tire forces possible, a significant portion of this friction is not utilized when tires are actuated using steer. However, with camber it is possible to generate up to 30% more lateral tire force. Therefore, active control of camber, in coordination with active steer and suspension, is used to maximize lateral force capability. The result is a more maneuverable vehicle with increased turning capacity.

The contributions of this dissertation follow two main threads. The first is active camber tires. Passenger car tires are not well-suited for use at high camber angles, and motorcycle tires do not exhibit large gains in peak lateral force by using camber as opposed to steer. To exploit fully the benefits of cambering, the active camber concept requires new, specialized tires. To accomplish this, a new, 2D variant of a brush tire model is developed that considers the force distribution in the contact patch in both lateral and longitudinal directions. This gives a more complete picture of how camber forces are generated than existing models, which typically ignore the lateral distribution. Similar to other brush models, this requires a model of the vertical force distribution in the contact patch. Several contact patches are measured and characterized using a new, semi-empirical contact patch model. The resulting 2D brush tire model is used to characterize existing motorcycle tires and to predict what design parameters need to be altered to generate a tire that does exploit fully the benefits of cambering.
The second thread of this dissertation is active camber suspension systems. Of course, a conventional suspension system will not suffice for the active camber concept: a simple mechanism connected to a steering wheel will not be sufficient to coordinate the steer and camber angles of all four wheels to maximize lateral force. A specialized, mechatronic suspension system is required that provides full control over the tire to maximize maneuverability. However, the design criteria presented by existing suspension design literature do not address active camber. Therefore, this dissertation articulates a clear set of design principles rooted in the design of mechatronic systems and applied to a kinematic model of the suspension system. By using the forward kinematics, inverse kinematics, and Jacobians of the model, the design principles are mapped into design criteria. By applying this process to conventional suspension systems, design criteria are developed that are similar to existing suspension design literature. By applying this process to a suspension system with active camber, active steer, and active vertical suspension, design criteria for the active camber concept are developed. These are then used to guide the design and construction of a prototype suspension system.

The prototype suspension system is attached to rollers on a chassis dynamometer, providing an experimental rolling road for testing. This is used to measure the performance of three different motorcycle tires. Not only does this data serve to validate the new tire model, but it also serves to demonstrate the capability of the suspension system as a research testbed. This can be used to further the development of an active camber concept, providing vehicles with more lateral force capability than anything else on the road. The result: a vehicle with extreme maneuverability.