A Theoretical Analysis of the Lateral Properties of Suspension Systems

Abstract

The suspension system of a vehicle interconnects its wheels and body and provides the means by which forces and movements are transferred from one to the other. The lateral properties of suspension systems are required for lateral handling and other suspension studies, and this paper contains a theoretical analysis of such properties for a range of suspension systems: viz. double wishbone, Macpherson strut, swing axle, beam axle, and beam axle-Panhard rod. The lateral motions of a wheel on a road, resulting from prescribed motions of the vehicle body, are derived for the range of suspension systems considered. Attention is concentrated on the two principal body motions of vertical and rolling velocity, and the resulting suspension movements are determined in terms of these suspension derivatives. In the case of the double wishbone suspension, the various types, each with differing characteristics, are distinguished. Regions are defined within which suspension systems will have certain properties, and the boundaries of these regions are determined. By a suitable choice of defining parameters, the Macpherson strut suspension is shown to be equivalent to a double wishbone suspension, and therefore the analysis of the double wishbone suspension is applicable to the Macpherson strut. By comparison with the double wishbone suspensions the remaining suspensions considered have only limited ability to vary their suspension characteristics. The effects of suspension design on the steady state roll angle, suspension jacking, and load transfer are investigated. A single end suspension is first considered, and the dependence of the roll angle and suspension jacking on the suspension derivatives is demonstrated. This dependence enables the significance of the various regions derived earlier to be stated. For the complete vehicle, the suspension derivatives again determine the roll angle, and in this case the wheel load transfers are also dependent, via the body transferred rolling moment, on the suspension derivatives. The suspension derivatives are required in handling studies and the method of analysis is capable of giving considerable insight into the general behaviour of suspension systems.