Lateral stability of an automobile.

The lateral movement and stability of an automobile are investigated by using an equivalent dynamic system which takes account of unsprung masses. The following problems are studied in the paper: (1) The transfer functions and the characteristic equations are derived for the cases of fixed and free control. The stability criteria of these systems are discussed. (2) The effects of various viscous dampings in the steering system are investigated, both theoretically and experimentally. (3) The effect of dry friction in the steering system is analysed by solving the equations of motion directly for the case of fixed control and using the describing function method for the case of free control. (4) The motion of a vehicle accelerated or decelerated from a steady circular turn is generally analysed and the characteristics of under steer, neutral steer and over steer are discussed by observing the radii of curvature of the trajectories of front and rear wheels. The effect of roll on the steer characteristics can be discussed by substituting the equivalent cornering coefficients complemented by rolling motion for the cornering coefficients of tires in the result obtained by disregarding the effect of roll. (5) The mathematical model representing the motion of an automobile suddenly disturbed by cross wind is proposed by theoretical and experimental studies using a large full scale wind tunnel and side wind blowers. In order to evaluate the sensitivity of a vehicle to cross wind, the cross wind sensitivity coefficient is defined in the paper. (6) In order to discuss the variation of parameters and decide the most effective parameter in the stages of design and experiment of an automobile, the method of sensitivity analysis is applied to the problems of vehicle dynamics. The sensitivity coefficient and equation are introduced in the generalized form, and some examples of application of this analysis are studied relating to a simple model of steering system and the test method on the handling performances in the Specification of Experimental Safety Vehicle (ESV) of Japan.