Wheelbase filtering and automobile suspension tuning for minimizing motions in pitch

Abstract

The rear to front stiffness tuning of the suspension system of a car is discussed, through reference to a half-car pitch plane mathematical model. New results relating to the frequency responses of the bouncing (heaving) and pitching motions of the carbody in following sinusoidal terrain are used to show more clearly than before that the pitch minimization mechanism, known since Olley's observations in the early 1930s, involves interference between the responses to the front and rear axle inputs. It is shown that interference with respect to the rotational motion implies reinforcement with respect to the translational motion, and vice versa. Phasor diagrams are constructed for particular cases to illustrate the mechanics. At higher vehicle speeds, Olley tuning is shown to bring a marked advantage in pitch suppression with very little disadvantage in terms of body accelerations. At lower speeds, not only does the pitch tuning bring with it large vertical acceleration penalties but also the suspension stiffnesses implied are impractical from an attitude control standpoint. The results shown can be employed to guide new suspension designs. Alternatively, corresponding computations can be carried out for new cases of interest, since they are easy to do and rapid.