The mass properties of a vehicle affect its’ motion in all directions, translational and rotational. Previously this author has dealt with how mass properties affect automotive longitudinal acceleration1 and automotive lateral acceleration2. Now a consideration is in order of how mass properties affect automotive vertical acceleration. Of course, lateral or longitudinal inputs can lead to vertical responses, etc., every aspect of a vehicle’s dynamics is interconnected with every other aspect, but it is convenient to divide up automotive dynamics as if the subject were purely a matter of independent accelerations in the longitudinal, lateral, and vertical directions.
Initially, this paper will investigate the significance of mass properties with regard to automotive ride (transmission of road shock & vibration) and road-holding (maintaining contact at the tire/road interface) through the use of simple, undamped, 1 DOF models. Later, the full story of how mass properties influence the bounce and pitch motions of the sprung mass will necessitate recourse to more complex 2 DOF models. The mass properties of greatest relevance to this investigation will prove to be the “sprung” mass, the “unsprung” masses, the “sprung” mass distribution (longitudinal, lateral, and vertical c.g. location), the rotational inertias of the rotating portions of the “unsprung” masses, and the “sprung” mass longitudinal and lateral mass moments of inertia.
The basic intent of this paper is to counter the commonly held simplistic concept of the role mass properties play in determining ride and road-contact. For those that have never undertaken any study of the matter, the general presumption seems to be that all that is required to achieve optimum performance is to minimize the weight and to obtain a balanced mass distribution. The reality is that there are many aspects to automotive performance, and what constitutes an optimum mass properties condition is generally a very complex matter which often necessitates difficult compromises. Tailoring some mass property parameters so as to achieve a desirable level of behavior with regard to one performance criteria will often adversely affect other performance criteria.
Although this paper is restricted to mass properties issues related to performance resulting from motion in the vertical direction, occasional reference will be made to those mass properties requirements necessitated by performance considerations associated with the longitudinal (acceleration, braking) and lateral (maneuver, roll-over, and directional stability) directions, as revealed in the previous investigations noted earlier. To do otherwise would be to work in a vacuum; the nature of reality tends to be such that all things are ultimately interrelated. To the fullest extent possible, the greater intent herein is to approach reality through the totality of the papers and articles written by this author on the subject of mass properties and automotive performance.