There are a number of automotive performance aspects which are associated with accelerations in the lateral direction: maneuver (transient and steady state), roll-over, and directional stability. For each of these automotive performance...
moreThere are a number of automotive performance aspects which are associated with accelerations in the lateral direction: maneuver (transient and steady state), roll-over, and directional stability. For each of these automotive performance aspects certain mass property parameters play significant roles; it is the intent of this paper to make explicit exactly how those mass property parameters affect each of those automotive performance aspects.
With regard to maneuver, the maximum lateral acceleration which can be attained in steady-state turning is an important index of performance and safety. The obtaining of high maximum lateral acceleration levels has inherent vehicle weight and center of gravity (longitudinal, lateral, and vertical) implications. However, before attaining a steady-state condition, a turning maneuver must first go through a transient phase. When the transient phase is included in the full maneuver picture, the previous list of significant vehicle mass properties parameters acquires two more members: the mass moments of inertia about the roll and yaw axes.
For modern passenger vehicles, the lateral acceleration point at which roll-over can occur is generally at a level significantly greater than the maximum lateral acceleration. That is, a modern car will tend to slide out of control long before there is a possibility of overturn. Accidents involving rollover generally occur because the vehicle was “flipped” by obstacles in the roadway, not because the vehicle traction was great enough to reach the critical lateral acceleration level. However, the level at which rollover could occur is still an important index of safety, and the most significant mass property for the determination of that level is the vertical center of gravity.
Lastly, there is the matter of directional stability, which has to do with the lateral tire traction force balance front-to-rear, and the front-to-rear “drift angle” relationship of the vehicle tires due to those forces. The lateral force/drift angle relationship is dependent upon normal load, so the most significant mass properties with regard to directional stability are the vehicle weight and static longitudinal and lateral weight distribution.
However, the static normal loads are dynamically modified in response to lateral directional “disturbance” forces. Such disturbances generate initial lateral inertial reactions at the vehicle c.g.; the consequent roll moment not only causes lateral changes in the normal load distribution, but also longitudinal changes due to the front-to-rear suspension roll resistance balance. Such changes readjust the initial lateral force/drift angle relationship front-to-rear, and thereby affect the lateral inertial reaction. If this reaction augments the effect of the original disturbance, then the vehicle is termed unstable or “oversteering”; if the reaction is such as to diminish the effect of the original disturbance, then the vehicle is termed stable or “understeering”. Therefore, for directional stability, the primary mass property parameters are the vehicle weight, and total weight distribution (longitudinal, lateral, and vertical).
