It is well understood and supported by more than a decade of statistical investigation that vehicles involved in on-road rollovers are generally out of control (e.g. spinout, side slide) prior to the roll, and that lateral stability is the primary vehicle parameter governing such occurrences. Although a rollover is unstable from lift-off to tip-over, an impulse of sufficient duration is required to input the work necessary to lift the center of gravity to a position over the leading wheels. The force in this impulse is, absent a geometric trip, limited to friction between the vehicle tires and the travelled surface. The coefficient of friction of a tire sliding laterally on dry pavement is generally at or below 0.9, although in certain conditions of unusual tire shoulder wear somewhat higher friction has been observed. Therefore vehicle side stability above this level (0.9 at GVW) is a reasonable design criterion for light vehicles. Statistical studies have been based on geometric stability (half track/center of gravity height) as an approximate indicator. However, the actual parameter of concern is, of course, the operating vehicle stability, including the effects of suspension and tire compliance. General Motors, approximately 20 years ago, found the typical operating stability of its vehicles to be about 75% of geometric stability. To facilitate testing for regulatory purposes, the presently favored technique (NHTSA, ECE) is the tilt table. An analytic model including detailed characterization of suspension components and tires is presented to quantitatively account for both suspension and tire compliance in reducing geometric stability to operating stability, and also to quantitatively relate tilt table stability to the operating stability of actual concern. Further, available empirical data shows that the effect of tire compliance is substantive and is of concern in tilt table design.
