- The static stability factor is, of course, derived on the basis of a rigid body, such as a block of wood or a brick.
- The analytical derivation of the factor assumes a purely lateral motion, without any rotation about a vertical axis (i.e., yawing), on a uniform high-friction surface.
Thus, individual vehicles with T/2H values of 1.20 and 0.94 respectively could have identical lateral acceleration values at incipient rollover, for the cited operating condition, if their design details produced the indicated extremes of performance:
(0.70)(1.20) = 0.84 (0.90)(0.94) = 0.84
The following is an excerpt from 1987 "Safety Issues Related to Mini-Cars from a Roadway Perspective",(11 megs!) Council, F.M., Reinfurt, D.W., McHenry, B.G. Pages 53-56, and Appendix E Details of the HVOSM runs
Vehicle parameters associated with rollover.
While other accident and HVOSM related efforts have examined issues related to roadway parameters, this specific HVOSM effort was designed to further examine vehicle parameters which might be related to increased rollover propensity.
- Past theory has suggested that a critical indicator of rollover propensity is the ratio of the half track width to the height of the center of gravity (T/2H).
- This HVOSM effort involved repeated runs involving eight vehicles ranging in weight from 1699 to 4450 lb (0.77 to 2.02 Mg).
- While the initial goal was to input a steering maneuver which would put the vehicle in a near-rollover position and to then modify various parameters to determine which were critical, the method had to be modified due to the difficulty of obtaining such a state on a flat area with a normal coefficient of friction.
- As a substitute, the vehicles were run from the roadway onto a flat, high friction surface and placed in a yawed (nontracking) attitude. The nominal friction value for the test surface was then increased until a rollover occurred. The vehicle parameters were then studied as they changed with this change in critical rollover friction.
This is most clearly shown by figures 3 and 4 below.Here, vehicle weight and then T/2H values are plotted against the critical friction value resulting in rollover. If either weight or T/2H was a perfect indicator of rollover, then one would expect a fairly straight-line relationship with very little variability. However, as can be seen from the figures, there is some variability, with both vehicle weight and T/2H deviating from an increasing slope. More pertinent to this effort, the maximum variability is at the lower weight and T/2H values, values pertinent to smaller vehicles. Figure 3 Figure 4
In a related set of runs, the center of gravity heights were changed in order to give all vehicles the same basic T/2H value. Simulation runs were then made to see if the critical friction factor remained constant.
- Results here also indicated that for the higher T/2H values (approximately 1.3) the critical friction coefficient for rollover was a constant function of the static stability factor (T/2h). However, for lower T/2H values of approximately 1.1, there was a great deal of deviation in the friction factors, again denoting the fact that T/2H is certainly not the only predictor of rollover potential.
- Whereas these analyses indicated a general trend that would explain larger cars having greater resistance to rollover, there does not appear to be any single variable in itself that would indicate why certain vehicles roll at a friction coefficients which are 65 to 70 percent of their static stability factors while others roll at 90 percent of their static stability factors.
There exists an inherent resistance of vehicles to roll which must be a function of certain other vehicle parameters which are yet to be defined.
The results of these HVOSM analyses were then utilized in the development of research plans.