1992-02-01

The Variation of Static Rollover Metrics With Vehicle Loading and Between Similar Vehicles 920583

This paper examines variability of two static rollover metrics, Static Stability Factor (SSF) and Tilt Table Ratio (TTR), due to vehicle loading and vehicle-to-vehicle variation. Variability due to loading was determined by measuring SSF and TTR for 14 vehicles/configurations at multiple loadings. Up to five loadings were used per vehicle/configuration tested. Vehicle-to-vehicle variability was studied by measuring SSF and TTR for ten unmodified vehicles of each of four make/models. Five baseline vehicles, as similar as was feasible, were tested. The other five test vehicles spanned the range of submodels and options available.
In general, both SSF and TTR decreased as occupants were added to a vehicle. The change in SSF and TTR per occupant was fairly consistent, with changes in TTR being more consistent. Placing ballast on the floor of the cargo compartment had a mixed effect on SSF, raising it for some vehicles and lowering it for others. TTR always decreased due to the addition of ballast. Even for vehicles that had a lowering of their center of gravity height (i.e., an increase in SSF) due to ballast, TTR still dropped.
Testing the baseline vehicles found that, for some make/models, vehicle-to-vehicle variability slightly exceeded the expected non-repeatability of the measurement equipment. The range of results is small enough that, while the vehicle-to-vehicle differences between static rollover metrics of supposedly identical vehicles may be real, it is probably not important.
For the non-baseline vehicles, significant differences in the static rollover metrics were seen due to tire size changes. Tires that raised a vehicle's ride height lowered its SSF and TTR. Increasing the width of a vehicle's tires raised its TTR while leaving its SSF essentially unchanged.

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