FIGURE 2-5 Tilt table test—tilt table ratio equals tangent of angle theta (θ).

duce a rollover than is the case with SSF. The flaw in the test is that as the table is tipped up, the total weight supported by the tires (perpendicular to the tilt table) drops, and the suspension tends to move into rebound (i.e., the suspension loads drop) and away from the curb equilibrium position. This in turn causes the vehicle center of gravity to move away from the tilt table, and thus makes the car more prone to rollover than it would be on a horizontal surface.

There is also a potential undesirable consequence of using TTR to assess rollover propensity. In some cases, measured TTR values can be increased by altering suspensions in a way that degrades vehicle directional response. In particular, best test results are obtained by having front and rear uphill wheels lift at the same time. This means vehicles with balanced front and rear roll stiffness will yield better TTR test results than otherwise similar vehicles with unequal roll stiffness, even though unequal roll couple distribution often produces improved dynamic performance (Federal Register 2001). Thus, a vehicle rating system that used TTR to rank rollover propensity could encourage undesirable design trade-offs and vehicles with inferior directional response characteristics.

The side pull test provides another static measure of vehicle rollover propensity; Figure 2-6 shows a schematic of a side pull test facility. In this case, test engineers pull the vehicle sideways with a horizontal force at the height of the vehicle’s center of gravity. If there were no compliance in the suspensions and tires, the force required to tip the vehicle over, divided by the weight of the vehicle, would be the same as the SSF. Because of suspension and tire