Stability 8

Let's move along to some more cases. This time only the raw data will be presented with the results given. Then, a chart will be introduced which graphs the formulae and allows a quick SF without having to use a calculator. Then, I will describe a practical, severe road test and finally make recommendations to those who have already purchased SUVs which can be proven to be patently unsafe and recommendations to the designers of automobiles.

Case 2
Compact Sport Utility Vehicle (Rear Wheel Drive)
Weight, curb 3100 lbs.
Load, max. 1050 lbs
Wheel track, T = 58 in.
Curb weight, H = 33 in.

Note that this SUV is very similar in outward configuration to the station wagon of Case 1, however there are significant differences. The most important item is empty c.g. height, H. This car is 8 inches higher than the wagon. The other important difference is that it has rear wheel drive as most SUVs. The weight is higher by 500 pounds as it may have a four-wheel drive option with all the extra mechanics and it may have a more robust frame too. The track is a little wider, but only by approximately one-half inch.

Calculations reveal that the loaded c.g. height, H is 36 in. Substituting the two variables, H and track T into formula (1), the SF of this SUV is 8%. By inspection of the above stability chart, this car must be rated as "Unsafe" . We will take another look at what behavior can be expected in the severe road test to be described later.

Next, I will present the Rollover Stability Chart, Fig. 7. To use this chart, simply plot the two variables, H and T on the Cartesian coordinates and read out the safety factor directly. No calculations are necessary. The chart assumes the value of µ to be 0.75, which corresponds to dry concrete.

I recommend the reader copy the above chart and print it for field reference. At the end of this presentation the Rollover Stability Chart will reappear with the two example cases plotted.