page Back one
|c.g. = center of gravity
||H = height of c.g. above
|SF = rollover safety factor
||SUV = sport utility vehicle
|T = wheel track, in.
Evaluation of a compact SUV
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
Compact Sport Utility Vehicle (Rear Wheel
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.
Rollover Stability Chart
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.