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Tanker · Center of Gravity, Surge, and Outage

A tanker and a flatbed are loaded to the same gross weight and driven down the same ramp. Why is the tanker the one more likely to roll over?

1.Its load rides high in a tall vessel instead of low across a deck, and the liquid can shift too, so its center of gravity is both high and able to move while turningCorrect answer
2.Its load weighs more per gallon than dry freight does in every single case, so the tanker is simply the heavier of the two vehicles on the road
3.Its tires carry less of the load than a flatbed's do, because a tank transfers weight into the frame rails rather than onto the axles beneath it
4.Its wheelbase is shorter than the flatbed's, and wheelbase alone is what governs how easily any heavy vehicle rolls over on a steep ramp

Explanation

The liquid sits high inside a single tall tank instead of lying low across a flat deck, which puts a large share of the load's weight well above the road, and because the cargo is liquid it can also shift, so the high center of gravity will not even stay in one place. That is a different problem from a tall dry load, which stays put once it is secured. The manual draws the practical conclusions: a tanker goes over far more readily than its weight alone would suggest, testing has found tankers turning over at the speeds posted for curves, and highway curves and ramps should therefore be taken well under those posted speeds. 49 CFR 383.119(f) makes the effects of road grade and curvature with filled, half-filled, and empty tanks a required knowledge area, but states no speed and no figure. The wrong options all look for the answer in the wrong property: the two vehicles were given the same weight to begin with, a tank's load reaches the road through the axles like any other, and wheelbase is not what differs between these two vehicles.

Source: 49 CFR 383.119(f)

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