Rollover Prevention: Practice Questions & Explanations

The vast majority of cargo tank rollovers are caused by driver error, most often excessive speed in curves. A tanker's high center of gravity and the surge of liquid cargo make it tip easily when entering a curve too fast. The posted curve speed on signs is for cars; tankers need to enter substantially slower, often 5 to 15 mph below the posted advisory speed. The risk is greatest in two situations: highway ramps (off-ramps and cloverleaf interchanges, where drivers underestimate the curve and over-rely on momentum) and city turns (where the curb-side wheels can come off the ground without the driver realizing). Even seemingly mild curves can roll a half-empty tanker due to liquid surge. Slow down before the curve, not in it.

A high center of gravity is the strongest predictor of rollover. Cargo loaded high in a trailer raises the center of gravity, which means smaller side forces (from a curve or a swerve) are enough to tip the vehicle. Tankers have inherently high centers of gravity. Stacking cargo high in box trailers — for example, light loads piled to the ceiling rather than spread on the floor — makes the same trailer much more prone to rollover than the same weight loaded low. Heavy cargo should always be loaded low. Even loads of similar weight can produce very different rollover thresholds depending on how they are arranged. Tall narrow loads roll more easily than short wide loads of equal weight.

The rollover threshold is the level of lateral acceleration (cornering force) at which a trailer will tip over. It is expressed in terms of gravity (g). A typical empty box trailer has a rollover threshold around 0.5g; a typical loaded box trailer around 0.4g; a typical loaded tanker around 0.35g; and a tanker with partial liquid load may be as low as 0.25g. A passenger car has a rollover threshold around 1.0g — well above the limits of road adhesion, so cars rarely roll. Trucks have thresholds within or below the limits of grip, which is why curves and lane changes that pose no rollover risk to cars can roll heavy vehicles. Drivers reduce rollover risk by reducing speed and avoiding sudden steering.

JACKKNIFE describes the dangerous folding of a tractor-trailer when the trailer slides sideways relative to the tractor. There are two main types with different causes: TRACTOR JACKKNIFE (drive axle jackknife): The tractor's rear drive axles lock (typically from overbraking on slippery pavement); the locked wheels can no longer provide directional control; the tractor's rear slides sideways while the trailer continues pushing straight; the tractor pivots at the fifth wheel;

Trailer jackknife: The trailer's brakes lock excessively (often from trailer brake imbalance or improper hand-valve use); the trailer slides sideways while the tractor continues forward; the trailer swings out from behind the tractor.

Common causes: Excessive brake force in a turn or on slippery pavement; improper use of the trailer hand valve (which applies only trailer brakes); brake imbalance between tractor and trailer; driving too fast for conditions; over-braking in a curve.

Prevention: Maintain balanced braking; avoid using the trailer hand valve except for pre-trip inspection; reduce speed before curves; use brakes smoothly and progressively; if skid begins, release brakes to restore directional control. ABS SYSTEMS dramatically reduce jackknife risk by preventing wheel lockup.