Truck Trailer Combination Basics and Rollover Risks
Truck Trailer Combination Basics and Rollover Risks
This domain tests the physics you must apply: weight distribution, center of gravity, and the speed at which a combination rolls over. Rollover is the most common serious crash type for combination vehicles, and speed and steering choices prevent almost all of them.
Core concepts
- The combination's center of gravity is high - a van trailer's cargo floor sits roughly 4 feet up, and a loaded trailer's CG may be 6 feet or more above the road.
- Rollover threshold: a loaded tractor-trailer can roll over at lateral acceleration as low as about 0.3-0.4 g, while a passenger car resists close to 1.0 g. That is why a ramp a car takes at 50 mph may roll a truck at 35-40 mph.
- Lateral force rises with the square of speed: at 40 mph in a curve you generate about 1.8 times the side force you generate at 30 mph.
- Weight distribution matters: a trailer loaded heavy at the top or to one side has a lower rollover threshold and can overload an axle.
- Load securement: cargo that shifts changes the CG and can push through a trailer wall.
Worked example. A ramp is posted at 30 mph and you enter at 45 mph. Side force scales as speed squared: (45/30) squared equals 2.25. You are asking the combination to resist more than twice the side load it was designed for, with a high-CG van trailer. The correct action is to slow to the advisory speed before the curve, not brake in it.
Traps that cost marks. Treating the posted speed as a target; braking hard mid-curve, which transfers weight and can trigger a rollover; carrying a top-heavy or shifting load without reducing speed; assuming a lightly loaded trailer resists rollover better - it often rides higher and rolls sooner.