Rollover Risks, Speed Control, and Safe Operating Practices
Rollover and speed-control questions test whether you understand why a combination vehicle turns over and how to prevent it before the curve arrives.
Rollover physics
- A loaded tractor-trailer has a high center of gravity. Centrifugal force in a turn increases with the square of speed and decreases as the turn radius grows.
- Rollover threshold is lower than a passenger car's. Posted curve speeds are often for cars; a tall or partially loaded combination may need to be slower.
- Liquid loads surge; a partly filled tank can shift weight and aggravate rollover.
- High loads, poor securement, and soft or uneven surfaces raise risk.
Speed control
- Brake before the curve; enter slowly and accelerate gently only after the trailer is through.
- On a downgrade, choose the gear before the grade and use engine braking with controlled brake applications.
- Keep space ahead. A common rule is one second per 10 feet of vehicle length under 40 mph, plus one second over 40 mph; a 60-foot rig at 55 mph needs about 7 seconds.
- Identify an escape route before you need it.
Worked example: A curve is posted 25 mph. A loaded tanker enters at 40 mph. Reasoning: doubling speed quadruples centrifugal force; 40 versus 25 is 1.6 times the speed, so about 2.6 times the force. The trailer's high center of gravity can reach rollover before the tires slide. Correct action is to slow to the posted speed or below before the curve, not brake in it.
Traps
- Treating a car advisory speed as your truck's limit.
- Braking hard mid-curve.
- Following a car at car distance.
- Ignoring liquid surge or a high, light load.