A sudden steering movement grows stronger as it travels back through a combination, so the trailer swings much harder than the tractor and can tip over. Make lane changes gradually and steer smoothly to prevent it.
A rig is more likely to tip when its weight rides high or when it goes through a turn too fast, so keeping cargo low and slowing down for turns both make a rollover less likely.
A low center of gravity makes a vehicle much harder to tip over, so cargo should sit as close to the trailer floor as possible. Stacking freight high, or putting light freight beneath heavy freight, raises the center of gravity and the risk of a rollover.
Low-riding cargo keeps the center of gravity down, which makes the rig much less likely to tip over in a turn or a sudden lane change. Stacking heavy freight high or loading it off to one side raises that risk instead.
An empty or lightly loaded trailer has little weight pressing its tires onto the road, so it has poor traction. Its wheels lock up more easily, and the trailer can slide sideways and swing out of line.
A trailer skid usually starts when the trailer wheels lock, so letting off the brakes allows them to roll and grip the road again; the trailer will then follow the tractor and straighten out. Using the trailer hand brake makes things worse, because braking the trailer wheels is what caused the skid.
Fix the drift the moment you see it by moving the top of the wheel in the same direction the trailer is drifting. Backing slowly keeps each correction small and easy to control.
Drive wheels normally lock because the driver brakes too hard. Once they slide, the trailer can push the tractor out of line, and the combination can fold into a jackknife within seconds.
In a turn, a vehicle's rear wheels usually track inside the path of its front wheels. This off-tracking grows as the vehicle or combination gets longer, which is why long rigs need extra room to turn.
Rollovers are prevented by keeping the center of gravity low, which means loading cargo low, and by slowing down for turns. Controlled braking is a way to stop in a straight line without locking the wheels; it is not a rollover-prevention measure.
A trailer that starts to slide often gives no warning you can feel in the cab, so the mirrors are where you will spot it first. Check them whenever you brake hard to make sure the trailer is still tracking behind you.
Without a trailer, a tractor has little weight on its drive wheels, and its stiff springs and strong brakes lock the wheels easily, so smooth stops are hard and tests show it can need more distance than a fully loaded tractor-semitrailer. Heavier vehicles actually need more braking power, and braking early is safer than waiting.
A combination vehicle tips over most easily when its center of gravity is high and it takes a turn too fast. Loading cargo as low as possible and slowing down for turns address both causes, so they are the two most important precautions.
Because of rearward amplification, often called the crack-the-whip effect, a sudden steering input grows larger at each trailer, so the last one swings farthest and is the most prone to rolling over. In a set of doubles, the rear trailer has twice the tractor's chance of tipping over.
Every question in this set has its own page with the correct answer and an explanation.
