Decreasing-Radius Loop Ramp With Residual Liquid in an Empty Tank
A driver is pulling an empty 5,500-gallon insulated cargo tank with about 900 gallons of residual liquid in the last compartment. The tank’s center of gravity is high, and the route requires a 270-degree loop ramp with a decreasing radius; the ramp is posted 20 mph and is wet from recent rain. The driver enters at 30 mph, and halfway through the curve the trailer’s left side begins to lift and the steering feels light as the residual liquid shifts to the outside of the curve. The tractor has a wheelbase of 220 inches, the trailer track width is 72 inches, and the driver is 15 feet from the outside guardrail. A box truck is alongside in the left lane, and the trailer is not equipped with a roll-stability system.
1. What is causing the trailer to begin to roll, and why does the posted 20 mph matter here?
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The trailer is beginning to roll because the high center of gravity and the lateral shift of the 900 gallons of residual liquid increase load transfer to the outside wheels. A tanker’s rollover threshold is lower than a dry van’s, and a decreasing-radius ramp tightens faster than the driver expects. The light steering is a warning that the front tires are unloading as the trailer rolls. The posted 20 mph is based on a vehicle that can hold the lane; entering at 30 mph uses more of the available friction, and the wet surface reduces it further. The residual liquid moving to the outside adds a lateral force at the worst time. The box truck alongside limits the driver’s escape space, so the driver cannot simply steer left.
2. What immediate action should the driver take, and what should the driver avoid doing?
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The driver should ease off the accelerator and hold a smooth, constant steering input; do not brake hard, because hard braking transfers more weight forward and can cause the rear tires to lose grip or the trailer to swing. Do not steer sharply toward the guardrail or the box truck. The driver should reduce speed gradually by engine drag and light braking after the roll motion is controlled, and should look through the curve to the exit. If the trailer continues to roll, the driver may need to let the vehicle drift slightly toward the outside of the lane while keeping the tractor straight, but should not make a violent correction. After the curve, the driver should stop in a safe place and inspect the load and tires.
3. How should this trip have been planned differently to reduce the rollover risk?
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The route should be planned so the tank is not carrying a partial residual load through a decreasing-radius ramp. If the tank must be moved empty, the residual should be drained or the compartment should be filled to eliminate the free surface. The driver should enter the ramp at or below 20 mph, in the correct lane, with no vehicle alongside if possible, and should brake before the curve. The company should identify the ramp as a rollover location and consider an alternate route. Pre-trip planning should include weather; wet pavement reduces the speed at which the tank can safely negotiate the curve. The driver should also check that the tank’s baffles, if any, are intact and that the residual is not in an unbaffled compartment.