Partial Ethanol Load on a Decreasing-Radius Ramp
You are assigned a 6,200-gallon insulated stainless-steel tanker with three compartments. Dispatch loads 3,000 gallons of ethanol (about 6.6 lb/gal), split as 1,000 gallons in each compartment, because the customer needs a split delivery later. The tractor is a 6x4 with a high-mounted sleeper, and the tank shell sits about 11 feet above the ground at its highest point. Your route includes a two-lane ramp posted 25 mph with a decreasing-radius curve and a 20-mph wind gusting across an open bridge. You approach at 35 mph in 9th gear; the engine is at 1,300 rpm, and the tank is not visibly leaking. The load is legal on axle weights, and the placard is correct. Your co-driver says the truck 'feels light' and wants you to speed up to get through the curve before traffic builds.
1. Estimate the product weight and explain how the partial load affects the tanker's center of gravity and rollover threshold.
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3,000 gal × 6.6 lb/gal ≈ 19,800 lb of ethanol, plus the tank and tractor. A 6,200-gal tank at 3,000 gal is about 48% full; splitting it as 1,000 gal in each of three compartments leaves a free surface in every compartment.
The liquid mass sits around the middle of the barrel, and the tank shell itself is already high, so the combined center of gravity is high. A partial fill is not automatically 'lighter and safer': the free surface can shift laterally in a curve, moving the center of gravity sideways and adding a rolling moment. A tanker's rollover threshold is lower than a van's; at 35 mph the lateral demand may exceed it even though axle weights are legal.
2. Is 35 mph acceptable? Use the curve geometry, speed-squared lateral force, and the posted 25 mph to justify your answer.
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No. If 25 mph is the safe posted speed for the curve, lateral acceleration scales with speed squared: (35/25)^2 = 1.96, so 35 mph roughly doubles the sideways force for the same path. A decreasing-radius curve tightens after you commit, which raises the demand further, and the crosswind adds a steady side load. The high center of gravity and the free-surface shift reduce the margin.
The correct plan is to be at or below 25 mph before the curve, in a lower gear, with the chassis settled. Do not 'speed up to get through.' If traffic is close behind, use turn signals and a legal lane position, not extra speed.
3. If traffic forces you to stop mid-curve, what sequence reduces rollover risk?
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Brake early and progressively while the steering is nearly straight; avoid a hard stab of the service brakes that transfers weight forward and unloads the rear. Keep both hands on the wheel and make one smooth steering input; do not saw the wheel.
If the tank starts to lean, release brake pressure momentarily to let the tires regain grip, then reapply gently. Accept stopping in the lane rather than swerving into the shoulder; a shoulder drop-off can trip a high-center-of-gravity tanker. After stopping, set the brake, warn traffic, and wait for a safe gap before moving. Do not accelerate hard to pull out while the liquid is still surging.