Stop-and-Go Surge in a Smooth-Bore Milk Tanker
You are driving a 6,200-gallon smooth-bore tanker carrying 3,000 gallons of milk-just under half full. The tank is a single compartment with no bulkheads. The route through downtown has repeated red lights and short acceleration lanes. At the first red light, you brake gently but the truck still lurches forward as the milk surges; you stop with the tractor's front bumper over the crosswalk. When the light turns green, the load surges rearward and the drive tires feel light for a moment. Your dispatcher asks you to 'make up time' because the milk is perishable. You have 12 more miles of this stop-and-go traffic before the highway.
1. Explain what causes the forward lurch at the red light and the light drive tires on acceleration, and why the lack of bulkheads matters.
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Milk has mass and momentum. When you brake, the tank slows but the liquid keeps moving forward until it hits the front bulkhead or shell, transferring a forward force to the truck; that is the lurch that pushes the bumper over the crosswalk. After the stop, the liquid can surge back. On acceleration, the liquid lags rearward, momentarily unloading the drive tires and reducing traction. A smooth bore has no internal bulkheads, so the entire 3,000 gallons can move as one mass; bulkheads would divide the tank and limit the distance the liquid can travel, though each compartment would still surge. At just under half full, there is a large free surface, so the surge is worse than in a full tank.
2. Describe a braking and acceleration technique that minimizes surge, including following distance and how to use the brake pedal.
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Increase following distance to at least 4 to 6 seconds and look ahead for stale green lights so you can stop gradually. Brake early with a light initial application, then increase pressure smoothly; avoid late, hard braking. Just before a complete stop, ease off slightly to let the liquid settle, then hold the brake-this is a controlled stop, not a coast. Accelerate smoothly and progressively; do not jackrabbit. If you must stop on a downgrade, use a lower gear and engine braking, and leave even more room. Use the brake pedal as a planning tool, not a reaction.
3. What should you tell dispatch about making up time, and what additional hazard does a downgrade or a sudden stop create?
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I would tell dispatch that the schedule cannot justify unsafe surge in downtown traffic; the safe response is to allow more time, not to drive aggressively. A downgrade makes surge worse because gravity also pushes the liquid forward, and a sudden stop can send the load forward hard enough to affect steering and braking. If the route includes a steep grade, I would select a lower gear before the grade, keep speed low, and avoid stopping unless necessary. If dispatch insists, I would document the request and follow safety, not the schedule.