Driving a Tank Vehicle: Surge, Sloshing, and Vehicle Stability
Liquid in a partly filled tank does not stay put. Every brake application, acceleration, and steering input sets it moving, and the exam tests whether you can predict which way the load will push and what that does to the vehicle.
The physics the test expects
- Surge: liquid moves opposite the vehicle's change in speed. Brake and the product runs forward against the front bulkhead and pushes the truck; accelerate and it runs rearward.
- Sloshing: side-to-side motion in a partly filled tank, produced by steering and by curves, shifting weight onto the outside tires.
- Free surface: the less full the tank (short of completely full), the more surface the liquid has to move across and the stronger the effect. A full tank moves least.
- Baffles: bulkheads with holes that break up the flow. They reduce surge but never eliminate it; a smooth-bore tank has the strongest surge.
Worked example
A smooth-bore tanker is stopped at a light and the driver accelerates hard to merge. As the truck speeds up, the liquid runs rearward, momentarily lightening the steer axle and cutting front traction. The driver feels the front end go vague and corrects with steering - and that correction starts a side slosh. The exam answer is to accelerate smoothly so the load stays settled, and to keep space ahead so braking is never abrupt.
Traps that cost marks
- Believing baffles "solve" surge rather than reduce it.
- Thinking a nearly empty tank is the safest load; a partly filled tank is the least stable.
- Reading the symptom as a steering or brake defect instead of a load effect.
- Assuming surge only matters when you stop, ignoring the rearward surge on acceleration.