2.1 Liquid Surge: Causes, Effects & Prevention
Key Takeaways
- The AAMVA manual defines liquid surge as movement of the liquid in partially filled tanks, which can have bad effects on handling.
- When the surge wave hits the end of the tank, it tends to push the truck in the direction the wave is moving.
- On a slippery surface such as ice, the wave can shove a stopped truck out into an intersection.
- To control surge, keep steady pressure on the brakes, do not release too soon, brake far in advance, and increase following distance.
- Side-to-side surge in curves and lane changes shifts weight toward the outside and can contribute to a rollover.
Surge is the reason the tank endorsement exists. A load of boxes stays where you secured it; a partly filled tank of liquid keeps moving after the truck changes speed or direction. The first item on the federal knowledge list for the endorsement (49 CFR 383.119(a)) is the causes, prevention, and effects of cargo surge on vehicle handling.
What Surge Is
The AAMVA model CDL manual (8.2.2) says liquid surge results from movement of the liquid in partially filled tanks, and that this movement can have bad effects on handling. Its example is a stop: the liquid surges back and forth, and when the wave hits the end of the tank, it tends to push the truck in the direction the wave is moving. If the truck is on a slippery surface such as ice, the wave can shove a stopped truck out into an intersection.
The physics is inertia. When the tires slow the vehicle, the liquid keeps going until the front of the tank stops it. Then it rebounds toward the rear, then forward again, until friction settles it.
Why a Partial Load Is the Problem
| Fill condition | Room for liquid to move | Mass that can move | What to expect |
|---|---|---|---|
| Nearly full (with required outage) | Very little | Large | Little surge, but a high center of gravity (Section 3.2) |
| Partly filled | A lot | Large | Strong surge in stops, starts, turns, and lane changes |
| Nearly empty | A lot | Small | Weak surge, but a light vehicle with less traction and longer stops (Section 4.2) |
A partial load combines two things surge needs: enough liquid mass to push the truck and enough empty space for that liquid to build speed. That is why dense products hauled as legal partial loads (Section 1.3) call for extra care.
Longitudinal Surge: Starting and Stopping
Longitudinal surge runs front to back.
- Stopping: liquid moves forward and hits the front head of the tank or compartment, pushing the truck ahead. If you ease off the brake as you come to a stop, the returning wave can roll the truck forward again.
- Starting: liquid moves toward the rear as you accelerate, then rebounds forward. Jerky starts make the rig lurch and can spin the drive wheels on slick surfaces.
- Speed changes on grades: the liquid shifts as the vehicle pitches up or down and as speed changes (Section 4.1).
Baffles reduce front-to-back surge; smooth bore tanks do not (Section 2.2).
Lateral Surge: Curves and Lane Changes
Lateral surge runs side to side. In a curve, the liquid moves toward the outside wall. That shifts weight to the outside wheels in the same direction the curve is already pushing the vehicle, on a vehicle whose center of gravity is already high. If you then steer back quickly, as in a double lane change, the liquid can slosh back the other way. Repeated quick steering inputs can build the slosh instead of letting it settle.
Baffles do not stop side-to-side surge. The manual says side-to-side surge can still occur in baffled tanks and can cause a roll over.
Surge Effects on Handling
| Driver action | Surge effect | Possible result |
|---|---|---|
| Braking to a stop | Wave pushes truck forward | Longer stop; truck rolls into intersection, especially on ice |
| Releasing brakes too soon | Returning wave moves truck | Truck creeps forward after stopping |
| Hard acceleration | Liquid shifts rearward then forward | Lurching; drive wheels may spin on slick roads |
| Entering a curve too fast | Liquid shifts to outside | Added rollover risk |
| Quick steer and counter-steer | Liquid sloshes side to side | Loss of control or rollover |
| Steering quickly while braking | Forward and sideways forces at once | The manual warns the vehicle may roll over |
Preventing Surge Problems: The Manual's Rules
The AAMVA manual (8.3.1 and 8.3.2) gives the habits the test expects:
- Drive smoothly. Because of the high center of gravity and surge, start, slow down, and stop very smoothly. Make smooth turns and lane changes.
- Keep a steady pressure on the brakes. Do not release too soon when coming to a stop.
- Brake far in advance of a stop and increase your following distance (Section 4.3).
- For a quick stop to avoid a crash, use controlled braking or stab braking (Section 4.2).
- Remember that steering quickly while braking may roll the vehicle over.
- Slow down before curves, then accelerate slightly through the curve (Section 4.1).
A stop at a signal, step by step
- Look far enough ahead to see the light change early (most good drivers look 12 to 15 seconds ahead).
- Start braking early with light, steady pressure.
- Increase pressure smoothly as speed drops.
- As you stop, keep steady pressure; do not release just because the speedometer reads zero.
- Hold the brakes until the vehicle is steady and the surge has settled, then start smoothly.
Surge and Road Surface
Surge force does not change on ice, but traction does. A push the tires can resist on dry pavement can slide the truck on a slick surface. The manual's intersection example is on ice for this reason. On wet or icy roads, combine the surge rules with the reduced speeds in Section 4.3.
Common Test Traps
- "Full tanks surge the most." Surge is a problem in partially filled tanks. Full tanks have little room to surge but a high center of gravity.
- "Baffles stop all surge." They slow front-to-back surge only.
- "Once stopped, surge is over." The wave can still push a stopped truck, so keep steady brake pressure.
According to the AAMVA CDL manual, what happens when the surge wave hits the end of the tank?
How does the manual say to control surge when coming to a stop?
In which condition is liquid surge the main handling concern?