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Driving a tanker: surge, baffles, outage and safe driving

Tank Vehicles (N)4 minManual section 8.2, 8.3

A tank of liquid is a moving load. It sits high and it sloshes. These two facts, the high center of gravity and liquid movement (surge), are why tanker driving needs special skill.

High center of gravity

A high center of gravity means a big part of the cargo weight sits far above the pavement. That makes the truck top-heavy, so it tips over easily. Tankers full of liquid are among the easiest trucks to roll.

Tests show tankers can turn over at the posted speed limits for curves. Take highway curves and on-ramp and off-ramp curves well below the posted speed.

The danger of liquid surge

Liquid surge is the movement of liquid in a partly filled tank. It hurts handling.

Example: when you stop, the liquid rushes forward and back. As the wave slams into the tank wall, it shoves the truck the same way the wave travels. On ice or another slick surface, that push can slide a truck that has already stopped into the intersection.

A tanker driver must know the handling of the vehicle very well.

Bulkheads, baffles and smooth bore tanks

  • Bulkheads are solid walls that divide one tank into several smaller tanks. When you load or unload them, watch weight distribution. Avoid loading so the front or the back of the truck ends up overweight.
  • Baffles are bulkheads with holes that let liquid flow through. They help control forward-and-back surge. But side-to-side surge can still happen, and that can cause a rollover.
  • Un-baffled tanks, also known as smooth bore tanks, have no inner walls at all, so nothing slows the liquid. Forward-and-back surge is very strong. They usually carry food products such as milk, because sanitation rules forbid baffles: baffles make the inside too hard to clean.

Be extremely cautious (slow and careful) with smooth bore tanks, especially when starting and stopping.

Outage and how much to load

Do not fill a cargo tank to the very top. A liquid grows in volume when it gets warmer, so leave space for it. This empty space is called outage.

Each liquid grows by its own amount, so each one needs its own outage. Learn the outage rule for every bulk liquid you carry.

A full tank of a dense (heavy) liquid, such as some acids, may be over the legal weight limits. So you often fill tanks with heavy liquids only part way.

How much to load depends on three things:

  • how much the liquid will expand in transit,
  • the weight of the liquid,
  • legal weight limits.

Safe driving rules for tankers

Drive smoothly. A tall, sloshing load punishes jerky driving, so start, slow and stop gently. Turn and change lanes gently too.

Control surge. Keep steady pressure on the brakes, and hold it until the truck is fully stopped (releasing early lets the wave push you forward). Brake far ahead of a stop and increase your following distance.

If you must stop fast to avoid a crash, use controlled braking or stab braking (see Section 2). Remember: steering quickly while braking can roll a tanker over.

Curves. Slow down before the curve, then accelerate slightly through it. The posted curve speed may be too fast for a tank vehicle.

Stopping distance. Know how much room you need. Wet roads double normal stopping distance. An empty tanker can need more distance to stop than a loaded one, because with less weight pressing down the tires grip less.

Skids. Do not over-steer, over-accelerate or over-brake. On tank trailers, if the drive wheels or trailer wheels skid, the vehicle may jackknife. Whenever a skid begins, your job is to get the wheels gripping again.

References and scope

The topic reference is listed below. Sources support verification; the lesson itself is here. The current content needs technical and language review before public launch.

Manual reference: 8.2, 8.3

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

Educational questions with explanations. The answer review also works without the interactive quiz.

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Review questions with answers

  1. Which two things make driving a liquid tanker require special skill?
    1. Long wheelbase and wide mirrors
    2. High center of gravity and liquid movement
    3. Low weight and short trailer
    4. Air brakes and a sleeper
    Show answer
    High center of gravity and liquid movement

    The two reasons: a tall center of gravity and sloshing (surge) liquid.

  2. A high center of gravity means:
    1. The load sits low near the axles
    2. The tank is longer than normal
    3. The brakes are weaker
    4. A large share of the cargo weight sits high off the road
    Show answer
    A large share of the cargo weight sits high off the road

    High center of gravity means much of the weight is high off the road, making the vehicle top-heavy.

  3. How should a tanker take highway curves and on-ramp or off-ramp curves?
    1. Well below the posted speed
    2. At exactly the posted speed
    3. Slightly above the posted speed
    4. At the highway speed limit
    Show answer
    Well below the posted speed

    Tests show tankers can roll over at posted curve speeds, so go well below them.

  4. Tests have shown that tankers cannot turn over if they stay at the posted speed for a curve.
    1. True
    2. False
    Show answer
    False

    False. Tankers can roll over at the posted curve speed.

  5. Liquid surge is caused by:
    1. A full tank with no room
    2. Too much air pressure in the tires
    3. Movement of liquid in a partly filled tank
    4. A loose fifth wheel
    Show answer
    Movement of liquid in a partly filled tank

    Surge comes from liquid moving in a partially filled tank.

  6. When the wave of liquid hits the end of the tank after you stop, it tends to:
    1. Shove the truck the same way the wave is going
    2. Pull the truck backward only
    3. Lift the front wheels off the road
    4. Have no effect on a stopped truck
    Show answer
    Shove the truck the same way the wave is going

    The wave pushes the truck the way the wave is moving; on ice it can shove a stopped truck into an intersection.

  7. On an icy road, what can liquid surge do to a tanker that has just stopped?
    1. Nothing, because the brakes are set
    2. Freeze the brakes
    3. Empty the tank
    4. Shove it out into an intersection
    Show answer
    Shove it out into an intersection

    On slippery surfaces the surge wave can push a stopped truck into an intersection.

  8. When loading and unloading a tank divided by bulkheads, the driver must pay attention to:
    1. Paint color
    2. Weight distribution
    3. Radio frequency
    4. Fuel economy
    Show answer
    Weight distribution

    Balance the weight so neither the front nor the rear is overloaded.

  9. How are bulkheads different from baffles?
    1. Bulkheads have holes; baffles are solid
    2. They are the same thing
    3. Bulkheads are solid walls that divide a tank into smaller tanks; baffles have holes that let liquid through
    4. Bulkheads are only used in food tanks
    Show answer
    Bulkheads are solid walls that divide a tank into smaller tanks; baffles have holes that let liquid through

    Bulkheads divide a tank into separate compartments; baffles are bulkheads with holes that slow surge.

  10. Baffles help control forward-and-back surge. What kind of surge can still occur and cause a rollover?
    1. Side-to-side surge
    2. Upward surge
    3. No surge at all
    4. Only surge when the tank is full
    Show answer
    Side-to-side surge

    Baffles slow front-to-back surge, but side-to-side surge can still happen and roll the vehicle.

  11. Un-baffled tanks (smooth bore tanks) usually carry:
    1. Gasoline
    2. Sand
    3. Compressed gas
    4. Food products such as milk
    Show answer
    Food products such as milk

    Smooth bore tanks usually haul food products like milk.

  12. Why do food tanks usually have no baffles?
    1. Baffles are too heavy
    2. Sanitation rules forbid them because they make the tank hard to clean
    3. Food does not surge
    4. Baffles cost too much
    Show answer
    Sanitation rules forbid them because they make the tank hard to clean

    Sanitation regulations forbid baffles because the inside of the tank would be hard to clean.

  13. In a smooth bore tank, forward-and-back surge is:
    1. Weaker than in a baffled tank
    2. The same as in a baffled tank
    3. Very strong
    4. Impossible
    Show answer
    Very strong

    With nothing inside to slow the liquid, front-to-back surge is very strong.

  14. When should you be most careful driving a smooth bore tanker?
    1. When starting and stopping
    2. Only on flat roads
    3. Only at night
    4. Only when the tank is empty
    Show answer
    When starting and stopping

    Be extremely cautious with smooth bore tanks, especially when starting and stopping.

  15. What is outage?
    1. The time a tank is out of service
    2. A leak at the outlet valve
    3. Room left in a tank for the liquid to expand
    4. The weight of an empty tank
    Show answer
    Room left in a tank for the liquid to expand

    Outage is the space left so liquid can expand as it warms. Never load a cargo tank totally full.

  16. Why must you never load a cargo tank totally full?
    1. Full tanks surge more
    2. Liquids expand as they warm
    3. It wastes fuel
    4. Inspectors cannot see inside
    Show answer
    Liquids expand as they warm

    Liquids expand when they get warmer, so you must leave outage.

  17. All liquids need the same amount of outage.
    1. True
    2. False
    Show answer
    False

    False. Different liquids expand by different amounts, so they need different outage. You must know the requirement.

  18. Which is NOT one of the three things that decide how much liquid to load into a tank?
    1. How much the liquid will expand in transit
    2. The weight of the liquid
    3. Legal weight limits
    4. The color of the liquid
    Show answer
    The color of the liquid

    The amount depends on expansion, the liquid's weight and legal weight limits.

  19. Why are tanks carrying dense liquids such as some acids often only partly filled?
    1. To reduce surge
    2. A full tank could exceed legal weight limits
    3. Acids do not expand
    4. To make unloading faster
    Show answer
    A full tank could exceed legal weight limits

    A full tank of a dense liquid may be over legal weight limits.

  20. Because of high center of gravity and surge, a tanker driver should:
    1. Brake hard and late
    2. Change lanes quickly to avoid traffic
    3. Start, slow down and stop very smoothly
    4. Accelerate quickly from stops
    Show answer
    Start, slow down and stop very smoothly

    Drive smoothly: smooth starts, slowdowns, stops, turns and lane changes.

  21. To help control surge when coming to a stop, you should:
    1. Keep steady pressure on the brakes and not release too soon
    2. Pump the brakes quickly
    3. Release the brakes just before the truck stops
    4. Shift to neutral and coast
    Show answer
    Keep steady pressure on the brakes and not release too soon

    Keep steady brake pressure and do not release too soon when stopping.

  22. Which is a good way to control surge in a tanker?
    1. Follow closely so you can draft
    2. Brake at the last moment
    3. Steer quickly while braking
    4. Brake far ahead of a stop and increase following distance
    Show answer
    Brake far ahead of a stop and increase following distance

    Brake far in advance and leave more following distance.

  23. If you must make a quick stop in a tanker to avoid a crash, which braking method should you use?
    1. Lock all wheels and steer hard
    2. Controlled or stab braking
    3. Pull the parking brake
    4. Jake brake only
    Show answer
    Controlled or stab braking

    Use controlled or stab braking; quick steering while braking can roll the tanker.

  24. What may happen if you steer quickly while braking in a tank vehicle?
    1. The tank will empty
    2. The brakes will fade instantly
    3. The vehicle may roll over
    4. Nothing, if the tank is baffled
    Show answer
    The vehicle may roll over

    Quick steering while braking can cause a tanker to roll over.

  25. The correct way to drive a tanker through a curve is to:
    1. Slow down before the curve, then accelerate slightly through it
    2. Brake hard in the middle of the curve
    3. Enter at the posted speed and coast
    4. Speed up before the curve
    Show answer
    Slow down before the curve, then accelerate slightly through it

    Slow before the curve and accelerate slightly through it; the posted speed may be too fast.

  26. How do wet roads affect normal stopping distance?
    1. No change
    2. Add 10 percent
    3. Cut it in half
    4. Double it
    Show answer
    Double it

    Wet roads double the normal stopping distance.

  27. Compared with a full tank vehicle, an empty tank vehicle:
    1. Always stops in half the distance
    2. May take longer to stop
    3. Cannot skid
    4. Has no surge risk on curves
    Show answer
    May take longer to stop

    With less weight on the tires, an empty tanker can need more room to stop.

  28. On a tank trailer, if the drive wheels or trailer wheels begin to skid, the vehicle may:
    1. Stop faster
    2. Gain traction
    3. Jackknife
    4. Self-correct
    Show answer
    Jackknife

    A skid of the drive or trailer wheels can cause a jackknife.

  29. What three driving errors can cause a tanker to skid?
    1. Over-steering, over-accelerating and over-braking
    2. Using mirrors, signals and horn
    3. Driving smoothly, slowly and carefully
    4. Checking gauges, tires and lights
    Show answer
    Over-steering, over-accelerating and over-braking

    Do not over-steer, over-accelerate or over-brake. When a skid starts, act to restore traction.