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Pulling double and triple trailers safely

Doubles and Triples (T)3 minManual section 7.1

Doubles and triples are less stable than other commercial vehicles and more things can go wrong. This lesson covers the eight areas the Doubles/Triples test asks about: rollover, crack-the-whip, inspection, looking ahead, space, bad conditions, parking, and ABS on converter dollies.

Rollover and crack-the-whip (7.1.1–7.1.2)

To avoid a rollover, steer smoothly and take it slow through corners, on-ramps, off-ramps and curves.

Important: a speed that is safe on a curve for a straight truck or a single-trailer rig may be too fast for doubles or triples. Slow down more.

Doubles and triples are more likely to turn over than other combinations because of the crack-the-whip effect (rearward amplification). The last trailer is the one most likely to turn over. A quick steering move at the front becomes a big, fast swing at the back.

Inspect, look ahead, manage space (7.1.3–7.1.5)

  • Inspect completely. With two or three trailers there are more critical parts to check. Check them all (see the inspection lesson).
  • Look far ahead. A double or triple has to be driven very smoothly or it can roll over or jackknife. Seeing problems early lets you slow down or change lanes little by little.
  • Manage space. Doubles and triples need more space: they are longer, and they cannot be turned or stopped suddenly. Allow more following distance, make sure gaps are big enough before entering or crossing traffic, and be sure you are clear at the sides before changing lanes.

Adverse conditions and parking (7.1.6–7.1.7)

Be extra careful in bad weather, slippery conditions and mountain driving. You have greater length and more dead axles (axles with no power) to pull with your drive axles than other drivers. That means more chance of skids and loss of traction.

Parking: do not get into a spot you cannot pull straight through. Learn how parking lots are laid out so you avoid a long and difficult escape — backing doubles or triples is extremely hard.

ABS on converter dollies (7.1.8)

Converter dollies built on or after March 1, 1998, must have antilock brakes. These dollies have a yellow ABS lamp on the left side of the dolly. That lamp is how you know the dolly has ABS.

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: 7.1

Official CDL handbook · Utah ↗StatesOur editorial approach

Put it into practice

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

Preparing your questions…

Review questions with answers

  1. Compared with other commercial vehicles, doubles and triples are:
    1. More stable
    2. Less stable
    3. Equally stable
    4. More stable only when empty
    Show answer
    Less stable

    Doubles and triples are less stable, and more things can go wrong.

  2. A speed that is safe on a curve for a single-trailer rig:
    1. Is always safe for triples
    2. Is too slow for doubles
    3. May be too fast for doubles or triples
    4. Only matters at night
    Show answer
    May be too fast for doubles or triples

    A safe curve speed for a straight truck or single trailer may be too fast for doubles or triples.

  3. To prevent a double or triple from rolling over, you should:
    1. Steer gently and go slowly around corners, ramps and curves
    2. Use the trailer hand valve in curves
    3. Speed up before ramps
    4. Brake hard in the middle of curves
    Show answer
    Steer gently and go slowly around corners, ramps and curves

    Steer smoothly and take corners, ramps and curves slowly.

  4. In a set of triples, which trailer is most likely to turn over in a quick lane change?
    1. The first trailer
    2. The middle trailer
    3. All are equally likely
    4. The last trailer
    Show answer
    The last trailer

    Because of the crack-the-whip effect, the last trailer is most likely to turn over.

  5. Why are doubles and triples more likely to turn over than other combinations?
    1. They have more brakes
    2. The crack-the-whip effect
    3. They are always overloaded
    4. They have no ABS
    Show answer
    The crack-the-whip effect

    The crack-the-whip effect makes doubles and triples more likely to turn over.

  6. Why does inspecting doubles and triples take more care?
    1. They use different tires
    2. Only the tractor needs checking
    3. There are more critical parts to check
    4. Inspections are not required
    Show answer
    There are more critical parts to check

    With two or three trailers there are more critical parts; check them all.

  7. Why should you look far ahead when pulling doubles or triples?
    1. So you can slow down or change lanes gradually
    2. To read the DOT number
    3. To keep the ABS lamp off
    4. So you can drive faster
    Show answer
    So you can slow down or change lanes gradually

    They must be driven very smoothly; looking far ahead lets you make gradual changes.

  8. Why do doubles and triples need more space than other commercial vehicles?
    1. They have wider mirrors
    2. They use more fuel
    3. They are required to drive in the left lane
    4. They are longer and cannot be turned or stopped suddenly
    Show answer
    They are longer and cannot be turned or stopped suddenly

    They are longer and can't turn or stop suddenly, so allow more following distance and bigger gaps.

  9. Before changing lanes with doubles, you should:
    1. Signal and move quickly
    2. Be certain you are clear at the sides
    3. Use the trailer hand valve
    4. Speed up by 10 mph
    Show answer
    Be certain you are clear at the sides

    Be certain you are clear at the sides before changing lanes, and use large gaps when entering or crossing traffic.

  10. Why are doubles and triples more likely to skid in bad weather and mountain driving?
    1. Their brakes are weaker
    2. They cannot use tire chains
    3. They have more length and more dead axles to pull with the drive axles
    4. They cannot use the foot brake
    Show answer
    They have more length and more dead axles to pull with the drive axles

    Greater length and more dead axles mean more chance of skids and loss of traction.

  11. When parking doubles or triples, you should:
    1. Avoid spots you cannot pull straight through
    2. Back into the tightest spot available
    3. Park on any slope
    4. Park across several car spaces
    Show answer
    Avoid spots you cannot pull straight through

    Do not get into a spot you cannot pull straight through; know how lots are laid out.

  12. Converter dollies built on or after March 1, 1998 must have:
    1. Spring brakes
    2. A sliding fifth wheel
    3. A spare tire
    4. Antilock brakes
    Show answer
    Antilock brakes

    Converter dollies made from March 1, 1998 on must have ABS.

  13. How can you tell a converter dolly has antilock brakes?
    1. It has red glad hands
    2. It has a yellow lamp on the left side
    3. It has a blue lamp on the right side
    4. It has two axles
    Show answer
    It has a yellow lamp on the left side

    These dollies have a yellow ABS lamp on the left side.

  14. True or False: When pulling doubles or triples, you must steer gently because of the crack-the-whip effect.
    1. True
    2. False
    Show answer
    True

    True. Gentle steering keeps the rear trailer from whipping and rolling over.