Doubles / Triples questions 81–100

Part of the 100-question Doubles / Triples pool. Each answer cites the FMCSA CDL Manual or the federal rule it comes from.

Practise these as a timed set

What must be verified about each trailer's doors before departure?
- ARemoved to reduce weight
- BLeft ajar for ventilation
- CClosed and latched securelyCorrect
- DSealed with tape only
Each trailer door must be closed and latched securely before departure so it remains secured while the combination is moving.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
How should the load be arranged within each trailer of a set?
- ALoose so it can settle
- BConcentrated at the rear of each
- CStacked entirely at the front
- DBalanced and secured in each unitCorrect
Each trailer handles as its own vehicle, so a badly loaded unit misbehaves regardless of how the others are packed.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
What should a driver do if a trailer in the set begins to skid?
- AEase off and stop braking hardCorrect
- BBrake harder to straighten it
- CAccelerate to pull it back
- DSteer sharply the other way
Locked wheels cannot steer, so releasing the brakes lets the trailer regain grip and fall back into line.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
Why must a driver watch all mirrors through a turn with triples?
- AThe mirrors fog in a turn
- BEach trailer tracks a different pathCorrect
- CThe dolly signals from the mirror
- DOnly the last trailer matters
Each trailer can follow a different path through a turn, so the driver must monitor all mirrors for off-tracking and clearance.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
What should a driver check about the dolly's tires specifically?
- AThe tread pattern is decorative
- BThe brand matches the trailer
- CMatched size and proper inflationCorrect
- DThey are softer than the others
Check dolly tires for matching size and proper inflation because tire condition affects stability and braking.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
Why must a set of doubles be checked for air leaks after coupling?
- ALeaks change the trailer order
- BThe engine idles rough with leaks
- CThe lights dim when air leaks
- DAir leaks can reduce the pressure needed for the trailer brakesCorrect
The emergency line supplies the trailer air tanks and controls the emergency brakes. A leak can reduce available pressure and cause the trailer brakes not to work correctly, so check every connection after coupling.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
How should a driver approach a curve with a loaded set of doubles?
- ABrake through the whole curve
- BSlow before entering itCorrect
- CAccelerate to settle the trailers
- DTake the curve wide and fast
Speed is lost before the turn because braking mid-curve is what pushes the rear trailer's wheels off the pavement.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
What should a driver do if a set must be stopped on the roadside?
- APark across the shoulder line
- BStop in the nearest travel lane
- CGet fully clear and set warnings outCorrect
- DLeave the set unmarked
When a set is stopped on the roadside, get the vehicle as far clear of traffic as safely possible and place the required warning devices.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
Why should a driver check the dolly's electrical cable at coupling?
- AIt grounds the air system
- BIt charges the dolly's brakes
- CIt powers the fifth wheel lock
- DThe rear trailer's lights depend on itCorrect
Power runs through each unit in turn, so a damaged cable at the dolly leaves everything behind it dark.
FMCSA 2005 CDL Testing System Model Manual (July 2014) · Section 7.3.1 — Inspecting Doubles and Triples; Oregon Manual § 7.3.1 cross-check · verified 2026-09-19
Before changing lanes while pulling doubles or triples, what must the driver be certain of?
- AThat the vehicle is clear at the sidesCorrect
- BThat the mirrors show no blind spots
- CThat the last trailer is empty
- DThat the lane ahead is one block clear
The manual says doubles and triples need more space, and to be certain you are clear at the sides before changing lanes. Mirrors also have blind spots they cannot show.
FMCSA Commercial Driver's License Manual (July 2014) · Section 7.1 Pulling Double/Triple Trailers (space); 2.4.2 Seeing to the Sides and Rear · verified 2026-09-25
Which endorsement is required to pull more than one trailer?
- AThe P endorsement
- BThe N endorsement
- CThe T endorsementCorrect
- DThe H endorsement
T covers doubles and triples and is earned by a knowledge test with no separate skills test. N is tank vehicles, P is passengers and H is hazardous materials.
49 CFR 383 · Section 7: Doubles and Triples · verified 2026-09-08
Where is the ABS lamp on a converter dolly built on or after March 1, 1998?
- AYellow lamp on the left side of the dollyCorrect
- BRed lamp on the right side of the dolly
- CGreen lamp on the dolly tongue
- DIn the tractor cab only
Dollies built on or after March 1, 1998 must have antilock brakes and have a yellow lamp on the left side of the dolly.
Oregon DMV Commercial Driver Manual · 7.1.8 Antilock Braking Systems on Converter Dollies · verified 2026-09-25
Why must the emergency line be charged before the rear trailer of a set will move?
- AIts spring brakes stay applied without airCorrect
- BIts service brakes will otherwise drag
- CThe dolly cannot steer without line pressure
- DThe lights will not work until it charges
Air pressure in the emergency line is what holds the spring brakes off. Until the rear unit is charged, those brakes remain locked and pulling against them can damage the drivetrain.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
What does a rearward amplification of 3.5 for triples mean?
- AThe last trailer can be rolled 3.5 times as easily as a five-axle tractorCorrect
- BThe last trailer swings 3.5 feet wider
- CTriples need 3.5 times the following distance
- DTriples brake 3.5 times faster
The manual says you can roll the last trailer of triples 3.5 times as easily as a five-axle tractor.
FMCSA Commercial Driver's License Manual (July 2014) · 6.1.2 Steer Gently / Figure 6.1 rearward amplification · verified 2026-09-25
How should a driver plan to turn a set of doubles around?
- AUncouple and turn each trailer separately
- BReverse the set into a side street
- CRoute the trip so backing is never neededCorrect
- DUse the shoulder to swing through
Backing doubles is difficult because the dolly articulates independently. Plan the route so backing is not needed when possible.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
In a combination with more than one trailer, which wheels off-track the most in a turn?
- AThe rear wheels of the last trailerCorrect
- BThe rear wheels of the tractor
- CThe front wheels of the tractor
- DThe dolly steering axle
Longer vehicles off-track more; with more than one trailer, the rear wheels of the last trailer off-track the most.
FMCSA Commercial Driver's License Manual (July 2014) · 6.1.x Off-tracking (Section 6.1 Driving Combination Vehicles Safely) · verified 2026-09-25
On a set of triples, what happens if the shut-off valves are left closed on a trailer ahead of the last one?
- AAir builds faster in the front trailer's tank
- BEvery trailer behind it loses its air supplyCorrect
- CThe low-air warning takes longer to clear
- DThe dolly's brakes release before the trailers
Shut-off valves stay open on every trailer except the very last one, where they are closed. Leaving one closed part-way down the set cuts the supply to everything behind it, so those spring brakes stay applied.
FMCSA Commercial Driver License Manual · Section 7: Doubles and Triples · verified 2026-09-08
In the doubles combination shown, a quick steering move affects the rear trailer:
- AOnly in reverse
- BLeast — it dampens motion
- CNot at all
- DMost severely because of rearward amplificationCorrect
Rearward amplification makes each trailing unit swing wider than the one ahead; abrupt steering that the tractor tolerates can flip the rear trailer of doubles.
FMCSA Commercial Driver's License Manual · Section 7 - Doubles and Triples · verified 2026-09-08
The rear trailer of a set has no spring brakes. Before coupling it, how can you set its emergency brakes?
- AConnect the emergency line, charge the trailer air tank, then disconnect the emergency lineCorrect
- BOpen the trailer air tank petcock and leave it open
- CConnect only the service line and hold the trailer hand valve on
- DLeave the air lines disconnected; the brakes apply without any air
Drive the tractor close, connect the emergency line, charge the trailer air tank, and disconnect the emergency line. This sets the trailer emergency brakes if the slack adjusters are correctly adjusted. Chock the wheels if you have any doubt about the brakes.
Oregon DMV Commercial Driver Manual · 7.2.1 Coupling Twin Trailers: Secure Second (Rear) Trailer · verified 2026-09-29
You have backed the converter dolly under the rear trailer of a set of doubles. Before you test the coupling, what should you do with that trailer's landing gear?
- ALeave it resting on the ground so it can hold the trailer steady during the pull test
- BRaise it slightly off the ground to prevent damage if the trailer movesCorrect
- CCrank it all the way up before you test and inspect the coupling
- DLower it firmly to take some weight off the dolly before the test
After you back the dolly under the rear trailer, raise the landing gear slightly off the ground. This prevents damage if the trailer moves. Then test the coupling by pulling against the second trailer's kingpin and check it visually. Raise the landing gear completely only after you connect the safety chains, air hoses and light cords, close the dolly air tank petcock and the second trailer's rear shut-off valves, and open the shut-off valves at the rear of the first trailer.
Oregon DMV Commercial Driver Manual · 7.2.1 Coupling Twin Trailers: Connect Converter Dolly to Rear Trailer · verified 2026-09-30

Doubles / Triples questions 81–100

What must be verified about each trailer's doors before departure?

How should the load be arranged within each trailer of a set?

What should a driver do if a trailer in the set begins to skid?

Why must a driver watch all mirrors through a turn with triples?

What should a driver check about the dolly's tires specifically?

Why must a set of doubles be checked for air leaks after coupling?

How should a driver approach a curve with a loaded set of doubles?

What should a driver do if a set must be stopped on the roadside?

Why should a driver check the dolly's electrical cable at coupling?

Before changing lanes while pulling doubles or triples, what must the driver be certain of?

Which endorsement is required to pull more than one trailer?

Where is the ABS lamp on a converter dolly built on or after March 1, 1998?

Why must the emergency line be charged before the rear trailer of a set will move?

What does a rearward amplification of 3.5 for triples mean?

How should a driver plan to turn a set of doubles around?

In a combination with more than one trailer, which wheels off-track the most in a turn?

On a set of triples, what happens if the shut-off valves are left closed on a trailer ahead of the last one?

In the doubles combination shown, a quick steering move affects the rear trailer:

The rear trailer of a set has no spring brakes. Before coupling it, how can you set its emergency brakes?

You have backed the converter dolly under the rear trailer of a set of doubles. Before you test the coupling, what should you do with that trailer's landing gear?