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FL CDL Combination study materials
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FL CDL Combination practice questions

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Question 1Combination Vehicles (CDL Class A knowledge area)

In a combination vehicle, what happens to the total stopping distance compared with a bobtail tractor?

  1. It stays about the same because the trailer brakes do most of the work
  2. It decreases until the trailer is fully loaded
  3. It decreases because more tires are on the road
  4. It increases because of the added weight and the trailer's push Correct answer.

Explanation

Adding a loaded trailer increases the gross weight that the brakes must stop, and the trailer's momentum pushes against the tractor during braking. The result is a longer stopping distance than a bobtail tractor needs at the same speed. Drivers must open following distance to compensate.

Question 2Combination Vehicles (CDL Class A knowledge area)

Before backing a tractor-trailer, the safest first step is to:

  1. Back quickly to reduce the time in a blind spot
  2. Rely on the backup alarm to warn nearby pedestrians
  3. Back toward the passenger side to use the convex mirrors
  4. Back toward the driver's side so the trailer stays visible in the mirrors Correct answer.

Explanation

Backing toward the driver's side keeps the trailer visible in the left mirror and lets the driver see the rear of the trailer with less blind area. Backing toward the right creates a much larger blind spot. Use a helper whenever possible and get out to check the area first.

Question 3Combination Vehicles (CDL Class A knowledge area)

When coupling a tractor to a trailer, which check confirms the fifth wheel has locked onto the kingpin?

  1. The landing gear has been fully raised
  2. The air lines are connected and charged
  3. The trailer lights come on when the pigtail is connected
  4. A gentle pull with the tractor in a low gear shows the trailer moving with it Correct answer.

Explanation

After coupling, a tug test with the tractor in a low gear while the trailer brakes are set verifies the jaws have closed around the kingpin. If the trailer does not move with the tractor, the coupling is secure. Visual inspection of the jaws from under the trailer is also required.

Question 4Combination Vehicles (CDL Class A knowledge area)

Which condition makes a tractor-trailer most likely to roll over during a turn?

  1. The trailer has new tires and the brakes are adjusted
  2. The load is centered and the speed is low
  3. The center of gravity is high and the speed in the turn is excessive Correct answer.
  4. The trailer is empty and the roads are wet

Explanation

Rollover risk rises with a high center of gravity, such as a loaded tanker or a tall dry van, combined with too much speed in a curve. The centrifugal force pushes the top of the load outward until the tires lift. Slowing before the curve is the driver's main defense.

Question 5Combination Vehicles (CDL Class A knowledge area)

What is the purpose of the trailer hand valve (trolley valve) in a combination vehicle?

  1. To bypass the tractor's service brakes during normal driving
  2. To release the tractor parking brakes
  3. To apply only the trailer brakes for a test or to hold the trailer Correct answer.
  4. To set the trailer spring brakes permanently

Explanation

The hand valve lets the driver apply the trailer service brakes independently, which is useful for checking that trailer brakes work and for holding a trailer during coupling. It must not be used for normal stopping because it does not apply the tractor brakes. Normal braking is done with the foot pedal.

Question 6Combination Vehicles (CDL Class A knowledge area)

The air line couplers between tractor and trailer are color-coded. Which statement about them is correct?

  1. Both lines are black and interchangeable
  2. The emergency line is green and the service line is yellow
  3. The lines are color-coded only on older trailers
  4. The service line is blue and the emergency line is red Correct answer.

Explanation

On standard combinations the service (control) line is blue and the emergency (supply) line is red. The colors help the driver connect them correctly during coupling. Cross-connecting them causes brake problems, so matching colors and checking the glad hands is part of the pre-trip.

Question 7Combination Vehicles (CDL Class A knowledge area)

When a combination vehicle's air pressure drops to the warning level, what should the driver expect and do?

  1. The trailer brakes release automatically and the driver continues
  2. The tractor engine shuts down to protect the compressor
  3. A buzzer and light warn the driver, who should stop and find the leak Correct answer.
  4. The spring brakes apply at full pressure immediately

Explanation

A low-air warning buzzer and light activate before pressure falls into the danger range. The driver should stop safely and locate the leak or fault before continuing. Ignoring the warning can lead to automatic spring brake application and loss of control.

Question 8Combination Vehicles (CDL Class A knowledge area)

Why should a driver check the trailer's rear wheels and mud flaps before a trip?

  1. To make sure nothing will strike the tires or be thrown into traffic Correct answer.
  2. To confirm the trailer is registered in the correct state
  3. To check that the trailer doors are locked
  4. To verify the trailer's cargo weight

Explanation

Loose mud flaps, debris, or damaged tires at the rear of the trailer can cause a blowout or be thrown into following traffic. A walk-around inspection catches these problems before they become roadside emergencies. The rear wheels also carry a large share of the trailer's weight and need visual inspection.

Question 9Combination Vehicles (CDL Class A knowledge area)

A driver is uncoupling a trailer on level ground. In what order should the steps be performed?

  1. Lower the landing gear, pull the fifth wheel release, then disconnect the air lines
  2. Disconnect the air lines, pull the fifth wheel release, then lower the landing gear
  3. Lower the landing gear, disconnect the air lines, then pull the fifth wheel release Correct answer.
  4. Pull the fifth wheel release, lower the landing gear, then disconnect the air lines

Explanation

The landing gear must be lowered and firm on the ground before the tractor is released from the trailer. The air lines and electrical cord are disconnected before the tractor pulls away. Pulling the release before the gear is down lets the trailer drop.

Question 10Combination Vehicles (CDL Class A knowledge area)

While driving a loaded combination, the driver notices the trailer beginning to swing side to side. The best response is to:

  1. Accelerate firmly to pull the trailer straight
  2. Release the accelerator and steer straight while slowing gradually Correct answer.
  3. Brake hard to stop the sway
  4. Steer sharply against the swing to correct it

Explanation

Trailer sway is best controlled by easing off the accelerator and holding the steering steady so the combination slows and the oscillation dies out. Hard braking or sharp steering can worsen the sway or cause a jackknife. Once stopped, the driver should check the load and tires.

Question 11Combination Vehicles (CDL Class A knowledge area)

What is off-tracking, and how should a driver allow for it when turning right?

  1. The tractor's front wheels follow a wider path, so the driver hugs the curb
  2. The trailer brakes drag in the turn, so the driver speeds up
  3. The trailer's rear wheels follow a shorter path, so the driver swings wide before the turn Correct answer.
  4. The fifth wheel shifts the load, so the driver checks the weight

Explanation

Off-tracking means the rear wheels of the trailer cut inside the path of the tractor's front wheels. In a right turn the driver must swing wide enough that the trailer's rear tires clear the curb and any objects near it. Watching the trailer's rear wheels in the right mirror helps confirm clearance.

Question 12Combination Vehicles (CDL Class A knowledge area)

During the pre-trip air brake test on a combination, the driver builds pressure and then shuts off the engine. What is being checked when the driver observes the gauges for one minute without touching the brake pedal?

  1. The compressor's cut-out pressure
  2. The slack adjuster travel
  3. The trailer spring brake release time
  4. The static leak rate of the charged system Correct answer.

Explanation

With the system charged and the engine off, watching the gauges for one minute reveals whether air is leaking from the charged system. A steady drop beyond the allowed rate indicates a leak that must be fixed. A separate check applies the brakes and watches for a leak in the applied system.

Question 13Combination Vehicles (CDL Class A knowledge area)

A combination vehicle has a trailer with spring brakes. If the emergency (supply) line is disconnected while the trailer is parked, what happens?

  1. The trailer service brakes release
  2. The trailer spring brakes apply automatically Correct answer.
  3. Nothing changes because the trailer is already parked
  4. The trailer brakes stay released until the hand valve is pulled

Explanation

The emergency line supplies the air that holds the trailer spring brakes released. If it is disconnected or loses pressure, the springs apply the brakes automatically. This is a fail-safe feature, but it also means the trailer will not roll until air is restored.

Question 14Combination Vehicles (CDL Class A knowledge area)

Why must the trailer's spring brakes be set before the driver performs the tug test during coupling?

  1. To charge the trailer air tanks faster
  2. To keep the trailer from rolling when the tractor pulls against it Correct answer.
  3. To release the tractor's parking brake
  4. To align the kingpin with the fifth wheel

Explanation

With the trailer brakes set, a gentle pull by the tractor will not move the trailer if the coupling is secure. If the jaws are not locked, the trailer will move or the tractor will pull free, revealing the problem. Setting the brakes also prevents the trailer from rolling during the test.

Question 15Combination Vehicles (CDL Class A knowledge area)

A driver must make a tight left turn with a long combination. Which practice best reduces the chance of striking a vehicle on the right?

  1. Begin the turn close to the left curb and accelerate through it
  2. Turn from the far right lane to give the trailer more room
  3. Rely on the convex mirrors alone for clearance
  4. Turn from the proper lane but watch the right side and allow for the trailer cutting the corner Correct answer.

Explanation

A long combination cuts the corner, so the driver should start from the correct lane and monitor the right side for the trailer's path. Turning from the wrong lane confuses other drivers and creates conflicts. Mirror checks plus a wide, slow turn reduce the risk of sideswiping.

Question 16Combination Vehicles (CDL Class A knowledge area)

What does the tractor protection valve do in a combination vehicle?

  1. It locks the fifth wheel jaws during braking
  2. It balances the load between tractor and trailer axles
  3. It limits tractor speed when the trailer is empty
  4. It closes the tractor's air supply to the trailer when pressure drops, protecting the tractor's brakes Correct answer.

Explanation

The tractor protection valve senses low air pressure and closes off the trailer supply, keeping the tractor's remaining air for its own brakes. It also helps apply the trailer spring brakes when the lines separate. This protects the tractor from losing all its air through a trailer leak.

Question 17Combination Vehicles (CDL Class A knowledge area)

A driver is inspecting a combination and finds the trailer's brake chambers have excessive pushrod travel. What is the correct action?

  1. Pump the brakes several times to take up the slack
  2. Increase air pressure at the compressor
  3. Continue the trip and note it on the post-trip report
  4. Adjust the slack adjusters or have the brakes repaired before driving Correct answer.

Explanation

Excessive pushrod travel means the brakes are out of adjustment and may not apply with full force. The defect must be corrected, either by adjusting automatic slack adjusters properly or by having the brakes serviced, before the vehicle is driven. Out-of-service criteria apply to brake adjustment.

Question 18Combination Vehicles (CDL Class A knowledge area)

When driving a combination vehicle on a downgrade, why should the driver select a low gear before starting down?

  1. To increase the trailer's air pressure
  2. To use engine braking and avoid overheating the service brakes Correct answer.
  3. To reduce the effect of off-tracking
  4. To keep the engine cool during the descent

Explanation

A low gear lets engine compression help control speed, reducing reliance on the service brakes. Continuous brake use on a long downgrade overheats the drums and causes brake fade. The gear should be chosen before the descent, since shifting may be difficult once the speed builds.

Question 19Combination Vehicles (CDL Class A knowledge area)

A driver is backing a trailer into a dock on the right side. The trailer begins to turn the wrong way. What should the driver do?

  1. Accelerate to bring the trailer around faster
  2. Pull forward to straighten the combination and start over Correct answer.
  3. Turn the steering wheel further in the same direction
  4. Continue backing and correct once the trailer is aligned

Explanation

If the trailer goes the wrong way, continuing to back usually makes the error worse. Pulling forward repositions the tractor and trailer so the driver can restart the maneuver with better alignment. Patience and small steering inputs make docking safer.

Question 20Combination Vehicles (CDL Class A knowledge area)

A driver must cross railroad tracks in a combination vehicle. Which action is most appropriate?

  1. Stop, then cross without shifting gears if the path is clear Correct answer.
  2. Stop on the tracks to check for a clear exit
  3. Cross slowly with the trailer brakes applied
  4. Shift to a higher gear while crossing to save fuel

Explanation

The driver should stop when required, confirm there is room on the far side, and then cross without shifting so the vehicle does not stall on the tracks. Never stop on the tracks. A combination needs more time and distance to clear the crossing.

Question 21Combination Vehicles (CDL Class A knowledge area)

A driver notices the trailer brakes are not releasing after the supply line is reconnected and pressure builds. Which is the most likely cause to investigate first?

  1. The trailer's spring brake chambers are defective
  2. The service line and emergency line are cross-connected Correct answer.
  3. The tractor's engine is idling too low
  4. The trailer is overloaded on the rear axle

Explanation

If the blue service line and red emergency line are swapped, the trailer may not receive supply air correctly and the spring brakes can stay applied. Matching the color-coded glad hands is part of proper coupling. If the lines are correct, the driver should look at the relay valve and chambers.

Question 22Combination Vehicles (CDL Class A knowledge area)

A driver must stop a combination quickly on a dry, straight road. Which technique gives the best control?

  1. Lock the tractor brakes and let the trailer push the combination straight
  2. Apply the trailer hand valve fully and steer to the shoulder
  3. Brake firmly with the foot pedal while keeping the wheels rolling and steering steady Correct answer.
  4. Downshift and rely on engine braking alone

Explanation

Firm, progressive braking with the foot pedal applies both tractor and trailer service brakes together, which is the balanced way to stop a combination. Keeping the wheels rolling preserves steering control and reduces the chance of a jackknife. The hand valve applies only trailer brakes and can cause the trailer to swing.

Question 23Combination Vehicles (CDL Class A knowledge area)

A driver is checking the fifth wheel after coupling and sees a gap between the apron and the fifth wheel plate. What does this most likely indicate?

  1. The trailer is too heavy for the tractor
  2. The air lines are connected backwards
  3. The fifth wheel is properly seated
  4. The kingpin is not fully locked in the jaws Correct answer.

Explanation

A visible gap between the trailer apron and the fifth wheel plate suggests the kingpin is riding high and the jaws have not closed around it. The driver should not move the vehicle until the coupling is corrected. A proper coupling shows the plate tight against the apron with the jaws closed.

Question 24Combination Vehicles (CDL Class A knowledge area)

A driver is descending a long grade when the service brakes begin to fade. What is the best course of action?

  1. Use the trailer hand valve to slow the combination
  2. Pump the brakes rapidly to restore them
  3. Pull over at the first safe place and let the brakes cool Correct answer.
  4. Shift to a lower gear and continue at the same speed

Explanation

Brake fade means the drums are overheated and braking force is dropping; continuing downhill can lead to a runaway. The driver should stop at the first safe spot and let the brakes cool. Shifting down or using the hand valve does not fix overheated brakes.

Question 25Combination Vehicles (CDL Class A knowledge area)

A combination vehicle's trailer is loaded with a heavy cargo placed at the rear of the trailer. How does this affect handling compared with the same weight placed over the trailer axles?

  1. It reduces weight on the drive axles and can cause the tractor to lose traction Correct answer.
  2. It has no effect as long as the total weight is under the limit
  3. It increases the load on the steering axle and improves steering
  4. It improves traction and shortens stopping distance

Explanation

Weight placed at the rear of the trailer acts like a lever that lifts weight off the tractor's drive axles. Less weight on the drive axles reduces traction for starting and can cause wheel spin or poor control. Loads should be distributed so the drive axles keep adequate weight.

Question 26Combination Vehicles (CDL Class A knowledge area)

In a combination vehicle, what is the primary purpose of the fifth wheel coupling?

  1. To lock the kingpin and transfer trailer weight to the tractor frame while allowing pivoting Correct answer.
  2. To keep the trailer air lines routed away from the exhaust
  3. To supply electrical power from the tractor to the trailer lights
  4. To apply the trailer service brakes through a mechanical linkage

Explanation

The fifth wheel is a horseshoe-shaped plate on the tractor that locks around the trailer kingpin. It carries the vertical load of the trailer's front end and forms the pivot point for turning. Air and electrical connections are handled by separate lines and cables.

Question 27Combination Vehicles (CDL Class A knowledge area)

What is the correct function of the glad hands on a combination vehicle?

  1. They couple the tractor and trailer air lines together Correct answer.
  2. They lock the landing gear in the raised position
  3. They secure the trailer doors during transit
  4. They hold the electrical cord in its socket

Explanation

Glad hands are the coupler fittings at the ends of the tractor and trailer air lines. They join the service and emergency lines so air can flow between the units. Couplers must be locked together and sealed with good rubber gaskets to prevent leaks.

Question 28Combination Vehicles (CDL Class A knowledge area)

When coupling a tractor to a trailer, at what height should the trailer apron sit relative to the fifth wheel before backing under?

  1. Well above the fifth wheel so the tractor can pass completely under
  2. Exactly level with the top of the tractor frame rails
  3. Slightly lower than the fifth wheel so the plate lifts the trailer
  4. Just at or slightly below the fifth wheel so the plate contacts the trailer firmly Correct answer.

Explanation

The trailer should be raised or lowered so its apron is just at or slightly below the fifth wheel height. Backing under then lets the plate contact the trailer and lift it slightly as the kingpin locks. If the trailer is too high, the tractor can strike the nose; if too low, the kingpin may hit the plate.

Question 29Combination Vehicles (CDL Class A knowledge area)

After coupling, which check confirms the kingpin is actually locked in the fifth wheel?

  1. Sound the horn and watch the trailer marker lights
  2. Check that the electrical cord hangs with slack
  3. Pull the tractor gently forward against the locked trailer brakes and watch for movement Correct answer.
  4. Release the trailer spring brakes and roll forward

Explanation

The tug test is performed with the trailer spring brakes set: the driver pulls gently forward and feels the tractor resist. If the kingpin is locked, the tractor will not pull out from under the trailer. Any forward creep means the coupling is not secure and must be corrected.

Question 30Combination Vehicles (CDL Class A knowledge area)

What is the purpose of the trailer landing gear during uncoupling?

  1. To support the front of the trailer once it is released from the tractor Correct answer.
  2. To raise the trailer so it can roll away from the tractor
  3. To lock the trailer brakes when air pressure is lost
  4. To level the trailer on a slope while driving

Explanation

Landing gear supports the trailer's front end after the tractor is removed. The legs must be lowered to firm ground and cranked down until they take the trailer's weight before the fifth wheel is released. They are not a lifting device for moving the trailer.

Question 31Combination Vehicles (CDL Class A knowledge area)

A driver is coupling and finds the trailer kingpin is slightly too far forward, so the tractor's fifth wheel jaws close on nothing. What is the safest correction?

  1. Drive forward and back under again with more speed to force the jaws closed
  2. Pull the release handle and manually close the jaws around the kingpin
  3. Lower the trailer onto the fifth wheel without moving the tractor
  4. Pull straight forward until the kingpin clears, then back under squarely Correct answer.

Explanation

If the jaws closed without the kingpin, the tractor must be pulled forward until the kingpin is clear of the plate. Then the driver backs under again squarely so the kingpin enters the throat and the jaws lock. Forcing the jaws or speeding up risks damage and a false coupling.

Question 32Combination Vehicles (CDL Class A knowledge area)

Why is it important to chock the trailer wheels before uncoupling on a slope?

  1. Chocks align the kingpin for the next coupling
  2. Chocks keep the landing gear from sinking into soft ground
  3. Chocks prevent the trailer from rolling if the spring brakes release or fail Correct answer.
  4. Chocks keep the trailer air lines from dragging on the ground

Explanation

On a grade, a released or faulty spring brake could let the trailer roll once the tractor is removed. Wheel chocks provide a positive mechanical stop independent of the air system. They should be placed on the downhill side before the trailer is released.

Question 33Combination Vehicles (CDL Class A knowledge area)

What does the service line on a combination vehicle control?

  1. The trailer spring brakes, which apply when the line loses pressure
  2. The trailer's electrical turn signals
  3. The tractor's parking brake only
  4. The trailer service brakes, which apply when the driver presses the brake pedal Correct answer.

Explanation

The service line carries the air pressure the driver applies through the foot valve to the trailer service brakes. The emergency line is the one that releases spring brakes when charged and applies them when pressure is lost. Keeping the two functions separate is essential to diagnosing brake problems.

Question 34Combination Vehicles (CDL Class A knowledge area)

A driver connects the air lines with the couplers crossed, so the service line feeds the emergency glad hand. What symptom is most likely when the brakes are released?

  1. The landing gear will not crank up
  2. The fifth wheel will not lock
  3. The trailer lights will not illuminate
  4. The trailer brakes will not release properly and pressure readings behave abnormally Correct answer.

Explanation

Crossed lines send supply air to the wrong circuit, so the trailer spring brakes may stay applied or release incorrectly. Gauge behavior and brake response will be abnormal until the lines are reconnected correctly. The lines are color-coded and should be matched, then checked by operation.

Question 35Combination Vehicles (CDL Class A knowledge area)

In a combination vehicle, what is the role of the trailer hand valve (trolley valve)?

  1. To set the tractor parking brake during pre-trip inspection
  2. To control the trailer's electrical marker lights
  3. To apply only the trailer brakes for a test or to straighten a skid Correct answer.
  4. To release the trailer spring brakes after coupling

Explanation

The hand valve applies air to the trailer service brakes only, independent of the tractor brakes. Drivers use it to check trailer brake operation and, in some situations, to help straighten a jackknife. It is not a parking brake and should not be used as one.

Question 36Combination Vehicles (CDL Class A knowledge area)

A driver must check that the trailer brakes hold when only the trailer spring brakes are applied. Which method is correct?

  1. Set the tractor parking brake and idle the engine
  2. Build pressure, shut off the engine, then pull the trailer air supply valve to apply the spring brakes and try to move Correct answer.
  3. Build pressure, release all brakes, and roll forward at low speed
  4. Disconnect the electrical cord and watch the lights

Explanation

With the system charged and the engine off, pulling the trailer air supply valve exhausts the emergency line and applies the trailer spring brakes. Gently attempting to move then shows whether the trailer brakes hold. This isolates the trailer brakes from the tractor brakes.

Question 37Combination Vehicles (CDL Class A knowledge area)

What is a jackknife in a combination vehicle?

  1. A load that has shifted to one side of the trailer
  2. A trailer that has lost a wheel and dragged on the axle
  3. A tractor and trailer folding against each other at an angle too sharp to recover Correct answer.
  4. A fifth wheel that has unlocked but stayed seated

Explanation

A jackknife occurs when the tractor and trailer articulate to an angle so sharp that the combination folds. It usually results from locking the drive or trailer brakes on a slippery surface or from oversteering. Prevention includes smooth braking and steering inputs and proper brake balance.

Question 38Combination Vehicles (CDL Class A knowledge area)

Why does a lightly loaded trailer tend to lock its brakes more easily than a fully loaded one?

  1. There is less weight pressing the tires to the road, so less brake force is needed to skid them Correct answer.
  2. The service line carries higher pressure when the trailer is empty
  3. The trailer's spring brakes apply automatically when the load is light
  4. Light trailers have larger brake chambers that apply more force

Explanation

Braking force is limited by tire-to-road friction, which depends on weight on the tires. An empty or lightly loaded trailer has less weight, so the same brake application can exceed available traction and lock the wheels. This is why drivers brake gently with empty trailers and why load-proportioning valves exist.

Question 39Combination Vehicles (CDL Class A knowledge area)

A driver is about towing a trailer whose brakes have been deliberately disabled. What is the correct understanding?

  1. It is acceptable if the trailer is empty and the trip is short
  2. It is allowed if the driver reduces speed by 10 mph
  3. It is allowed if the tractor has ABS
  4. It is illegal and unsafe because the combination cannot stop within required distance Correct answer.

Explanation

Federal and state rules require trailers over a certain weight to have working brakes, and a combination without trailer brakes cannot meet stopping-distance requirements. Operating with disabled trailer brakes is both a violation and a serious safety hazard. The trailer must be repaired before it is moved on the road.

Question 40Combination Vehicles (CDL Class A knowledge area)

What is the purpose of the tractor protection valve in relation to a trailer breakaway?

  1. It shuts off the tractor engine if the trailer rolls away
  2. It applies the tractor parking brake when the trailer is disconnected
  3. It automatically closes the tractor's air lines when trailer pressure drops, protecting the tractor's air supply Correct answer.
  4. It locks the fifth wheel jaws when the trailer separates

Explanation

The tractor protection valve senses a large pressure loss, such as a separated or broken trailer line, and closes off the tractor's air supply. This keeps enough air in the tractor to stop it. The trailer's spring brakes apply from its own loss of supply air.

Question 41Combination Vehicles (CDL Class A knowledge area)

Why must the trailer's emergency (supply) line be connected before the service line when hooking up?

  1. It is the only line that carries air to the trailer
  2. It prevents the electrical cord from shorting out
  3. It ensures the trailer lights work before the brakes
  4. It charges the trailer system so the spring brakes release and the trailer can be moved safely Correct answer.

Explanation

The emergency line supplies air that releases the trailer spring brakes and charges the trailer tanks. Connecting it first lets the trailer build pressure so the brakes release in a controlled way. The service line is connected afterward, and both couplers are checked for leaks.

Question 42Combination Vehicles (CDL Class A knowledge area)

A combination vehicle's trailer has a sliding tandem axle. What is the main reason a driver adjusts its position?

  1. To raise the trailer's landing gear off the ground
  2. To change the trailer's color-coded air line routing
  3. To increase the trailer's brake chamber size
  4. To shift weight between the tractor drive axles and the trailer axles for legal and handling balance Correct answer.

Explanation

Sliding the trailer tandems changes how much weight rests on the tractor's drive axles versus the trailer axles. Drivers adjust it to comply with axle weight limits and to keep enough weight on the drives for traction. The change also affects steering axle loading and overall handling.

Question 43Combination Vehicles (CDL Class A knowledge area)

What is the correct way to test that the trailer spring brakes will apply if the emergency line is pulled?

  1. With the engine running, press the foot brake and watch the gauges
  2. With the engine off, release the tractor parking brake and roll
  3. With the system charged, pull the trailer air supply valve and listen for the spring brakes to set, then try to move gently Correct answer.
  4. With the system charged, disconnect only the electrical cord

Explanation

Pulling the trailer air supply valve exhausts the emergency line, which should set the trailer spring brakes. Listening for the application and gently testing for movement confirms the emergency braking function. This is a required part of the combination pre-trip inspection.

Question 44Combination Vehicles (CDL Class A knowledge area)

When a combination vehicle is driven around a curve, why does the trailer's rear end swing outward?

  1. The trailer has a wider track than the tractor
  2. The trailer's brakes drag on one side during the turn
  3. The trailer's tires are larger than the tractor's tires
  4. The trailer follows a shorter path than the tractor because it pivots at the fifth wheel Correct answer.

Explanation

Because the trailer pivots at the fifth wheel, its wheels track inside the path of the tractor's front wheels. In a turn, the rear of the trailer sweeps toward the outside of the curve. Drivers must allow extra room for this off-tracking to avoid hitting curbs or other vehicles.

Question 45Combination Vehicles (CDL Class A knowledge area)

A driver must make a right turn into a narrow street with a long combination. Which action best prevents the trailer's rear wheels from striking the curb?

  1. Accelerate through the turn to shorten the trailer's path
  2. Swing wide to the left before turning, then turn right Correct answer.
  3. Stay close to the right curb throughout the turn
  4. Turn sharply as soon as the tractor clears the corner

Explanation

Swinging wide before the turn gives the trailer room so its rear wheels track inside the tractor's path without hitting the curb. Turning too early or staying tight to the curb causes the trailer to cut the corner. The driver should also check mirrors throughout the turn.

Question 46Combination Vehicles (CDL Class A knowledge area)

What is the safest way to back a combination vehicle that has no trailer, such as a bobtail tractor?

  1. Back slowly and use mirrors, remembering that a bobtail tractor steers differently from a loaded combination Correct answer.
  2. Back quickly so the tractor does not drift
  3. Back only with the trailer attached
  4. Back with the hazard lights off to avoid confusing traffic

Explanation

A bobtail tractor has no trailer to stabilize or pivot, and its rear wheels react more sharply to steering. Backing slowly with constant mirror checks keeps it controlled. The driver should get out and look if visibility is uncertain.

Question 47Combination Vehicles (CDL Class A knowledge area)

A driver is backing a trailer into a dock and the trailer begins to turn the wrong way. What is the correct response?

  1. Pull forward to straighten out, then resume backing Correct answer.
  2. Accelerate to correct the angle
  3. Crank the steering wheel further in the same direction
  4. Get out and push the trailer straight

Explanation

When the trailer starts to go the wrong way, continuing to back worsens the angle. Pulling forward repositions the tractor and trailer so the driver can start again with better alignment. Small steering corrections made early prevent large errors.

Question 48Combination Vehicles (CDL Class A knowledge area)

Why should a driver avoid using the trailer hand valve to hold a combination vehicle at a stoplight?

  1. It prevents the tractor brakes from releasing
  2. It drains the electrical system
  3. It releases the spring brakes
  4. It applies only the trailer brakes and can cause the trailer to skid or the combination to jackknife Correct answer.

Explanation

The hand valve applies only the trailer brakes, leaving the tractor brakes free. On a slippery surface the trailer wheels can lock and swing the trailer around, causing a jackknife. The foot brake applies all brakes together and is the correct way to hold at a stop.

Question 49Combination Vehicles (CDL Class A knowledge area)

A combination vehicle's air pressure warning buzzer sounds while driving. What is the correct immediate action?

  1. Increase speed to reach a service facility sooner
  2. Turn off the buzzer and keep driving
  3. Continue to the next exit and check the pressure there
  4. Stop as soon as it is safe and identify the cause before continuing Correct answer.

Explanation

The low-pressure warning signals that air pressure is falling toward the level where spring brakes will apply. The driver should stop in a safe place and find the cause, such as a leak or a failed compressor. Continuing to drive risks a sudden loss of braking.

Question 50Combination Vehicles (CDL Class A knowledge area)

What is the purpose of a trailer's spring brakes in a breakaway situation?

  1. They release automatically so the trailer can roll clear
  2. They apply automatically when the emergency line loses pressure, stopping the trailer Correct answer.
  3. They lock the trailer's landing gear
  4. They apply only when the driver uses the hand valve

Explanation

Spring brakes are held off by air pressure from the emergency line. If the trailer separates and the line breaks, pressure is lost and heavy springs apply the brakes automatically. This failsafe design stops the trailer even when no driver is present.