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Kansas CDL Combination study materials
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Kansas CDL Combination practice questions

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Question 1Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

For a combination vehicle with a tractor protection valve, at what air pressure should the trailer supply line be closed off to protect the tractor's air supply?

  1. Around 60 to 80 psi
  2. Around 5 to 10 psi
  3. Around 100 to 120 psi
  4. Around 20 to 45 psi Correct answer.

Explanation

The tractor protection valve is designed to close the trailer supply line when system pressure drops to roughly 20 to 45 psi. This keeps enough air in the tractor tanks so the driver can still brake the power unit if the trailer separates or its line ruptures. Above that range the valve stays open and supplies the trailer normally.

Question 2Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

You are coupling a tractor to a semitrailer. After backing under the trailer, what must happen before you pull the tractor forward to test the coupling?

  1. Raise the landing gear fully
  2. Pull the release handle and lock it
  3. Open the trailer air supply valve
  4. Set the trailer brakes and tug gently against them Correct answer.

Explanation

A proper coupling check requires the trailer brakes to be set, then a gentle forward tug with the tractor to confirm the fifth wheel jaws are locked around the kingpin. If the trailer holds the tractor back, the coupling is secure. Pulling forward before this test can let the trailer drop if the jaws never closed.

Question 3Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

Which condition indicates that the fifth wheel jaws have properly locked around the kingpin after coupling?

  1. The release handle moves freely in and out
  2. The kingpin is visible below the fifth wheel plate
  3. A gentle tug in low gear fails to separate the units Correct answer.
  4. The trailer apron sits level with the tractor frame

Explanation

The definitive check is a tug test: with the trailer brakes set, pull gently in a low gear and confirm the trailer resists. A locked jaw assembly grips the kingpin so the trailer cannot slide off the plate. Visual checks alone are unreliable because the jaws can appear closed while not fully engaged.

Question 4Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the correct order when uncoupling a tractor from a loaded semitrailer?

  1. Set the trailer brakes, lower the landing gear, release the fifth wheel, then pull out Correct answer.
  2. Lower the landing gear, set the trailer brakes, then release the fifth wheel and pull out
  3. Set the trailer brakes, release the fifth wheel, lower the landing gear, then pull out
  4. Release the fifth wheel, lower the landing gear, set the trailer brakes, then pull out

Explanation

Set the trailer parking brakes first so the trailer cannot roll, then lower the landing gear to take the weight before releasing the fifth wheel. Only after the trailer is supported and braked do you pull the tractor clear. Doing these steps out of order risks a dropped or rolling trailer.

Question 5Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

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

  1. To apply the trailer brakes only, for testing and for holding the units steady Correct answer.
  2. To supply air to the trailer tires
  3. To release the trailer spring brakes for parking
  4. To apply all brakes on the combination at once

Explanation

The hand valve controls only the trailer service brakes, which is useful for checking that trailer brakes work and for holding the rig during some maneuvers. It should not be used for parking because it can leak off. Normal braking is done with the foot pedal, which signals both tractor and trailer.

Question 6Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is a key reason to avoid using the trailer hand valve for parking a combination vehicle?

  1. It disables the trailer spring brakes
  2. It can leak air and release over time Correct answer.
  3. It overheats the trailer drums
  4. It applies only the tractor brakes

Explanation

The hand valve holds the trailer with service brake air pressure, which can slowly leak away and let the trailer roll. Parking brakes, by contrast, are spring-applied and fail safe when air is lost. That is why the trailer parking brake control, not the hand valve, is used when leaving the vehicle.

Question 7Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When backing a tractor-semitrailer, what happens to the trailer's direction of travel as you turn the steering wheel?

  1. The trailer pivots opposite to the tractor's steering direction Correct answer.
  2. The trailer steers independently of the tractor
  3. The trailer moves in the same direction as the tractor's front
  4. The trailer follows the tractor's path exactly

Explanation

Because the trailer pivots at the fifth wheel, turning the tractor's front wheels one way swings the trailer the opposite way. To back into a dock on your right, you steer left to push the trailer's rear right. Understanding this reversed response is essential for controlled backing.

Question 8Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When backing a tractor-semitrailer, which technique helps you keep the trailer under control?

  1. Back quickly so the trailer has no time to drift
  2. Turn the wheel sharply and hold it until the trailer is in place
  3. Watch the trailer in the mirrors and make small corrections early Correct answer.
  4. Rely on the backup camera and ignore the mirrors

Explanation

Backing a combination vehicle requires constant mirror checks and small, early steering corrections; large corrections cause the trailer to swing and jackknife. Backing slowly gives you time to react. Mirrors remain the primary tool because they show the trailer's full length and clearance.

Question 9Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is off-tracking in a combination vehicle?

  1. The rearward creep of a parked combination on a grade
  2. The trailer tires following a path inside the tractor's front wheels during a turn Correct answer.
  3. The loss of air pressure in the trailer line
  4. The tendency of the trailer to sway at highway speed

Explanation

Off-tracking is the tendency of the rear wheels of a long combination to cut a shorter path than the front wheels during a turn. The longer the wheelbase, the more the rear wheels track inside. Drivers must swing wide enough to keep the trailer's rear wheels from hitting curbs or vehicles.

Question 10Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When making a right turn in a long combination vehicle, why should you swing wide and check your right mirror?

  1. To increase engine rpm for the turn
  2. To keep the trailer brakes cool
  3. To reduce the turning radius of the tractor
  4. To keep the trailer's rear wheels from cutting the corner and striking objects Correct answer.

Explanation

Because the trailer's rear wheels track inside the tractor's path, a tight right turn can drag them over the curb or into a pedestrian or parked car. Swinging wide and watching the right mirror lets you confirm the trailer clears the corner. This is the practical response to off-tracking.

Question 11Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the effect of a trailer's spring brakes when air pressure in the trailer system drops to a low level?

  1. They apply only when the hand valve is used
  2. They remain in whatever state they were last set
  3. They apply automatically as a fail-safe Correct answer.
  4. They release fully so the wheels roll freely

Explanation

Trailer spring brakes are held off by air pressure and apply by spring force when pressure drops, which is why they are a fail-safe system. A major air loss therefore sets the trailer brakes automatically. This protects against a runaway trailer if the supply line is severed.

Question 12Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

You are preparing to drive a combination vehicle and the low air pressure warning comes on. What should you do?

  1. Release the parking brakes and drive slowly
  2. Build air pressure back up before moving and check for leaks Correct answer.
  3. Switch off the warning and proceed
  4. Continue driving but avoid heavy braking

Explanation

A low air pressure warning means the system cannot safely supply the brakes, so you must let the compressor build pressure to normal before moving. If pressure will not build or leaks off, the vehicle needs repair. Driving with low air risks losing braking ability entirely.

Question 13Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

During a pre-trip inspection of a combination vehicle, which item confirms the trailer air lines are properly connected?

  1. The glad hands are locked together with no audible leak Correct answer.
  2. The fifth wheel is greased
  3. The landing gear is raised
  4. The trailer lights are turned off

Explanation

Glad hands must be firmly locked and the coupler seals sound so supply and service air reach the trailer without leaking. A leak at the connection can cause slow air loss and weak trailer braking. Checking for secure lock and no hiss confirms a proper connection.

Question 14Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the correct way to test that the trailer service brakes are working during a pre-trip check?

  1. Rev the engine and listen for the compressor
  2. Use the trailer hand valve or have a helper apply the brake while you watch the wheels Correct answer.
  3. Release the parking brakes and roll forward
  4. Open the trailer air supply valve

Explanation

Applying the trailer service brakes with the hand valve, or having a helper press the pedal, lets you watch or feel whether the trailer wheels hold. This confirms the service line and brake chambers function. It should be done with the unit secured so it cannot roll unexpectedly.

Question 15Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

Why does a combination vehicle need a longer stopping distance than a single-unit truck of similar weight?

  1. Air brakes are weaker than hydraulic brakes
  2. The trailer has no brakes of its own
  3. The combination is heavier and its brakes must stop more mass Correct answer.
  4. The tractor's brakes are disconnected when towing

Explanation

Stopping distance grows with total weight, and a loaded combination carries far more mass than a single unit. Even with trailer brakes working, the extra weight and the push of the trailer lengthen the distance needed. Drivers must allow greater following distance to compensate.

Question 16Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the main danger of a trailer swing (rearward jackknife) during braking?

  1. The fifth wheel jaws release the kingpin
  2. The trailer's rear swings out and can strike other vehicles or leave the road Correct answer.
  3. The trailer air line ruptures
  4. The trailer brakes lock and release rapidly

Explanation

A trailer swing happens when the trailer's rear wheels lose traction and the trailer rotates outward around the fifth wheel. The swinging trailer can hit adjacent traffic or drag the combination off the roadway. Smooth braking and avoiding sudden steering inputs help prevent it.

Question 17Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

How can a driver reduce the risk of a jackknife when braking a combination vehicle on a slippery surface?

  1. Brake hard and steer sharply at the same time
  2. Downshift to a lower gear and slam the brakes
  3. Brake smoothly and avoid sudden steering while braking Correct answer.
  4. Release the trailer air supply while braking

Explanation

Jackknifes often occur when the drive or trailer wheels lock while the vehicle is turning, so smooth, progressive braking without hard simultaneous steering keeps the wheels rolling and the units aligned. Sudden inputs on slick pavement break traction. Antilock brakes help, but smooth technique remains essential.

Question 18Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the purpose of the trailer air supply (emergency) line in a combination vehicle?

  1. To supply air that holds off the trailer spring brakes and to trigger them on separation Correct answer.
  2. To inflate the trailer tires
  3. To carry the service brake signal to the trailer
  4. To power the trailer's interior lights

Explanation

The supply line charges the trailer's air tanks and holds the spring brakes released during normal operation. If the line separates, pressure drops and the trailer spring brakes apply automatically. This fail-safe design stops a detached trailer.

Question 19Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

If a trailer's service line separates from the tractor while driving, what should you expect?

  1. The tractor's engine will shut down
  2. The trailer lights will flash a warning
  3. The trailer brakes will not apply and the trailer will coast
  4. The trailer spring brakes will apply and the trailer will stop Correct answer.

Explanation

Losing the service line alone does not set the trailer brakes, but the supply line usually separates too, dropping pressure and applying the spring brakes. The trailer then stops on its own. This fail-safe behavior limits the danger of a breakaway.

Question 20Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What does the red trailer parking brake control knob do when pulled out?

  1. Releases the trailer spring brakes
  2. Applies the trailer spring brakes by exhausting supply air Correct answer.
  3. Applies only the trailer service brakes
  4. Opens the tractor's air tanks to the atmosphere

Explanation

Pulling the red knob exhausts air from the trailer supply line, which lets the spring brakes apply and holds the trailer parked. Pushing it in restores supply air and releases the springs. This is the proper parking control for the trailer.

Question 21Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When driving a combination vehicle, why should you check your mirrors frequently, especially before changing lanes?

  1. To monitor the trailer's tire pressure
  2. To verify the fifth wheel grease level
  3. To confirm the trailer is tracking properly and to see vehicles beside the trailer Correct answer.
  4. To watch the landing gear position

Explanation

Mirrors show whether the trailer is following straight and reveal vehicles in the long blind spots alongside and behind the trailer. Because a combination is long, a lane change requires more clearance than a car. Frequent mirror checks prevent sideswipes and trailer drift.

Question 22Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the proper procedure for a brake check after driving through deep water?

  1. Accelerate hard to throw off the water
  2. Continue at highway speed without braking
  3. Set the parking brakes and wait for them to dry
  4. Apply the brakes gently while moving slowly to dry them Correct answer.

Explanation

Water on the brake linings reduces friction, so you should test the brakes gently at low speed to dry them and confirm they grip. If braking feels weak, keep testing until normal. Never assume brakes are fine after a water crossing.

Question 23Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

How does the weight of a load affect the handling of a combination vehicle on a curve?

  1. Load weight has no effect on cornering
  2. A heavy load shortens the turning radius
  3. A heavy load lowers the center of gravity and always improves cornering
  4. A high, heavy load raises the center of gravity and increases rollover risk Correct answer.

Explanation

A tall, heavy load raises the combination's center of gravity, so the outward force in a curve can tip the trailer or tractor more easily. Drivers must slow down before curves, especially with high loads. Weight placement and height both matter for stability.

Question 24Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What does the term "crack-the-whip" describe in a combination vehicle?

  1. The sound of a loose air line
  2. The rear of the trailer swinging sideways with increasing force as the units move Correct answer.
  3. A method of tightening the fifth wheel
  4. A brake adjustment procedure

Explanation

Crack-the-whip occurs when a small steering movement at the tractor is amplified into a large side-to-side swing at the rear of a long combination, especially at speed. The rear can swing into other lanes or off the road. Smooth, small steering inputs prevent it.

Question 25Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When should you use the trailer air supply valve to charge the trailer's air tanks?

  1. Before connecting the glad hands
  2. While the vehicle is moving at highway speed
  3. Only after the trailer spring brakes have been manually released
  4. After coupling, once the lines are connected and the tractor is running Correct answer.

Explanation

After the air lines are connected and the tractor's compressor is running, opening the supply valve charges the trailer tanks so the spring brakes can release. Connecting lines before charging prevents whipping hoses and ensures proper pressure. The trailer should not be moved until pressure is up.

Question 26Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

In a combination vehicle, what is the primary function of the tractor protection valve?

  1. It balances air pressure between the two trailer service lines
  2. It limits air pressure in the tractor's front brake circuit
  3. It closes the tractor's air lines to the trailer when pressure drops too low Correct answer.
  4. It regulates the trailer's spring brake release timing

Explanation

The tractor protection valve senses system pressure and closes off the tractor's trailer air supply and service lines when pressure falls below a safe level. This preserves the tractor's remaining air for its own brakes and lets the trailer brakes apply. It is a safety cutoff, not a regulator of front brake pressure or release timing.

Question 27Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

You are coupling a tractor to a semitrailer. After backing under the trailer, what should you do before raising the landing gear?

  1. Lower the trailer apron onto the fifth wheel plate
  2. Lock the fifth wheel and tug-test the tractor against the trailer Correct answer.
  3. Connect the air lines and electrical cord, then supply air to the trailer
  4. Release the trailer spring brakes and pull forward slightly

Explanation

The correct coupling sequence locks the fifth wheel jaw around the kingpin and then performs a pull test with the trailer brakes set to confirm the lock. Only after the coupling is verified should you connect lines and raise the landing gear. Tug-testing before raising the gear prevents the trailer from dropping if the lock failed.

Question 28Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What does the yellow (trailer air supply) knob do when pushed in on a tractor with charged air tanks?

  1. It isolates the trailer from the tractor's electrical system
  2. It applies the trailer service brakes only
  3. It charges the trailer air tanks and releases the trailer spring brakes Correct answer.
  4. It dumps all trailer air to the atmosphere

Explanation

Pushing in the yellow trailer air supply knob allows tractor air to flow to the trailer, charging its tanks and releasing the spring (parking) brakes so the trailer can roll. Pulling it out shuts off supply and causes the trailer spring brakes to set. It does not apply the service brakes or affect electrical circuits.

Question 29Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When should you use the trailer hand valve while driving a combination vehicle?

  1. To make routine stops on the highway
  2. To hold the vehicle on a hill while the clutch is engaged
  3. To test the trailer brakes at low speed before a trip Correct answer.
  4. To slow the vehicle when the foot brake fails

Explanation

The hand valve is intended for testing trailer brakes at low speed, such as during a pre-trip check. It should not be used for normal stopping or parking because it applies only the trailer brakes and can cause a skid or jackknife. Relying on it for hill holding or highway stops is unsafe.

Question 30Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

Which condition most increases the risk of a rollover in a combination vehicle?

  1. Traveling at the posted speed limit on a straight road
  2. Keeping the trailer air tanks fully charged
  3. Using the engine brake on a downgrade
  4. Carrying a light load with a high center of gravity Correct answer.

Explanation

Rollover risk rises when the center of gravity is high and the load is light relative to the vehicle's size, because the vehicle is easier to tip in a curve or sudden maneuver. Speed through a turn and load placement are the key factors. Straight-road speed, engine braking, and charged air tanks do not increase rollover tendency.

Question 31Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the correct order for uncoupling a tractor-semitrailer?

  1. Lower the landing gear, disconnect the air lines, release the fifth wheel, then pull out Correct answer.
  2. Pull the tractor out first, then lower the landing gear and disconnect the lines
  3. Release the fifth wheel, then set the trailer brakes, then lower the landing gear
  4. Disconnect the air lines first, then set the trailer brakes, then lower the landing gear

Explanation

Proper uncoupling lowers the landing gear to support the trailer, disconnects the air and electrical lines, releases the fifth wheel, and then pulls the tractor clear. Setting the trailer brakes and chocking wheels should happen before pulling the pin. Doing steps out of order can drop the trailer or damage lines.

Question 32Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

Why is a combination vehicle more likely to roll over in a curve than a single-unit truck of the same weight?

  1. It cannot use an engine brake on curves
  2. Its longer combination has a higher center of gravity and greater lateral force at the trailer Correct answer.
  3. Its brakes are less effective at low speed
  4. Its steering ratio is slower

Explanation

A tractor-trailer has a higher center of gravity and the trailer's mass generates lateral force that can tip the unit outward in a turn. The longer wheelbase and articulation also make the combination less forgiving of excessive speed in a curve. Brake effectiveness, engine braking, and steering ratio are not the primary reason.

Question 33Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When you must stop on a railroad crossing in a combination vehicle, what should you do?

  1. Get out and check the crossing, then back off the crossing if possible Correct answer.
  2. Sound the horn and proceed quickly
  3. Stop and wait for a signal to change
  4. Lower the landing gear to stabilize the trailer

Explanation

If a combination vehicle stalls or must stop on the tracks, the driver should get clear and, if possible, move the vehicle off the crossing by backing or pulling forward. Never remain on the tracks waiting. If the vehicle cannot be moved, call for help and warn approaching trains.

Question 34Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the purpose of the trailer spring brakes in a combination vehicle?

  1. To provide normal service braking during stops
  2. To balance braking between tractor and trailer
  3. To apply automatically when trailer air pressure is lost Correct answer.
  4. To hold the trailer only while loading

Explanation

Trailer spring brakes are held off by air pressure and apply automatically when trailer air pressure drops, such as during a breakaway or line separation. This fail-safe design stops the trailer if it separates from the tractor. They are not the normal service brakes and do not balance braking.

Question 35Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

During a pre-trip inspection, how do you check that the trailer spring brakes apply when air is lost?

  1. Step on the foot brake and watch the gauge
  2. Push the yellow knob in and check for air flow
  3. Open the drain cocks on the trailer tanks
  4. Pull the yellow knob out and listen for the trailer brakes to set Correct answer.

Explanation

Pulling out the trailer air supply knob shuts off air to the trailer, causing the spring brakes to apply; you should hear them set and the trailer should not roll. This verifies the fail-safe function. Pushing the knob in charges the trailer, and draining tanks or applying the foot brake tests different functions.

Question 36Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the correct way to check the trailer service brakes during a pre-trip inspection?

  1. Release the tractor parking brake and roll forward
  2. Pull the yellow knob and listen for the spring brakes
  3. Use the trailer hand valve or have an assistant apply the foot brake while you watch the trailer wheels Correct answer.
  4. Open the trailer tank drains and watch for air loss

Explanation

The trailer service brakes are checked by applying them with the hand valve (or an assistant's foot brake) and observing that the trailer wheels hold or that the brake chambers move. This confirms the service circuit works. Pulling the yellow knob tests spring brakes, and draining tanks tests air supply.

Question 37Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

Why should you avoid using the trailer hand valve to park a combination vehicle?

  1. It disables the tractor's parking brake
  2. It prevents the trailer spring brakes from ever applying
  3. It drains the trailer air tanks completely
  4. It applies only the trailer brakes and can allow the combination to roll or skid Correct answer.

Explanation

The hand valve applies only the trailer service brakes and does not hold the tractor, so the combination may roll or skid. Parking should be done with the tractor parking brake and trailer spring brakes. The hand valve also does not drain all trailer air or disable other brakes.

Question 38Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When backing a tractor-semitrailer, what should you do if you lose sight of your spotter?

  1. Continue backing slowly using your mirrors
  2. Sound the horn and keep backing
  3. Speed up to finish the maneuver quickly
  4. Stop immediately and do not move until you see the spotter again Correct answer.

Explanation

If you lose sight of the spotter, stop at once and wait until you can see them again. Backing blind in a combination vehicle risks crushing someone or hitting an object. Continuing, honking, or speeding up increases the danger.

Question 39Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the safest way to turn the steering wheel when backing a tractor-semitrailer to the right?

  1. Turn the wheel left and accelerate
  2. Turn the wheel left, then follow the trailer with the tractor Correct answer.
  3. Turn the wheel right, then quickly straighten it
  4. Turn the wheel right and hold it steady

Explanation

To back a trailer to the right, you turn the steering wheel left so the tractor's rear moves right and pushes the trailer right. You then follow the trailer with the steering to keep it tracking. Turning right initially would move the trailer the opposite way.

Question 40Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

Why should you check your mirrors frequently when driving a combination vehicle?

  1. To check the fuel gauge
  2. To watch the tachometer
  3. To adjust the radio
  4. To monitor the trailer's position and detect sway or shifting loads Correct answer.

Explanation

Frequent mirror checks let you monitor the trailer's tracking, spot sway or a shifting load, and confirm clearance before lane changes. They also help you detect a tire or brake problem early. Radio, fuel gauge, and tachometer are monitored through the dash, not the mirrors.

Question 41Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

How does a heavy load affect the handling of a combination vehicle on a curve?

  1. It lowers the center of gravity and makes the vehicle more stable
  2. It has no effect on curve handling
  3. It reduces the trailer's weight on the tires
  4. It raises the center of gravity and increases the outward tipping force Correct answer.

Explanation

A heavy load, especially one stacked high, raises the center of gravity and increases the lateral force that pushes the vehicle outward in a curve, raising rollover risk. Drivers must slow down more for curves when heavily loaded. It does not make the vehicle more stable or reduce tire loading.

Question 42Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What should you do if a trailer's service line separates from the tractor while driving?

  1. The trailer spring brakes will apply and bring the trailer to a stop Correct answer.
  2. The trailer service brakes will not apply, so stop carefully using the tractor brakes
  3. The tractor will lose all braking
  4. The trailer will accelerate

Explanation

A service line separation causes a loss of trailer air supply, which triggers the trailer spring brakes to apply automatically and stop the trailer. The tractor retains its own brakes. The trailer will not accelerate, and the tractor does not lose all braking.

Question 43Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

Why must you inspect the fifth wheel and kingpin area before coupling a tractor to a semitrailer?

  1. To ensure the locking jaws are open and the kingpin is undamaged Correct answer.
  2. To verify the electrical cord is connected
  3. To confirm the trailer is level
  4. To check the trailer's tire pressure

Explanation

Before coupling, the fifth wheel locking jaws should be open and the kingpin and mounting area free of damage so the jaws can close and lock properly. A damaged kingpin or closed jaws can cause the trailer to separate. Leveling, tire pressure, and electrical connection are checked separately.

Question 44Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What happens to the trailer's direction of travel when you turn the steering wheel while backing a tractor-semitrailer?

  1. The trailer moves only when the hand valve is used
  2. The trailer moves in the same direction as the steering input
  3. The trailer does not move
  4. The trailer moves opposite to the steering input Correct answer.

Explanation

When backing a combination vehicle, the trailer moves opposite to the direction you turn the steering wheel: turning left moves the trailer's rear to the right. This reversed response is why backing requires practice and small corrections. The trailer does move and does not depend on the hand valve.

Question 45Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

On a combination vehicle with dual air lines, which color identifies the service line that carries the brake application signal from the tractor to the trailer?

  1. Red
  2. Black
  3. Green
  4. Blue Correct answer.

Explanation

The service line on a dual-line air brake system is the blue line, and it carries the air pressure signal that tells the trailer relay valve to apply the trailer service brakes. The emergency line is red and supplies air to charge the trailer tanks and trigger emergency braking if it separates.

Question 46Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

What is the purpose of the glad hands on a combination vehicle?

  1. They hold the trailer landing gear in the raised position
  2. They seal the trailer doors during transport
  3. They lock the trailer kingpin into the fifth wheel
  4. They couple the tractor and trailer air lines together Correct answer.

Explanation

Glad hands are the quick-coupling fittings on the ends of the tractor and trailer air lines that join the two halves of the air system. They must be locked together with their seals intact so air does not leak out of the service or emergency line.

Question 47Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

When the trailer air supply (red) knob pops out on its own while you are driving a combination vehicle, what has most likely happened?

  1. The trailer has lost air pressure in the emergency line Correct answer.
  2. The trailer service brakes have been fully released
  3. The tractor parking brakes have been automatically set
  4. The trailer has been disconnected from the electrical circuit

Explanation

The tractor protection valve pops the red knob out when emergency line pressure drops below a safe level, which typically means a separation or major leak. This automatically applies the trailer spring brakes so the trailer cannot roll free.

Question 48Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

You are backing under a semitrailer and the trailer starts to slide sideways instead of being lifted. What is the most likely cause?

  1. The trailer brakes are released and the trailer is being pushed Correct answer.
  2. The kingpin is greased excessively
  3. The landing gear is cranked too high
  4. The fifth wheel is too far forward on the tractor

Explanation

If the trailer is not held by its brakes, the tractor's rear wheels push the trailer sideways instead of sliding under it. You should set the trailer brakes, or use chocks, before backing under so the trailer stays put.

Question 49Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

After coupling, you tug-test the tractor against the trailer with the trailer brakes set. What does a satisfactory test look like?

  1. The tractor stops without moving the trailer Correct answer.
  2. The trailer rolls forward a few feet
  3. The tractor stalls immediately
  4. The fifth wheel handle pops out

Explanation

A proper tug test is done in a low gear against the set trailer brakes; the tractor should feel resistance and stop without the trailer moving. This confirms the locking jaws have closed around the kingpin. If the trailer moves or the tractor pulls free, the coupling is not secure.

Question 50Combination Vehicle Knowledge (CDL general knowledge and combination vehicles)

In a combination vehicle, what does the trailer hand valve (trolley valve) control?

  1. The tractor drive axle brakes only
  2. Both the tractor and trailer service brakes
  3. The trailer service brakes only Correct answer.
  4. The trailer spring brakes only

Explanation

The hand valve applies only the trailer service brakes, which is why it is useful for testing the trailer brakes during inspection. It does not apply the tractor brakes, and it should not be used for parking because it can leak off over time.