CDL Combination Vehicles Practice Test — 40 Free Questions (Unofficial)

1. A tall load shifts a rig's mass upward. What risk increases?

• A. A lower rollover tendency because mass is farther above the road
• B. Rollover
• C. Less sensitivity to abrupt steering at the same speed
• D. Greater lateral stability during the same turn

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Answer: B. Rollover

A higher center of gravity reduces rollover stability.

2. A trailer swings sharply after an abrupt lane change. What is the safer steering principle?

• A. Use smooth, gradual inputs
• B. Make a rapid counter-steer each time the trailer moves
• C. Use only the trailer brake as a routine correction
• D. Accelerate while adding sharper steering

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Answer: A. Use smooth, gradual inputs

Gentle steering reduces rearward amplification.

3. Trailer tires cut inside the tractor's path during a turn. What is this called?

• A. Delay between control application and brake response
• B. Off-tracking
• C. Trailer wheels following a wider outside path only
• D. Loss of tire traction during braking

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Answer: B. Off-tracking

Rear wheels can follow a shorter inside path.

4. Before reversing into an unfamiliar bay, what should be checked?

• A. Only overhead clearance at the starting point
• B. Only whether the tractor fits the entrance
• C. The route and surrounding clearance
• D. Only the center of the trailer in one mirror

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Answer: C. The route and surrounding clearance

Inspecting the path helps identify obstacles.

5. A backing path becomes uncertain in the mirrors. What is appropriate?

• A. Keep backing slowly without confirming the path
• B. Rely only on the last clear mirror image
• C. Accelerate to shorten time in the blind area
• D. Stop and inspect again

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Answer: D. Stop and inspect again

Reinspect when visibility leaves uncertainty.

6. A tractor's lower fifth wheel can slide unintentionally on its mounting. Which requirement is implicated?

• A. Adequate articulation of the trailer tow-bar
• B. Automatic engagement of the kingpin jaws alone
• C. Transmission of the trailer ABS malfunction signal
• D. Positive prevention of shifting on the frame

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Answer: D. Positive prevention of shifting on the frame

The lower fifth wheel needs a positive anti-shifting device.

7. Which finding is unacceptable on a fifth-wheel installation?

• A. Adequate bolted attachment with secure plates
• B. Equivalent secure mounting hardware
• C. Cracking caused by mounting stresses
• D. An approved frame installation without deformation

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Answer: C. Cracking caused by mounting stresses

The installation must not crack, warp or deform the frame.

8. A separable fifth wheel locks only after a person installs a temporary clamp. What feature does the federal rule require instead?

• A. A manual clamp installed only after the driver finishes coupling
• B. Automatic application of the locking devices on coupling
• C. A warning indicator used instead of automatically applying the lock
• D. A lock applied only when the load reaches the stated capacity

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Answer: B. Automatic application of the locking devices on coupling

Readily separable fifth-wheel locking devices must apply automatically on coupling.

9. When may the fifth-wheel locking mechanism permit separation under its design requirement?

• A. When a positive manual release is activated
• B. When ordinary cornering forces reach the coupling
• C. When trailer weight is removed from the coupling
• D. When the tractor protection valve changes position

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Answer: A. When a positive manual release is activated

The lock must prevent separation unless positively released.

10. A fifth-wheel adapter can separate despite the main lock remaining closed. Is the lock rule satisfied?

• A. Yes; adapter retention is unnecessary when the kingpin jaws remain closed
• B. Yes; an empty trailer removes the adapter retention requirement
• C. Yes; the adapter need only resist separation during straight-line travel
• D. No; the adapter also must prevent unintended separation

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Answer: D. No; the adapter also must prevent unintended separation

The rule explicitly includes any adapter used with the lock.

11. A fifth-wheel location puts excessive weight on one axle and impairs steering. What must its placement support?

• A. Safe axle weight distribution and maneuvering
• B. Minimizing coupling distance without considering axle load distribution
• C. Meeting overall gross weight regardless of individual axle loading or steering
• D. Placing the coupling solely to maximize load on one axle

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Answer: A. Safe axle weight distribution and maneuvering

Position must distribute weight properly without unsafe effects on steering or braking.

12. The upper fifth-wheel half is loosely fastened to the trailer. Which statement is correct?

• A. Upper-half attachment may remain loose if the locking jaws close
• B. Only the tractor frame attachment needs the required security
• C. Upper-half attachment requirements apply only at maximum trailer load
• D. It needs security at least equivalent to the required lower-half installation

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Answer: D. It needs security at least equivalent to the required lower-half installation

The upper half must be fastened with the specified level of security.

13. A mechanic substitutes undersized mounting bolts without equivalent security. What is the concern?

• A. Aligned bolt holes establish compliance regardless of mounting strength
• B. Kingpin jaw engagement compensates for inadequate mounting fasteners
• C. The installation may not satisfy fifth-wheel mounting requirements
• D. Mounting adequacy can be deferred until a periodic inspection after operation

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Answer: C. The installation may not satisfy fifth-wheel mounting requirements

Mounting hardware must be adequate or provide equivalent security.

14. A full trailer's tow-bar is rated below the weight it draws. Which requirement is unmet?

• A. Adequacy for the empty trailer weight rather than the weight drawn
• B. Adequacy supplied by the secondary chain instead of the tow-bar
• C. Adequacy only for stationary support instead of the drawn load
• D. Structural adequacy for the drawn weight

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Answer: D. Structural adequacy for the drawn weight

A full-trailer tow-bar and attachments must carry the drawn weight adequately.

15. A tow-bar connection binds during normal turning. What characteristic is required?

• A. Adequate articulation without excessive slack
• B. Articulation only after disconnecting the hitch
• C. A rigid joint that prevents all turning movement
• D. Unlimited slack to maximize articulation

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Answer: A. Adequate articulation without excessive slack

The attachment must allow turning movement without excessive looseness.

16. A full trailer's hitch has no device to prevent accidental separation. What is missing?

• A. A locking device
• B. A retaining arrangement for the air-hose connection alone
• C. An inspection reminder instead of a device preventing separation
• D. A secondary chain used instead of the primary hitch locking device

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Answer: A. A locking device

The tow-bar connection requires protection against accidental separation.

17. Why must a full trailer have a secondary safety connection to the towing vehicle frame?

• A. To permit an under-strength primary connection
• B. To replace the trailer service brakes
• C. To retain the trailer if the tow-bar fails or disconnects
• D. To carry the normal load instead of the tow-bar

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Answer: C. To retain the trailer if the tow-bar fails or disconnects

The safety device guards against complete breakaway after tow-bar failure.

18. A safety cable has far more slack than turning requires. Does it meet the slack rule?

• A. Yes; additional slack always improves breakaway retention
• B. Yes; sufficient cable strength eliminates the slack rule
• C. No; slack must be limited to what proper turning needs
• D. Yes; the slack rule applies only when parked

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Answer: C. No; slack must be limited to what proper turning needs

Unnecessary slack is not permitted by the safety-device requirement.

19. A failed tow-bar can hit the road despite the safety chains. Which required function is missing?

• A. Preventing the tow-bar from dropping to the ground
• B. Transmitting the service-brake signal after separation
• C. Providing ordinary articulation at the primary hitch
• D. Supporting the normal trailer load in place of the primary hitch

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Answer: A. Preventing the tow-bar from dropping to the ground

The safety arrangement must also keep a failed tow-bar off the ground.

20. An air hose pulls tight during an ordinary tractor-trailer turn. What does the installation need?

• A. Enough length only when the tractor points straight ahead
• B. Enough length and flexibility for normal movement without damage
• C. A rigid line across every moving joint
• D. A connection stretched tight at maximum normal articulation

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Answer: B. Enough length and flexibility for normal movement without damage

Brake hoses must accommodate normal motion safely.

21. A brake hose rubs against a sharp frame edge. Which hazard must installation prevent?

• A. Chafing and mechanical damage
• B. Heat damage caused solely by exhaust contact
• C. Air loss caused solely by a defective valve seat
• D. Insufficient articulation at the primary fifth-wheel lock

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Answer: A. Chafing and mechanical damage

Hoses must be secured against chafing and other mechanical damage.

22. A brake hose touches a hot exhaust component. Is that acceptable?

• A. Yes; secure attachment by itself removes the need for heat protection
• B. Yes; the rule protects only hydraulic hoses from hot components
• C. Yes; contact is acceptable if it occurs only during normal turns
• D. No; installation must prevent contact with high-temperature sources

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Answer: D. No; installation must prevent contact with high-temperature sources

Heat exposure from exhaust or other hot sources must be prevented.

23. A connected brake fitting audibly leaks. What condition does the rule require?

• A. An attachment free of leaks that impair performance
• B. A visual attachment check without evaluating leakage
• C. Permission to leak whenever the trailer is empty
• D. A faster compressor to compensate for an unresolved leak

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Answer: A. An attachment free of leaks that impair performance

Connections must not leak or otherwise adversely affect braking performance.

24. A brake line is pinched by a new bracket. What defect should be corrected?

• A. Normal reduction in air demand that substitutes for an adjustment
• B. An acceptable restriction if no external leak is audible
• C. A concern only when the constriction also disconnects the fitting
• D. A constriction that can affect braking

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Answer: D. A constriction that can affect braking

Connections and tubing must not be constricted or mechanically damaged.

25. A coiled nonmetallic brake tube has no protective guard at its fitting. Which feature should be checked against section 393.45(c)?

• A. The required protected straight segment and spring guard
• B. A tightly bent unguarded end segment at the connector
• C. A rigid clamp constricting the air passage near the fitting
• D. A connection that relies on the unsupported coil to prevent every kink

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Answer: A. The required protected straight segment and spring guard

The rule specifies a guarded end segment to prevent kinking at the fitting.

26. A driver assumes a brake hose connection is satisfactory because it looks connected. What additional condition matters?

• A. Whether compressor recovery alone compensates for unresolved leakage
• B. Whether visual attachment alone proves adequate airflow
• C. Whether the fitting has been tightened without checking for restriction
• D. Whether leaks or other restrictions impair performance

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Answer: D. Whether leaks or other restrictions impair performance

An attached connection can still leak or constrict brake airflow.

27. A braked trailer breaks away. What must its breakaway system do?

• A. Apply only after a delayed manual command
• B. Apply the trailer brakes automatically and immediately
• C. Apply only while the tractor remains connected
• D. Release the trailer brakes to avoid wheel lock

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Answer: B. Apply the trailer brakes automatically and immediately

Automatic immediate application is required for a trailer subject to the brake requirement.

28. For a trailer subject to section 393.43(d), how long must breakaway braking remain applied at minimum?

• A. 15 minutes
• B. One minute
• C. Ten minutes
• D. Five minutes

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Answer: A. 15 minutes

The federal minimum applied period is fifteen minutes.

29. An air-braked tractor protection device operates automatically at 30 psi. Is that pressure within the stated federal interval?

• A. No; the required value is exactly 20 psi
• B. Yes, within 20–45 psi
• C. No; the required interval is 45–60 psi
• D. No; the required interval is 60–90 psi

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Answer: B. Yes, within 20–45 psi

Thirty psi lies within the required automatic operation interval.

30. What must remain possible for the towing vehicle after a braked trailer breaks away?

• A. Preserving only the ability to stop with a parking brake
• B. Keeping service stopping dependent on a still-connected trailer
• C. Venting the towing vehicle's remaining service-brake supply
• D. Stopping with the towing vehicle's service brakes

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Answer: D. Stopping with the towing vehicle's service brakes

The protection system must preserve the towing vehicle's ability to stop.

31. An air-braked towing vehicle provides automatic emergency trailer braking but no driver-operable means. What is missing?

• A. A readily operable manual emergency control
• B. A manual control that can replace and defeat automatic activation
• C. A written emergency procedure without an operable manual control
• D. A manual control usable only from outside the driver's operating position

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Answer: A. A readily operable manual emergency control

The rule requires both automatic and driver-operable means.

32. A manual control can disable the required automatic emergency activation. Is this permitted?

• A. Yes; manual selection may defeat automatic emergency activation
• B. Yes; automatic activation applies only without a driver
• C. Yes; an empty trailer removes that protection requirement
• D. No; the manual means must not prevent the automatic means

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Answer: D. No; the manual means must not prevent the automatic means

The manual arrangement must not defeat automatic activation.

33. Why does a trailer emergency valve guard against air flowing back toward the tractor?

• A. To protect the trailer brake supply reservoir when tractor pressure drops
• B. To transfer the trailer reservoir's air back to the tractor after pressure loss
• C. To make a separate trailer emergency-braking function unnecessary
• D. To replace the service-brake control circuit during ordinary braking

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Answer: A. To protect the trailer brake supply reservoir when tractor pressure drops

No-bleed-back or equivalent protection preserves the trailer brake air supply.

34. An ordinary air-braked truck tractor was manufactured in 2000. Which federal requirement generally applies?

• A. The March 1998 threshold that applies to non-tractor air-braked vehicles
• B. ABS under the post-March 1997 tractor provision
• C. A threshold based on the date the tractor was first sold
• D. An exemption based solely on the tractor being used without a second trailer

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Answer: B. ABS under the post-March 1997 tractor provision

The tractor manufacturing date is after the applicable March 1997 threshold; stated exceptions still matter.

35. An ordinary air-braked semitrailer was manufactured in 2000. Which statement matches the federal ABS date provision?

• A. The March 1997 threshold specifically for truck tractors
• B. It falls after the March 1998 non-tractor threshold
• C. A threshold based on the trailer's first registration date
• D. An exemption based solely on operating below the trailer's rated load

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Answer: B. It falls after the March 1998 non-tractor threshold

The rule applies to non-tractor air-braked CMVs manufactured on or after March 1, 1998, subject to exceptions.

36. A 2002 tractor is required to report an air-braked trailer's ABS malfunction in the cab. What connection supports this?

• A. A tractor-only fault circuit with no connection for trailer signals
• B. A trailer exterior lamp without the required signal path to the cab
• C. An electrical malfunction-signal circuit to the towed vehicle
• D. A pneumatic service-brake connection without an electrical malfunction circuit

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Answer: C. An electrical malfunction-signal circuit to the towed vehicle

The provision requires the capability and connection to transmit the trailer ABS malfunction signal.

37. A 2002 semitrailer subject to the ABS rule has no means to signal an ABS malfunction to the towing vehicle. What is missing?

• A. A service-pressure indicator without an ABS fault-signaling circuit
• B. A trailer lamp with no required circuit connection to the towing vehicle
• C. The required malfunction circuit and connection means
• D. An arrangement that disables the trailer fault signal whenever connected

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Answer: C. The required malfunction circuit and connection means

Covered trailers manufactured from March 2001 need a malfunction-signaling circuit and connection.

38. An ordinary air-braked converter dolly was manufactured in 2000. Which ABS provision generally applies?

• A. The March 1997 threshold specifically for truck tractors
• B. A threshold based on the date the dolly was purchased
• C. The March 1998 threshold for air-braked CMVs other than truck tractors
• D. An exemption based solely on the dolly having no cargo-carrying deck

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Answer: C. The March 1998 threshold for air-braked CMVs other than truck tractors

49 CFR 393.55(c)(2) expressly includes converter dollies among covered non-tractor vehicles, subject to stated exceptions.

39. A covered 2002 trailer is designed to tow another air-braked trailer. What ABS signal capability must it provide in addition to reporting its own malfunction?

• A. Report only its own fault and block all downstream signals
• B. Send downstream faults only to its own exterior lamp
• C. Use trailer brake pressure as a substitute for electrical fault signals
• D. Transmit malfunction signals from the trailer it tows to the vehicle ahead

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Answer: D. Transmit malfunction signals from the trailer it tows to the vehicle ahead

49 CFR 393.55(d)(3) requires a covered towing trailer to pass the ABS malfunction signal of the trailer it tows forward.

40. A manufacturer date is unknown when assessing ABS applicability. What should be verified before assuming a date-based exemption?

• A. The vehicle type, brake system, manufacture date and applicable exceptions
• B. The first registration date without checking manufacture date or vehicle category
• C. The current ownership date and whether the vehicle is loaded
• D. The model name alone without checking brake system or regulatory exceptions

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Answer: A. The vehicle type, brake system, manufacture date and applicable exceptions

The regulatory thresholds differ by vehicle type and system, so those facts must be established.