South Carolina SCDMV CDL General Knowledge Practice Test 7
- Questions50
- Correct to pass40
- Passing score80%
- Minimum age18 years
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1. At 55 mph, about how far does a vehicle travel during a typical 3/4 second perception time plus 3/4 second reaction time, before the brakes even begin to work?
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Answer C: About 120 feet
Each 3/4 second at 55 mph covers about 60 feet, so perception and reaction together use up roughly 120 feet before braking starts.
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2. How is free play in the steering wheel measured during the inspection?
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Answer B: By turning the wheel until the front wheels just start to move and noting the movement
Play is the movement at the rim that produces no movement at the road wheels, so it is measured by turning until the wheels first respond.
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3. Under federal rules, what happens to a commercial driver found with any detectable amount of alcohol while operating a commercial vehicle?
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Answer D: The driver is placed out of service for 24 hours
Any measurable alcohol takes a commercial driver off the road for 24 hours, and a concentration of 0.04 percent or more brings a disqualification.
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4. Why is backing toward the driver side preferred?
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Answer C: You can watch the rear of the vehicle directly through the side window
Backing to the left lets the driver look out the window and see the rear of the vehicle directly instead of relying on a mirror alone.
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5. Which set of maneuvers makes up the basic control skills part of the commercial driving test?
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Answer D: Straight line backing, offset backing, parallel parking and alley docking
The basic control skills test is carried out off the road and consists of low speed exercises such as straight line backing, offset backing, parallel parking and the alley dock.
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6. A straight truck has a long body behind the rear axle. What happens to that overhang in a sharp left turn?
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Answer D: It swings out to the right and can strike a vehicle or an object on that side
Body that hangs behind the rear axle swings the opposite way to the turn, so the tail of the truck sweeps out into the next lane or over the curb.
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7. How should the steering wheel be held when driving a large vehicle?
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Answer D: Firmly with both hands, placed on opposite sides of the wheel
Both hands on opposite sides give the leverage and control needed if a tire drops off the pavement or the wheel is jolted.
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8. What should you check about the fuel tank during the walkaround?
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Answer A: That it is securely mounted, the cap is on tight and there are no leaks
A loose tank or a missing cap can spill fuel, and any fuel leak is a fire hazard that makes the vehicle unsafe to drive.
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9. What information does an inspection decal on a commercial vehicle carry?
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Answer B: The date of the inspection, where the inspection report is kept, and information identifying the vehicle
The decal shows when the inspection was done, the name and address of the entity that holds the inspection report, and which vehicle it belongs to, so an officer can confirm the inspection at a glance.
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10. How should a commercial driver treat the speed posted for a curve?
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Answer B: As a limit set for cars in good conditions, so a heavy or tall load calls for a lower speed
Posted curve speeds are based on passenger vehicles in good conditions, and a tall or heavy load can roll over well below that speed.
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11. You are making a right turn and cannot complete it without swinging into the oncoming lane. An oncoming car is approaching. What should you do?
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Answer A: Wait and let the oncoming vehicle pass before completing the turn, and do not back up
The oncoming vehicle has the lane; backing up in traffic is dangerous, so hold your position and finish the turn once the way is clear.
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12. What do you check about the fire extinguisher during the pre-trip?
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Answer C: That it is securely mounted, charged and that you know how to operate it
An extinguisher must be held in its bracket, show a charged gauge and be understood by the driver, because a fire leaves no time to learn.
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13. A car is following you far too closely. What is the safest response?
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Answer A: Open up more space ahead so you can slow gradually, avoid sudden changes, and do not speed up
Extra room in front lets you slow smoothly instead of braking hard, which is exactly what the driver behind cannot cope with.
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14. Which road users are hardest to see at night?
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Answer C: Pedestrians and cyclists in dark clothing
People on foot or on a bicycle carry no lights of their own in many cases, so they appear only when the headlights reach them.
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15. How does a load carried high in the trailer affect safe speed through a curve?
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Answer B: It raises the rollover risk, so the speed must be lower
A high center of gravity lets the vehicle tip long before the tires lose their grip, so height of the load directly limits curve speed.
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16. Under the federal air brake standard, the low air pressure warning must come on before reservoir pressure drops below what level?
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Answer A: Before pressure falls below 60 psi
Federal Motor Vehicle Safety Standard 121 requires the warning to operate while service reservoir pressure is still above 60 psi, which leaves enough air to bring the vehicle to a normal stop.
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17. You are given a ratchet strap with a deep cut along one edge. What should you do?
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Answer C: Do not use it; a damaged tiedown cannot be relied on to hold its rated load
Cuts, burns, knots and broken stitching weaken a strap far below its marked working load limit, so it must be taken out of service.
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18. Why should the radiator cap never be removed while the engine is hot?
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Answer A: The cooling system is under pressure and scalding coolant can be sprayed out
A hot cooling system holds pressurized coolant above its normal boiling point, so releasing the cap can throw boiling liquid and steam at whoever is holding it.
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19. Why does extra weight make a long downgrade more dangerous even when the brakes are in good order?
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Answer B: Gravity adds to the momentum and the brakes heat up faster
The heavier the vehicle, the more energy the brakes must turn into heat, and heat is exactly what makes brakes fade on a long descent.
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20. Your axle weights are legal, but the tires on that axle are rated below the weight it is carrying. What controls?
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Answer B: The lower figure, so the tires may not be loaded beyond their marked rating
You may never exceed the rating marked on a tire, even when the axle limit would allow more, because an overloaded tire builds heat and can fail.
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21. What does the working load limit stamped on a chain or strap describe?
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Answer B: The maximum load the device is rated to hold in normal use
The working load limit is the rated safe capacity for everyday use, deliberately set well below the force that would break the device.
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22. What do the U-bolts on a leaf spring suspension do?
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Answer A: They clamp the spring assembly to the axle so it cannot move
U-bolts hold the spring pack tight against the axle, and a loose or broken U-bolt lets the axle slide out of place under load.
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23. What is wrong with resting your foot on the clutch pedal between shifts?
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Answer B: It slowly wears the clutch out, because the pressure plate is never fully engaged
Any weight on the pedal partly disengages the clutch, so it slips and wears; the foot belongs off the pedal once the shift is finished.
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24. Which of these is NOT on the federal list of parts and accessories the driver must be satisfied are in good working order before driving?
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Answer D: The seat cushion upholstery
The federal pre-trip rule names brakes, steering, lights and reflectors, tires, horn, wipers, mirrors, coupling devices, wheels and rims, and emergency equipment; seat upholstery is not on that list.
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25. Why is braking while in a curve dangerous in a heavy vehicle?
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Answer A: The wheels lock more easily and the vehicle can skid
Tires already using their grip to turn have little left for braking, so hard braking in a curve locks the wheels; slow down before the curve instead.
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26. Which factors determine a safe speed for a long, steep downgrade?
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Answer C: The steepness and length of the grade, the weight of the vehicle and cargo, and the road and weather conditions
Gravity, weight and traction all decide how fast a heavy vehicle can safely descend, which is why signs often post lower speeds for trucks on grades.
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27. You are stopped in traffic on a steep upgrade behind another truck. How much space should you leave in front?
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Answer A: Enough that you can pull away without rolling back into the vehicle behind you
Room ahead lets you get moving smoothly on a grade instead of having to hold the vehicle perfectly still or roll back toward the traffic behind.
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28. How long must a motor carrier keep a driver vehicle inspection report and the certification of repairs?
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Answer B: Three months
The original report and the certifications attached to it are kept on file for three months from the date the report was prepared.
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29. A vehicle carries placards on its front, rear and both sides. What is the main purpose of those placards?
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Answer C: To tell emergency responders and other people what hazard the cargo presents
Placards warn everyone around the vehicle, especially emergency responders, about the kind of hazard the cargo presents.
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30. Before raising the hood to inspect the engine compartment, what should you do?
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Answer B: Shut off the engine, set the parking brake and chock the wheels if needed
The engine compartment is inspected with the engine off and the vehicle secured so nothing can move or turn while your hands are inside.
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31. With water standing on the road, hydroplaning can begin at speeds as low as what?
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Answer D: 30 mph
When enough water covers the surface, the tires can ride up on it and lose contact at speeds as low as 30 mph.
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32. What is the minimum tread depth required in every major groove of a steering axle tire?
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Answer D: 4/32 inch
Federal rules set a higher tread minimum for front steering tires than for other positions because a steering tire failure is far harder to control.
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33. Which combination of warning devices meets the requirement for a stopped commercial vehicle?
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Answer A: Three reflective triangles, or six fusees, or three liquid burning flares
A vehicle may carry three bidirectional reflective triangles, six fusees or three liquid burning flares as its warning devices.
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34. You need fuel and the canopy over the pumps is low. What is the safe way to handle the clearance?
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Answer B: Stop and check the height at the entrance and at the way out before committing
Clearance can differ at the entry, under the canopy and at the exit, and a sloping approach changes the effective height, so both ends have to be checked.
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35. You hear air escaping from a trailer tire during the pre-trip. What should you do?
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Answer C: Have it repaired or replaced before the vehicle is driven
An audible leak means the tire is failing now, and federal rules do not allow a vehicle to be operated on a flat or leaking tire.
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36. When is a road surface usually most slippery during a rain shower?
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Answer A: Just after the rain starts, when water mixes with oil and dirt on the surface
The first rain lifts oil and dust into a slick film that has not yet been washed away, so the road is more slippery than it will be later.
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37. What must the driver confirm about the required emergency equipment before a trip?
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Answer A: That it is in place, in working order and ready to be used
Federal rules require the driver to be satisfied that the emergency equipment is present and ready, because it is needed at the worst possible moment.
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38. For a modern truck with low-friction parts and a streamlined shape, which gear should be used going down a grade?
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Answer D: A gear lower than the one needed to climb the same grade
Newer trucks roll more freely than older ones, so the old rule of using the climbing gear no longer holds; a lower gear is needed for enough engine braking.
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39. What does the driver check in the final step of the seven-step inspection?
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Answer A: Hydraulic brake leaks where those brakes are fitted, then the parking brake and the service brakes
The last step is done with the engine started: test for hydraulic leaks on vehicles with hydraulic brakes, then prove that the parking brake holds and that the service brakes stop the vehicle.
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40. Which of these is a major cause of skids in heavy vehicles?
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Answer C: Over-braking, over-steering, over-acceleration or driving too fast for conditions
Skids happen when the tires are asked for more grip than the road can give, which happens through braking, steering or accelerating too hard or simply too much speed.
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41. A car ahead of you keeps drifting within its lane and moving unevenly. What should you assume?
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Answer C: The driver may be distracted, lost or impaired
Unsteady lane keeping is a classic clue to a driver who is not in full control, so the safe response is to keep back and give plenty of room.
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42. What is the minimum tread depth for tires in positions other than the steering axle?
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Answer A: 2/32 inch
Drive and trailer tires must keep at least 2/32 inch of tread in a major groove, measured away from tie bars and fillets.
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43. What are you checking when you look at the batteries during the engine compartment inspection?
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Answer D: That they are securely mounted, the connections are tight and free of corrosion, and the caps are in place
Loose or corroded batteries can fail or short out, so the mount, cables, terminals and vent caps are all part of the check.
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44. Cargo carried inside the cab must never do which of these?
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Answer C: Block the driver view, restrict arm or leg movement, or block an emergency exit
Anything inside the cab must leave the driver a clear view, free movement of the arms and legs, and an unobstructed way out of the vehicle.
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45. Your vehicle is stopped just past a hill or curve that hides it from approaching traffic. How should the warning devices be adjusted?
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Answer A: Move the rearmost device farther back, up to 500 feet, so drivers are warned before the obstruction
When a hill or curve blocks the view, the farthest device is moved back beyond the obstruction so approaching drivers see the warning in time.
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46. What is the most common cause of a front-wheel skid, where the vehicle keeps going straight even though you turn the wheel?
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Answer B: Driving too fast for the conditions
A front-wheel skid usually comes from too much speed for the traction available; the only cure is to let the vehicle slow down and stop turning or braking so hard.
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47. Why do cracked spring hangers or badly worn shackles matter?
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Answer A: They allow the axle to shift out of its correct position under the vehicle
Hangers and shackles hold the spring, and therefore the axle, in place, so when they fail the axle can move and the vehicle may become uncontrollable.
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48. Two vehicles of exactly the same length make a right turn, one gently and one very tightly. What is the effect on off-tracking?
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Answer A: Turning more sharply increases off-tracking, so the tight turn needs more room
Off-tracking depends on both the length of the vehicle and the sharpness of the turn; a tighter corner brings the rear wheels further inside the path of the front wheels.
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49. What frame defects are you looking for during the walkaround?
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Answer A: Cracks, bends or broken cross members
The frame carries the whole load, so cracked or bent rails and broken cross members are serious structural defects.
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50. In dense city traffic you cannot see 12 seconds ahead because other vehicles block the view. What is the best response?
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Answer A: Slow down and open up more space in front of you
When you cannot see far enough ahead, extra following distance buys back the reaction time that the blocked view has taken away.