CDL Test Practice Exams & CDL Training Materials
Combination Vehicles CDL Practice Test
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Taking the CDL test will be much easier after going through our practice CDL tests here online. This is the Combination Vehicles portion of the CDL written practice exam. These questions are very similar to the ones you'll find on the actual CDL written test you'll be taking to get your CDL license. Follow along with the test questions below and read through the explanations that follow each CDL test question to get some better insights into the CDL exam and the trucking industry itself.
1. Combination vehicles are usually heavier, longer and require more driving skill than single commercial vehicles.
A. True
B. False
⊞ Answer:
A. True
This answer is true in most cases, but I will say that driving dump trucks safely, day in and day out is extrememly difficult also - far more than people would guess.
2. More than half of truck driver deaths in crashes are the result of truck rollovers. Trucks turn over easier when fully loaded and are:
A. Two times more likely to roll over in a crash than empty rigs.
B. Five times more likely to roll over in a crash than empty rigs.
C. Ten times more likely to roll over in a crash than empty rigs.
⊞ Answer:
C. Ten times more likely to roll over in a crash than empty rigs.
The center of gravity is much higher in a loaded tractor trailer than in an empty tractor trailer.
3. The following two things will help prevent rollovers.
A. Keep the cargo close to the front and drive slowly around turns.
B. Keep the cargo as close to the ground as possible and drive slowly around turns.
C. Keep the cargo to the back of the rig and drive the speed limit.
⊞ Answer:
B. Keep the cargo as close to the ground as possible and drive slowly around turns.
Again, you want the center of gravity to be low, and of course go easy around those turns - especially the off-ramps, where excessive speed causes a lot of tractor trailers to roll.
4. When you turn suddenly while pulling doubles, which trailer is most likely to turn over?
A. The rear trailer is twice as likely to turn over as the tractor.
B. The front trailer is twice as likly to turn over as the tractor.
C. Each trailer is equally likely to turn over as is the tractor.
⊞ Answer:
A. The rear trailer is twice as likely to turn over as the tractor.
This is the whipping effect - the rear trailer gets whipped around the corner, making it far more likely to tip.
5. Why should you not use the trailer hand brake to straighten out a jackknifing trailer?
A. Because the brake handle is too hard to reach.
B. Because the brakes on the trailer wheels cause the skid in the first place.
C. Because the brakes on the trailer wheels will not respond to the hand brake.
⊞ Answer:
B. Because the brakes on the trailer wheels cause the skid in the first place.
Well, first of all, not too many company trucks come with a trailer brake handle anymore. However, locking up the trailer wheels will cause the trailer to want to slide out and around the tractor. The CDL manual refers to that as a "trailer jacknife". A "tractor jacknife" is when you lock up the drive tires on the tractor and the tractor spins out sideways as the trailer continues to push forward.
6. What is off tracking?
A. When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels.
B. When a vehicle goes around a corner, and the driver pulls off the road.
C. When a vehicle goes around a curve and turns back the other way.
⊞ Answer:
A. When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels.
What they are referring to is that the trailer tires make a sharper turn around a curve than the tracor tires, which is why you have to swing wide when going around a turn. This is not to be confused with "dog tracking" which is when the trailer tandems are out of alignment and the trailer does not track directly behind the tractor when driving in a straight line. And this is not to be confused with "cat-tracking" - which is the act of purposely running over a cat.
Yeah, I just totally made that last one up.
7. What is a "trailer jacknife?"
A. When the tires on a trailer blow out.
B. When the wheels of a trailer lock up and the trailer swings around.
C. When the wheels of a trailer lock up and the trailer pulls the tractor to a stop.
⊞ Answer:
B. When the wheels of a trailer lock up and the trailer swings around.
This is the type of jacknife they were referring to in question 5. They should have specified that.
8. Should you swing wide to the left before starting a 90 degree turn to the right?
A. Yes
B. No
⊞ Answer:
B. No
The answer should be "no, not if you can help it." In the real world, things do not always go according to the book. You don't want to swing wide to the left if you can help it because cars will sometimes try to pass you on the right if you leave them room to do it. But sometimes the road, parking lot, or driveway you are turning onto isn't wide enough to permit you to make the turn without first swinging way out to the left. That's just the reality of it. But on your CDL test, the correct answer is B.
9. Why should you not use the trailer hand valve while driving?
A. Because it won't work as well as the foot brake.
B. Because you should use the parking brake.
C. Because of the danger of making the trailer skid.
⊞ Answer:
C. Because of the danger of making the trailer skid.
Again, most trucks don't have this feature anymore. But if you get one that does, there is very rarely any use for it. You're best off just leaving it alone. It can get you into a bad situation in a hurry, especially if you're a new driver.
10. Describe what the trailer air supply control does.
A. It is used to supply the trailer with air, shut the air off, and put on the trailer emergency brakes.
B. It is a yellow, six sided knob used to control the tractor protection valve.
C. It is a device used to keep the trailer behind the tractor.
⊞ Answer:
A. It is used to supply the trailer with air, shut the air off and put on the trailer emergency brakes.
Basically, it controls the parking brake for the trailer. When you push it in, it supplies air to the trailer and releases the parking brake. When you pull it out, the air is released from the trailer, the air supply to the trailer is cut off, and the parking brakes apply.
11. Describe what the service line is for:
A. To carry air to the parking brake to control the trailer brakes.
B. To carry air to a relay valve that allows the trailer brakes to be applied quickly.
C. To carry air to the cab of the rig to cool it.
⊞ Answer:
B. To carry air to a relay valve that allows the trailer brakes to be applied quickly.
Basically, this is the air line that is controlled by the brake pedal.
12. What are other names for the Service Air Line?
A. Control line or signal line
B. Straight line or control line
C. Curved line or signal line
⊞ Answer:
A. Control line or signal line
Or, in the trucker's world, "blue air line". How's that for getting straight to the point and keeping it simple? NEXT!
13. What is the emergency air line?
A. Air lines connecting the parking brake to the trailer brakes.
B. Air lines that control the emergency brakes of the tractor.
C. Air lines that control the emergency brakes on combination vehicles.
⊞ Answer:
C. Air lines that control the emergency brakes on combination vehicles.
Emergency brakes are also known as parking brakes. Same exact thing. When you pull the parking brake, air to the emergency air line is cut off and all of the air is realeased from the trailer which applies the emergency brake, or parking brake - same thing.
14. What is the emergency air line for?
A. To engage the front brakes of the tractor in case of an emergency.
B. To engage the trailer brakes in case of air pressure loss.
C. To have extra air pressure in case you need it.
⊞ Answer:
B. To engage the trailer brakes in case of air pressure loss.
I don't like this question - but you may find it on the CDL test. The emergency air line supplies air to the trailer to release the trailer brakes. If you lost air pressure to the trailer through the emergency air line the spring pressure in the spring brake will apply the trailer brakes. Air from the emergency line does not engage the trailer brakes. A loss of air to this system will engage the trailer brakes. Poorly worded question, but you have to be aware of it.
15. Why should you use chocks when parking a trailer without spring brakes?
A. Because you don't want the trailer rolling down a hill.
B. Because if the air pressure leaks away or down, there will be no brakes.
C. Because some states require chocking of trailer wheels.
⊞ Answer:
B. Because if the air pressure leaks away or down, there will be no brakes.
Ok, another poor question. First of all, I've never heard of a trailer that doesn't have spring brakes. They must have existed at one time or this question wouldn't exist. But I've never heard of such a beast. But again, know this answer in case it appears on your CDL test.
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