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Section 6: Combination Vehicles

This section provides information needed to pass the tests for combination vehicles (tractor-trailer, doubles , triples, straight truck and trailer). The information provides only the minimum knowledge needed for driving common combination vehicles. You should also study Section 7 if you need to pass the exams for doubles/triples.

6.1 Driving Combination Vehicles

Combination vehicles are usually heavier, longer and require more driving skill than single commercial vehicles. This means that drivers of combination vehicles need more knowledge and skill than drivers of single vehicles. In this section, important safety factors are discussed that apply specifically to combination vehicles.

Rollover Risks

More than half of truck driver deaths in accidents are the result of truck rollovers. When more cargo is piled up in a truck, the “center of gravity” moves higher up from the road. The truck becomes easier to turn over. Fully loaded rigs are 10 times more likely to roll over in a crash than empty rigs.

Two things will help prevent rollover: keeping the cargo as close to the ground as possible, and going slowly while turning. Keeping cargo low is even more important in combination vehicles than in straight trucks. Also, keep the load centered on your rig. If the load is to one side so it makes a trailer lean, a rollover is more likely. Make sure your cargo is centered and spread out as much as possible. (Cargo distribution is covered in detail in Section 3.)

Rollovers can happen when you turn too fast. Drive slowly around corners, on ramps and off ramps. Avoid quick lane changes, especially when fully loaded.

Rearward Amplification

Trucks with trailers have a dangerous “crack-the-whip” effect. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many accidents where only the trailer has overturned.

"Rearward amplification" causes the crack-the-whip effect. Figure 6-1 below shows eight types of combination vehicles and the rearward amplification each has in a quick lane change. Rigs with the least crack-the-whip effect are shown at the top, and those with the most at the bottom. Rearward amplification of 2.0 on the chart means that the rear trailer is twice as likely to turn over as the tractor. You can see that triples have a rearward amplification of 3.5. This means you can roll the last trailer of triples 3.5 times as easily as a 5-axle tractor-semi.

Figure 6-1
Influence of Combination Type
on Rearward Amplification

Steer Gently

Steer gently and smoothly when pulling trailers. If you make a sudden movement with your steering wheel, you could tip over. Follow far enough behind other vehicles - at least 1 second for each 10 feet of your vehicle length, plus another second if going over 40 mph. Look far enough down the road to avoid being surprised and having to make a sudden lane change. At night, drive slow enough to see obstacles with your headlights before it is too late to change lanes or stop gently. Slow down to a safe speed before going into a turn.

Brake Early

Control your speed whether fully loaded or empty. Large combination vehicles take longer to stop when they are empty than when they are fully loaded. When lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels. Your trailer can swing out and strike other vehicles. Your tractor can jackknife very quickly (see Figure 6-2 to the right). You also must be very careful about driving “bobtail” tractors (tractors without semitrailers). Tests show that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight.

In any combination rig, allow lots of following distance and look far ahead so you can brake early. Do not be caught by surprise and have to make a “panic” stop.

Prevent Trailer Skids

When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a “trailer jackknife” (see Figure 6-3 to the right).

Following is the procedure for stopping a trailer skid:

  • Recognize the skid. The earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors. Any time you apply the brakes hard, check the mirrors to make sure the trailer is staying where it should be. Once the trailer swings out of your lane, it is very difficult to prevent a jackknife.
  • Stop using the brake. Release the brakes to get traction back. Do not use the trailer hand brake (if you have one) to “straighten out the rig.” This is the wrong thing to do because the brakes on the trailer wheels caused the skid in the first place. Once the trailer wheels grip the road again, the trailer will start to follow the tractor and straighten out.

Turn Wide

When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels. This is called “offtracking” or “cheating.” Figure 6-4 to the right shows how offtracking causes the path followed by a tractor-semi to be wider than the rig itself. Longer vehicles will offtrack more. The rear wheels of the powered unit (truck or tractor) will offtrack some, and the rear wheels of the trailer will offtrack even more. Steer the front end wide enough around a corner so the rear end does not run over the curb, pedestrians, other vehicles, etc. However, keep the rear of your vehicle close to the curb. This will stop other drivers from passing you on the right.

If you cannot complete your turn without entering another traffic lane, turn wide as you complete the turn (see Figures 6-5-A and 6-5-B below). This is better than swinging wide to the left before starting the turn because it will keep other drivers from passing you on the right. If drivers pass on the right, you may collide with them when you turn.

Figure 6-4
Off-Tracking in a 90-degree turn

Figures 6-5-A and 6-5-B
Making a Proper Right Turn

There are three main points to take away from this paragraph:

  • More than half of truck driver deaths in accidents are the result of truck rollovers.
  • A higher center of gravity increases the chances of a rollover.
  • Fully loaded rigs are 10 times more likely to roll over in a crash than empty rigs.
The next couple paragraphs will discuss the "crack-the-whip" effect. While you don't need to memorize anything particular, you should be able to understand what "crack-the-whip" refers to.
Figure 6-1 does not need to be memorized. But you should have a general understanding of what type of what trailer setup experiences the most rearward amplification, thus, leading to a higher "crack-the-whip" effect.

It is very important to memorize proper following distance:

  • Maintain at least 1 second of following distance for each 10 feet of your vehicle length.
  • Add 1 second for speeds above 40mph.

Example 1: A 40ft vehicle traveling 30mph needs at least 4 seconds of following distance.

Example 2: A 40ft vehicle traveling 60mph needs at least 5 seconds of following distance.

We've seen this statement a few times now and it shows up on the written exam fairly often. Always remember to slow down before going into a turn.
Quite a few people get this incorrect on the written exam. But it's true, empty trucks actually take longer to stop than fully loaded trucks.
You do not need to memorize the actual definition of off-tracking, but you should read the rest of this paragraph and refer to Figure 6-4 to have an overall understanding of what off-tracking refers to. A question or two about off-tracking may show up on your written exam.
Take a look at figures 6-5-A and 6-5-B and make sure you understand how a right turn should be properly completed. The rest of this paragraph explains why a turn should be made in such a way. It's important for you to understand this turning method and why it's used.

CDL:

Commercial Driver's License (CDL)

A CDL is required to drive any of the following vehicles:

  • Any combination of vehicles with a gross combined weight rating (GCWR) of 26,001 or more pounds, providing the gross vehicle weight rating (GVWR) of the vehicle being towed is in excess of 10,000 pounds.
  • Any single vehicle with a GVWR of 26,001 or more pounds, or any such vehicle towing another not in excess of 10,000 pounds.
  • Any vehicle, regardless of size, designed to transport 16 or more persons, including the driver.
  • Any vehicle required by federal regulations to be placarded while transporting hazardous materials.

Bobtail:

"Bobtailing" means you are driving a tractor without a trailer attached.

Combination Vehicle:

A vehicle with two separate parts - the power unit (tractor) and the trailer. Tractor-trailers are considered combination vehicles.

Doubles:

Refers to pulling two trailers at the same time, otherwise known as "pups" or "pup trailers" because they're only about 28 feet long. However there are some states that allow doubles that are each 48 feet in length.

TWIC:

Transportation Worker Identification Credential

Truck drivers who regularly pick up from or deliver to the shipping ports will often be required to carry a TWIC card.

Your TWIC is a tamper-resistant biometric card which acts as both your identification in secure areas, as well as an indicator of you having passed the necessary security clearance. TWIC cards are valid for five years. The issuance of TWIC cards is overseen by the Transportation Security Administration and the Department of Homeland Security.

HOS:

Hours Of Service

HOS refers to the logbook hours of service regulations.

OWI:

Operating While Intoxicated

Review Questions - Click On The Picture To Begin...

Of all truck driver deaths, how many of them are caused as a result of truck rollovers?
  • More than 1/4
  • Rollover accidents rarely result in death
  • More than half
  • More than 3/4

Quote From The CDL Manual:

More than half of truck driver deaths in accidents are the result of truck rollovers

Next
When will a truck have the longest stopping distance?
  • When a truck is empty
  • When a truck is 1/2 loaded
  • When a truck is 1/4 loaded
  • When a truck is fully loaded

Quote From The CDL Manual:

Control your speed whether fully loaded or empty. Large combination vehicles take longer to stop when they are empty than when they are fully loaded. When lightly loaded, the very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels. Your trailer can swing out and strike other vehicles. Your tractor can jackknife very quickly. You also must be very careful about driving "bobtail" tractors (tractors without semitrailers). Tests show that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight.

Prev
Next
Which of the following can help reduce the chance of a truck rollover?
  • All of these can help reduce the chance of a truck rollover
  • At night, drive slow enough to see obstacles with your headlights before it is \ too late to change lanes or stop gently
  • Look far enough down the road to avoid being surprised and having to make a sudden lane change
  • Slow down to a safe speed before going into a turn

Quote From The CDL Manual:

Steer gently and smoothly when pulling trailers. If you make a sudden movement with your steering wheel, you could tip over. Follow far enough behind other vehicles (at least 1 second for each 10 feet of your vehicle length, plus another second if going over 40 mph). Look far enough down the road to avoid being surprised and having

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What is Off-Tracking?
  • Describes why the trailer always follows the same path as the tractor during turns
  • When the trailer wheels take a more inside path than the tractor wheels during a turn
  • When a trailer jackknife is to the point the trailer makes contact with the tractor
  • When the tractor wheels take a more inside path than the trailer wheels during a turn

Quote From The CDL Manual:

When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels. This is called "offtracking" or "cheating." Offtracking causes the path followed by a tractor-semi to be wider than the rig itself. Longer vehicles will offtrack more. The rear wheels of the powered unit (truck or tractor) will offtrack some, and the rear wheels of the trailer will offtrack even more. Steer the front end wide enough around a corner so the rear end does not run over the curb, pedestrians, other vehicles, etc. However, keep the rear of your vehicle close to the curb. This will stop other drivers from passing you on the right.

TruckingTruth's Advice:

Offtracking is why trucks and large vehicles must swing wide. You often see the bumper stickers which say "this vehicle makes wide right turns" and the reasoning for that is offtracking. Longer vehicles have to swing wide to avoid running the rear of the vehicle (or trailer) up onto the grass, curb, or striking pedestrians or people.

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Next
Which statement is true regarding driving a truck without a trailer (driving a bobtail)?
  • It takes a truck with no trailer longer to stop than a tractor-trailer loaded to maximum gross weight
  • A truck with no trailer will be very easy to control during an emergency stop
  • A truck with no trailer attached will stop very quickly
  • Trucks without trailers typically have more traction and drive better in adverse weather conditions

Quote From The CDL Manual:

You also must be very careful about driving "bobtail" tractors (tractors without semitrailers). Tests show that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight.

TruckingTruth's Advice:

Driving a bobtail (truck with no trailer) can be very dangerous. Trucks were designed to have trailers attached so driving will be difficult when a trailer is not attached.

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Next
When does a trailer jackknife most often occur?
  • When the trailer is fully loaded to maximum weight
  • All trailers, regardless of weight or cargo placement, have an equal chance of jackknifing
  • When the trailer is empty
  • When the trailer is loaded to half of its maximum weight

Quote From The CDL Manual:

When the wheels of a trailer lock up, the trailer will tend to swing around. This is more likely to happen when the trailer is empty or lightly loaded. This type of jackknife is often called a "trailer jackknife"

Prev
Next
Which type of combination vehicle experiences the worst type of Rearward Amplification?
  • Trucks pulling 1 trailer
  • Trucks pulling 2 trailers
  • Trucks pulling 3 trailers
  • All trucks, regardless of how many trailers, experience the same amount of Rearward Amplification

Quote From The CDL Manual:

The rear trailer is twice as likely to turn over as the tractor. You can see that triples have a rearward amplification of 3.5. This means you can roll the last trailer of triples 3.5 times as easily as a 5-axle tractor semi.

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What is Rearward Amplification?
  • Another term for a jackknifing trailer
  • The "crack-the-whip" effect of a trailer when a sudden lane change or quick left or right movement is made
  • When turning, the trailer makes a tighter turning radius than the tractor
  • The momentum generated by the trailer of a vehicle during long or steep downgrades

Quote From The CDL Manual:

Rearward Amplification - Trucks with trailers have a dangerous "crack-the-whip" effect. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many accidents where only the trailer has overturned.

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Next
When you turn suddenly while pulling doubles, which trailer is most likely to turn over?
  • Both trailers are equally as likely to turn over
  • The front trailer
  • The tractor is more likely to turn over than either trailer
  • The rear trailer

Quote From The CDL Manual:

Trucks with trailers have a dangerous "crack-the-whip" effect. When you make a quick lane change, the crack-the-whip effect can turn the trailer over. There are many accidents where only the trailer has overturned.

"Rearward amplification" causes the crack-the-whip effect. The rear trailer is twice as likely to turn over as the tractor. You can roll the last trailer of triples 3.5 times as easily as a 5-axle tractor-semi.

TruckingTruth's Advice:

When pulling more than 1 trailer, the rear-most trailer is always at a higher risk of overturning than the front trailer.

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What does the term "Bobtail" refer to?
  • A fully loaded truck and trailer
  • A tractor without a trailer attached
  • A truck and trailer that is not loaded
  • A parked trailer with no truck attached

Quote From The CDL Manual:

You also must be very careful about driving "bobtail" tractors (tractors without semitrailers). Tests show that bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight.

Prev
Next
Fully loaded trucks are how many times more likely to roll over in a crash than an empty truck?
  • Twice as likely
  • Four times as likely
  • Three times as likely
  • Ten times as likely

Quote From The CDL Manual:

More than half of truck driver deaths in accidents are the result of truck rollovers. When more cargo is piled up in a truck, the "center of gravity" moves higher up from the road. The truck becomes easier to turn over. Fully loaded rigs are 10 times more likely to roll over in a crash than empty rigs.

TruckingTruth's Advice:

Rollover accidents are extremely serious! Slow down before your turns, not during the turn. You can always speed up through a turn, but you may not be able to slow down enough to avoid an accident.

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Next
What are the two main types of a jackknife?
  • Tractor jackknife and trailer jackknife
  • Drive wheel jackknife and trailer wheel jackknife
  • Steer jackknife and drive jackknife
  • Front jackknife and rear jackknife

Quote From The CDL Manual:

Tractor Jackknife: Rear tractor wheels locked up or spinning.

Trailer Jackknife: Trailer wheels locked and sliding

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Next
What is a way to properly stop a trailer skid?
  • Try to stop the truck as quickly as possible by pressing as hard as you can into the brake pedal
  • There is no way to stop a trailer skid once it begins
  • Release the brakes to get traction back
  • Use the trailer hand brake to help straighten out the trailer

Quote From The CDL Manual:

Following is the procedure for stopping a trailer skid:

1. Recognize the skid. The earliest and best way to recognize that the trailer has started to skid is by seeing it in your mirrors. Any time you apply the brakes hard, check the mirrors to make sure the trailer is staying where it should be. Once the trailer swings out of your lane, it is very difficult to prevent a jackknife.

2. Stop using the brake. Release the brakes to get traction back. Do not use the trailer hand brake (if you have one) to "straighten out the rig." This is the wrong thing to do because the brakes on the trailer wheels caused the skid in the first place. Once the trailer wheels grip the road again, the trailer will start to follow the tractor and straighten out.

TruckingTruth's Advice:

Make certain you glance in your mirrors while stopping in slick conditions. It is very important to catch a jackknife early. The more the trailer swings into a jackknife, the harder it will be to recover.

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