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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