6. Combination Vehicles 

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

6.1 Driving Combination Vehicles Safely

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, we talk about some important safety factors that apply specifically to combination vehicles.

6.1.1 Rollover Risks

More than half of truck driver deaths in crashes 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 tum over. Fully loaded rigs are ten times more likely to roll over in a crash than empty rigs.

The following two things will help you prevent rollover--keep the cargo as close to the ground as possible, and drive slowly around turns. 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 Section 3 of this manual.)

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

6.1.2 Steer Gently

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

"Rearward amplification" causes the crack-the-whip effect. Figure 6.1 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 in 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 five-axle tractor.

Influence of Combination Type on Rearward Amplification, long description is at end of document under heading "Long descriptions"
Figure 6.1: Influence of Combination Type on Rearward Amplification

Steer gently and smoothly when you are pull­ing trailers. If you make a sudden movement with your steering wheel, your trailer 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 slowly 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.

6.1.3 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. You also must be very careful about driving "bobtail" tractors (tractors without semitrailers). Tests have shown 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. Don't be caught by surprise and have to make a "panic" stop.

6.1.4 Railroad-highway Crossings

Railroad-highway crossings can also cause problems, particularly when pulling trailers with low underneath clearance.

These trailers can get stuck on raised crossings:

  • Low slung units (lowboy, car carrier, moving van, possum-belly livestock trailer).
  • Single-axle tractor pulling a long trailer with its landing gear set to accommodate a tandem-axle tractor.

If for any reason you get stuck on the tracks, get out of the vehicle and away from the tracks. Check signposts or signal housing at the crossing for emergency notification information. Call 911 or other emergency number. Give the location of the crossing using all identifiable land­marks, especially the DOT number, if posted.

6.1.5 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 6.2.

Trailer Jacknife, where the cab continues in the line of travel and the back trailer wheels are locked-up and sliding left into oncoming traffic
Figure 6.2: Trailer Jacknife

The procedure for stopping a trailer skid is: 

  • 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's 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 since 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. 

*From R.D. Ervin, R.L. Nisconger, C.C. MacAdam, and P.S. Fancher, "Influence of size and weigh variables on the stability and control properties of heavy trucks, "University of Mich­igan Transportation Research Institute, 1983.

Off tracking in a 90 degree turn where the cab is on the leftmost side of the lane and also this is the path followed by the outside trailer tire, the cargo is on the rightmost side of the lane which is the path followed by the innermost tire
Figure 6.3: Off Tracking in a 90 degree turn

6.1.6 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 6.3 shows how offtracking causes the path followed by a tractor 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. If there is more than one trailer, the rear wheels of the last trailer will offtrack the most. Steer the front end wide enough around a corner so the rear end does not run over the curb, pedestrians, 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. 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. See Figure 6.4.

Jag handle (where you go onto the other side of the road) is incorrect, however Button hook, where you go onto the other side of the road that you are turning into is correct
Figure 6.4

6.1.7 Backing with a Trailer.

Backing with a Trailer.

When backing a car, straight truck, or bus, you turn the top of the steering wheel in the direction you want to go. When backing a trailer, you turn the steering wheel in the opposite direction. Once the trailer starts to turn, you must turn the wheel the other way to follow the trailer.

Whenever you back up with a trailer, try to position your vehicle so you can back in a straight line. If you must back on a curved path, back to the driver's side so you can see. See Figure 6.5.

Backing a trailer: turn wheel right for the trailer to go left, turn wheel left to make the trailer go right
Figure 6.5
Look at Your Path.

Look at your line of travel before you begin. Get out and walk around the vehicle. Check your clearance to the sides and overhead, in and near the path your vehicle.

Use Mirrors on Both Sides.

Check the out­side mirrors on both sides frequently. Get out of the vehicle and re-inspect your path if you are unsure.

Back Slowly.

This will let you make corrections before you get too far off course.

Correct Drift Immediately.

As soon as you see the trailer getting off the proper path, correct it by turning the top of the steering wheel in the direction of the drift.

Pull Forward.

When backing a trailer, make pull-ups to re-position your vehicle as needed.

Test Your Knowledge - Subsection 6.1

  • What two things are important to prevent rollover?
  • When you turn suddenly while pulling doubles, which trailer is most likely to turn over?
  • Why should you not use the trailer hand brake to straighten out a jackknifing trailer?
  • What is offtracking?
  • When you back a trailer, you should position your vehicle so you can back in a curved path to the driver's side. True or False?
  • What type of trailers can get stuck on railroad-highway crossings?

These questions may be on your test. If you can't answer them all, re-read subsection 6.1.