This section covers
Driving combinations
Combination vehicle air brakes
Inspecting combinations
This section provides information needed to pass the test for combination vehicles (tractor-trailer, doubles, triples, straight truck and 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-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.
Rollover Risk
More than half of truck driver’s deaths and 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 turn over. Fully loaded rigs are 10 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 a low center on your rig. If the load is to one side it makes a trailer lean, a rollover is more likely. Make sure the cargo is centered and spread out as much as possible. (Cargo distribution is covered in section 3 of this manual.)
Rollovers happen when you turn too fast. Drive slowly around corners, on ramps, and off ramps. Avoid quick lane changes, especially when fully loaded.
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 turn the trailer over. There are many accidents were only the trailer has overturned.
"Rearward amplification" causes the crack-the-whip effect. Figure 6 -- 1 shows eight types of combination vehicles and their 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 the 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-axel tractor-semi.
Influence of combination type or 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 one second for each ten 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, try the 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.
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 jacknife very quickly (figure 6-- 2). 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 break early. Don't 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." This is shown in figure 6-3.
The procedure for stopping a trailer skids is as follows:
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 he applied the brakes hard, check the mirrors to make sure that 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.
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 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 off 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 the corner so the rear and 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 (figure 6.5). 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 might collide with them when you turn.
Safety First!
Jayson Zamora
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