Page 55 - Georgia Commercial Drivers Manual 2020
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6SECTION
COMBINATION 6.1 – Driving Combination change, the crack-the-whip effect can turn the
VEHICLES Vehicles Safely trailer over. There are many accidents where
only the trailer has overturned.
This Section Covers Combination vehicles are usually heavier,
longer, and require more driving skill than “Rearward amplification” causes the crack-
Driving single commercial vehicles. This means that the-whip effect. Figure 6.1 shows eight types of
Combinations.............. 6.1 drivers of combination vehicles need more combination vehicles and the rearward ampli-
Combination Vehicle knowledge and skill than drivers of single fication each has in a quick lane change. Rigs
Air Brakes................... 6.2 vehicles. In this section, we talk about some with the least crack-the-whip effect are shown
Antilock Brake important safety factors that apply specifically at the top and those with the most, at the bot-
Systems...................... 6.3 to combination vehicles. tom. Rearward amplification of 2.0 in the chart
Coupling and means that the rear trailer is twice as likely to
Uncoupling................ 6.4 6.1.1 – Rollover Risks turn over as the tractor. You can see that triples
Inspection More than half of truck driver deaths in have a rearward amplification of 3.5. This means
Combinations............. 6.5 you can roll the last trailer of triples 3.5 times
crashes are the result of truck rollovers. When as easily as a five-axle tractor.
This section provides information needed more cargo is piled up in a truck, the “center of
to pass the tests for combination vehicles gravity” moves higher up from the road. The Steer gently and smoothly when you are pull-
(tractor-trailer, doubles, triples, straight truck truck becomes easier to turn over. Fully loaded ing trailers. If you make a sudden movement
with trailer). The information is only to give you rigs are ten times more likely to roll over in a with your steering wheel, your trailer could tip
the minimum knowledge needed for driving crash than empty rigs. over. Follow far enough behind other vehicles
common combination vehicles. You should (at least 1 second for each 10 feet of your vehicle
also study Section 7 if you need to pass the The following two things will help you pre- length, plus another second if going over 40
test for doubles and triples. vent rollover--keep the cargo as close to the mph). Look far enough down the road to avoid
ground as possible, and drive slowly around being surprised and having to make a sudden
turns. Keeping cargo low is even more impor- lane change. At night, drive slowly enough to
tant in combination vehicles than in straight see obstacles with your headlights before it is
trucks. Also, keep the load centered on your too late to change lanes or stop gently. Slow
rig. If the load is to one side so it makes a trailer down to a safe speed before going into a turn.
lean, a rollover is more likely. Make sure your
cargo is centered and spread out as much as 6.1.3 – Brake Early
possible. (Cargo distribution is covered in Sec- Control your speed whether fully loaded or
tion 3 of this manual.)
empty. Large combination vehicles take longer
Rollovers happen when you turn too fast. to stop when they are empty than when they
Drive slowly around corners, on ramps, and are fully loaded. When lightly loaded, the very
off ramps. Avoid quick lane changes, especially stiff suspension springs and strong brakes give
when fully loaded. poor traction and make it very easy to lock up
the wheels. Your trailer can swing out and strike
6.1.2 – Steer Gently other vehicles. Your tractor can jackknife very
Trucks with trailers have a dangerous “crack- quickly. You also must be very careful about
driving “bobtail” tractors (tractors without
the-whip” effect. When you make a quick lane semitrailers). Tests have shown that bobtails
INFLUENCE OF COMBINATION TYPE ON REARWARD AMPLIFICATION
Image courtesy ©iStockphoto.com/MCCAIG Figure 6.1
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