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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 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 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.

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. They take longer to stop than a tractor-semitrailer loaded to maximum gross weight.

In any combination truck, tractor, or semi, 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.

Multiple-Choice Questions:

Question #313 (1 of 6)

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Why do large combination vehicles take longer to stop when they are empty than when they are fully loaded?

  • The very stiff suspension springs and strong brakes give poor traction and make it very easy to lock up the wheels.
  • The air pressure that builds up in the secondary air system when the trailer is empty can cause the wheels to lock up
  • Hard braking with an empty trailer will exceed the tire load limit
  • All these are correct

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.

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Question #312 (2 of 6)

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You should do which of the following to help avoid rollovers?

  • At night, drive slowly enough to see obstacles with your headlights before it is too late to change lanes or stop gently
  • Steer gently and smoothly when you are pulling trailers
  • All these are correct
  • Slow down to a safe speed before going into a turn

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.

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Question #314 (3 of 6)

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Which of the following is true about stopping a bobtail tractor (tractors without semitrailers)?

  • Bobtails stop far more quickly using the engine brake instead of the foot brake, especially on slick roads
  • Bobtails will stop more quickly than a truck with a loaded trailer, but only if the air release valve is open in the secondary air system
  • Bobtails can be very hard to stop smoothly. They take longer to stop than a tractor-semitrailer loaded to maximum gross weight.
  • Bobtails have very little weight, so they stop faster than a loaded trailer

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. They take longer to stop than a tractor-semitrailer loaded to maximum gross weight.

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Question #309 (4 of 6)

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If you make a quick lane change, what can happen?

  • All these are correct
  • The steering knuckle can buckle under the pressure
  • The "crack-the-whip" effect can turn the trailer over
  • The cargo's center of gravity can raise as much as 22%

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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Question #311 (5 of 6)

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Rearward amplification of 2.0 in the chart means:

  • The rear trailer is twice as likely to turn over as the tractor.
  • The tractor is twice as likely to turn over as the trailer
  • None of these are correct
  • The rear trailer is half as likely to turn over as the tractor

Rearward amplification of 2.0 in the chart means that the rear trailer is twice as likely to turn over as the tractor.

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Question #310 (6 of 6)

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What causes the "crack-the-whip" effect?

  • All these are correct
  • High-point thrusting
  • Rearward amplification
  • Low tire pressures

"Rearward amplification" causes the crack-the-whip effect.

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