District of Columbia CDL: Double & Triple Trailers Practice Test

Not quite. The answer is 20 to 45 psi.

The tractor protection valve keeps air in the tractor or truck brake system should the trailer break away or develop a bad leak. The tractor protection valve is designed to close automatically when the air pressure falls into the range specified by the manufacturer, typically 20–45 psi.

Not quite. The answer is step on and off the brake pedal several times.

To test the tractor protection valve, charge the trailer air brake system, turn off the engine, and lower the air pressure by stepping on and off the brake pedal several times. The tractor protection valve control (also called the trailer air supply control) should pop out (or go from the “normal” to the “emergency” position) when the air pressure falls into the range specified by the manufacturer (typically 20–45 psi).

Not quite. The answer is slightly raised.

To test the coupling of the dolly to the second trailer, you'll pull the tractor forward so the dolly will pull against the kingpin of the second trailer. Before doing so, raise the trailer's landing gear slightly. This will prevent damage if the trailer moves, and prevent the trailer from falling onto the ground if the dolly pulls away from the trailer.

Not quite. The answer is You must drive a double or triple very smoothly to avoid a rollover or jackknife.

Take special care when pulling two or three trailers. Doubles and triples are more likely to turn over than other combination vehicles because of the "crack-the-whip" effect. You must steer gently when pulling trailers. To prevent the trailers from rolling over, you must steer gently and go slowly around corners, on-ramps, off-ramps, and curves.

Not quite. The answer is a coupling device that connects a semitrailer to the rear of a tractor-trailer.

The converter dolly connects a semitrailer to the rear of a tractor-trailer, forming a double or triple combination.

Not quite. The answer is to lower its landing gear.

After you have uncoupled the rear trailer from the converter dolly, you need to uncouple the converter dolly from the front trailer. You do this as follows: 1. Lower the landing gear of the dolly. 2. Disconnect the safety chains. 3. Apply the dolly's spring brakes or chock its wheels to keep the dolly from moving. 4. Release the pintle hook connecting the dolly to the trailer ahead of it. 5. Slowly pull clear of the dolly.

Not quite. The answer is Some do; some don't.

All converter dollies built since 1975 are required to have spring brakes. Older converter dollies that don't have spring brakes have emergency air brakes, which work from the air stored in the trailer air tank.

Not quite. The answer is Go to the rear of the second trailer and open the emergency line shut-off valve.

To check whether air is being supplied to the second trailer, go to the rear of the trailer and open the emergency line shut-off valve. You should hear air escaping, indicating that the entire system is charged up to that point. (For a triple combination vehicle, perform this procedure with the third trailer.)

Not quite. The answer is A sudden movement with the steering wheel can result in a tipped-over rear trailer.

Because of rearward amplification (the "crack-the-whip" effect), the rearmost trailer is much more prone to rolling over. Avoid sudden steering changes that could cause the rearmost trailer to roll over.

Not quite. The answer is that you can pull straight through.

When parking a long vehicle, you should look for a parking space that you can go straight through rather than back out of. Also note the layout of the parking lot and your parking space's location within it so you won't have difficulty exiting the parking space later.

Not quite. The answer is Doubles/triples flip over from quick steering movements more easily than do many other vehicles.

Because of rearward amplification (the "crack-the-whip" effect), steering changes affect the rearmost trailer much more than they affect the front trailer or the tractor. Avoid quick or sharp steering changes that could cause the rearmost trailer to roll over.

Not quite. The answer is make things look smaller and farther away than they really are.

A convex mirror has a wider field of view than a flat mirror. However, objects viewed in a convex mirror look smaller and farther away. Remember this when you're trying to judge the distance of an object that you're viewing in a convex mirror.

Not quite. The answer is 3

In this context, articulation or pivoting means movement between two units separated by a joint, such as between a tractor and a trailer. The standard or STAA double combination in the CDL handbook has 3 points of articulation: (1) the fifth wheel of the tractor; (2) the pintle hook of the front trailer; (3) the fifth wheel of the converter dolly.

Not quite. The answer is Check for normal air pressure. Release the parking brakes. Pull forward slowly. Apply the trailer brakes, using the hand control if so equipped.

Checking your trailer's service brakes is a four-step process: 1. Check for normal air pressure. 2. Release the parking brakes. 3. Pull your vehicle forward slowly. 4. Apply the trailer brakes, using the hand control (trolley valve) if so equipped. If the trailer brakes are properly connected and working, you should feel them come on at this point.

Not quite. The answer is a yellow lamp on the left side.

A converter dolly with antilock brakes is required to have a yellow ABS lamp on its left side. All converter dollies built since March 1, 1998 have antilock brakes.

Not quite. The answer is Older models may lack spring brakes.

All converter dollies built since 1975 are required to have spring brakes. All converter dollies built since 1998 are required to have antilock brakes as well.

Not quite. The answer is Hook the converter dolly to the front semitrailer.

To couple standard doubles, the first step is to correctly position the dolly in front of the second semitrailer. You can do this by hand or by picking up the dolly with the first semitrailer. Either way, the dolly must be connected to the first semitrailer before proceeding further. Once this is done, back the tractor, first semitrailer, and dolly until you back the dolly underneath the second semitrailer. Only after you've checked and tested the coupling, and connected all the air lines and electrical lines, should you raise the landing gear.

Not quite. The answer is foot pedal.

The trailer hand valve (also called the Trolley Valve or Johnson Bar) operates only the trailer brakes, not the tractor brakes. Hence, use it only when you want to test the trailer brakes. Don't use it in normal driving because braking the trailers and not the tractor can cause a trailer to skid. The foot brake sends air to all the brakes on the vehicle – the tractor and all trailers. There is much less danger of causing a skid or jackknife when using just the foot brake.

Not quite. The answer is The pintle eye of the dolly should be in place in the pintle hook of the trailer.

A pre-trip inspection of a double or triple combination vehicle includes an inspection of each converter dolly in the vehicle. Among other things, check the coupling of the dolly to the trailer in front. Make sure that the pintle eye of the dolly is in place in the pintle hook of the trailer, and the pintle hook is latched closed.

Not quite. The answer is the vehicle is more likely to roll over or jackknife.

A double or triple combination vehicle is more likely to roll over or jackknife from sudden braking or steering changes. Look far enough ahead that you can slow down or change lanes gradually when necessary.