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Flashcards in Combination Vehicles Deck (21):

How do you confirm that air is going to all breaks in your trailers?

Opening in the emergency line shut off valve And then the Service line valve The rear of the last trailer And listening for Air escaping each time.


How much space should there be betweenThe upper and lower fifth wheel After coupling.




Refers to pulling two trailers at the same time, otherwise known as "pups" or "pup trailers"


Rollover risks

More than half of truck driver deaths in accidents 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.


How to prevent a rollover

keeping the cargo as close to the ground as possible, and going slowly while turning.

keep the load centered on your rig

Avoid quick lane changes, especially when fully loaded.


Rearward Amplification

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.

Triple 27ft trailer trailer setup experiences the most rearward amplification, thus, leading to a higher "crack-the-whip" effect.



Steer gently and smoothly

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

At night, drive slow enough to see obstacles with your headlights

Slow down to a safe speed before going into a turn.



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.


Bobtail tractors

tractors without semitrailers

bobtails can be very hard to stop smoothly. It takes them longer to stop than a tractor-semitrailer loaded to maximum gross weight.


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. - trailer jackknife

Recognize the skid -watch trailer in your mirrors

Stop using the brake - Release the brakes to get traction back.
Once the trailer wheels grip the road again, the trailer will start to follow the tractor and straighten out.

Do not use the trailer hand brake because the brakes on the trailer wheels caused the skid in the first place.


Turn wide and off tracking

When a vehicle goes around a corner, the rear wheels follow a different path than the front wheels.

offtracking causes the path followed by a tractor-semi to be wider than the rig itself.

The rear wheels of the powered unit (truck or tractor) will offtrack some, and the rear wheels of the trailer will offtrack even more

Longer vehicles will offtrack more.


Steer the front end wide enough around a corner so the rear end does not run over the curb

keep the rear of your vehicle close to the curb. This will stop other drivers from passing you on the right.


Trailer hand valve

The trailer hand valve works the trailer brakes.

The trailer hand valve should be used only to test the trailer brakes. Do not use it in driving because of the danger of making the trailer skid.

Never use the hand valve for parking because all the air might leak out, unlocking the brakes (in trailers that do not have spring brakes.) Always use the parking brakes when parking.


Tractor Protection Valve

keeps air in the tractor or truck should the trailer break away or develop a bad leak. The tractor protection valve is controlled by the “trailer air supply” control valve in the cab.

The control valve allows you to open and shut the tractor protection valve.

The tractor protection valve will close automatically if air pressure is low (in the range of 20 to 45 psi).

When the tractor protection valve closes, it stops any air from going out of the tractor. It also lets the air out of the trailer emergency line. This causes the trailer emergency brakes to come on.


Trailer Air Supply Control

trailer air supply control on newer vehicles is a red 8-sided knob used to control the tractor protection valve.

Push it in to supply the trailer with air, and pull it out to shut the air off and put on the trailer emergency brakes.

The valve will pop out, thus closing the tractor protection valve, when the air pressure drops into the range 20 to 45 psi.


Service air line (control line) - blue

The service line controls the "regular" brakes when you use the brake pedal.

Depending on how hard you press the foot brake or hand valve, the pressure in the service line will similarly change.

connected to relay valves. These valves allow the trailer brakes to be applied more quickly than would otherwise be possible.


Emergency air line (supply line) - red

has two purposes: (1) To supply air to the trailer air tanks, and
(2) to control the emergency brakes on combination vehicles.

Loss of air pressure in the emergency line causes the trailer emergency brakes to come on.

The pressure loss could be caused by a trailer breaking loose, thus tearing apart the emergency air hose or by a hose, metal tubing or other part that breaks, letting the air out.

When the emergency line loses pressure, it also causes the tractor protection valve to close (the air supply knob will pop out).

When the tractor protection valve closes, the red "emergency brake" valve in the cab of the truck will pop out. This will stop any air from flowing out of the tractor and into the trailer. That way, if there is a major air leak leading to air pressure loss in the system, the trailer brakes will activate before the tractor brakes, helping you keep some control of the vehicle.


Hose Couplers (Glad Hands)

Glad hands" are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer.

glad hands have rubber seals which prevents air from escaping. The rubber seals must be checked to be sure they are in good condition to prevent air leaks.

When connecting the glad hands, press the two seals together with the couplers at a 90-degree angle to each other.

dead end” or dummy couplers to which the hoses may be attached when they are not in use. This will prevent water and dirt from getting into the coupler and the air lines.

glad hands can sometimes be locked together (depending on the couplings). It is very important to keep the air supply clean.


Glad hands - crossed air lines

If you do cross the air lines, supply air will be sent to the service line instead of going to charge the trailer air tanks.

Air will not be available to release the trailer spring brakes (parking brakes).

If the spring brakes do not release when you push the trailer air supply control, check the air line connections.

Always test the trailer brakes before driving with the hand valve or by pulling the air supply (tractor protection valve) control.

Pull gently against them in a low gear to make sure the brakes work.


Trailer Air Tanks

Each trailer has one or more air tanks.

They are filled by the emergency (supply) line from the tractor.

They provide the air pressure used to operate trailer brakes.

It is important not to let water and oil build up in the air tanks. If you do, the brakes may not work correctly. Each tank has a drain valve on it, and you should drain each tank every day.


Shut-off Valves

Shut off valves are only used on trailers designed to be towed in conjunction with other trailers

The shut off valves, when in the open position, allows air to flow out to another trailer. If you're only pulling one trailer, the shut off valves should always be closed or else the air will simply escape out the back of the trailer.

If you're pulling more than one trailer, all valves should be open except the very last trailer, which should have all valves in the closed position.


Fifth wheel

The "Fifth Wheel" is the plate and locking mechanism on the tractor which the trailer rest on and locks into.