COMBINATION — SAFE HANDLING
Why do combination vehicles require more driving skill than single vehicles?
Their added length, weight, and pivot points make maneuvering and control more difficult.
What should you do to your speed before making a tight turn with a trailer?
Slow down before entering the turn.
What trailer part is captured by the fifth-wheel locking jaws?
The kingpin.
After coupling, should there be a gap between the trailer’s upper plate and the fifth wheel?
No. The plates should be fully in contact.
What are glad hands?
Couplers that connect the tractor’s and trailer’s air lines.
What supports the front of a semitrailer after the tractor is removed?
The landing gear.
What powers an air-brake system?
Compressed air.
What does PSI stand for?
Pounds per square inch.
What should you do if the low-air warning activates while driving?
Stop safely as soon as possible and have the problem corrected.
What is brake lag?
The delay between pressing the brake pedal and air pressure applying the brakes.
What happens to rollover risk when a load’s center of gravity is raised?
Rollover risk increases.
When beginning to back a trailer, which direction do you initially turn the steering wheel?
Opposite the direction you want the trailer to move.
Before coupling, how should the tractor be positioned relative to the trailer?
Straight in front of the trailer.
What part of the kingpin must the fifth-wheel jaws lock around?
The kingpin’s shank, not its lower head.
Which trailer air line is normally identified by red?
The emergency or supply line.
What kind of surface should you choose before uncoupling a trailer?
Firm ground that can support it, preferably level.
What compresses the air.
The air compressor.
Before driving a vehicle with a dual air system, what minimum pressure should be built in both systems?
At least 100 psi.
Why should wheels be chocked when parking brakes must be released for a stationary air-brake test?
To prevent the vehicle from rolling.
Why should you avoid repeatedly pumping air brakes during normal braking?
Each application uses air and can unnecessarily reduce the available pressure.
What is offtracking?
During a turn, the rear wheels follow a path inside the path of the front wheels.
Once a backing trailer begins moving in the desired direction, what should you do with the steering?
Steer back toward the trailer’s direction to follow it and keep the angle from becoming too sharp.
Why must the trailer be secured against movement before coupling?
To prevent it from rolling as the tractor backs underneath.
What is the purpose of a gentle tug test with the trailer brakes applied?
To help check that the coupling holds. It does not replace a visual inspection.
Which trailer air line is normally identified by blue?
The service or control line.
Why should the tractor frame remain under the trailer briefly after the fifth wheel clears the kingpin?
To catch or support the trailer if the landing gear fails or the ground gives way.
What component controls when the compressor starts and stops pumping air?
The governor.
According to Minnesota’s CDL manual, the low-air warning must activate before pressure falls below what level?
55 psi.
For a single vehicle, what is the maximum acceptable air loss in one minute with the engine off, parking brakes released, and service brake fully applied after the initial pressure drop?
No more than 3 psi in one minute.
What causes brake fade on a long downgrade?
Excessive heat from overusing the brakes reduces their effectiveness.
What is the “crack-the-whip” effect?
A quick steering movement becomes more severe toward the rear of a combination, increasing the risk of trailer rollover.
Why is swinging left before a right turn dangerous?
Another vehicle may enter the opening on your right and be trapped or struck during the turn.
At what height should the trailer be positioned for fifth-wheel coupling?
Low enough that the tractor raises the trailer slightly as it backs underneath.
Before driving, how far should the landing gear be raised?
Fully raised, with the crank handle secured.
What are the emergency air line’s two main jobs?
Supplying the trailer’s air tanks and controlling the trailer’s emergency brakes.
What is the purpose of the tractor protection valve?
To protect the tractor’s air supply if the trailer breaks away or develops a major air leak.
Why must water and oil be removed from air tanks?
They can damage the system, and water can freeze and interfere with braking.
Within what general pressure range should the parking-brake valve pop out as air is depleted?
20–45 psi, according to the manufacturer’s specification.
For a tractor and one trailer, what is the maximum acceptable air loss in one minute under the same applied-brake test conditions?
No more than 4 psi in one minute.
With the correct low gear selected on a long downgrade, how should you use the service brakes to control speed?
Apply them firmly enough to slow about 5 mph below the chosen safe speed, release them, and repeat when speed returns to that limit.
Does an empty tractor-trailer always stop in less distance than a loaded one?
No. An empty combination can have less traction and may take longer to stop.
What should you do if the trailer starts drifting too far off your intended backing path?
Stop and pull forward to realign before continuing.
What can happen if the trailer is too high when you attempt to couple?
The kingpin may pass over the fifth wheel or fail to lock correctly, creating a false or unsafe coupling
What should you verify about the fifth-wheel release handle and any safety latch after coupling?
The release handle is in its locked position and the safety latch, if equipped, is secured.
What happens to the trailer’s emergency brakes when emergency-line pressure is lost?
The trailer’s emergency brakes apply.
Why must you not use the trailer hand valve as a parking brake?
Air can leak away and release the trailer brakes, allowing the trailer to roll.
What holds spring brakes released during normal driving?
Air pressure holds back the powerful springs.
With the engine at operating RPM, a typical dual air system should build from 85 to 100 psi within how long?
45 seconds. Systems with larger-than-minimum air tanks may take longer.
During a slow-speed service-brake check, the vehicle pulls to one side. What might that indicate?
A braking problem, such as unequal braking or a brake needing adjustment or repair.
During emergency braking in a vehicle equipped with ABS, should you pump the brake pedal?
No. Apply steady braking pressure as needed; ABS automatically controls impending wheel lockup.