CDL Combination Vehicles flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Brake lag distance

The distance traveled during the roughly 0.4-second delay before trailer air brakes apply after the pedal is pressed; it adds to total stopping distance for air-braked combinations.

Total stopping distance

The sum of perception distance, reaction distance, brake lag distance, and braking distance; for a loaded tractor-trailer it can exceed the length of a football field at highway speed.

Following distance rule

Allow one second per 10 feet of vehicle length below 40 mph, plus one extra second above 40 mph; a 60-foot combination therefore needs about 7 seconds at highway speed.

Tractor protection valve

A valve that closes to keep tractor air from escaping when trailer pressure drops below about 45 psi, simultaneously applying the trailer spring brakes.

Supply and control lines

The emergency (supply) line charges trailer reservoirs and is red; the service (control) line carries brake applications and is blue on a standard two-line system.

Low-air warning threshold

The warning buzzer and light must activate before system pressure falls below 60 psi; the tractor protection valve should pop out between 20 and 45 psi.

Static air leak test

With the engine off and the pedal held fully applied for one minute, leakage should not exceed 3 psi on a single vehicle or 4 psi on a combination.

Spring brakes

Parking and emergency brakes held off by air pressure; they apply automatically when pressure is lost, so a vehicle below about 45 psi cannot be moved.

Slack adjuster free play

Pushrod travel with the brakes released and applied should be no more than about 1 inch; more means the brakes are out of adjustment and stopping distance increases.

Fifth-wheel jaws check

After backing under the trailer, visually confirm the jaws are closed around the kingpin and the release lever is secured with its safety latch before the pull test.

Pull test

With the trailer brakes set, tug forward gently after coupling; if the trailer does not move and the tractor does not separate, the kingpin is locked in the jaws.

High hitch

A condition where the trailer is too high and the kingpin rests on top of the closed jaws instead of locking inside them; the tractor can separate from the trailer in motion.

Uncoupling sequence

Park level, set the tractor brake, chock wheels, lower the landing gear, disconnect lines and cord, release the jaws, then pull straight forward slowly.

Landing gear check

Before pulling out, confirm the landing gear is fully raised and its crank handle secured; on soft ground use a block under the foot to prevent settling.

Combination brake inspection

Check slack adjusters, drums, hoses, and couplings, then run the static leak test, low-air warning test, and tractor protection valve test before each trip.

Tire tread minimums

At least 4/32 inch on front steer tires and 2/32 inch on all other tires; dual tires must not touch and should be matched in diameter within about 1/4 inch.

Steering wheel play

Free play should not exceed about 10 degrees on a 20-inch steering wheel; more indicates worn steering components and a failed inspection item.

Moving brake test

At about 5 mph, apply the brakes firmly; the vehicle should stop without pulling to one side and the steering wheel should not turn.

Off-tracking

The difference between the path of the front wheels and the rear wheels; the trailer cuts the corner on a turn, roughly 2 feet for a 45-foot trailer on a 90-degree turn.

Swing-out

The outward swing of the tractor's front overhang during a turn; it can strike a vehicle or pedestrian on the outside of the turn, so watch the outside mirror.

Tractor pivot point

The combination pivots at the tractor's rear drive axle, not at the fifth wheel; the trailer pivots around the kingpin, so trailer path follows the drive-axle path plus off-tracking.

Right-turn procedure

Approach wide in the lane, keep the tractor near the left side, and begin the turn when the front wheels pass the corner; turning early makes the trailer cut the corner.

Rear-wheel skid recovery

Release the brakes and steer in the direction the rear is sliding; steering opposite the skid increases rotation and can produce a jackknife.

Front-wheel skid recovery

Release the brakes so the front tires roll again and can steer; a locked front tire will not respond to steering input until traction returns.

ABS use in a combination

Apply firm continuous pressure and do not pump the brakes; the pedal may pulse as the system cycles, and steering control is maintained during hard braking.

Trailer skid response

If the trailer swings out independently, release the brakes and steer to keep the tractor aligned with the trailer, then reapply the brakes gently.

Jackknife

A fold at the coupling: a drive-axle jackknife occurs when drive tires lose traction, a trailer jackknife when trailer brakes lock and the trailer swings around.

Trailer sway causes

Improper load distribution, too little tongue weight, worn tires, or crosswinds; load at least 10-15 percent of total weight on the tongue for a conventional trailer.

Rollover threshold

Lateral acceleration at which a combination rolls, about 0.3-0.4 g for a typical loaded tractor-trailer; it is far below the tire grip limit, so curves feel safe before they roll.

Load placement rule

Place the heaviest cargo over or near the trailer axles, not behind the rear axle; rear placement reduces kingpin load and increases sway and jackknife risk.

Doubles and triples endorsement

A special endorsement required to drive twin or triple trailers; these combinations have rearward amplification, greater off-tracking, and longer stopping distances than a single semitrailer.

Cargo tiedown rule

Aggregate working load limit of tiedowns must be at least 50 percent of cargo weight, with at least 1,100 pounds per tiedown for heavy articles; tiedowns must be within 4 feet of the cargo ends.