NC CDL Combination Vehicles flashcards

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

Rollover threshold of a loaded rig

A loaded tractor-trailer tips at roughly 0.4 g of lateral acceleration, about half a passenger car's limit. Posted curve speeds assume cars, so enter ramps and curves well below the advisory speed.

Centrifugal force versus speed

Centrifugal force rises with the square of speed, so doubling speed through a curve quadruples the tipping force. A modest speed reduction yields a large rollover margin.

Trailer center of gravity effect

The trailer's center of gravity controls rollover risk. High loads, dense partial loads, and hanging loads raise the center of gravity and lower the speed at which the trailer tips.

Brake fade on downgrades

Repeated or continuous braking heats drums and shoes and reduces friction, causing fade. Select a low gear before the descent and use intermittent firm braking to control speed.

Escape ramp use

Escape ramps are for brake failure. Commit early, keep the vehicle straight, and expect the gravel bed to stop the truck through rolling resistance rather than wheel braking.

Trailer sway correction

Release the accelerator and hold the steering wheel firmly without steering against each swing. Brake only after the trailer settles, then slow down before continuing.

Sway triggers

Sway is triggered or worsened by speeds above roughly 40 to 45 mph, improper tongue or kingpin weight, crosswinds, and abrupt steering. Slowing down is the primary remedy.

Off-tracking definition

The trailer's rear wheels follow a shorter path than the tractor's front wheels. Sharper turns and longer wheelbases increase the cut, so swing wide before turning.

Rearward amplification

In doubles and triples the last trailer swings wider than the first. A steering input can be amplified two to three times at the rear, so use lower speeds.

Empty trailer sway risk

Sway is most dangerous when the trailer is empty or lightly loaded. Empty combinations need lower speeds and gentler steering than loaded ones.

Fifth-wheel jaw check

Confirm the jaws are fully closed around the kingpin, the release handle is locked, and the safety latch or pin is engaged. A partly closed jaw can release under load.

Tug test

After coupling, pull gently against the trailer with the trailer brakes set. The tractor must not move, which confirms the kingpin is locked in the fifth wheel.

Gladhand color coding

The emergency or supply line is red and the service line is blue. Crossed lines cause trailer brakes to release or drag, so match colors and check function.

Combination air leak limit

With the engine off and the system charged, a loss of more than 3 psi per minute in a tractor-trailer combination indicates a leak that must be repaired before driving.

Landing gear stowage

After coupling and raising the landing gear, the legs must be fully up and the crank handle secured. A partially raised leg can strike the road on dips.

Slack adjuster pushrod check

With brakes released, push the slack adjuster by hand. Movement of more than about 1 inch indicates excessive pushrod travel, meaning the brakes need adjustment.

Coupling sequence

Inspect the fifth wheel and kingpin, back under slowly, listen for the click, tug test, connect air lines and electrical cord, then raise the landing gear.

Uncoupling sequence

Chock the trailer wheels, lower the landing gear to firm ground, disconnect the electrical cord and air lines, release the fifth-wheel handle, then pull forward slowly.

Fifth-wheel release hazard

Never pull the release handle with the trailer brakes released or wheels unchocked. The trailer can roll and the tractor can tip when the kingpin separates.

Gladhand stowage

After uncoupling, cap the gladhands and stow the electrical cord. Dirt and moisture in open couplers contaminate the air system and cause brake faults.

Tractor protection valve

The valve cuts off trailer air supply if pressure drops to roughly 20 to 45 psi, preserving tractor brakes. The trailer spring brakes then apply automatically.

Trailer hand valve caution

The trailer hand valve applies only the trailer brakes. It is used for testing, not normal stopping, because applying trailer brakes alone can cause a jackknife.

Static air leak test

With a fully charged system and the engine off, pressure loss should not exceed 2 psi per minute for a single vehicle or 3 psi per minute for a combination.

Low-pressure warning point

The low-pressure warning must activate before pressure drops below 60 psi. Spring brakes apply automatically at about 20 to 45 psi, stopping the vehicle.

Trailer ABS fault lamp

A trailer ABS fault lamp means trailer ABS is disabled, but normal trailer brakes still function. Note the defect and have the system serviced.

ABS braking technique

With ABS active, hold firm steady pressure on the brake pedal and steer. Do not pump the brakes, which defeats the system and lengthens stopping distance.

Pintle hook inspection

The drawbar eye must sit fully in the hook with the latch closed and the safety pin installed. Check the eye for wear and cracks before moving.

Converter dolly checks

The dolly carries the second trailer's kingpin. Check its fifth wheel, safety chains, and air lines, and confirm shutoff valves are open to supply air to the rear trailer.

Safety chain crossing

On a pintle or dolly, cross the safety chains and attach them so the drawbar cannot drop to the road if the coupling fails.

Doubles and triples speed

Doubles and triples need lower speeds on curves and lane changes because rearward amplification increases with each added trailer and with speed.

Axle weight limits

Typical limits are 12,000 lb on the steer axle, 34,000 lb on tandem drive axles, and 34,000 lb on trailer tandems, with 80,000 lb gross on the Interstate System.

Bridge formula

The bridge formula limits weight on any group of axles based on axle count and the distance between the first and last axle. Spreading the load reduces the per-axle penalty.

Tiedown working load limit

The working load limit of a tiedown assembly is its lowest rated component. The aggregate of all tiedowns must be at least 50 percent of the cargo weight.

Minimum tiedown count

Use one tiedown for articles up to 5 feet and 1,100 lb, two for articles longer than 5 feet or over 1,100 lb, and two for articles 10 feet or longer.

Load recheck interval

Check the load and tiedowns within the first 50 miles of a trip, then every 3 hours or 150 miles, and after any emergency stop.