NC CDL Combination Vehicles cheat sheet

Rollover threshold: a loaded tractor-trailer tips at roughly 0.4 g lateral acceleration, about half a car's limit; posted curve speeds assume cars, so enter ramps and curves well below the advisory speed.

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

The trailer's center of gravity is the controlling factor: high loads, partial loads of dense cargo, and hanging loads raise CG and lower the tipping threshold.

Braking distance for a combination grows with weight and is longer than a bobtail's; at 55 mph a fully loaded tractor-trailer needs roughly 100 yards more stopping distance than a passenger car.

Brake fade: repeated or continuous braking heats drums and shoes and reduces friction; control downgrade speed by selecting a low gear before the descent and using intermittent firm braking.

Escape ramps are for brake failure: commit to the ramp early, keep the vehicle straight, and expect a gravel bed that stops the truck by rolling resistance.

Speed on downgrades is governed by the gear, not the brake pedal: a gear that holds the load at a steady speed without service brakes preserves braking capacity for emergencies.

Off-tracking and swept path widen in tight turns; swing wide before the turn, and check mirrors for the trailer's rear wheels clearing curbs and poles.

Trailer sway (snaking) is corrected by releasing the accelerator and holding the steering wheel firmly; do not steer against each swing, and brake only after the trailer settles.

Sway is triggered or worsened by speed above about 40-45 mph, improper tongue or kingpin weight, crosswinds, and abrupt steering; slowing down is the primary remedy.

Off-tracking: the rear wheels of a trailer follow a shorter path than the tractor's front wheels; the sharper the turn and the longer the wheelbase, the greater the cut.

Rearward amplification: in doubles and triples the last trailer swings wider than the first; a 27-foot triple can amplify a steering input two to three times at the rear.

Rearward amplification grows with speed and with the number of trailers, so doubles and triples require lower speeds on curves and lane changes than a single trailer.

In a turn, the tractor's rear wheels track inside the front wheels by roughly the wheelbase squared divided by twice the turn radius; longer tractors cut more.

When changing lanes, wait until the last trailer's wheels clear the lane line; the rear unit swings opposite the direction of the steer input during the maneuver.

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 inspection: check that jaws are fully closed around the kingpin, the release handle is in the locked position, and the safety latch or pin is engaged.

Tug test: after coupling, pull gently against the trailer with the trailer brakes set; the tractor must not move, confirming the kingpin is locked.

Air lines: connect the emergency (supply, red) line to the emergency gladhand and the service (blue) line to the service gladhand; crossed lines cause trailer brakes to release or drag.

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

Landing gear: after coupling and raising, the legs must be fully up and the crank handle secured; a partially raised leg can strike the road on dips.

Check the trailer apron and fifth-wheel plate for cracks, missing bolts, and grease; a cracked fifth-wheel mounting is an out-of-service defect.

Electrical cord: the pigtail must be plugged in, secured with the dummy coupler, and not dragging; check that brake, turn, and marker lights work on the trailer.

Slack adjusters: with the brakes released and pushed by hand, a slack adjuster that moves more than about 1 inch indicates excessive pushrod travel and needs adjustment.

Coupling order: inspect the fifth wheel and kingpin, back under slowly, hear the click, tug test, then connect air lines and electrical cord, then raise landing gear.

Before backing under, set the trailer brakes, align the tractor squarely, and ensure the trailer kingpin height matches the fifth-wheel throat to avoid riding over the pin.

Uncoupling order: 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.

Never pull the fifth-wheel release handle with the trailer brakes released or the wheels unchocked; the trailer can roll and the tractor can tip.

After uncoupling, cap the gladhands and stow the electrical cord; dirt and moisture in open couplers contaminate the air system.

Landing gear must contact the ground before the tractor is pulled out; lowering gear after release drops the trailer onto the frame.

When backing, use the mirrors and get out to check alignment if unsure; a spotter or a second look is cheaper than a bent kingpin.

Release the tractor parking brake only after confirming the trailer brakes are set and the wheels are chocked; the combination can roll on any grade.

The trailer has its own air tank fed by the tractor's supply line; the tractor protection valve cuts off the trailer air supply if pressure drops to about 20-45 psi to preserve tractor brakes.

The trailer hand valve (trolley valve) applies only the trailer brakes; it is used for testing, not for normal stopping, because it can cause a jackknife.

Static leak test: with a fully charged system and 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 must activate before pressure drops below 60 psi; the spring brakes apply automatically at about 20-45 psi.

Antilock brake systems (ABS) on tractor and trailer work independently; a trailer ABS fault lamp means trailer ABS is disabled but normal trailer brakes still function.

With ABS active, hold firm steady pressure on the brake pedal and steer; do not pump the brakes, which defeats the system.

The tractor protection valve and trailer emergency line must be connected for the trailer spring brakes to release; a disconnected line sets the trailer brakes.

Check that the trailer brake lights and ABS lamp work during the pre-trip; an inoperative trailer ABS lamp is a defect to note.

Pintle hook coupling: the drawbar eye must sit fully in the hook with the latch closed and the safety pin or cotter installed; check the eye for wear and cracks.

Pintle hook combinations have more articulation and less stability than fifth-wheel combinations; keep speed down and avoid sharp steering at speed.

Doubles and triples: the converter dolly carries the second trailer's kingpin; check the dolly's fifth wheel, safety chains, and air lines before moving.

Converter dolly air lines must be connected to the trailer ahead and to the trailer behind; a dolly with a closed shutoff valve leaves the rear trailer without brakes.

Safety chains on a pintle or dolly must be crossed and attached so the drawbar cannot drop to the road if the coupling fails.

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

The shutoff valves at the rear of each trailer must be open to supply air to the following trailer and closed only when no trailer is attached.

Backing a doubles or triples combination is generally prohibited or avoided; plan the route so the vehicle only moves forward.

Axle weight limits: steer axle typically 12,000 lb, tandem drive axles 34,000 lb, and trailer tandem axles 34,000 lb; gross vehicle weight is limited to 80,000 lb on the Interstate System without a permit.

Bridge formula limits the weight on any group of axles based on the number of axles and the distance between the first and last axle; spreading the load reduces the per-axle penalty.

Cargo securement: aggregate working load limit of tiedowns must be at least 50 percent of the cargo weight, and the cargo must be immobilized against forward, rearward, and lateral movement.

Minimum tiedowns: one for articles up to 5 feet and 1,100 lb, two for articles longer than 5 feet but under 10 feet or over 1,100 lb, and at least two for articles 10 feet or longer.

Tiedown working load limit is the lowest rated capacity in the assembly; use the marked rating on the chain, strap, or binder, not an estimate.

Weight distribution: keep the kingpin load within the tractor's rating and avoid overloading the steer axle; sliding the fifth wheel or trailer tandems shifts weight between axles.

High center of gravity cargo raises rollover risk; place heavy items low and centered, and never exceed the trailer's rated payload.

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