CDL Combination Vehicles cheat sheet

A combination vehicle stops more slowly than a single unit: total stopping distance is the sum of perception distance, reaction distance, brake lag distance, and braking distance; at 55 mph a loaded tractor-trailer needs roughly the length of a football field or more on dry pavement.

Brake lag on an air-braked combination is about 0.4 second before the trailer brakes begin to apply, so following distance must exceed the car-only rule; the CDL manual guidance is one second per 10 feet of vehicle length at speeds under 40 mph, plus one extra second above 40 mph.

Off-tracking on a 90-degree turn is approximately 2 feet for a 45-foot trailer and grows roughly 1 foot per additional 10 feet of wheelbase; swing-out at the front of the tractor is the mirror problem and requires watching the outside rearview mirror through the turn.

Rearward amplification: the last trailer in a multi-trailer set swings wider and faster than the tractor, so a lane change or curve entry that is gentle in the cab can produce a rollover or lane departure at the rear pup.

Speed on curves should be reduced before entering, not during; a combination rolls over when lateral acceleration exceeds roughly 0.3-0.4 g for a high-center-of-gravity load, which is far below the tire breakaway limit.

Downgrades require selecting a gear low enough to hold the descent without service brakes; use engine braking and stab braking (apply firmly, release when speed drops, repeat) rather than continuous light pressure that overheats drums.

A combination vehicle must be brought to a complete stop at railroad crossings unless the crossing is exempt; stop no closer than 15 feet and no farther than 50 feet from the nearest rail, and do not shift gears while crossing.

When passing, allow extra time and distance because of the vehicle's length and slower acceleration; return to the right lane only when the trailer's rear is visible in the mirror with clearance.

A tractor-trailer air system is split: the tractor has its own reservoirs and the trailer has separate reservoirs; the tractor protection valve closes and prevents air loss from the tractor if trailer pressure falls below about 45 psi, which also applies the trailer spring brakes.

The supply line (emergency line) charges the trailer reservoirs and carries the trailer-protection signal; the control line (service line) carries the service-brake application. In a standard two-line glad-hand system the couplers are color-coded: blue for service/control, red for emergency/supply.

Normal system pressure is 100-125 psi; the low-air warning buzzer and light must activate before pressure drops below 60 psi, and the tractor protection valve should pop out (trailer brakes apply) between 20 and 45 psi.

The compressor must build pressure from 85 to 100 psi within 45 seconds at fast idle on a typical system; a slower build indicates a compressor, governor, or leak problem.

A static leak test: with the engine off, release the service brake and hold the pedal fully applied for one minute; leakage should not exceed 3 psi for a single vehicle or 4 psi for a combination.

The spring brakes (parking/emergency brakes) apply automatically when air pressure is lost; they are held off by air pressure, so a vehicle with less than about 45 psi cannot be moved until pressure builds.

The trailer hand valve (johnson bar) applies only the trailer service brakes and must never be used for parking; it is used for testing trailer brake operation and for controlled stops on slick surfaces in some older procedures.

Slack adjusters should have no more than about 1 inch of free play at the pushrod with the brakes released and applied; excessive free play means the brakes are out of adjustment and stopping distance increases.

Before coupling, check the fifth-wheel jaws are open, the mounting bolts are secure, the fifth wheel is greased, the trailer kingpin is not damaged, and the trailer is at the correct height so the tractor's fifth wheel will contact the trailer plate without lifting it.

Backing under the trailer: the tractor should be aligned straight with the trailer, and after the kingpin engages the jaws, a pull test (tug forward with the trailer brakes set) confirms the lock; the release lever should be in the locked position and a visual check under the trailer should show the jaws closed around the kingpin.

After coupling, connect the air lines (red emergency to red, blue service to blue) and the electrical cord, then raise the landing gear fully and secure the crank handle; check that the trailer is supported by the fifth wheel, not the landing gear.

The trailer air supply (red) knob must be pushed in to charge the trailer system; the trailer hand valve test confirms the trailer brakes apply and release, and the tractor protection valve should not pop with the system charged.

Uncoupling sequence: park on level ground, set the tractor parking brake, chock the trailer wheels, lower the landing gear to firm ground, disconnect the air lines and electrical cord, release the fifth-wheel jaws, and pull straight forward slowly.

The fifth-wheel release lever must be secured with the safety latch after coupling; an unlatched lever can allow the jaws to open and the trailer to separate under load.

The kingpin must be fully engaged: the gap between the tractor and trailer should be closed, and the tractor frame should be under the trailer nose; a high hitch (trailer too high) leaves the kingpin resting on top of the jaws rather than locked.

Landing gear should be lowered until it bears the trailer weight before pulling out; on soft ground use a block under the foot to prevent the trailer from settling and the kingpin from being pulled out of the fifth wheel.

The pre-trip inspection for a combination includes the tractor engine compartment, coupling system, trailer, and the brake system; the coupling system check is unique and includes the fifth wheel, kingpin, air lines, electrical cord, and landing gear.

Walk-around items on the trailer: landing gear fully raised and crank secured, mud flaps present, lights and reflectors intact, doors and hinges secure, and the rear bumper/underride guard in place.

Air brake inspection sequence: check slack adjusters, drum/rotor condition, hoses and couplings, then perform the leak test (static, 3 psi single / 4 psi combination), the low-air warning test, and the tractor protection valve test.

Tire inspection: check tread depth (at least 4/32 inch on front steer tires, 2/32 inch on other tires), sidewall condition, and inflation; dual tires must not touch and should be matched in diameter within about 1/4 inch to avoid overload.

Lighting check requires a helper or reflective surface: headlights, turn signals, four-way flashers, brake lights, clearance lights, and trailer marker lights; the seven-way cord must be seated and not chafed.

The in-cab inspection covers the steering wheel play (no more than 10 degrees on a 20-inch wheel), the gauges (oil, air, voltmeter), the horn, the wipers, the mirrors, and the seat belt; the parking brake must hold the vehicle.

The brake test while moving: at about 5 mph, apply the brakes firmly; the vehicle should stop without pulling to one side, and the steering wheel should not turn.

After the inspection, confirm the trailer air lines are not crossed and do not rub against the tractor frame or exhaust; chafed lines are a common cause of roadside failures.

Off-tracking is the difference between the path of the front wheels and the path of the rear wheels; on a 90-degree turn the rear wheels cut the corner, so the driver must swing wide to keep the trailer tires out of the curb and the opposing lane.

Swing-out is the opposite problem at the front of the tractor: the tractor's front overhang swings outside the turn, so a car or pedestrian on the outside of the turn can be struck; watch the outside mirror and the front corner.

The rule for a right turn: approach wide in the lane, keep the tractor close to the left side of the lane, and begin the turn when the tractor's front wheels pass the corner; do not start turning early or the trailer will cut the corner.

The pivot point is the rear axle of the tractor (the drive tandems), not the fifth wheel; the trailer pivots around the kingpin, and the tractor pivots around its rear axle, so the trailer's path is determined by the drive-axle path plus off-tracking.

For a 53-foot trailer, off-tracking on a 90-degree turn is roughly 2-3 feet; for a 45-foot trailer it is about 2 feet; for a 28-foot pup trailer it is about 1 foot.

In a multi-trailer set, the last trailer tracks even further inside the turn than a single trailer, and the rear trailer's tires may cross the centerline on a tight curve; plan the turn to keep the rear trailer in its lane.

Tracking on a straight road is affected by load distribution and tire pressure; a trailer that tracks to one side may have a dragging brake, a mismatched tire, or a bent axle.

When backing a combination, the trailer moves opposite the steering direction at the bottom of the wheel; use the mirrors, go slow, and use a spotter when possible.

A skid occurs when the tires lose traction; the three types are front-wheel skid (steering lost, vehicle goes straight), rear-wheel skid (rear swings out, vehicle rotates), and all-wheel skid (vehicle slides sideways).

For a rear-wheel skid, release the brakes and steer in the direction the rear is sliding; do not steer opposite the skid, because that increases the rotation.

For a front-wheel skid, release the brakes to let the front tires regain traction and steer where you want to go; if the front tires are locked, the vehicle will not respond to steering until they roll again.

For an all-wheel skid, release the brakes and steer gently; the goal is to get the wheels rolling so the tires can regain grip, then brake again with less force.

Anti-lock braking systems (ABS) prevent wheel lockup and allow steering during hard braking; the driver should apply firm continuous pressure and not pump the brakes, and the pedal may pulse.

On a combination, the trailer can skid independently of the tractor; if the trailer swings out, release the brakes and steer to keep the tractor aligned with the trailer, then reapply brakes gently.

Skids are more likely on wet, icy, or gravel surfaces, at high speed, and with an empty or lightly loaded trailer because there is less weight on the drive axles; reduce speed before the hazard.

Recovery from a jackknife is difficult once the angle exceeds about 45 degrees; prevention by braking early, keeping the trailer brakes working, and avoiding sudden steering is the effective strategy.

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

Trailer drift (trailer sway) is caused by improper load distribution, insufficient tongue weight, worn tires, or crosswinds; load the trailer with at least 10-15 percent of the total weight on the tongue for a conventional trailer.

Rollover occurs when lateral acceleration exceeds the roll threshold; a high-center-of-gravity load (tall, dense cargo) rolls at a lower speed, so reduce speed before curves and avoid sudden steering.

The rollover threshold for a typical loaded tractor-trailer is about 0.3-0.4 g; this is much lower than the tire grip limit, so a curve that feels safe can still roll the vehicle.

Load the trailer so the heaviest cargo is over or near the trailer axles, not at the rear; cargo placed behind the rear axle reduces kingpin load and increases sway.

Use engine braking and gentle steering inputs on slick roads; sudden braking or steering can lock the trailer brakes and start a trailer jackknife.

Maintain trailer brake adjustment and tire pressure; a dragging brake or a soft tire can cause the trailer to pull to one side and drift.

In a jackknife, release the brakes to let the wheels roll and steer to straighten the combination; if the angle is too large, the vehicle may not recover, so prevention is the primary control.

Common configurations: tractor-semitrailer (5th wheel), doubles (twin trailers), triples, and B-train (a semitrailer with a fifth wheel at its rear that carries a second semitrailer).

Doubles and triples require a special endorsement (T for doubles/triples) and have additional handling concerns: rearward amplification, off-tracking, and a longer stopping distance.

A B-train's second trailer is coupled to the first trailer's rear fifth wheel, so the combination has two articulation points and a different off-tracking pattern than a standard doubles set.

Cargo securement rules: aggregate working load limit of tiedowns must be at least 50 percent of the cargo weight for cargo not requiring a cover, and at least 1,100 pounds per tiedown for heavy articles; the number of tiedowns depends on cargo length and weight.

The cargo must be secured to prevent shifting; use tiedowns with a working load limit (WLL) marked on the strap or chain, and inspect for wear, cuts, and broken fittings before use.

For a flatbed, the front tiedown should be within 4 feet of the front of the cargo and the rear tiedown within 4 feet of the rear; cargo longer than 10 feet needs additional tiedowns at intervals of no more than 10 feet.

A covered load (tarp) is required for cargo that could blow off or cause a hazard; the cover must be secured so it does not flap or tear.

Special configurations such as a stinger-steered pole trailer or a lowboy have different coupling and load-height concerns; check bridge laws, height clearances, and axle weight limits before routing.