Kentucky CDL Combination cheat sheet

A combination vehicle's trailer follows inside the tractor's path in a turn; the longer the wheelbase and trailer, the wider the off-tracking sweep, so swing wide before starting the turn.

Stopping distance grows with weight and speed: at highway speed a loaded tractor-trailer needs substantially more distance than a passenger car, so increase following distance to at least one second per 10 feet of vehicle length below 40 mph.

The trailer's air supply is fed through the tractor protection valve; if trailer pressure drops below about 45 psi the valve closes and the trailer brakes apply, so watch the gauge during long downgrades.

Rearward amplification means the last trailer in a multi-trailer set swings wider and faster than the tractor in a lane change; avoid abrupt steering at highway speed.

Surging occurs when the trailer pushes the tractor after a stop; hold the brake pedal firmly until the trailer brakes set, then release.

Coupling a loaded trailer raises the tractor's rear frame; check that the landing gear is fully raised and the fifth-wheel jaws are locked before moving.

Snap rolls happen when the trailer's rear wheels lift during a hard turn; they occur at lower speeds than most drivers expect, especially with a high center of gravity.

Use the trailer hand valve only for testing trailer brakes before a trip; never use it to stop a moving combination because it can cause a jackknife.

The tractor supplies air to the trailer through the glad hands; the emergency (supply) line is typically the one with the larger fitting and is color-coded, while the service line carries the brake application signal.

A dual air brake system has a primary circuit (rear brakes) and a secondary circuit (front brakes); a loss of pressure in one circuit still leaves partial braking, signaled by a low-pressure warning.

The low-air warning buzzer and light must activate before pressure drops below 60 psi, and the spring brakes must fully apply by about 20-45 psi.

The tractor protection valve closes automatically when trailer air pressure falls to about 20-45 psi, sealing the tractor's air supply and applying the trailer spring brakes.

Build air pressure to the governor cut-out (typically 120-140 psi) before moving; the compressor should cut back in around 100 psi.

Perform the static leak test: with the engine off and full pressure, apply the brakes and hold; loss should not exceed about 3 psi per minute for a single vehicle or 4 psi for a combination.

The alcohol evaporator or air dryer removes moisture; drain the air tanks daily because water in the lines can freeze and disable trailer brakes in cold weather.

A tractor with a trailer air supply valve in the "emergency" position will not release the trailer spring brakes until pressure builds above the valve's threshold.

ABS prevents wheel lockup by modulating brake pressure at each wheel; it does not shorten stopping distance on all surfaces and does not replace safe following distance.

A trailer ABS malfunction lamp (usually amber, on the trailer's left rear or dash) that stays on means the trailer ABS is disabled; the trailer still has normal brakes but without antilock protection.

On a combination vehicle, the tractor and trailer ABS are separate systems; a tractor ABS lamp staying on does not necessarily mean the trailer ABS has failed.

During an ABS event, hold steady pressure on the brake pedal and steer; do not pump the brakes, because pumping defeats the system's modulation.

ABS works only when the wheels are actually locking; on dry pavement a light application may not trigger it, so absence of ABS noise is not a failure.

The ABS lamp should come on at startup and go out after a few seconds; if it stays on, the system has logged a fault and should be checked before the trip.

ABS does not change the fact that a combination vehicle can still jackknife or roll over; it improves steering control during hard braking, not stability limits.

Trailer ABS requires a constant power supply through the seven-wire cable; a damaged cable can disable trailer ABS without any warning on the dash.

Before coupling, check that the fifth-wheel jaws are open, the mounting bolts are secure, the trailer kingpin is not damaged, and the trailer floor is at the correct height - the trailer should be raised so the tractor's fifth wheel contacts the trailer's apron.

Back under the trailer slowly and squarely; after the kingpin engages, tug-test by pulling forward gently with the trailer brakes set to confirm the jaws are locked.

After coupling, visually check that the locking jaws are closed around the kingpin and that the safety latch is engaged; a high-mounted release handle must be in the locked position.

Connect the glad hands, then the electrical cable, then raise the landing gear fully and secure the crank handle before releasing the trailer brakes.

Uncoupling order: set the trailer brakes, lower the landing gear, disconnect the electrical cable and glad hands, pull the fifth-wheel release handle, then pull forward slowly.

Never uncouple on a slope without chocking the trailer wheels; the landing gear is not designed to hold a loaded trailer on an incline.

A trailer that is too low will be struck by the tractor's fifth wheel and can damage the kingpin; a trailer too high will be pushed forward instead of coupled.

After coupling, check that the trailer's air lines are not crossed (emergency to emergency, service to service) and that they have enough slack to turn without dragging.

The pre-trip inspection covers three areas: the tractor engine compartment and cab, the tractor-trailer connection, and the trailer; each has specific items that must be checked and named aloud on the CDL skills test.

Check the fifth-wheel mounting bolts, the locking jaws, the safety latch, the kingpin, the air lines, the electrical cord, the landing gear, and the mud flaps at the coupling point.

Trailer inspection includes the frame, the sliding tandem lock pins, the suspension, the brake chambers and slack adjusters, the wheels and tires, the lights, and the rear impact guard.

Measure slack adjuster free play: with the brakes released, pull the slack adjuster by hand; more than about 1 inch of movement at the push rod indicates adjustment is needed.

Check tire tread depth (at least 4/32 inch on front tires and 2/32 inch on other tires) and look for cuts, bulges, and mismatched duals.

Test the service brakes, the parking brake, and the trailer emergency brakes separately; the trailer emergency brakes should apply when the air lines are disconnected.

Check that all lights work: headlights, turn signals, four-way flashers, brake lights, clearance lights, and the trailer's rear identification lamps.

Inspect the load securement and the trailer doors or tarps; an unsecured load or open door is an out-of-service defect.

A converter dolly has its own axle, wheels, brakes, and a fifth wheel that carries the front of the rear trailer; it must be coupled and checked like any other trailer.

When coupling a dolly, set the dolly's spring brakes, back the tractor's rear trailer to the dolly's fifth wheel, and confirm the jaws lock; then connect the dolly's air lines and electrical cord.

Converter dollies are the most common source of jackknife and offtracking problems in double and triple trailer combinations because they add articulation points.

The dolly's air lines must be connected to the rear trailer and to the tractor's supply; a dolly with no air supply will not have working brakes.

Check the dolly's safety chains and the locking pins on the dolly's drawbar before moving; a dolly that separates from the lead trailer is a catastrophic failure.

Doubles and triples require a special endorsement and are prohibited on many Kentucky routes; check the approved network before routing a multi-trailer combination.

The rear trailer of a set tracks inside the lead trailer's path; allow extra room on right turns and when entering or leaving fuel stops.

When backing a set of doubles, the dolly can fold; backing should be avoided when possible, and if necessary, back slowly with a spotter.

Brake smoothly and progressively; a hard application on a combination can lock the tractor's drive wheels and cause a jackknife before ABS intervenes.

On a downgrade, select a gear low enough that the engine brake and the service brakes together hold the speed; do not ride the brakes, which can fade them.

If the trailer starts to push the tractor (trailer swing), release the brake pedal briefly to let the wheels roll and regain traction, then reapply.

A drive-wheel skid is corrected by releasing the brakes and steering in the direction of the skid; a trailer skid is corrected by releasing the brakes and steering to keep the trailer behind the tractor.

Use the trailer hand valve only to test trailer brakes; using it to stop a moving combination can cause the trailer to swing out.

Stopping distance increases with speed squared; doubling speed roughly quadruples the distance needed to stop.

The engine brake (Jake brake) should be used on long downgrades but may be prohibited in some municipalities; check local signage.

When stopped on a hill, use the parking brake and the trailer brakes; holding the vehicle with the clutch or service brakes alone can overheat the brakes.

Offtracking is the difference between the path of the front wheels and the path of the rear wheels; it increases with wheelbase and with the sharpness of the turn.

On a right turn, swing wide to the left before turning, then bring the tractor close to the right curb as the trailer follows; watch the trailer's right rear wheels in the mirror.

The trailer's rear wheels cut the corner; a common error is clipping the curb or striking a pole with the trailer's rear tires.

When backing, use a spotter and GOAL (get out and look); back toward the driver's side when possible and keep the trailer's rear in view.

Backing a combination requires small steering inputs; oversteering causes the trailer to swing out of control quickly.

At intersections, check the clearance on both sides; a long combination may not clear a light or sign that a car would pass under.

Use the mirrors constantly; the trailer's wheels and the rear of the trailer should be visible, and the mirrors should be adjusted before moving.

A combination vehicle has a larger turning radius and a longer stopping distance; plan lane changes and turns well in advance.

The center of gravity rises with load height; a high center of gravity lowers the speed at which a rollover can occur in a turn.

Rollover occurs when the lateral force in a turn exceeds the vehicle's stability limit; the critical speed decreases as the center of gravity rises and as the turn tightens.

Load the heaviest cargo low and centered over the axles; an off-center or top-heavy load increases the risk of a rollover and of a load shift.

Secure the load with tiedowns rated for the weight; the number of tiedowns depends on the cargo length and weight, and the aggregate working load limit must be at least half the cargo weight.

Check the load after the first few miles and periodically; a shifted load changes the vehicle's handling and can cause a rollover.

Slow down before a curve or ramp, not in it; braking in a curve transfers weight and reduces stability.

A trailer with a high, heavy load may sway; if sway starts, ease off the accelerator and avoid hard steering or braking.

Check the trailer's axle weight limits and the bridge formula; overloading an axle increases rollover risk and is a violation.