LA CDL Combination flashcards

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

Fifth wheel jaws

The locking mechanism that grips the trailer kingpin. After coupling, verify the jaws are fully closed around the kingpin, the release lever is locked, and no gap exists between the fifth wheel and trailer apron.

Tug (tilt) test

After backing under the trailer, pull gently forward with the trailer brakes released and the tractor brakes set. If the tractor moves and stops firmly, the kingpin is locked; a clunk or creep means recheck the coupling.

Trailer air line order

Connect the emergency (supply) line first, then the service line. The emergency line charges the trailer tanks and applies trailer spring brakes automatically if it separates, preventing a runaway trailer.

Landing gear check

Confirm the landing gear is fully raised, the crank handle is secured in its holder, and the gear does not drag. A partially raised handle can strike the road, a curb, or a pedestrian.

Uncoupling on a grade

Park on firm level ground whenever possible, set the parking brakes, and chock the trailer wheels before lowering the landing gear. Never uncouple on a slope without chocks and a secure tractor.

Kingpin inspection

The kingpin must be firmly attached to the trailer apron with no bends, cracks, or excessive wear. A damaged kingpin can fail and let the trailer separate from the tractor while moving.

Trailer light check

After coupling, test the clearance lights, brake lights, turn signals, and four-way flashers on the trailer. A loose electrical cord or bad ground can leave the trailer invisible at night.

Coupling alignment

Back the tractor squarely under the trailer so the kingpin enters the fifth wheel centered. Angled approach can bend the kingpin, damage the jaws, or leave the trailer resting on the tractor frame.

Off-tracking

The tendency of a combination vehicle's rear wheels to follow inside the path of the front wheels. Longer wheelbases and trailers increase off-track, so turns require a wider swing and mirror checks.

Swing-out

During a tight turn, the rear of the tractor or the front corner of the trailer swings outward beyond the path of the tractor. Pedestrians and vehicles beside the tractor can be struck by this swing.

Right turn setup

Approach in the left portion of the right lane, swing wide enough for the trailer to clear the curb, and watch the right mirror through the turn for vehicles or pedestrians in the off-track zone.

Following distance rule

Under 40 mph, allow one second for each 10 feet of vehicle length plus one second. Above 40 mph, add one more second. A 60-foot combination needs at least seven seconds below 40 mph.

Downgrade control

Select a low gear before starting down, use engine braking and retarders to hold speed, and brake firmly on straight sections. Never coast in neutral or ride the service brakes continuously.

Curve speed

Reduce speed before entering a curve, not during it. Braking while turning uses traction the tires need for steering and can cause a trailer skid or jackknife.

Trailer push in panic stop

In a hard stop, the trailer can push the tractor forward and sideways, especially if the trailer brakes are weak or out of adjustment. Balanced trailer braking is essential for straight stops.

Lane change gap

A combination needs a larger gap than a car because the trailer must clear the lane before returning. Signal early, check both mirrors, and confirm the trailer is visible before moving over.

Static leak test

With the engine off and the system charged, release the parking brakes and hold the service brake for one minute. Pressure loss must not exceed 3 psi single or 4 psi combination.

Applied leak test

With the engine off and brakes fully applied, watch the gauge for one minute. A drop of more than 3 psi in a combination vehicle indicates an air leak that must be repaired before driving.

Low-air warning

The warning buzzer and light must come on before pressure drops below 60 psi. If the warning does not activate, the low-air warning system is defective and the vehicle must be taken out of service.

Spring brake test

With the engine off, pump the brake pedal to reduce pressure. Trailer spring brakes should set near 45 psi and tractor spring brakes between 20 and 45 psi. Both must apply and hold.

Tractor protection valve

This valve seals the tractor air system if the trailer separates or the supply line breaks. Test it by closing the trailer supply valve: the trailer brakes must set and the tractor must hold pressure.

Compressor build-up

With the engine at operating rpm, pressure must rise from 85 to 100 psi within 45 seconds in a combination vehicle. Slower build-up means the compressor or governor needs service.

Governor cut-in and cut-out

The compressor stops pumping at about 120 to 125 psi (cut-out) and resumes at about 100 psi (cut-in). Pressure must never exceed the safety valve setting of about 150 psi.

Trailer brake hand valve

With the trailer supply connected and charged, use the hand valve or trailer brake control to apply trailer brakes alone. They must engage and hold while tractor brakes stay released.

Bridge formula

A federal and state formula limiting the weight carried on a group of axles based on the distance between them. More axles or greater spacing allow more weight within gross limits.

California axle limits

Without a permit: 12,000 lb on the steering axle, 20,000 lb on each drive axle, 20,000 lb on each trailer axle, and 80,000 lb gross combination weight on state highways.

Aggregate tiedown strength

The total working load limit of all tiedowns must be at least half the cargo weight, and the frontmost tiedown must prevent forward movement. Check securement again after the first 50 miles.

Load placement

Place heavy freight low, centered, and over or slightly ahead of the drive axles. This keeps weight off the steer axle, lowers the center of gravity, and reduces rollover risk on curves.

Rollover threshold

A loaded combination can roll over at lateral acceleration below 0.4 g. Ramp speeds posted for cars may be too fast for a loaded truck, so slow down before the curve, not in it.

Liquid surge

Partially filled tanks let liquid shift suddenly during turns and braking, raising the center of gravity and pushing the vehicle sideways. Keep tanks full or empty and slow down in curves.

Rear overhang flag

Cargo extending more than 4 feet beyond the rear of the trailer needs a red or orange flag in daylight and a red light at night. Cargo must never block lights, reflectors, or the plate.

Securement recheck

Inspect tiedowns and cargo within the first 50 miles, then every 150 miles or 3 hours, and after any hard braking or evasive maneuver. Shifting cargo can change axle loads and handling.