OR 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. Jaws must close fully around the kingpin, and a gentle forward tug with trailer brakes set confirms a secure coupling.

Kingpin override

A failed coupling in which the trailer kingpin rides over the closed fifth wheel jaws instead of locking in. Caused by a trailer set too low during hookup.

Trailer air line colors

The service line is blue and the emergency (supply) line is red; the electrical cord is green. Correct connection ensures trailer brakes release and apply with the tractor.

Landing gear procedure

Before uncoupling, lower the landing gear until it firmly contacts the ground. This supports the trailer and prevents damage to the gear, frame, and tractor.

Off-tracking

The tendency of a trailer's rear wheels to follow a shorter path than the tractor's front wheels. It requires wider, later turns to keep trailer tires off curbs.

Combination following distance

Allow at least one second per 10 feet of vehicle length, plus one second when speed exceeds 40 mph. A 60-foot unit at highway speed needs about 7 seconds.

Trailer swing-out

The tractor's rear end swings in the opposite direction of a turn. Drivers must leave clearance near poles, signs, and parked vehicles during tight turns.

Backing with a spotter

Use a spotter or get out and look before backing. Agree on hand signals and keep the spotter visible in the mirror; back toward the driver's side when possible.

Static leak test limits

With full pressure, engine off, and brakes released, loss should not exceed 3 psi per minute for a single vehicle or 4 psi for a combination. With brakes applied, 3 psi per minute.

Low-air warning point

The low-air warning must activate before pressure falls below 60 psi. Spring brakes apply automatically when pressure drops to between 20 and 45 psi.

Tractor protection valve

A valve that closes the trailer supply line when tractor pressure falls to about 20-45 psi, preventing total air loss if the trailer separates.

Slack adjuster free play

With brakes released, pull the slack adjuster by hand. More than about 1 inch of free play at the pushrod indicates adjustment is needed.

Trailer hand valve

A control that applies only the trailer brakes. It is used to test trailer brake function and to control trailer swing on slippery surfaces, not as a parking brake.

Drive-wheel skid correction

Stop accelerating, ease off the throttle, and steer in the direction the rear is sliding. Do not brake hard, which can worsen the skid.

All-wheel skid correction

Release the brake pedal to let the wheels roll and regain steering, then reapply brakes gently once traction returns.

ABS on combination vehicles

Anti-lock brakes prevent wheel lockup and maintain steering control during hard braking, but they do not shorten stopping distance on every surface.

Hydroplaning response

At highway speed on wet pavement, tires can ride on a film of water, eliminating steering and braking. Ease off the throttle and avoid hard braking until contact returns.

Jackknife cause

The trailer pushes the tractor's drive wheels sideways, folding the unit. It is most likely during hard braking on downgrades, slick surfaces, or with a light trailer.

Jackknife recovery

Release the brakes and steer to follow the trailer. Do not hold the brake pedal down or fight the steering wheel while the trailer is pushing.

Rollover threshold

The lateral acceleration at which a vehicle tips. Loaded combination vehicles typically roll over well below 1 g, so sharp steering at speed is dangerous.

Center of gravity height

A high, heavy load raises the center of gravity and lowers the rollover threshold, requiring slower curve speeds and gentler steering inputs.

Drive axle traction

Too little weight on the tractor's drive axles reduces traction and increases jackknife risk. The trailer should carry enough weight to keep drive axles properly loaded.

Aggregate WLL rule

The total working load limit of all tie-downs must be at least half the cargo weight for general cargo, and at least the full weight where the rules require it.

Tie-down count rule

One tie-down for articles up to 5 feet and 1,100 lb; two for longer or heavier articles; additional tie-downs every 10 feet beyond the first 10 feet.

Cargo re-inspection

Inspect cargo and tie-downs within the first 50 miles, then every 3 hours or 150 miles, and after any emergency stop or rough road.

Tie-down hardware condition

Straps must be free of cuts, fraying, and knots; chains free of cracks; binders and ratchets locked and secure. Damaged hardware must be replaced.

Gross combination weight

The total weight of tractor, trailer, cargo, fuel, and driver. It must not exceed the GCWR or the sum of the axle ratings.

Fifth wheel position effect

Moving the fifth wheel forward adds weight to the steer axle and removes it from the drive axles; moving it rearward does the opposite.

Trailer tandem sliding

Sliding the trailer tandems rearward transfers weight to the trailer axles and reduces weight on the tractor's drive axles; sliding forward does the opposite.

Trailer nose loading

A trailer loaded too light on the nose can sway and track poorly; too heavy on the nose can overload the steer axle and reduce steering traction.

Backing steering direction

Steer the top of the steering wheel in the direction you want the trailer to go. Use small corrections and pull forward to reposition if the trailer gets too far out of line.

Level tracking

The tractor and trailer should sit level when loaded. An excessively high or low fifth wheel changes axle loading and can cause poor handling.