OR CDL Combination cheat sheet

Pre-trip inspection order for a combination unit: coupling system, air lines and electrical cord, landing gear, lights, brakes, tires, wheels and rims, then the tractor engine compartment and in-cab controls.

Fifth wheel jaws must fully close around the kingpin; test by tugging the tractor gently forward with the trailer brakes set - a locked kingpin holds, a bad coupling separates.

Trailer height at coupling: trailer apron should contact the fifth wheel and lift it slightly; too low causes kingpin override, too high causes a high hook that hides a failed lock.

After coupling, verify the release arm is in the locked position and a safety latch (if equipped) is over the arm; then raise landing gear fully and secure the crank handle.

Air line colors: service line is blue (or black on some older units), emergency/supply line is red; the electrical cord is green. Crossed lines cause trailer brakes to release or stay applied incorrectly.

Uncoupling sequence: park on level ground, set parking brakes, chock wheels, lower landing gear to firm ground contact, disconnect air lines and electrical cord, pull the release handle, then pull tractor slowly forward.

Landing gear must be lowered until it touches the ground before the fifth wheel is released; dropping a loaded trailer on unextended gear damages the gear, frame, and tractor.

Check the trailer air supply with the tractor protection valve: with the red knob pulled out the trailer brakes should set and the trailer supply line should lose pressure.

Off-tracking: the rear wheels of a trailer follow a shorter path than the tractor's front wheels, so a right turn must be started later and swung wider to keep the trailer tires off the curb.

Rule of thumb for low-speed turns: pull forward until the tractor's rear axle is roughly even with the corner before turning, and watch the trailer tires in the mirrors throughout the turn.

Stopping distance for a combination vehicle is longer than for a bobtail or single unit at the same speed; brake application must begin earlier and be smoother to avoid trailer push.

Following distance: allow at least one second per 10 feet of vehicle length plus one second for speeds over 40 mph - about 7 seconds for a 60-foot combination at highway speed.

Speed on curves and ramps must be reduced before entering; a combination vehicle's high center of gravity and off-tracking make mid-curve braking and steering corrections hazardous.

When backing, use a spotter or get out and look; if a spotter is used, agree on hand signals and keep the spotter visible in the mirror. Back toward the driver's side where possible.

Swing-out: the tractor's rear end swings wide in the opposite direction of a turn, so leave clearance for the front of the tractor when turning near poles, signs, or parked vehicles.

Downshift before long downgrades and use engine braking with intermittent brake applications; continuous light braking overheats drums and glazes linings.

Static leak test: with engine off, full pressure, and brakes released, pressure should not drop more than 3 psi in one minute for a single vehicle or 4 psi for a combination; with the brake pedal held down, no more than 3 psi per minute.

Low-air warning must activate before pressure falls below 60 psi; the parking (spring) brakes must automatically apply when pressure drops to between 20 and 45 psi.

The tractor protection valve closes the trailer supply line when tractor pressure drops to about 20-45 psi, preventing a total loss of air from a separated trailer.

Build pressure to governor cut-out (about 120-125 psi), then fan the brakes to check that the compressor recovers; the low-air warning and spring brake application points should both be verified.

Tractor and trailer must be charged to normal pressure before a road test; with the red trailer supply knob pushed in, trailer brakes release, and pulling it out applies them.

Slack adjuster check: with brakes released, pull the slack adjuster by hand - more than about 1 inch of free play at the pushrod indicates adjustment is needed; automatic slack adjusters still require inspection.

Trailer hand valve (if equipped) applies only the trailer brakes and is used to test trailer brake function and to control trailer swing on slick surfaces; it is not a parking brake.

After any brake work or coupling, test the brakes by moving slowly and applying the brakes firmly to confirm both tractor and trailer brakes engage and the unit stops straight.

A skid occurs when tires lose traction; the two main types are drive-wheel (power) skids and all-wheel (braking or steering) skids.

To correct a drive-wheel skid: stop accelerating, ease off the throttle, and steer in the direction the rear of the vehicle is sliding - do not brake hard, which can worsen the skid.

To correct an all-wheel skid caused by braking: release the brake pedal to let the wheels roll and regain steering control, then reapply brakes gently once traction returns.

ABS on tractors and trailers prevents wheel lockup but does not shorten stopping distance on all surfaces; it maintains steering control during hard braking.

If the trailer begins to swing out during braking, release the brakes to let the trailer straighten, then reapply; holding the brakes can cause a jackknife.

On slippery surfaces, reduce speed before curves and bridges and avoid abrupt steering, acceleration, or braking; the combination vehicle's length multiplies the effect of any sudden input.

Hydroplaning: at highway speeds 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.

If the rear wheels of the tractor skid, steer into the skid; if the front wheels skid, straighten the wheel and let the vehicle slow before steering again.

A jackknife happens when the trailer pushes the tractor's drive wheels sideways, folding the unit; it is most likely during hard braking on a downgrade, on a slick surface, or when the trailer is lightly loaded relative to the tractor.

Prevention: brake early and smoothly, keep the trailer brakes properly adjusted, and avoid hard braking while turning; release the brakes if the trailer begins to push.

Rollover threshold for a loaded combination vehicle is typically well below 1 g; sharp steering at speed on a curve or a sudden lane change can roll the unit even without braking.

Load height raises the center of gravity and lowers the rollover threshold; high, heavy loads require slower speeds in curves and gentler steering inputs.

Speed reduction in curves: enter slower than the posted advisory speed when loaded high or when the surface is wet, and accelerate gently only after the trailer is straight.

Load distribution affects jackknife risk: too little weight on the tractor's drive axles reduces traction, so the trailer should carry enough weight to keep drive axles properly loaded.

Ramps and off-ramps are common rollover locations because of decreasing radius; watch the speed and avoid braking hard while turning.

If a jackknife begins, release the brakes and steer to follow the trailer; do not fight the wheel or hold the brake pedal down.

Aggregate working load limit (WLL) of all tie-downs must be at least half the cargo weight for cargo not requiring a specific securement standard, and at least the full weight where the rules require it.

Minimum number of tie-downs: one for articles up to 5 feet and 1,100 lb, two for articles longer than 5 feet or heavier than 1,100 lb, and additional tie-downs every 10 feet of length beyond the first 10 feet.

Each tie-down must be attached to the vehicle's securement points and to the cargo; tie-downs must be tight and the attachment points must be rated for the load.

Cargo must be contained, immobilized, or secured so it cannot shift, tip, or fall; covers and tarps must be secured to prevent flapping and load exposure.

Front-end structure or headboard is required for logs, and for other cargo the front securement must prevent forward movement under braking.

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

Tie-down hardware must be in good condition: no cuts, fraying, or knots in straps; chains free of cracks; binders and ratchets locked and secure.

Weight of the load must be distributed so axles are not overloaded; check axle and gross weight limits before departure and adjust the load position if needed.

Gross combination weight (GCW) is the total weight of the tractor, trailer, cargo, fuel, and driver; it must not exceed the GCWR or the sum of the axle ratings.

Fifth wheel position controls weight distribution between the tractor's steer and drive axles: moving the fifth wheel forward adds weight to the steer axle and removes it from the drive axles.

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

Kingpin-to-tandem distance and axle spacing determine how much weight each axle carries; check the bridge formula and state axle-spacing rules when loading near limits.

A trailer loaded too light on the nose can cause trailer sway and poor tracking; a trailer loaded too heavy on the nose can overload the steer axle and reduce steering traction.

Trailer height and 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.

When backing a trailer, 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.

Check tire pressure and tread before every trip; uneven tire pressure across an axle changes weight distribution and can cause a pull or uneven braking.