Arkansas CDL Combination Vehicles Test cheat sheet

A combination vehicle needs more stopping distance than a single unit: total stopping distance = perception distance + reaction distance + brake lag distance + braking distance.

Brake lag for a typical air brake system is about 0.4 second; at 55 mph that alone covers roughly 32 feet before the brakes even begin to work.

Offtracking: the rear wheels of a turning combination follow a shorter path inside the front wheels, so the trailer cuts the corner and can strike curbs or pedestrians.

Swing-out: the rear of the tractor and the front of the trailer swing wide in the opposite direction of the turn; allow extra clearance on the outside of the turn.

Safe following distance rule: one second per 10 feet of vehicle length at speeds under 40 mph, plus one extra second for speeds above 40 mph.

A 60-foot combination at 55 mph needs at least 7 seconds of following distance under ideal conditions.

Rollover threshold: a loaded combination can tip at a lateral acceleration near 0.4 g, so posted curve speeds must be obeyed or reduced.

When backing a combination, steer the bottom of the steering wheel toward the direction you want the trailer to go; correct drift early with small inputs.

Air pressure buildup must reach at least 85 psi within 45 seconds at fast idle for a typical tractor-trailer; a slower buildup signals a compressor or leak problem.

Static leak test: with the engine off and full pressure, loss must not exceed 3 psi in one minute for a single vehicle or 4 psi per minute for a combination.

Applied leak test: hold a full brake application for one minute; leakage must not exceed 4 psi for a combination vehicle.

Low-air warning must activate by 60 psi; the spring brakes must fully apply between 20 and 45 psi.

The tractor protection valve closes and cuts off air to the trailer when pressure falls into the 20-45 psi range, preserving tractor brakes and triggering a trailer emergency application.

The red trailer air supply (emergency) knob must be pushed in to charge the trailer and pulled out to set the trailer spring brakes; the yellow tractor parking knob controls the tractor spring brakes.

The trailer hand valve (trolley valve) applies only the trailer brakes and must never be used for parking or for normal stopping because it can cause a jackknife.

Total air loss at the glad hands or a broken line causes the trailer emergency spring brakes to apply automatically through the emergency line.

ABS prevents wheel lockup and maintains steering control during hard braking; it does not shorten stopping distance on all surfaces and does not replace safe following distance.

A tractor ABS lamp that stays on after the bulb check indicates a fault in the tractor ABS; the system reverts to standard braking.

A trailer ABS lamp (usually amber, mounted on the trailer or dash) that stays on indicates a trailer ABS fault; the trailer still has normal brakes.

If only the trailer ABS lamp is on, the tractor ABS is still functional; if both are on, the whole combination may be without ABS.

During an ABS event, hold firm steady pressure on the brake pedal and do not pump the brakes; pumping defeats the system.

ABS on a combination may activate on one unit only; the combination can still be steered and stopped, so maintain pedal pressure and steer.

Required ABS lamp check: lamps should come on at start-up and go out after a few seconds once the vehicle exceeds about 4 mph.

Before coupling, check that the trailer kingpin is not damaged, the fifth wheel jaws are open and greased, and the trailer is at the correct height so the tractor will not strike the landing gear.

After backing under, the tractor should be raised slightly by the trailer, then pull forward twice to test that the fifth wheel jaws have locked around the kingpin.

A tug test alone is not proof of coupling; a visual check of the closed jaws and a pull test with the trailer brakes set are both required.

Connect the emergency (red) line first when charging, and disconnect the service (blue) line first when uncoupling to avoid a trailer rollaway.

After coupling, raise the landing gear fully and secure the crank handle; the tractor frame should clear the trailer apron.

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

The fifth wheel release handle must be secured with its safety latch after coupling; an unlatched handle can allow the trailer to separate.

Pre-trip inspection of a combination includes the coupling system: fifth wheel, kingpin, locking jaws, safety latch, and the air/electric lines.

Check that the trailer apron is not so low that the tractor frame or cab will strike it during a turn, and not so high that the kingpin cannot seat.

Inspect glad hands for cracked seals and check that lines are not crossed; emergency is red, service is blue.

Check the trailer landing gear for cracks, bent legs, and missing feet or crank handle; gear must be fully raised before moving.

Verify the trailer air brake system charges to full pressure, the low-air warning works, and the trailer spring brakes apply and release.

Check the electrical cord and trailer lights: clearance, brake, turn, and ABS lamp function.

Inspect tires for tread depth (4/32 inch minimum on front steer tires, 2/32 inch on other tires) and for matching duals and proper inflation.

Verify cargo is secured and the trailer is loaded so the weight is distributed within axle and gross limits before moving.

Right turns: swing wide to the left to keep the trailer off the curb, but do not swing into oncoming traffic; start the turn from as far right as practical.

Left turns: allow extra room because the trailer offtracks to the right; do not cut the corner.

A combination vehicle's turning radius is determined by the wheelbase of the tractor plus the distance to the trailer axles; longer combinations need wider turns.

Backing a combination requires the trailer to be straight; if it is not, pull forward to straighten rather than trying to correct a large angle.

Use a spotter when backing; if no spotter is available, get out and check the area before backing.

When driving through a curve, reduce speed before entering and use steady throttle through the curve; braking hard in a curve can cause a rollover or skid.

The rear of the trailer swings out in the opposite direction of a turn; watch the trailer's rear when turning near parked cars or poles.

When passing, allow extra distance because a combination is longer and slower to accelerate; signal early and return to the lane only when the trailer is clear.

A drive-wheel skid (jackknife) occurs when the tractor's rear wheels lock and the trailer pushes the tractor sideways; release the brakes and steer in the direction of the skid to regain control.

A trailer skid occurs when the trailer wheels lock and the trailer swings out; release the brakes and steer in the direction the trailer is sliding.

A front-wheel skid occurs when the steering axle locks; release the brakes and steer in the direction you want to go, then straighten when traction returns.

Jackknife prevention: use the trailer hand valve only for testing, brake smoothly, and avoid hard braking on slippery surfaces.

If a trailer breaks away, the emergency line separates and the trailer spring brakes apply automatically; do not try to stop the trailer by hand.

If the brakes fail, downshift, use the engine brake or retarder, and look for an escape ramp or a safe off-road area; do not pump the brakes.

In a brake emergency, the emergency (spring) brakes will apply automatically when air pressure is lost; this is a controlled stop, not a total brake failure.

If a tire blows out, hold the steering wheel firmly, keep the vehicle straight, and slow gradually; do not brake hard.

The cargo securement system must withstand a forward force of 0.8 g, a rearward force of 0.5 g, and a lateral force of 0.5 g of the cargo weight.

Aggregate working load limit of tiedowns must be at least 50 percent of the cargo weight for cargo that could roll or shift.

Cargo must be secured so it cannot shift, and the tiedowns must be attached to the vehicle frame or to points with adequate strength.

Weight distribution: the steer axle, drive axles, and trailer axles each have a rated capacity; overloading any axle is a violation even if gross weight is legal.

The bridge formula limits the weight that can be carried on any group of axles based on the distance between them; shorter axle spacing means lower allowable weight.

Sliding the fifth wheel forward transfers weight to the steer axle; sliding it rearward transfers weight to the drive axles.

A trailer loaded with too much weight behind the trailer axles can cause the trailer to sway; keep the load centered over the trailer axles.

Cargo must not obscure lights, reflectors, or the license plate, and must not extend beyond the legal overhang without proper flags and lights.