Alabama CDL Combination Vehicles cheat sheet

A combination vehicle's trailer follows a shorter path than the tractor, so the trailer cuts corners and swings wide; the longer the combination, the greater the difference.

Total stopping distance = perception distance + reaction distance + brake lag distance + braking distance; at 55 mph this is roughly the length of a football field for a loaded rig.

Brake lag distance for air brakes at 55 mph is about 32 feet before the brakes even begin to apply.

The heavier the combination, the longer the stopping distance; weight added to the trailer also increases the chance of trailer swing and jackknife.

Drive so you can stop within the distance you can see ahead; posted speed limits are for cars in good conditions, not for loaded combinations.

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

When the trailer breaks away or air pressure is lost, the emergency relay valve applies the trailer spring brakes automatically.

Minimum safe following distance for a combination vehicle is one second for each 10 feet of vehicle length at speeds under 40 mph, plus one extra second above 40 mph.

With the engine running and governor cut-in reached, build pressure to governor cut-out (typically 120-140 psi), then shut the engine off and watch the gauge.

Static leak test: pressure loss must not exceed 3 psi in one minute for a single vehicle or 4 psi in one minute for a combination vehicle.

Applied leak test: fully apply the foot brake for one minute; leakage must not exceed 4 psi per minute for a combination vehicle.

Low-air warning must activate before pressure drops below 60 psi; the wig-wag or buzzer signals the driver to stop safely.

Spring (emergency) brakes must fully deploy between 20 and 45 psi as pressure drops; the tractor protection valve then closes to protect tractor air.

The tractor protection valve keeps air in the tractor when the trailer breaks away or the trailer air supply is pulled out, and it closes at about 20-45 psi.

The safety valve on the air compressor releases at about 150 psi to prevent over-pressurization of the system.

Compressor build-up must raise pressure from 85 to 100 psi within 45 seconds at normal governed engine speed.

Inspect the fifth wheel jaws and the kingpin for cracks, bends, or wear before coupling; a worn or open jaw is the most common cause of trailer separation.

Back under the trailer slightly low so the tractor frame lifts the trailer, then pull forward gently twice to test the lock; the trailer should not move.

After coupling, check that the locking jaws fully close around the kingpin shank, the release handle is in the locked position, and a pull test confirms the lock.

Connect the air lines and electrical cord; the emergency (red) line goes to the trailer emergency glad hand and the service (blue) line to the service glad hand.

Raise the landing gear fully after coupling, then check that the trailer is supported by the fifth wheel and not the landing gear.

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

Chock the trailer wheels or confirm the trailer spring brakes are set before uncoupling; a trailer without brakes set can roll when the tractor is pulled away.

The fifth wheel should be greased and the kingpin free of rust; a dry fifth wheel causes steering resistance, and a bent kingpin prevents proper lock.

Walk-around inspection covers coupling system, air lines, electrical cord, landing gear, mud flaps, lights, tires, and the trailer frame.

Check that the locking jaws are closed around the kingpin and that the release handle is secured with the safety latch.

Look for air line chafing or leaks at the glad hands; a leaking line can prevent the trailer brakes from releasing or applying fully.

Confirm the electrical cord is plugged in, the cord is not dragging, and the trailer lights (turn, brake, tail) work with the tractor.

Inspect tires for tread depth, cuts, bulges, and proper inflation; dual tires must not touch and should be matched in size.

Check the landing gear is fully raised, the crank handle is secured, and the gear is not damaged or bent.

Verify the trailer frame is not cracked, the crossmembers are intact, and the mud flaps are present and not dragging.

After the walk-around, perform the air brake tests (static leak, applied leak, low-air warning, and spring brake test) before moving.

Offtracking is the difference between the path of the tractor's front wheels and the path of the trailer's rear wheels; it increases with vehicle length and turn sharpness.

When turning, swing wide enough so the trailer's rear wheels clear the inside curb, but not so wide that the tractor enters an adjacent lane.

Make right turns from the rightmost lane and, if needed, from a position that allows the trailer to track clear; do not turn from the left lane across traffic.

Before changing lanes, check mirrors on both sides, signal early, and confirm the trailer is clear; the trailer may still be in the old lane after the tractor has moved.

Use the mirrors to monitor the trailer's position during turns and lane changes; the trailer's rear can swing into another lane.

Turn signals must be canceled after the turn; a combination vehicle may need a wider turning radius, so plan the turn before entering the intersection.

When entering a curve, slow down before the curve; a combination vehicle's high center of gravity and length make it prone to rollover if speed is too high.

Backing a combination vehicle requires a spotter or a walk-around check; if you cannot see the trailer's rear, use a helper or get out and check.

Back slowly and use a spotter whenever possible; if no spotter is available, get out and check the area behind the trailer before backing.

When backing, steer the tractor in the direction opposite the desired trailer movement: turn the wheel right to make the trailer go left.

Back from the driver's side whenever possible; backing from the blind side is more dangerous because the trailer is not visible in the mirrors.

Use the mirrors continuously while backing; the trailer's rear can drift out of view, so stop and re-check if visibility is lost.

Park on level ground when possible; if parking on a grade, set the parking brakes, use wheel chocks, and turn the front wheels toward the curb.

Before parking, set the trailer spring brakes and the tractor parking brake; do not rely on air pressure alone to hold the vehicle.

When parking, leave enough space to pull out; a combination vehicle needs a larger turning radius to exit a parking space.

Pull forward after parking to confirm the trailer is not blocking traffic and that the landing gear is not on soft ground.

A combination vehicle's high center of gravity makes it prone to rollover, especially in curves and on ramps; reduce speed before entering a curve.

Rollover threshold is the lateral acceleration at which the vehicle tips; it decreases as the center of gravity rises and as the load shifts.

The posted ramp speed is for cars; a loaded combination may need to travel well below the posted speed to avoid rollover.

Loads must be secured and centered; an off-center or high load raises the center of gravity and increases rollover risk.

Use engine braking and downshifting on downgrades; do not ride the brakes, which can cause brake fade and loss of control.

Brake fade occurs when brakes overheat; on long downgrades, select a lower gear and use the brakes intermittently to control speed.

The faster you drive, the longer the stopping distance and the greater the rollover risk; reduce speed in rain, snow, and on curves.

If the trailer starts to swing, release the brakes and steer in the direction of the skid; do not use the trailer hand valve to correct a swing.