Mississippi CDL Combination cheat sheet

A combination vehicle stops shorter than a single unit but takes longer to accelerate; the trailer's brakes and weight push the tractor during hard braking, so following distance must be longer than for a straight truck.

The rule of seconds: at speeds under 40 mph allow at least one second per 10 feet of vehicle length, and add one extra second for speeds above 40 mph. A 60-foot rig at 55 mph needs at least 7 seconds.

Offtracking increases with wheelbase and with the number of articulation points; a tractor-semitrailer's rear wheels cut inside the front wheels' path, and doubles cut more sharply at the second pivot.

Swing-out is the opposite effect: the rear of the tractor and the front corner of the trailer swing wide in the direction opposite the turn, striking objects beside the vehicle.

Brake lag on an air-braked combination is roughly two-tenths of a second before trailer brakes apply after the pedal is pressed; at 55 mph that is about 16 feet of travel before any braking force.

A lightly loaded trailer brakes harder than a loaded one and can lock its wheels; bobtail and empty-trailer combinations require gentler pedal pressure and lower speed.

Trailer sway (whipping) is corrected by easing off the accelerator and holding the wheel steady; braking hard or steering sharply amplifies the oscillation.

Rearward amplification means the last trailer in a multi-trailer set swings farther than the tractor in a rapid lane change; the more articulation points, the greater the amplification.

A compressor charges the supply tank; the supply tank feeds the wet tank and the foot valve. A leak or failure in the supply circuit is announced by the low-air warning at about 60 psi and by the spring brakes applying near 45 psi.

The low-air warning must activate before pressure drops below 60 psi, and the tractor protection valve must close and pop the trailer supply valve between 20 and 45 psi.

The governor cuts the compressor in near 100 psi and out near 125 psi; a gauge that never reaches the cut-out or pumps down with the engine off indicates a leak or a weak compressor.

The static leak test: with the engine off, release the spring brakes, charge to governor cut-out, and hold the pedal down; loss must not exceed 3 psi per minute for a single vehicle or 4 psi per minute for a combination.

The applied leak test: hold a full brake application for one minute with the engine off; loss must not exceed 4 psi per minute for a single vehicle or 6 psi per minute for a combination.

The tractor protection valve prevents air loss from the tractor if the trailer breaks away; the trailer emergency (supply) line and the service line are distinct, and crossing them causes the trailer brakes to drag or fail.

Spring brakes are held off by air pressure and apply mechanically when pressure is lost; never cage a spring brake and drive without confirming the brake is released and functional.

Slack adjusters should have no more than about one inch of free play at the pushrod; excessive travel means the brakes are out of adjustment and stopping distance increases.

Coupling order: inspect the fifth wheel and kingpin, back under until the fifth wheel contacts the trailer, and confirm the tractor is centered on the trailer before the trailer is raised.

After backing under, the trailer must be raised slightly to take weight off the landing gear, then the tractor is backed fully until the fifth wheel locks; a firm tug in low gear with the trailer brakes set confirms the lock.

The pull test: with the trailer brakes set and the tractor in a low forward gear, pull gently; the tractor must not move if the jaws are locked. A clunk or movement means the coupling failed.

The locking jaws must close around the kingpin shank, not the head; visually check that the release handle is in the locked position and a safety latch is engaged.

Connect the emergency (supply) line first and the service line second; charge the trailer system, then check that both lines hang with slack so they do not separate in a turn.

Raise the landing gear fully after coupling and secure the crank handle; a partially raised gear can strike the ground or a railroad crossing.

Uncoupling order: set the trailer brakes, lower the landing gear to firm ground, disconnect the lines, pull the release handle, and pull the tractor slowly forward until clear of the kingpin.

Before uncoupling, chock the trailer wheels if the ground is sloped, and confirm the landing gear is rated for the load; a dropped trailer damages the kingpin and nose.

The walk-around begins at the coupling: check the fifth wheel plate for grease and cracks, the kingpin for wear, and the locking jaws for full closure around the shank.

Check the air lines for chafing, cracks, and proper connection, and confirm the electrical cord is plugged and the trailer lights work on brake, turn, and marker circuits.

Inspect the landing gear for bent legs, missing shoes, and a secure crank; check the mud flaps and the rear impact guard for damage.

Tire inspection includes tread depth (at least 4/32 inch on steering tires, 2/32 inch on other tires), sidewall condition, and proper inflation; dual tires must not touch.

Check the brake system with the engine running: build pressure to governor cut-out, then verify the low-air warning, the spring brake application, and the tractor protection valve function.

Check the coupling with a tug test and verify the trailer brakes hold; then check the lights, the air lines, and the trailer's spring brakes release properly.

The in-cab inspection includes the gauges, the low-air warning buzzer or light, the parking brake, the service brake, and the steering play (no more than 10 degrees free play at the rim).

A pre-trip inspection must be documented and defects corrected before driving; an out-of-service defect such as a cracked spring or an inoperative brake disqualifies the vehicle.

Right turns require a wide swing to the left before the turn, then a tight track to keep the trailer's rear wheels off the curb; watch the trailer's right side in the mirror throughout.

Left turns require the tractor to stay close to the center line and the trailer to track inside; a wide left swing can put the trailer over the curb on the inside of the turn.

Before any lane change, signal, check both mirrors, and confirm the trailer's rear is clear; a combination vehicle's blind spots are larger and the trailer can hide a vehicle beside it.

Backing a tractor-semitrailer: use the mirrors, back slowly, and steer the trailer by turning the wheel in the direction you want the trailer to go; correct small errors early.

When backing, use a spotter if available; if not, get out and check the area behind the trailer before backing, because the trailer blocks the view.

The pivot point for a semitrailer is the fifth wheel; for a double, the second pivot is the converter dolly, so the rear trailer responds more slowly and swings wider.

A 90-degree backing maneuver requires pulling forward to reposition; do not attempt to force a jackknife angle, which can damage the landing gear or the tractor's cab.

When turning, the rear wheels of the trailer track inside the front wheels; allow for offtracking by starting the turn later and wider than in a car.

The center of gravity rises with load height; a high center of gravity lowers the speed at which a combination will roll over in a turn, so posted curve speeds are maximums, not targets.

Rollover occurs when lateral force exceeds the vehicle's stability; the critical speed falls as the load's center of gravity rises and as the track width narrows.

Load the heaviest cargo low and centered over the axles; shifting weight to one side increases the rollover risk in that direction.

Check axle weight limits: steer axle, drive axles, trailer axles, and gross vehicle weight must each be within the legal limit; overloading an axle increases braking distance and tire failure risk.

The bridge formula limits the weight per axle group based on the distance between axles; a short wheelbase cannot legally carry as much weight as a long one.

Secure the load with an adequate number of tiedowns; the aggregate working load limit must be at least half the cargo weight, and the load must not shift in a turn or a stop.

A trailer loaded tail-heavy reduces weight on the drive axles and causes poor traction and steering; check the load distribution before driving.

In a curve, slow down before entering; braking while turning transfers weight forward and can induce a jackknife or a rollover.

A drive-axle skid (jackknife) is corrected by releasing the brake and steering in the direction the rear of the tractor is sliding; hard braking while the wheels are locked worsens the jackknife.

A trailer skid is corrected by releasing the brake and steering to keep the tractor aligned with the trailer; do not steer sharply against the trailer's swing.

A front-wheel skid is corrected by releasing the brake and steering in the direction you want to go; locked front wheels cannot steer.

Threshold braking (braking just short of lockup) preserves steering control; with ABS, hold firm pedal pressure and steer, and do not pump the brake.

If the trailer brakes fail, the tractor's brakes alone can stop the combination but require much longer distance; downshift and use engine braking to slow.

In an emergency stop with a combination, apply the brakes firmly and keep the wheel straight; steering while braking hard can cause a jackknife.

If a tire blows, hold the wheel firmly, ease off the accelerator, and avoid hard braking; brake gently after the vehicle slows and steer to the shoulder.

Escape ramps are for brake failure; steer onto the ramp and do not attempt to stop on the downgrade with a failed brake system.

A double (twin trailer) uses a converter dolly with its own fifth wheel and brakes; the dolly must be coupled and locked, and its air lines connected before driving.

A triple trailer set has two converter dollies and the greatest rearward amplification; it requires the longest following distance and the gentlest lane changes.

A tanker's liquid load shifts with motion, raising the center of gravity and producing a surge that pushes the vehicle in a turn or stop; baffled tanks reduce but do not eliminate surge.

A flatbed load must be secured to the deck with chains or straps and covered if it could shift or blow off; an uncovered load can strike other vehicles.

A refrigerated trailer carries a fuel tank and a compressor; check the fuel level and the unit's operation, and keep the unit's exhaust clear of the cargo.

A livestock trailer's load shifts with the animals' movement, so drive smoothly and check the animals at stops; the trailer's high sides increase wind resistance.

An auto transporter has a low center of gravity when loaded but a high one when empty; the ramps and tie-downs must be secured before moving.

A converter dolly's air lines must be connected and its brakes tested; an unbraked dolly can cause the rear trailer to push the tractor in a stop.