Mississippi CDL Combination flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Following distance rule

At speeds under 40 mph allow one second per 10 feet of vehicle length; above 40 mph add one second. A 60-foot rig at 55 mph needs at least seven seconds.

Offtracking

The tendency of a combination vehicle's rear wheels to cut inside the path of the front wheels in a turn. It increases with wheelbase and with the number of articulation points.

Rearward amplification

The tendency of the last trailer in a multi-trailer combination to swing farther sideways than the tractor during a rapid lane change. More articulation points increase the effect.

Brake lag

The delay of roughly two-tenths of a second between pressing the brake pedal and the trailer brakes applying on an air-braked combination, adding about 16 feet of travel at 55 mph.

Trailer sway correction

Ease off the accelerator and hold the steering wheel steady. Hard braking or sharp steering inputs amplify the oscillation and can cause a jackknife or rollover.

Low-air warning threshold

The warning buzzer or light must activate before air pressure drops below 60 psi, giving the driver time to stop before the spring brakes apply near 45 psi.

Tractor protection valve

A valve that closes and pops the trailer supply valve between 20 and 45 psi, preventing air loss from the tractor if the trailer separates or an air line breaks.

Applied air leak test

With the engine off and a full brake application held for one minute, air loss must not exceed 4 psi for a single vehicle or 6 psi for a combination vehicle.

Static air leak test

With the engine off, spring brakes released, and the system at governor cut-out, hold the pedal down; loss must not exceed 3 psi per minute single or 4 psi per minute combination.

Slack adjuster free play

The pushrod should move no more than about one inch before the brake applies. Excessive travel means the brakes are out of adjustment and stopping distance increases.

Governor cut-in and cut-out

The compressor resumes pumping near 100 psi and stops near 125 psi. A gauge that never reaches cut-out or pumps down with the engine off indicates a leak.

Coupling tug test

With the trailer brakes set and the tractor in a low forward gear, pull gently. If the tractor does not move, the locking jaws are closed around the kingpin shank.

Kingpin lock check

Visually confirm the locking jaws close around the kingpin shank rather than the head, and that the release handle is in the locked position with the safety latch engaged.

Air line connection order

Connect the emergency (supply) line first and the service line second. Charge the trailer system and check that both lines hang with slack for turns.

Uncoupling sequence

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

Landing gear after coupling

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

Fifth wheel inspection

Check the fifth wheel plate for grease and cracks, the kingpin for wear, and the locking jaws for full closure around the shank before backing under the trailer.

Steering free play limit

During the in-cab inspection, steering wheel free play should not exceed about 10 degrees at the rim. More play indicates worn steering components.

Tire tread minimums

At least 4/32 inch tread depth on steering tires and 2/32 inch on all other tires. Dual tires must not touch, and sidewalls must be free of damage.

Trailer light check

Confirm the electrical cord is plugged and the trailer's brake, turn, and marker lights operate. A failed circuit disqualifies the vehicle from service.

Right turn technique

Swing wide to the left before the turn, then track tight to keep the trailer's rear wheels off the curb. Watch the trailer's right side in the mirror throughout the turn.

Backing a semitrailer

Use the mirrors, back slowly, and turn the wheel in the direction you want the trailer to go. Correct small errors early and use a spotter when one is available.

Blind spot on lane change

A combination vehicle has larger blind spots and the trailer can hide a vehicle beside it. Signal, check both mirrors, and confirm the trailer's rear is clear before moving.

Left turn tracking

Keep the tractor close to the center line and let the trailer track inside the turn. A wide left swing can put the trailer over the curb on the inside.

Rollover critical speed

The speed at which lateral force in a turn exceeds the vehicle's stability. It falls as the load's center of gravity rises and as the track width narrows.

Load placement rule

Place the heaviest cargo low and centered over the axles. Shifting weight to one side raises the rollover risk in that direction and can overload an axle.

Bridge formula

A limit on the weight per axle group based on the distance between axles. A short wheelbase cannot legally carry as much weight as a long one.

Cargo tiedown requirement

The aggregate working load limit of the tiedowns must be at least half the cargo weight, and the load must not shift in a turn or a stop.

Drive-axle skid recovery

Release the brake and steer in the direction the rear of the tractor is sliding. Hard braking while the wheels are locked worsens the jackknife.

ABS braking technique

Hold firm, steady pedal pressure and steer; do not pump the brake. The system modulates pressure to prevent wheel lockup while preserving steering control.

Converter dolly

A coupling device with its own fifth wheel and brakes that connects a second trailer to a tractor. Its air lines must be connected and its brakes tested before driving.

Liquid load surge

A tanker's liquid shifts with motion, raising the center of gravity and pushing the vehicle in a turn or stop. Baffled tanks reduce but do not eliminate the surge.