Mississippi CDL Combination key terms

The horseshoe-shaped coupling plate mounted on the tractor frame that locks around the trailer kingpin, carrying the trailer's weight and allowing it to pivot as the combination turns.

The hardened steel pin hanging beneath the trailer's front that slides into the fifth wheel and is gripped by the locking jaws to hold the trailer to the tractor.

The two curved metal pieces inside the fifth wheel that close around the kingpin shank; the release handle and safety latch must be in the locked position before you drive.

The flat plate under the front of the trailer surrounding the kingpin; it must sit flush on the fifth wheel with no gap, or the trailer is not properly coupled.

After coupling, you pull gently forward against the trailer with the trailer brakes set to confirm the kingpin is locked and will not pull loose from the fifth wheel.

The red dash valve that applies only the trailer brakes independently of the tractor, used to check coupling, test trailer brakes, and perform a tug test.

A valve that closes automatically when trailer air pressure is lost or the lines separate, sealing the tractor's air supply so the rig can still brake and the trailer brakes apply.

The coupling fittings that join the tractor's air lines to the trailer's; the emergency line is usually red and the service line blue, and they must be sealed and matched correctly.

The air line that carries the driver's brake application signal to the trailer relay valve; it is normally charged only when the brake pedal or hand valve is applied.

The air line that keeps the trailer supply charged and signals the trailer to apply its spring brakes if pressure drops or the line separates.

A trailer valve that uses the low-pressure service line signal to send a large volume of stored air to the trailer brake chambers quickly, speeding brake response.

Brake chambers with a powerful spring that applies the brakes when air pressure is released; they set automatically on low pressure and release only when air builds up.

The lever arm connecting the brake chamber pushrod to the brake camshaft; excessive pushrod travel means the brakes are out of adjustment and stopping distance grows.

The air cylinder that converts air pressure into pushrod force to apply the wheel brakes; combination vehicles use both service and spring sections in each chamber.

Loss of braking power from overheated drums and linings after repeated or prolonged braking, common on long downgrades when brakes are not used correctly.

A technique for downgrades: apply the brakes firmly to reduce speed about 5 mph, then release and let the brakes cool before applying again, rather than riding them.

A retarding device that uses engine compression to slow the vehicle without wheel brakes, helping control speed on grades and reduce fade; it does not replace service brakes entirely.

Choosing a low enough gear before starting down a grade so the engine and retarder hold a safe speed; you should be in the gear before the descent begins.

Side-to-side whipping of the trailer, often caused by too little weight on the tractor's rear axles, improper loading, or steering corrections made too sharply.

The tendency of the rear wheels of a long combination to follow a path inside the front wheels, so the trailer cuts corners and can strike curbs, signs, or vehicles.

The total width of road the combination occupies as it turns, including the path of the rear trailer wheels; wider than the path of the tractor alone.

The turn where off-tracking is most dangerous; swing wide enough to keep the trailer wheels clear of the curb, and check the right mirror throughout the turn.

A person positioned outside the vehicle to guide a backing maneuver; agree on hand signals and stop immediately if you lose sight of the spotter.

Get Out And Look: the practice of stopping, exiting, and walking around a combination before backing when space or clearance is uncertain, because mirrors leave blind spots.

An area around the combination the driver cannot see in the mirrors, especially directly behind the trailer and along the right side; large vehicles have much larger blind spots.

Moving to another lane only after signaling, checking mirrors, and confirming the trailer has cleared the old lane; combination vehicles need more time and space than cars.

The gap you keep behind the vehicle ahead; for a loaded combination at highway speed, one second for each 10 feet of vehicle length plus one second is the standard rule.

The total distance traveled from perception to a full stop, made up of perception, reaction, and braking distance; it grows with speed, weight, and poor brake condition.

Placing cargo so axle loads stay within limits and enough weight sits on the tractor's drive axles to keep traction and stability without overloading any axle.

Center of gravity: the point where the vehicle's weight balances. A high or off-center CG makes a rollover more likely during turns, curves, and evasive steering.

A tip-over caused by centrifugal force in a turn or by a sudden swerve; the main defense is slowing before the curve and avoiding sharp steering at speed.

Movement of cargo during driving that changes the vehicle's balance, can break restraints, and can push the combination into a rollover or loss of control.

Loss of tire grip on the road, caused by braking too hard, accelerating too fast, or steering too sharply for the surface and speed.

A wheel that stops rotating and slides because brake force exceeds available traction; locked wheels give no steering control and can cause the trailer to swing out.

Folding of the combination at the coupling, where the tractor and trailer swing toward each other; often follows locked drive or trailer wheels during hard braking.

Antilock braking system: sensors and valves that release and reapply pressure to prevent wheel lockup during hard braking, helping you keep steering control.

The trailer's own braking system, applied through the service line and spring brakes; a trailer with weak or inoperative brakes greatly increases stopping distance.

The process of connecting tractor and trailer: align the fifth wheel and kingpin, back under until jaws lock, then connect air lines, electrical cord, and check the coupling.

The reverse of coupling: park on level ground, set the trailer parking brakes, lower the landing gear, disconnect lines, then pull the release handle and drive clear slowly.

The retractable legs under the front of the trailer that support it when uncoupled; they must be fully raised before driving away or they will be damaged.

The systematic walk-around check of coupling, air lines, brakes, lights, tires, and load before driving; it is required and is the driver's best defense against breakdowns.

The sequence of checks for leaks, pressure buildup, warning devices, and spring brake operation that confirms the air system is safe before a trip.

The patterned rubber surface of the tire; minimum tread depth rules apply, and damaged or unevenly worn tires can cause blowouts and loss of control on a combination.