Michigan CDL Combination key terms

A systematic walk-around check of the tractor, trailer, coupling, brakes, lights, tires, and cargo performed before driving, so defects are found while the vehicle is stationary rather than discovered at highway speed.

The horseshoe-shaped plate on the tractor that locks around the trailer kingpin to carry the trailer load and allow pivoting during turns. Its jaws must be closed and the release handle secured.

The vertical steel pin under the trailer nose that drops into the fifth wheel and holds the trailer to the tractor. It must be fully seated, not resting on top of the jaws.

The two curved metal pieces inside the fifth wheel that close around the kingpin. After coupling, they should grip the pin with no gap and the safety latch should be engaged.

After coupling, pull forward gently with the trailer brakes set; the tractor should not move. If it does, the kingpin is not locked and the trailer could separate.

The retractable legs under the trailer nose that support it when uncoupled. They must be fully raised before driving and firmly on the ground before the tractor is pulled out.

The sequence of aligning the tractor with the trailer, backing under it, engaging the fifth wheel, connecting air lines and electrical cord, raising landing gear, and testing the connection before driving.

The reverse of coupling: park on firm level ground, set trailer brakes, lower landing gear, disconnect air and electrical lines, release the fifth wheel, then pull the tractor slowly away.

The couplers that join the tractor and trailer air lines. They must be locked, sealed, and free of leaks; crossing the service and emergency lines disables trailer braking and emergency protection.

The air line that carries the driver's brake application signal to the trailer. A break or disconnect here causes a slow, controlled trailer brake release rather than an emergency application.

The air line that supplies air to the trailer and triggers automatic trailer braking if pressure is lost or the line separates. Also called the supply line.

A dash-mounted control that applies only the trailer brakes, used to test the trailer brake system and to straighten or hold a trailer during certain maneuvers.

Brake chambers that apply automatically when air pressure drops, stopping the vehicle if pressure is lost. They must have enough pressure to release before the vehicle can move.

A buzzer and light that activate before air pressure falls too low to stop the vehicle safely, warning the driver to stop and find the leak.

With the system charged and engine off, watch the gauges for a minute; pressure loss beyond the allowed amount means a leak that must be fixed before driving.

The engine-driven pump that builds and maintains air pressure in the brake system. It must raise pressure to the governor cutoff within a reasonable time.

The amount of pushrod travel in each brake chamber. Too much travel means brakes apply late or weakly, so they must be checked and adjusted to stay within limits.

The tendency of a trailer's rear wheels to cut inside the path of the tractor's front wheels during a turn. The longer the wheelbase, the wider the path the driver must allow.

The total width of road the tractor and trailer occupy through a turn, including the rear of the trailer swinging outward and the wheels cutting inward.

The spot beside the tractor where the driver begins the turn so the trailer follows without hitting the curb. It is set further ahead than in a car to allow for off-tracking.

Swing wide left before turning right, keep the trailer wheels clear of the curb, and watch the trailer's rear in the mirrors. Do not turn so wide that you enter oncoming lanes.

A skid in which the tractor and trailer fold toward each other, often from overbraking or abrupt steering on slick pavement. Recovery requires releasing the brakes and steering into the skid.

A skid in which the trailer's rear slides sideways out of line. Correct by releasing the brakes and steering so the tractor follows the trailer's direction.

Stop braking, look and steer where you want to go, and avoid overcorrecting. On a slippery surface, gentle steering and throttle let tires regain grip before you resume speed.

Loss of stopping power when brakes overheat from repeated or prolonged use, common on long downgrades. Prevent it by gearing down and using engine braking instead of riding the brakes.

Using a lower gear so the engine slows the vehicle on downgrades, sparing the service brakes from heat and fade. It is the main speed control on long grades.

The controlled speed chosen before a long descent, low enough that a single brake application can hold it. You cannot safely slow down after the grade has already built speed.

Choosing a gear before a grade or turn so the engine and transmission match the load and slope. Starting too high risks stalling, rolling backward, or losing control.

In a manual tractor, engage and release the clutch smoothly and never coast with it held in; coasting removes engine braking and control, especially on grades and turns.

The regular scheduled check of brakes, tires, lights, hoses, and coupling hardware that catches wear before it becomes a roadside breakdown or a crash.

The minimum groove depth required for safe traction and water displacement. Worn or unevenly worn tires on a combination vehicle increase blowout and hydroplaning risk.

The inflation level checked cold on every tire, including trailer tires. Wrong pressure causes heat, uneven wear, and blowouts, and dual tires must match closely.

Placing cargo so axle loads stay within limits and the tractor keeps enough weight on the drive axles for traction and steering control.

The maximum load allowed on each axle and axle group. Overloading any axle, even if gross weight is legal, damages roads and reduces braking and steering control.

The total loaded weight of the tractor, trailer, cargo, fuel, and people. It must stay within the rating for the combination and any bridge or road limits.

Using tiedowns, straps, and blocking to keep cargo from shifting, falling, or spilling. Shifting load changes axle weight and can make a combination vehicle unstable in traffic.

The gap kept behind the vehicle ahead, longer for a loaded combination vehicle because it needs far more distance to stop than a car.

Scanning far ahead and checking mirrors to spot trouble early, such as brake lights, merging traffic, or road debris, and planning an escape before it becomes an emergency.

The clear area kept around the vehicle on all sides so there is room to steer or stop if another driver makes a mistake.