Illinois CDL Combination key terms

The difference between the path followed by the front wheels of a combination vehicle and the path followed by the rear trailer wheels, which cut inside the curve during a turn.

The total width of road a combination vehicle occupies as it moves through a turn, measured from the outside of the front tractor path to the inside of the rear trailer path.

The imaginary line a driver tracks from the tractor's outside rear duals through the turn to judge where the trailer wheels will travel and stay clear of curbs or lanes.

The tendency of lateral motion at the tractor to be magnified at the rear of a long trailer, so a small steering input can swing the trailer's tail much farther sideways.

Side-to-side movement of the trailer's rear end caused by steering, road crown, wind, or braking, which can strike objects or move the unit out of its lane.

The fifth wheel location where the trailer pivots on the tractor; the trailer rotates around this point, so the rear of the trailer swings opposite the direction the tractor nose turns.

The horseshoe-shaped coupling plate mounted on the tractor frame that locks around the trailer kingpin and carries the trailer's weight while allowing it to pivot.

The heavy steel pin mounted under the trailer's front that locks into the fifth wheel jaws and holds the trailer to the tractor.

The locking mechanism inside the fifth wheel that closes around the kingpin; the release handle must be latched and the jaws closed before the vehicle moves.

After coupling, pulling the tractor gently forward against the locked trailer brakes to confirm the fifth wheel jaws hold the kingpin and the trailer does not separate.

The procedure of connecting a tractor to a trailer: aligning the fifth wheel with the kingpin, backing under, locking the jaws, and verifying the connection before driving.

The procedure of separating a tractor from a trailer by lowering landing gear, releasing the fifth wheel handle, and pulling the tractor slowly out from under the trailer.

The retractable support legs under the front of a trailer that hold it upright when it is not attached to a tractor; they must be fully raised before driving.

The coupler fittings on the ends of the tractor and trailer air hoses that lock together to supply brake air and trailer air systems.

The flexible hoses connecting tractor and trailer air systems; they must be coupled, supported so they do not drag, and free of leaks before driving.

The air line that carries the driver's brake application signal to the trailer relay valve; it is also called the control or supply line in some systems.

The air line that keeps the trailer spring brakes released; if it loses pressure or separates, the trailer brakes apply automatically.

Air-released, mechanically applied parking and emergency brakes on the tractor and trailer that set automatically when air pressure drops or the emergency line separates.

A trailer valve that receives the service line signal and quickly delivers stored air to the trailer brake chambers, speeding brake response on long combinations.

The delay between pressing the brake pedal and brakes applying at the wheels, caused by air traveling through lines; it is longer on combination vehicles and requires greater following distance.

The trailer system that automatically applies the trailer spring brakes if the trailer separates from the tractor and the emergency line parts.

The engine-driven pump that builds and maintains air pressure in the brake system; the gauge should build pressure to the governed limit at the required rate.

The dash gauge showing brake system air pressure; it must build to normal operating range and hold pressure when the engine is off and brakes released.

A check with the engine off and brakes released that air pressure does not drop faster than the allowed rate, indicating leaks in the system.

A check with the brakes fully applied and engine off that pressure does not drop faster than the allowed rate, confirming the applied brake system holds air.

The buzzer and light that activate when brake air pressure falls below a set threshold, warning the driver to stop before spring brakes apply automatically.

A check that the parking brake will hold the vehicle by applying it and gently pulling against it in a low gear to confirm it stops the combination.

The systematic walk-around check of a combination vehicle before driving, covering coupling, air lines, brakes, lights, tires, and secure cargo.

The use of tiedowns, straps, and blocking to keep cargo from shifting; shifting cargo can cause rollover or loss of control in a combination vehicle.

A crash in which a combination vehicle tips onto its side, most often caused by taking a curve or evasive maneuver too fast for the load and road.

The point where a vehicle's weight is concentrated; a high or shifting center of gravity makes a combination vehicle more likely to roll over in turns.

Movement of cargo or liquid within a trailer during turns or braking, which can raise the center of gravity or push the vehicle sideways toward rollover.

The practice of slowing before entering a curve, because a combination vehicle cannot safely carry the speed a car can and rollover risk rises sharply with speed.

A loss of control in which the tractor and trailer fold at the fifth wheel, often caused by locking the drive wheels or steering too sharply while braking.

Loss of traction at the trailer wheels that lets the trailer swing sideways out of line with the tractor, a step toward jackknifing.

A condition in which braked wheels stop rotating and slide, reducing steering control and directional stability, especially on wet or slick pavement.

The technique of releasing the brakes, steering in the direction you want to go, and re-applying brakes gently to regain traction and control.

An electronic system that modulates brake pressure to prevent wheel lockup during hard braking, helping the driver keep steering control.

The space kept behind the vehicle ahead; for a combination vehicle it must be greater than for a car because of brake lag and longer stopping distance.

The total distance traveled from perceiving a hazard to a full stop, including perception, reaction, and braking distance, which is longer for heavy combinations.

The skill of scanning ahead and recognizing developing risks, such as stopped traffic, tight curves, or crossing trains, early enough to act safely in a long vehicle.

A place where a highway crosses train tracks; a long combination must have enough room beyond the tracks to clear them completely before starting across.

Confirming that the space beyond a railroad crossing is large enough to hold the entire combination vehicle before entering the crossing, so the trailer never stops on the rails.

Moving a combination vehicle in reverse, which requires using mirrors, getting out to check, and turning the wheel opposite the direction you want the trailer to go.

Get Out And Look: the practice of stopping and walking around a combination vehicle to check clearance before and during backing maneuvers.

A person who guides a driver during backing or parking, using agreed hand signals and staying visible in the driver's mirrors.

Moving a combination vehicle to another lane after signaling, checking mirrors, and confirming the trailer is clear of vehicles in the new lane.

An area around a combination vehicle that the driver cannot see in the mirrors, larger than for a car and requiring extra mirror checks and caution.

Setting the flat and convex mirrors so the sides of the tractor and trailer and the adjacent lanes are visible, reducing blind spots before driving.