KS CDL General Knowledge key terms

The maximum total weight a single vehicle is designed to carry safely, including its own chassis, body, fuel, passengers, and cargo. It is set by the manufacturer and shown on the vehicle certification label.

The maximum allowable combined weight of a towing vehicle plus its trailer or trailers, including all cargo, fuel, and occupants. It is specified by the manufacturer and used to judge whether a combination is overweight.

The load carried by a single axle or axle group, measured at the scale. Overloaded axles are a common cause of failed inspections and can make steering or braking unsafe even when total weight is legal.

How cargo weight is spread across axles and along the vehicle. Shifting load forward or backward changes steering and traction, and improper distribution can overload one axle while leaving another underused.

The system of straps, chains, binders, and tiedowns that keeps cargo from shifting, falling, or blowing off. Cargo must be secured against forward, rearward, and sideways movement before the vehicle is driven.

The combined strength rating of all tiedowns used to secure a load. It must be at least half the cargo weight, and each tiedown must be attached and tensioned so the load cannot shift.

A sturdy barrier placed behind the cab of a flatbed or open-top trailer to stop cargo from sliding forward into the cab during a sudden stop or crash. It must be strong enough to contain the load.

A systematic walk-around check of lights, tires, brakes, fluids, coupling devices, and cargo performed before driving. It is required for commercial drivers and is the best defense against breakdowns and out-of-service violations.

A braking system that uses compressed air to apply the brakes. Air pressure pushes a diaphragm in the brake chamber, which moves a pushrod and slack adjuster to apply the foundation brakes.

The engine-driven pump that builds and maintains air pressure in the brake system. It is governed to stop pumping at a set pressure and restart when pressure drops, keeping the system charged while driving.

A valve that controls when the air compressor pumps and when it unloads. It cuts the compressor out at the maximum pressure and back in at a lower pressure, preventing over-pressurization of the system.

Storage reservoirs that hold compressed air for braking and auxiliary use. They collect moisture and oil, so they must be drained regularly, and they supply the reserve needed to stop if the compressor fails.

A pressure-relief valve on the air tank that opens automatically if system pressure rises too high, protecting tanks and lines from rupture. It is usually set near 150 psi and should never be plugged.

The sealed unit where air pressure acts on a diaphragm to push a rod that applies the brake. A ruptured diaphragm or leaking chamber weakens braking and must be repaired before driving.

The rod that transfers force from the brake chamber diaphragm to the slack adjuster. Its travel is measured during inspection; too much travel means the brakes are out of adjustment or a component is worn.

The lever that converts pushrod motion into rotation of the brake camshaft, applying the brakes. Its adjustment determines how far the pushrod travels and must be checked often on air-braked vehicles.

A light and buzzer that activate when air pressure falls below a safe level, typically around 60 psi. If it comes on while driving, stop safely and find the leak before continuing.

A parking and emergency brake that uses a powerful spring to apply the brakes when air pressure is released. It sets automatically if air pressure is lost, which is why it is also called an emergency brake.

A sequence of checks for leaks, compressor buildup, warning devices, and parking brake holding power, performed before a trip. It confirms the system can build and hold pressure and stop the vehicle safely.

With the engine off, brakes released, and full pressure in the tanks, the driver watches the gauge for a set time. A pressure drop beyond the allowed limit indicates a leak in the system.

With full pressure and the brake pedal held down, the driver watches the gauge for a pressure drop. Excess loss shows a leak in the applied-brake circuit, such as a bad chamber or hose.

A check that the parking brake, often a spring brake or driveline brake, holds the vehicle when applied. The vehicle should not move when the brake is set and the engine is revved gently in gear.

Loss of stopping power caused by overheating, common on long downgrades when brakes are used continuously. Downshifting and using engine braking help prevent it; fading brakes may require a stop to cool.

A device that uses engine compression to slow the vehicle, reducing reliance on the service brakes on long downgrades. It is often called a Jake brake and may be restricted in some areas by noise rules.

A technique of applying the brakes firmly to slow well below the safe speed, then releasing them to let them cool, rather than riding them down a grade. It helps avoid brake fade on long descents.

The combination of fifth wheel, kingpin, and related hardware that connects a tractor to a semitrailer. Proper coupling, locking, and a pull test are required before the combination is moved.

The horseshoe-shaped plate on the tractor that locks around the trailer kingpin and supports the front of the trailer. Its jaws must be closed and locked, and the plate must be greased for smooth turning.

The vertical pin under the front of a semitrailer that locks into the fifth wheel jaws. A worn or damaged kingpin can let the trailer separate, so it is checked during pre-trip inspection.

The retractable legs supporting the front of an uncoupled semitrailer. They must be fully raised before driving and firmly on the ground when parked, and the crank handle should be secured.

A valve that closes the tractor's air supply to the trailer if the trailer breaks away or a line ruptures. It keeps enough air in the tractor to stop it and applies the trailer emergency brakes.

The coupling fittings that connect the tractor's air lines to the trailer's. They must be locked together with seals in good condition, and the lines must not cross or drag on the ground.

The air line that supplies the trailer's emergency and parking brake system. If it separates or loses pressure, the trailer brakes apply automatically, which is why it is often called the emergency line.

The tendency of the rear wheels of a long combination to follow a shorter path than the front wheels when turning. Drivers must swing wide and watch mirrors to avoid hitting curbs or other vehicles.

The outward sweep of the rear of a long trailer during a turn, which can strike nearby objects or vehicles. The driver must allow extra room and check mirrors throughout the turn.

The way the last trailer in a multi-trailer combination swings farther sideways than the tractor during a turn or lane change. It makes doubles and triples harder to control and demands slower speeds and wider turns.

A special endorsement, marked H on the license, required to drive a vehicle carrying placarded amounts of hazardous materials. It requires a background check and a knowledge test.

A diamond-shaped warning sign placed on the outside of a vehicle to identify the hazard class of its cargo. Placards warn emergency responders and the public and must match the material being carried.

The document describing a hazardous materials shipment, including the proper shipping name, hazard class, UN number, quantity, and emergency response information. It must be kept within reach and shown to inspectors.

One of nine categories used to group hazardous materials by the type of danger they present, such as flammable, corrosive, or radioactive. The class number appears on placards and shipping papers.

A four-digit identification number assigned to a specific hazardous material or group of materials. It appears on shipping papers and often on orange panels or placards to help responders identify the substance.

A reference carried by hazardous materials drivers that lists initial response actions by guide number for spills, fires, and exposures. It helps drivers and responders decide on isolation distances and protective measures.