CDL key terms

The maximum total weight a single vehicle is built to carry safely, counting the vehicle itself plus fuel, passengers, and cargo. It is set by the manufacturer and shown on the certification label.

The maximum combined weight allowed for a towing vehicle and its trailer or trailers, including all cargo, fuel, and people. The manufacturer sets this limit for the whole combination.

The amount of weight resting on one axle and its tires. Every axle has legal and design limits, and overloading one axle can cause tire failure or brake problems.

A federal rule that sets the maximum weight allowed on a group of axles. The limit depends on how many axles are in the group and how far apart they are spaced.

A commercial driver's license that allows operating a combination vehicle with a gross combination weight rating over 26,000 pounds, when the towed unit is over 10,000 pounds.

An extra authorization added to a CDL that lets the driver operate a special vehicle type or haul specific cargo, such as air brakes, tankers, doubles and triples, or hazardous materials.

An engine-driven pump that pulls in outside air and forces it into the brake system's storage tanks, building the pressure needed to apply the brakes.

A control device that shuts off the compressor when tank pressure reaches the cut-out level and turns it back on when pressure drops to the cut-in level.

A metal tank that stores compressed air for the brake system. Each vehicle usually has one or more tanks holding enough air to make several brake applications.

A spring-loaded valve on the air tank that opens automatically if pressure rises too high, releasing air to protect the tank and lines from damage.

A round chamber at the wheel that uses air pressure against a rubber diaphragm and push rod to apply the foundation brakes when the driver presses the pedal.

The lever arm that connects the brake chamber push rod to the brake camshaft. It multiplies force and takes up the gap as brake linings wear.

A drum brake on heavy vehicles where a rotating S-shaped cam pushes brake shoes outward against the inside of the brake drum to slow the wheel.

A brake that uses a powerful compressed spring to apply the brakes when air pressure is lost or when the parking brake is set. Air pressure releases it.

A light and buzzer that comes on when brake system air pressure drops below a set level, warning the driver to stop before the spring brakes apply.

The horseshoe-shaped coupling plate mounted on the rear of a tractor. It locks around the trailer's kingpin and lets the trailer pivot as the rig turns.

A heavy vertical pin mounted under the front of a trailer. It locks into the tractor's fifth wheel to hold the trailer and allow it to pivot.

The metal couplers that connect the air brake hoses between the tractor and trailer. Two are used: one for the service line and one for the emergency line.

A valve that seals off the tractor's air supply and keeps its brakes working if the trailer breaks away or an air line separates.

A lever on the steering column or dash that lets the driver apply only the trailer brakes, used to test the trailer brakes and check for a firm hold.

The retractable support legs under the front of a trailer. They hold the trailer up when it is not attached to a tractor.

A small frame with its own axle, fifth wheel, and air lines that is used to hook a second trailer behind the first when pulling doubles or triples.

The tendency of the rear wheels of a long vehicle to follow a tighter path than the front wheels, making the rear swing toward the inside of a turn.

A wall or partition inside a tank that is partly open, used to slow the forward and backward surging of liquid so the vehicle stays stable.

The empty space left above a liquid in a tank. It allows the product to expand as it warms and gives room for movement so the tank is not overfilled.

The front-to-back movement of liquid inside a tank caused by starting, stopping, or turning. It can push a vehicle forward at a stop light or make it unstable.

A diamond-shaped warning sign placed on the outside of a vehicle to show the hazard class of the cargo it carries, such as flammable or corrosive.

The paperwork that travels with a hazardous materials load. It lists the material, hazard class, quantity, and emergency contact information for responders.

One of nine categories used to group hazardous materials by the kind of danger they pose, such as explosives, flammable liquids, or corrosives.

A reference book kept in the vehicle that tells drivers and emergency crews how to respond to a spill or fire involving hazardous materials.

A marked line near the front of a bus showing where standing passengers may not pass. Nothing may be placed in front of it, and the driver must be able to see it.

A written report the driver fills out after a trip listing any defects found on the vehicle. It is given to the carrier so repairs can be made before the vehicle is used again.

A check of the vehicle's critical parts, including tires, brakes, lights, coupling devices, and cargo securement, done before driving to catch problems that could cause a crash.

When tires ride on a thin layer of water instead of the road, causing loss of steering and braking. It happens most at higher speeds or with worn or underinflated tires.

The clear distance kept around the vehicle so there is room to stop or steer away if traffic ahead suddenly slows or something enters the lane.

Braking as hard as possible without locking the wheels or causing a skid. It gives the shortest stop while keeping steering control.

A technique for vehicles without antilock brakes where the driver presses the brake pedal hard, then releases it, repeating the cycle to avoid locking the wheels.

A loss of traction where the tires slide instead of rolling. The driver should stop braking, steer in the direction the vehicle needs to go, and avoid harsh pedal input.