California CDL Doubles and Triples Endorsement Test key terms

A short frame with one or two axles and a fifth-wheel plate that attaches to the rear of one trailer so a second trailer can be coupled behind it, forming a double or triple combination.

The horseshoe-shaped coupling plate on a tractor or dolly that locks around a trailer's kingpin and lets the trailer pivot as the combination turns.

The sturdy vertical steel pin under a trailer's nose that locks into the fifth-wheel jaws and holds the trailer to the towing vehicle.

The two curved metal pieces inside the fifth wheel that close around the kingpin; the locking lever and safety latch must hold them shut before you drive.

A pull test done after coupling in which you gently pull forward with the trailer brakes set to confirm the kingpin is locked and the trailer will not slide off.

The retractable legs under a trailer's front that support it when uncoupled; they must be fully raised before towing and firmly on the ground when parked.

The interlocking couplers that join the tractor's air hoses to the trailer's service and emergency lines, sealed with rubber gaskets to prevent air leaks.

The air hose that carries the driver's brake application pressure to the trailer brakes; a break or disconnect there causes a slow, controlled loss of trailer braking.

The air hose that supplies reservoir pressure and signals a breakaway; if it separates, the trailer's protection valve applies the spring brakes automatically.

Brakes held open by heavy springs and released by air pressure, so they set automatically when air pressure drops or the trailer separates from the towing vehicle.

The system that sets a trailer's spring brakes if it separates from the towing vehicle, preventing a runaway trailer; it depends on a charged air reservoir.

A trailer valve that speeds brake response by sending reservoir air directly to the brake chambers when it receives a signal from the service line.

The setting that keeps each brake chamber's pushrod stroke within its legal limit; on doubles and triples every axle's brakes must be checked and kept in adjustment.

The systematic walk-around check of coupling devices, hoses, lights, tires, brakes, and cargo done before driving, required by law and essential with multiple trailers.

The check performed at the end of a run for damage, air leaks, loose fasteners, shifted cargo, and tire or brake problems so defects are found before the next trip.

The colored hoses connecting tractor and trailer air systems; they must be supported, free of chafing, and connected so they do not drag or kink on turns.

A heavy hook-and-latch coupling on a dolly or trailer rear that grabs a drawbar eye, used to tow a following trailer in some double and triple setups.

The rigid bar with an eye at its end that connects a towed trailer or dolly to the pintle hook of the vehicle ahead.

Backup chains that connect a towed unit to the towing vehicle so the unit stays attached if the primary hitch fails; they must be crossed and properly rated.

The tendency of a trailer's rear wheels to cut inside the path of the front wheels on a turn, which is greater with multiple trailers and requires wider swings.

The total width of roadway a turning combination occupies, from the outside front corner to the inside rear trailer wheels; it grows with trailer length and count.

The distance from the center of a turn to the vehicle's path; doubles and triples need a larger radius so trailers do not strike curbs, poles, or opposing traffic.

How well a trailer follows directly behind the towing vehicle in a straight line; poor tracking from misaligned axles or overload causes sway and tire wear.

A slight extra outward movement of the tractor before a turn, or a small lane offset, used to keep rear trailer wheels clear of curbs and obstacles.

Side-to-side whipping of trailers that worsens as speed rises and can lead to loss of control; it is caused by poor loading, steering input, or wind.

A severe side-to-side motion of the rear trailer that can amplify through the combination; the correct response is to slow gradually, not brake hard or steer sharply.

A sudden rollover caused by rapid steering or excessive speed that throws the vehicle's weight sideways faster than the suspension can handle, common with high-center-of-gravity loads.

A lever that lets the driver apply only the trailer brakes; used carefully it can straighten a swaying combination, but it must not be used while turning or in slippery conditions.

Loss of tire grip that lets the vehicle slide; with multiple trailers recovery is harder, so the driver should release hard braking and steer smoothly in the direction of the skid.

A loss of control in which the tractor and trailer fold sharply toward each other, often triggered by locked drive wheels or hard braking on a slippery surface.

Loss of braking power from overheated drums or shoes after repeated or prolonged use, such as long downgrades; gearing down helps prevent it in heavy combinations.

An emergency technique of applying the brakes firmly until wheels nearly lock, then releasing fully and repeating, used only when ABS is not available.

An electronic system that rapidly modulates brake pressure to keep wheels from locking; the driver should brake firmly and steer normally while the system works.

The gap kept behind the vehicle ahead; with doubles and triples it should be at least one second per ten feet of combination length plus extra for speed and conditions.

The safety margin of empty space kept around the whole combination, including ahead, behind, and to both sides, so there is room to react and stop.

Choosing and holding a safe speed for curves, downgrades, traffic, and weather; doubles and triples need lower entry speeds because they are long, heavy, and slow to stop.

The total distance traveled from perception to a full stop; it grows with weight and speed, and multiple trailers need more room than a single trailer rig.

Using tiedowns, straps, chains, and blocking to keep cargo from shifting or falling; the number and strength of tiedowns depend on the load's weight and length.

Placing weight so each axle and trailer stays within its legal limit and the combination stays stable; uneven loads cause poor tracking, sway, and axle overloads.

The point where the load's weight is balanced; a high or off-center point makes a combination easier to roll over and harder to steer in curves or evasive moves.

The maximum legal load allowed on a single axle, tandem group, or gross combination; exceeding it is unsafe and can bring fines and out-of-service orders.

The manufacturer's maximum safe total weight for a vehicle or combination, including the vehicle, fuel, driver, and cargo; it must never be exceeded.

A federal rule limiting the weight carried on any group of axles based on the number of axles and the distance between them, meant to protect highway bridges.

The license endorsement required to tow two or three trailers, obtained by passing a knowledge test and holding the proper class of commercial license.

The step-by-step procedures for connecting or separating tractor, dollies, and trailers, including chocking wheels, aligning the kingpin, and checking the lock after each connection.

Inspecting each tire for tread depth, cuts, bulges, and proper inflation; with many axles, one bad tire can cause a blowout and loss of control in a long combination.