A tractor or truck pulling one or more trailers.
Tractor-trailers, doubles, triples and a straight truck with a trailer are all combinations. They are heavier and longer, need more skill, and roll over more easily in quick turns.
Example: Pulling a loaded trailer, you slow down before the ramp because a combination vehicle tips more easily.
Also called: combination vehicles
The power unit built to pull a semitrailer on its fifth wheel.
A truck tractor carries no cargo itself; the trailer does. The fifth wheel on the tractor's frame takes the trailer's kingpin.
Does not mean: Not a farm tractor; in CDL English a tractor is the truck that pulls a semitrailer.
Example: You line the tractor up straight in front of the trailer before backing under it.
Also called: truck tractor
A trailer with no front axle that rests on the tractor's fifth wheel.
When uncoupled it stands on its landing gear. Drivers often just say trailer. Older trailers may have no spring brakes, so chock the wheels when parking.
Example: You lower the landing gear until it takes the weight, then pull the tractor out from under the semitrailer.
Also called: semi-trailer
The greased plate on the tractor that locks onto the trailer's kingpin.
Its jaws close around the kingpin. The plate must be greased, or friction between tractor and trailer makes steering hard. Before coupling it should tilt down toward the rear with the jaws open.
Does not mean: Not the RV fifth-wheel camper; here it is the coupling plate on a truck tractor.
Example: You go under the trailer with a flashlight to check the fifth wheel is locked before raising the landing gear.
Diagram coming
A fifth wheel that can move along the tractor frame to shift weight.
Locking pins hold it in place. On the walk-around check that the slide is undamaged, nothing is missing and the pins are locked. It must not sit so far forward that the cab or frame hits the trailer in turns.
Example: Before leaving you confirm the slider pins are locked so the fifth wheel cannot move while you drive.
Diagram coming
The steel pin under the trailer nose that the fifth wheel grips.
Check it is not bent, broken or worn. When coupling, the jaws must close around the right part of the pin, and there must be no gap between trailer and fifth wheel.
Does not mean: Not a crime boss or a film title; in trucking the kingpin is the trailer's coupling pin.
Example: You back under slowly in low reverse so the fifth wheel does not slam into the kingpin.
Diagram coming
Also called: king pin
The metal plate around the kingpin that rests on the fifth wheel.
Its visible part must not be bent, cracked or broken. The trailer should lie flat on the fifth wheel with no gap between the apron and the plate.
On the test
The apron appears in the pre-trip coupling check: questions ask what you inspect around the kingpin and what a gap means.
Example: Shining a light along the trailer nose, you confirm the apron sits flat on the fifth wheel with no gap.
Diagram coming
Also called: trailer apron
The jaws inside the fifth wheel that close around the kingpin.
They must grip the kingpin's shank, not its head. If the kingpin sits on top of closed jaws there will be a gap and the trailer can come loose.
Example: Before uncoupling you back gently against the locked trailer brakes to take pressure off the locking jaws.
Diagram coming
Also called: fifth wheel jaws
The handle that opens the fifth wheel jaws to free the kingpin.
To uncouple, raise the handle's lock and pull it to the open position. In the walk-around it must be properly seated with the safety latch engaged.
Example: Standing clear of the rear tractor wheels, you pull the release arm out to unlock the fifth wheel.
Diagram coming
Also called: release handle
Pins that lock a sliding fifth wheel or sliding tandem in place.
Some parts of a tractor or trailer can slide to a new spot. Locking pins hold them there. Every pin must be in place and locked before you drive.
Example: Before leaving the yard, you crouch at the fifth wheel and confirm every locking pin is seated, not hanging loose.
Diagram coming
Crank-down legs that hold up the front of an uncoupled semitrailer.
A trailer has no front wheels. When you drop it, the landing gear holds the nose up. You crank it down before uncoupling and all the way up before driving.
Example: After the tug test, you crank the landing gear fully up and clip the handle so it cannot swing into traffic.
Diagram coming
Hooking a tractor to a trailer, or one trailer to another.
Coupling means hooking up. You back the fifth wheel under the trailer until the kingpin locks. Then you connect the air lines and electrical cord and check the lock by sight and by a gentle pull.
Example: You stop backing the moment the jaws grab the kingpin, set the brakes, and climb out to look under the trailer.
Separating a tractor from its trailer, or a trailer from its dolly.
Uncoupling means dropping the trailer. You lock the trailer brakes, lower the landing gear, disconnect the lines, unlock the fifth wheel and pull out slowly.
Example: This old trailer has no spring brakes, so you chock its wheels before you start cranking the landing gear down.
Two axles mounted close together that share one end's load.
Most trailers and many tractors have two rear axles side by side. Together they are called tandems. They carry more weight than one axle could.
Example: At the scale you see the trailer tandems are over their limit and plan to shift some weight forward.
Diagram coming
Also called: tandem, tandems
A trailer tandem that unlocks and slides along rails to shift weight.
The trailer's rear axles sit on a rail. Pull the release handle, and the axles slide forward or back. Moving them changes how much weight each axle carries.
On the test
Knowledge questions rarely name it; on the pre-trip inspection you must show the release arm is secured and the locking pins locked.
Example: At the scale you pull the tandem release, roll the trailer forward two holes, and relock the pins.
Diagram coming
Also called: tandem release
A fifth-wheel axle unit that lets one trailer tow another.
A semitrailer has no front wheels. The dolly gives it some. You hook the dolly to the first trailer with a pintle hook, then back it under the second trailer.
Example: You wheel the empty dolly by hand into line with the second trailer's kingpin before backing the first trailer onto it.
Diagram coming
Also called: converter gear
A latching hook that holds a dolly's or trailer's drawbar eye.
The dolly's tow bar ends in a steel ring. That ring drops over the pintle hook on the back of the front trailer. A latch then closes over it.
Example: You lift the lock pin, drop the dolly eye over the hook, and push the latch closed until the handle sits flush.
Diagram coming
A tractor pulling two trailers, the second on a converter dolly.
Doubles means two trailers behind one tractor. The second trailer rides on a dolly. The rig is longer, less stable and needs the T endorsement.
Example: Pulling doubles onto the ramp, you slow down earlier than you would with a single trailer because the rear trailer swings most.
Also called: double trailers
A tractor pulling three trailers, each extra one on a converter dolly.
Triples means three trailers in a row. They are the longest and least stable combination you can drive, and the last trailer whips the most.
Example: Coupling triples, you build the rig from the front: tractor to first trailer, then a dolly and trailer twice more.
Also called: triple trailers
Rear wheels following a tighter path than front wheels in a turn.
In a turn, the back wheels cut inside the front wheels. The longer the vehicle, the more they cut in. The last trailer of a rig cuts in the most.
Does not mean: Off-tracking is the normal geometry of a turn, not a skid or a loss of control.
Example: Turning right at a tight corner, you swing the tractor wide late in the turn so the trailer tires clear the curb.
Diagram coming
Also called: offtracking, cheating
A skid that folds the tractor and trailer toward each other.
Picture a pocket knife folding. When the trailer wheels lock and slide, the trailer swings around toward the cab. It can also start when the tractor's drive wheels lose grip.
Does not mean: On the test, jackknife is the folding skid of a combination vehicle, not a folding knife.
Example: Braking hard on a wet ramp with an empty trailer, you check your mirror and see the trailer starting to drift sideways.
Diagram coming
Also called: jackknifing, trailer jackknife, tractor jackknife
A crash in which a truck or trailer tips onto its side.
A tall, heavy load raises the center of gravity. Turn too fast and the rig tips over. Keep cargo low and centered, and slow down before curves and ramps.
Example: On a cloverleaf ramp with a tall load of appliances, you drop well below the posted speed before the curve begins.
Also called: roll over
The cable that powers the trailer's lights and ABS from the tractor.
Drivers call it the pigtail because it coils. You plug it into the trailer socket and close the safety catch. When unhooked, hang it plug-down so water stays out.
Example: After plugging in the cord, you snap the safety catch shut and make sure the cord cannot rub on the catwalk.
Also called: pigtail, electrical line
The service and emergency hoses carrying air from tractor to trailer brakes.
Every trailer gets two air hoses. The service line tells the trailer brakes how hard to apply. The emergency line fills the trailer tanks and sets the brakes if it breaks.
Example: Hooking up in the dark, you match the colored glad hands by feel and run your hand along each hose for cuts.
Diagram coming
Also called: air hoses
A gentle pull against locked trailer brakes to prove the coupling locked.
After you back under the trailer, you lock the trailer brakes and ease the tractor forward in low gear. If the trailer holds you back, the lock is good. The manuals describe this step but do not use the name.
On the test
Questions describe the step rather than the name: pulling gently against the trailer brakes after coupling, with the landing gear slightly raised.
Does not mean: Not a medical or firearms term; in CDL use it is the pull check after coupling.
Example: With the air supply knob pulled out, you let the clutch up slowly and feel the trailer brakes hold the whole rig.
The axle whose wheels get engine power and push the vehicle forward.
The engine turns the drive axle through the drive shaft. On a tractor it is the rear axle or axles. How much weight rests on it decides how well it grips.
Example: Bobtailing in snow, your drive axle has almost no weight on it, so you pull away gently to keep the wheels from spinning.
Also called: drive axles
