CDL Combination Vehicles Study Guide 2026

Driving combinations safely

Combination vehicles are heavier and longer than single vehicles, and they need more skill to drive. The combination vehicles knowledge test is needed for a Class A CDL. The number of questions varies by state (for example, 20 in Washington and 30 in Utah), and you need 80% to pass.

Rollovers

More than half of truck driver deaths in crashes come from rollovers. A fully loaded rig is about 10 times more likely to roll over in a crash than an empty one. To prevent rollovers:

• Keep the cargo as low as possible, centered, and spread out.
• Go slowly around corners, on-ramps and off-ramps.
• Avoid quick lane changes, especially when fully loaded.

Rearward amplification

A quick steering move is magnified at the back of the rig. This crack-the-whip effect is called rearward amplification, and it can tip over the rear trailer. A rearward amplification of 2.0 means the rear trailer is twice as likely to turn over as the tractor. With triples, the last trailer can roll about 3.5 times as easily as a five-axle tractor-semitrailer.

Steer gently and smoothly. Look far enough ahead that you don't need sudden lane changes, and leave at least 1 second for every 10 feet of length, plus 1 second over 40 mph.

Braking and skids

An empty combination can take longer to stop than a loaded one. Its stiff springs and strong brakes give poor traction, so wheels lock easily. A bobtail tractor (no trailer) is also hard to stop smoothly. Brake early and leave plenty of space.

A trailer jackknife happens when the trailer wheels lock and the trailer swings around. It is more likely with an empty or lightly loaded trailer. Check your mirrors any time you brake hard. If the trailer starts to skid, release the brakes so the wheels can grip again. Do not use the trailer hand brake to straighten out; the trailer brakes caused the skid.

Turns and offtracking

In a turn, the rear wheels follow a shorter path than the front wheels. This is offtracking. Longer vehicles offtrack more, and the rear wheels of the last trailer offtrack the most.

This is called a button hook turn. Swinging left before the turn (a jug-handle turn) is wrong, because cars may try to pass you on the right.

Railroad crossings

Low-slung trailers such as lowboys, car carriers, moving vans and possum-belly livestock trailers can get stuck on raised crossings. If you are stuck, get out and away from the tracks, and call 911 with the crossing's location and DOT number.

Backing with a trailer

• Turn the steering wheel the opposite way from where you want the trailer to go. Once it starts to turn, turn the wheel back to follow it.
• Back in a straight line when you can. If you must back on a curve, back toward the driver's side.
• Walk around and check your path, back slowly, and use both outside mirrors.
• To correct drift, turn the top of the wheel toward the drift. Use pull-ups to reposition.

Combination vehicle air brakes

A combination has extra air brake parts to control the trailer. Study the air brakes guide as well as this section.

• If you cross the air lines, a trailer with spring brakes will not release its brakes.
• Use dummy couplers to keep dirt and water out of unused lines.
• Drain the trailer air tanks every day.
• Trailers built before 1975 do not have to have spring brakes. Chock their wheels when you park, because the stored air can leak away.
• Trailers and converter dollies built on or after March 1, 1998 must have antilock brakes, with a yellow lamp on the left side of the trailer.
• Combination leakage limits: less than 3 psi a minute with the brakes released, and no more than 4 psi a minute with them applied.

Coupling and uncoupling

Coupling the wrong way can let a trailer come loose on the road. Follow the steps in order every time.

1. Inspect the fifth wheel. It should be securely mounted, greased, tilted down toward the rear, with the jaws open and the release handle in the automatic lock position. If it slides, make sure it is locked.
2. Inspect the area and chock the trailer wheels (or make sure the spring brakes are on).
3. Line up the tractor directly in front of the trailer. Never back under at an angle; you could push the trailer sideways and break the landing gear.
4. Back slowly until the fifth wheel just touches the trailer.
5. Secure the tractor: parking brake on, transmission in neutral.
6. Check trailer height. The trailer should be low enough that it is raised slightly when the tractor backs under it.
7. Connect the air lines: emergency to emergency, service to service.
8. Supply air to the trailer and check for crossed lines by applying and releasing the trailer brakes with the engine off.
9. Lock the trailer brakes by pulling out the air supply knob.
10. Back under the trailer slowly in the lowest reverse gear until the kingpin locks.
11. Check the connection: raise the landing gear slightly and pull gently forward with the trailer brakes locked.
12. Secure the vehicle: neutral, parking brake, engine off, take the key.
13. Inspect the coupling: no space between the upper and lower fifth wheel, the jaws closed around the shank (not the head) of the kingpin, and the locking lever in the lock position with the safety latch over it.
14. Connect the electrical cord and check that lines will not rub on moving parts.
15. Raise the landing gear fully, starting in low range. Secure the crank handle and check clearance in turns.
16. Remove and store the wheel chocks.

A gap between the upper and lower fifth wheel means the kingpin may be sitting on top of the jaws. Do not drive until the problem is fixed.

Before uncoupling

Park in a straight line on firm, level ground and set the parking brakes. Chock the trailer wheels if the trailer has no spring brakes. Then follow your manual's uncoupling steps in order.

Inspecting a combination vehicle

Add these checks to step 5 of the seven-step inspection.

• Lower fifth wheel: securely mounted, not damaged, enough grease, no space between upper and lower parts, jaws around the kingpin shank, release arm in place with the safety latch engaged.
• Upper fifth wheel: glide plate securely mounted and the kingpin not damaged.
• Air and electrical lines: connected, not leaking, with enough slack for turns and no damage.
• Landing gear: fully raised, not damaged, and the crank handle secured.

Make sure a sliding fifth wheel is not so far forward that the tractor frame hits the landing gear, or the cab hits the trailer, during turns.

Combination air brake tests