How many questions are on the combination vehicles test?

Your state sets the number of questions, the time limit and the passing score. Check your state's CDL manual.

Do I also need to study Section 7 on doubles and triples?

Only if you need the doubles and triples test. The manual says to study Section 7 if you need to pass that test. It is a separate test from combination vehicles.

CDL Combination Vehicles Test: What's on It

Q: Who needs the combination vehicles test?

Anyone who wants to drive a combination vehicle, like a tractor-trailer. The manual says the test is required if you want to drive combination vehicles. Class A is the license for heavy combinations.

The CDL combination vehicles test checks what you know about driving, braking, coupling and inspecting a truck with a trailer. You need it if you want to drive combination vehicles, such as a tractor-trailer.

The manual covers the minimum knowledge for tractor-trailers, doubles, triples and a straight truck with a trailer. Your state sets the number of questions and the passing score. Check your state's CDL manual.

Who needs the combination vehicles test?

The manual defines Class A as any combination with a GVWR of 26,001 or more pounds, when the vehicle being towed is over 10,000 pounds. If that is your truck, plan on this test. If your truck has air brakes, you also take the Air Brakes test.

Rollover and the crack-the-whip effect

The manual says more than half of truck driver deaths in crashes are from rollovers. Loaded rigs are ten times more likely to roll over in a crash than empty rigs.

Two things help prevent rollover. Keep cargo as close to the ground as you can, and drive slowly around turns. Keep the load centered too.

The crack-the-whip effect comes from rearward amplification. In a quick lane change, the trailer can tip over. A rearward amplification of 2.0 means the rear trailer is twice as likely to turn over as the tractor. Triples have 3.5. That means you can roll the last trailer of triples 3.5 times as easily as a five-axle tractor.

Steer gently and smoothly. Follow at least 1 second for each 10 feet of your vehicle length, plus 1 more second over 40 mph.

Trailer skids and jackknife

When the trailer wheels lock up, the trailer swings around. It's more likely when the trailer is empty or lightly loaded. The manual calls it a "trailer jackknife."

To stop a trailer skid:

Spot it early in your mirrors. Check them any time you brake hard.
Let off the brake to get traction back.
Do not use the trailer hand brake to straighten the rig.

Large combinations take longer to stop empty than loaded. A bobtail tractor, with no semitrailer, can take longer to stop than a rig loaded to maximum weight.

Wide turns and off-tracking

The rear wheels follow a different path than the front wheels. That's off-tracking. Longer vehicles off-track more, and the last trailer off-tracks the most.

Steer the front end wide enough that the rear doesn't hit the curb or pedestrians. Keep the rear close to the curb so other drivers can't pass on the right.

Air brakes on combination vehicles

Tractor protection valve. It keeps air in the tractor if the trailer breaks away or leaks badly. It closes on its own when air pressure is low, in the range of 20 to 45 psi. That lets the air out of the trailer emergency line, and the trailer emergency brakes come on.

Trailer air supply control. On newer vehicles it's a red eight-sided knob. Push it in to supply the trailer with air. Pull it out to shut the air off and put on the trailer emergency brakes.

Service line and emergency line. Every combination has both. The service line (signal line) carries air set by the foot brake or hand valve. The emergency line (supply line) fills the trailer air tanks and controls the emergency brakes. Losing pressure in it puts the trailer emergency brakes on.

Glad hands. The manual says colors are sometimes used. Blue is for service lines and red is for emergency (supply) lines. Cross the lines and supply air goes to the service line, so the trailer spring brakes won't release.

Trailer hand valve. Use it only to test the trailer brakes. Don't use it while driving, because it can cause a skid. Never use it for parking.

Coupling and uncoupling steps

The manual lists 16 coupling steps and 10 uncoupling steps for tractor-semitrailers. This is the coupling order in short form:

Inspect the fifth wheel. Jaws open, no damaged parts, plate greased, sliding fifth wheel locked.
Check that the area is clear. Chock the trailer wheels or make sure its spring brakes are on.
Put the tractor directly in front of the trailer. Never back under at an angle.
Back slowly until the fifth wheel just touches the trailer.
Set the tractor parking brake and put the transmission in neutral.
Check trailer height. The tractor should raise it slightly as you back under.
Connect the emergency line, then the service line.
Push in the air supply knob. Wait for normal pressure and check for crossed lines.
Pull the air supply knob out to lock the trailer brakes.
Back under in the lowest reverse gear. Stop when the kingpin locks.
Raise the landing gear slightly off the ground. Pull gently forward to test the lock.
Put the transmission in neutral, set the parking brakes, shut off the engine and take the key.
Inspect the coupling. No space between the upper and lower fifth wheel, jaws around the shank, lever locked, safety latch in place.
Plug in the electrical cord and check both air lines and the cord for damage.
Raise the landing gear all the way up. Never drive with it part way up.
Remove the wheel chocks and store them.

Inspection points for combinations

Use the seven-step inspection from Section 2, plus the combination checks. On the lower fifth wheel, check for secure mounting, enough grease, and no visible gap. The locking jaws must be around the shank of the kingpin, not the head. Check the air and electrical lines and the landing gear too.

For brakes, the manual has four tests:

Air flows to all trailers. Push in the air supply knob. At the rear of the last trailer, open the emergency line valve, then close it. Open the service line valve (with the trailer hand brake or foot brake on). You should hear air escape from both.
Tractor protection valve. With the engine off, pump the brake pedal. The knob should pop out at the maker's range, usually 20 to 45 psi.
Trailer emergency brakes. Pull the air supply control and tug gently on the trailer.
Trailer service brakes. Move forward slowly and apply the hand valve.

All shut-off valves must be open, except the ones at the back of the last trailer.

How to study it

Read Section 6 of the manual. At the end of each subsection, answer its "Test Your Knowledge" questions out loud. Then drill with exam-style questions built from the official CDL manual.