Which two habits do the most to prevent a combination vehicle from rolling over?
What to know for the Combination Vehicles test
Each set draws 20 of our 74 questions
The combination vehicles knowledge test covers coupling and uncoupling, trailer air brakes, inspection and safe handling of a tractor-trailer. It is required for a Class A CDL. Federal rules set the pass mark at 80 percent, and failing it means no Class A license can be issued.
Last checked against the handbook and 49 CFR on .
Who takes this test
The test covers three areas: coupling and uncoupling a tractor and semi-trailer, the inspection steps unique to combinations, and general operating practice including air brakes. 49 CFR 383.111
An applicant who scores under 80 percent on the combination portion cannot be issued a Group A CLP or CDL. 49 CFR 383.135
Because combination rigs normally use air brakes, expect to need the air brake test as well. CDL Handbook 6.2
Rollover, skids and turning
A fully loaded rig is about ten times as likely to roll over in a crash as an empty one, and keeping cargo low and slowing for turns are the two main defenses. CDL Handbook 6.1
Rearward amplification, nicknamed the crack-the-whip effect, makes the last trailer swing harder than the tractor in a quick lane change, so steer smoothly. CDL Handbook 6.1
A bobtail tractor with no trailer stops more slowly than a loaded tractor-trailer, and an empty trailer locks its wheels easily because the suspension is stiff. CDL Handbook 6.1
To stop a trailer skid, spot it in the mirrors, release the brakes to regain traction, and never use the trailer hand valve to straighten the rig. CDL Handbook 6.1
Off-tracking means the rear wheels follow a tighter path than the front wheels, and the last trailer off-tracks the most, so swing the front end wide while keeping the back of the rig near the curb. CDL Handbook 6.1
When backing a trailer, turn the top of the steering wheel the opposite way from the one you would use for a straight truck, and back toward the driver side when you must use a curve. CDL Handbook 6.1
Trailer air brake parts
Combination vehicle air brake components
Part
What it does
Service air line (blue)
Carries the foot-brake signal to the trailer relay valves
Emergency air line (red)
Charges the trailer tanks; losing its pressure applies trailer emergency brakes
Trailer air supply control
Red eight-sided knob; push in to feed the trailer, pull out to apply trailer brakes
Tractor protection valve
Closes at about 20 to 45 psi to keep tractor air from escaping
Trailer hand valve
For testing trailer brakes only, never for driving or parking
Glad hands
Couplers that join the lines; seal rubber must be clean
Crossed air lines send supply air into the service line, so the trailer spring brakes will not release when you push in the air supply knob. CDL Handbook 6.2
Trailers and dollies built before 1975 need not have spring brakes, so chock their wheels when parked. CDL Handbook 6.2
On a multi-trailer train, the valves at the tail of the final trailer stay closed and every other shut-off valve stays open. CDL Handbook 6.2
Any trailer or converter dolly manufactured from 1 March 1998 onward must have ABS, with a yellow malfunction lamp on the left side. CDL Handbook 6.3
Coupling a tractor-semitrailer
Inspect the fifth wheel: jaws open, plate greased, tilted toward the rear, and handle in the automatic lock position. CDL Handbook 6.4
Chock the trailer wheels or confirm the spring brakes are on, then line the tractor up straight in front of the trailer, never at an angle. CDL Handbook 6.4
Connect the emergency and service lines, push the air supply knob in, and wait for normal pressure. CDL Handbook 6.4
Pull the air supply knob out to lock the trailer brakes, then back slowly in the lowest reverse gear until the kingpin locks. CDL Handbook 6.4
Tug-test the connection by pulling gently forward, then crawl under the trailer with a flashlight and confirm there is no gap between upper and lower fifth wheel. CDL Handbook 6.4
Raise the landing gear fully, since driving with it partly down can catch on railroad tracks. CDL Handbook 6.4
Combination Vehicles test: common questions
Do I need the combination vehicles test for a Class A CDL?
Yes. Failing the combination portion (under 80 percent) means a Group A CLP or CDL cannot be issued. 49 CFR 383.135
What is the difference between the service line and the emergency line?
The service line carries the brake pedal signal to the trailer. The emergency (supply) line fills the trailer air tanks, and if it loses pressure the trailer emergency brakes apply and the tractor protection valve closes. CDL Handbook 6.2
Why should the trailer hand valve not be used while driving?
Applying only the trailer brakes can make the trailer skid or jackknife. The foot brake sends air to every brake, so it is much safer. Use the hand valve only to test trailer brakes. CDL Handbook 6.2
What is off-tracking?
Off-tracking is the way a vehicle's rear wheels follow a different, tighter path than its front wheels in a turn. Longer rigs off-track more, and the last trailer of a multi-trailer rig off-tracks the most. CDL Handbook 6.1
How do I check for crossed air lines?
After connecting the glad hands, shut off the engine, work the trailer brakes on and off and listen for them moving. If the trailer brakes do not apply and release properly, the lines may be crossed. CDL Handbook 6.4