OR CDL Combination study guide
OR CDL Combination · Work through your exam, one topic at a time.
The Oregon CDL Combination Vehicles knowledge test is the written exam a driver must pass to add the Class A combination-vehicle endorsement path to an Oregon commercial learner permit or commercial driver license. It is aimed at people who plan to drive tractor-trailers, doubles, or other vehicles with a towed unit, and who therefore need to show they understand coupling, inspection, air brakes, and safe handling before taking a CDL skills test. The OR CDL Combination study guide has 7 sections and about 2,198 words.
01Combination Vehicle Inspection and Coupling/Uncoupling
Combination-vehicle inspection questions test whether you can verify a tractor-trailer as a system: fifth wheel, kingpin, air lines, landing gear, and the order of coupling steps. Expect sequence questions ("what comes next?") and defect questions.
349 words, about 2 min02Driving Combination Vehicles Safely (Turns, Braking, Speed, Space)
A tractor-trailer is two or more vehicles hinged together. The trailer's rear wheels track inside the tractor's path (off-tracking), and the last trailer in a set swings out more than the first (rearward amplification).
329 words, about 2 min03Air Brakes and Brake System Checks on Combination Vehicles
Combination vehicles have a service line (blue) and an emergency line (red). The tractor protection valve closes the tractor's air supply if trailer pressure falls, and the trailer supply valve controls the emergency line.
338 words, about 2 min04Skid Control
A skid is loss of traction. In combinations, overbraking, too much speed for the curve, and abrupt steering are the usual causes.
304 words, about 2 min05Jackknifing and Rollover Prevention
A jackknife happens when the drive wheels lose traction: the trailer pushes the tractor around in a brake jackknife, or the tractor pulls away from the trailer in a power jackknife. A rollover happens when centrifugal force in a curve overturns a high-center-of-gravity load.
292 words, about 1 min06Cargo Securement
Cargo must be secured so it cannot shift enough to affect steering or stability. The federal standard requires the aggregate working load limit (WLL) of all tie-downs to be at least 50% of the cargo weight, and sets the number of tie-downs by cargo length and weight.
298 words, about 1 min07Weight Distribution and Trailer Handling
A combination must be legal by gross combination weight rating (GCWR), each axle's rating, tire rating, and the bridge formula. The kingpin load is carried by the tractor's drive axles and fifth wheel; the steering axle needs enough weight to steer but not more than its rating.
288 words, about 1 min