OR CDL Combination common mistakes
OR CDL Combination · Understand the traps and how to avoid them.
During the pre-trip walk-around, the candidate checks the fifth-wheel jaws by looking only at the handle position and never verifies that the locking jaws have actually closed around the kingpin shank.
The handle can be in the locked position while the jaws are open, especially if the coupling was done too high or too low and the trailer is riding on top of the jaws. Candidates memorize 'pull the handle, look at it' without understanding what the jaws must physically do.
What to do instead
Get under the trailer with a light and confirm the jaws are closed around the kingpin, the release handle is in the locked position, and the safety latch is engaged. Then do the two-part tug test: pull forward gently in low gear against the trailer brakes, then set the tractor brakes and pull the trailer forward gently against them. State out loud what each check proves.
The candidate skips or rushes the air brake system checks on the combination unit, particularly the static leak test and the tractor protection valve check, treating them as a formality before the drive.
Leak-down and tractor protection checks take time and require watching the gauge for a full minute, and nervous candidates shortcut the wait or forget the sequence. Many have only practiced on a single-unit truck where the trailer protection test does not exist.
What to do instead
Build a fixed verbal sequence: build pressure to governor cut-out, shut off the engine, confirm no drop beyond the allowable limit in one minute with brakes released and then applied, then test the tractor protection valve by fanning off trailer air and confirming the trailer spring brakes set and the tractor air lines seal. Rehearse the sequence until the order is automatic.
The candidate performs the coupling procedure in the wrong order, such as backing under the trailer before the trailer is at the correct height, or failing to chock the wheels and check the trailer brakes before coupling.
Coupling is taught as a list of steps and candidates memorize the list without understanding why each step prevents a specific failure, so under pressure they reorder or drop steps like the height check or the trailer brake test.
What to do instead
Learn coupling as cause-and-effect pairs: chock wheels and test trailer brakes so the trailer cannot roll; match height so the kingpin enters the jaws rather than striking the trailer frame; back slowly and squarely; tug test; then connect air lines and electrical cord and check for chafing. Practice the full sequence aloud in the same order every time.
When uncoupling, the candidate releases the fifth-wheel jaws and pulls out without first lowering the landing gear to firm ground and ensuring the trailer will not drop or tip.
Uncoupling is treated as an afterthought after the driving test, and candidates forget that an empty trailer on soft ground or with the gear too high will drop hard or tip forward when the tractor pulls away.
What to do instead
Follow the fixed uncoupling order: park on firm level ground, chock wheels, lower the landing gear to solid contact, disconnect and secure the air lines and cord, release the tractor parking brake only as needed, pull the handle, then pull straight out slowly. Verify the gear is supporting weight before releasing the jaws.
The candidate swings wide on right turns but cuts the corner on left turns, or fails to track the trailer wheels and lets them off the pavement or over a curb.
Drivers apply the 'swing wide' rule only to right turns and underestimate off-tracking, where the rear trailer wheels follow a shorter path than the tractor. The effect is worse with a long wheelbase and a short trailer.
What to do instead
Before every turn, check mirrors on both sides and remember that the trailer's rear wheels track inside the tractor's path. On left turns, pull forward far enough that the trailer clears the corner before turning; on right turns, swing wide enough that the trailer stays off the curb. State the off-tracking principle aloud during practice.
The candidate brakes hard in a curve or on a downgrade with a lightly loaded or empty trailer, or relies on engine braking alone without knowing the trailer brake condition.
Candidates transfer single-unit habits to combination vehicles and do not account for the trailer pushing the tractor, brake lag, or the way a lightly loaded trailer locks its wheels and swings out. They also neglect the rule against hard braking in curves.
What to do instead
Slow down before the curve or the crest, brake smoothly and progressively while the rig is straight, and release before entering the turn. On downgrades, select a low gear before starting down and use light steady brake applications rather than a single hard one. Practice stopping distances loaded and empty.
The candidate follows too closely and leaves no space ahead, especially when the trailer is empty and stopping distance is shorter but the trailer is more prone to bounce and wheel lock.
Candidates judge following distance by feel rather than by the time-and-speed rule, and they forget that a combination vehicle needs more room to stop and that an empty trailer behaves differently from a loaded one.
What to do instead
Use the one-second-per-ten-feet rule at speeds under 40 mph and add a second for speeds above that, plus extra time in rain, at night, or with an empty trailer. Say the count out loud during practice drives and adjust for the trailer's load state.
The candidate does not recognize the early signs of a jackknife or trailer swing and reacts by braking hard or steering sharply, making the skid worse.
Panic reactions override training, and candidates have not practiced the recovery sequence for a drive-wheel skid, a trailer skid, or a front-wheel skid, so they treat all skids the same.
What to do instead
Learn to identify which wheels are skidding: for a drive-wheel skid, release the brakes and steer in the direction the tractor needs to go; for a trailer skid, release the brakes and steer to keep the trailer behind the tractor; for a front-wheel skid, release the brakes and steer straight. Practice recovery in a controlled lot and say the sequence aloud.
The candidate loads or accepts a trailer with the weight concentrated at the rear or on one side, and does not check axle and kingpin weight limits before driving.
Candidates focus on total gross weight and ignore distribution, so the trailer is tail-heavy, the steer axle is light, or one side is overloaded, all of which change handling and can cause a rollover or jackknife.
What to do instead
Before driving, confirm the load is centered, spread evenly, and secured, and check that axle weights and the kingpin load are within limits. Know that too much weight at the rear lightens the steer axle and reduces traction, and that a high center of gravity raises rollover risk. Use a scale when in doubt.
The candidate secures cargo with too few tiedowns, with tiedowns that are not rated for the load, or with the aggregate working load limit below the required level, and does not check the securement after the first few miles.
Candidates treat securement as a box to tick and do not connect the number and strength of tiedowns to the weight and type of cargo, and they forget that loads settle and shift in transit.
What to do instead
Match tiedowns to the cargo: use the required number for the length and weight, confirm the aggregate working load limit meets the rule for the load, and use edge protection and proper knots or hardware. Stop and re-check tension and position after the first few miles and periodically thereafter.