MN CDL Combination flashcards

31 cards. Read the term and the definition here. The deck at the top of this page is optional.

Fifth wheel jaws

The locking mechanism that grips the trailer kingpin. After coupling, the jaws must be fully closed around the kingpin shank, the release handle must be locked, and a pull test must confirm the lock.

Tug test

A coupling verification step: with the trailer parking brake set, pull forward gently in a low gear. The tractor should not move and the fifth wheel jaws should stay locked around the kingpin.

Apron-to-fifth wheel gap

The space between the trailer upper coupler apron and the fifth wheel top plate. After coupling there should be no visible gap, which would mean the kingpin is not seated in the jaws.

Trailer air line colors

Standard coding: the emergency or supply line is red and the service line is blue. Crossed connections can cause trailer brakes to release or drag unexpectedly.

Landing gear after coupling

The trailer support legs must be fully raised after coupling, with the crank handle secured in its holder, so the gear clears the ground and the trailer tires.

Uncoupling sequence

Park on firm level ground, set the trailer parking brake, lower the landing gear, disconnect and secure the air and electrical lines, release the fifth wheel jaws, then pull the tractor clear slowly.

Slack adjuster free play

With the brakes released, a properly adjusted slack adjuster pushed by hand should move no more than about one inch at the push rod. Excess movement means the brakes need adjustment.

Combination air loss limits

With a fully charged system and the engine off, leakage with brakes released should not exceed about 2 psi per minute; with brakes applied, about 3 psi per minute.

Off-tracking

The tendency of a combination vehicle's rear wheels to follow a shorter path than the front wheels, which is why right turns require swinging wide and watching the trailer's rear tires.

Right turn in a combination

Swing wide to the left before turning, keep the tractor positioned so the trailer's right rear tires clear the curb, and finish the turn in the far right lane.

Left turn in a combination

Begin from the rightmost lane or a designated turn lane, swing slightly right before turning, and keep the trailer from cutting the corner across the centerline.

Backing toward driver side

Back toward the left whenever possible so the trailer stays visible in the left mirror. If a right-side back is unavoidable, use a spotter or get out and look.

Steering wheel while backing

Hold the bottom of the steering wheel and turn it the direction you want the trailer to go. Correct small deviations early because the combination responds slowly.

Following distance rule

Allow at least one second per 10 feet of vehicle length below 40 mph, plus one extra second above 40 mph. A 60-foot combination at 55 mph needs about 7 seconds.

Lane change blind spot check

Mirror checks plus a head check toward the side you are moving to. A combination vehicle has larger blind spots, so signal early and move smoothly.

Curve entry speed

Enter a curve slower than the posted advisory speed when the load is high or the surface is wet, because a high center of gravity raises rollover risk.

Air pressure build-up check

With the system charged and the engine at operating rpm, pressure should rise from about 85 to 100 psi within 45 seconds in a dual air system.

Low-air warning threshold

The warning buzzer or light must activate before pressure drops below 60 psi, and the spring brakes should apply automatically at roughly 20 to 45 psi.

Tractor protection valve

A valve that closes automatically if trailer air lines separate, trapping tractor air so the tractor can still brake. The trailer emergency line then applies the trailer spring brakes.

Trailer hand valve

A control that operates the trailer brakes only. It is used for testing trailer brakes, not for normal stopping, because it does not apply the tractor brakes.

Brake fade

Loss of braking power from overheated drums and linings after repeated or prolonged braking. Downshift before a long downgrade and use firm intermittent braking instead.

Stab braking

A downgrade technique: apply the brakes firmly to slow to a safe speed, release fully to let them cool, then repeat. Continuous light pressure overheats the brakes.

Parking on a grade

Apply the parking brake, place the transmission in the lowest reverse or first gear, and use wheel chocks. Spring brakes provide the primary holding force, not air pressure.

Skid recovery basics

Stop braking, keep both hands on the wheel, look and steer where you want to go, and do not accelerate until the wheels regain traction and the vehicle is straight.

Rear-wheel skid

The trailer swings out. Steer into the skid by turning the wheel toward the direction the rear is sliding, and release the brakes so the wheels can roll again.

Front-wheel skid

Steering feels light and the vehicle keeps going straight. Release the brakes and let the front wheels roll until steering control returns before applying brakes again.

Jackknife cause

A jackknife risk rises when the tractor drive wheels lock while the trailer keeps rolling. Balanced braking and avoiding hard braking on slick surfaces reduce this risk.

Hazard scanning interval

Scan 12 to 15 seconds ahead, watch brake lights several vehicles ahead, and check mirrors every 5 to 8 seconds for overtaking traffic and trailer tracking.

Railroad crossing rule

Never stop on the tracks. Before starting across, confirm there is enough room on the far side for the entire combination vehicle to clear the crossing.

Bridge height check

Know the loaded height of the combination, including any raised equipment, and check posted clearance signs before committing to a route with an overpass.

Reflective triangle placement

Set out warning triangles within 10 minutes of stopping: at 10, 100, and 200 feet behind the vehicle, or closer when stopped on a curve or hill.