Iowa CDL Combination Vehicles Test flashcards

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

Fifth wheel inspection

Check that the jaws are locked around the kingpin, the locking lever is secured, the mounting bolts are tight, and there is no gap between the fifth wheel and the trailer apron.

Kingpin and apron check

Inspect the kingpin for cracks, bends, or wear and the apron for cracks or missing fasteners; a damaged kingpin can let the trailer separate from the tractor.

Tractor protection valve test

With air pressure built up, release the parking brake and confirm the tractor protection valve closes and applies the trailer emergency brakes when pressure drops; it prevents air loss.

Landing gear inspection

Landing gear must be fully raised, the crank handle secured, and the legs free of cracks or bends; damaged or missing parts can let the trailer drop.

Trailer brake test

Use the hand valve or trailer brake control to apply only the trailer brakes; walk back and listen for air leaks while a helper confirms the brakes hold.

Tug test

After coupling, pull forward gently in low gear; if the trailer does not move, the fifth wheel jaws are locked, but still visually verify the jaws.

Crossed air lines

Connecting the service line to the emergency glad hand and the emergency line to the service glad hand; this can release trailer brakes when they should apply.

Coupling height

Back until the fifth wheel just touches the trailer apron; too low can damage the tractor frame, and too high can miss the kingpin.

Uncoupling sequence

Park on level ground, chock the wheels, lower the landing gear, disconnect the air lines, release the fifth wheel, and pull forward slowly.

Fifth wheel release

Pull the release handle to unlock the jaws; never pull it while the trailer is moving or before the landing gear supports the trailer weight.

Static air leak test

With the system fully charged and the engine off, pressure loss should not exceed 3 psi per minute for a single vehicle or 4 psi per minute for a combination.

Applied air leak test

With the brakes fully applied and held, air loss should not exceed 4 psi per minute for a single vehicle or 6 psi per minute for a combination.

Low air pressure warning

A warning buzzer or light must activate before pressure falls below 60 psi, giving the driver time to stop safely before spring brakes apply.

Tractor protection valve

It should pop out and apply the trailer emergency brakes when air pressure drops to between 20 and 45 psi, preventing tractor air loss.

Slack adjuster check

With the brakes released and the system charged, pull or push each slack adjuster by hand; movement of more than 1 inch indicates adjustment or repair is needed.

Following distance formula

At speeds under 40 mph, allow one second per 10 feet of vehicle length; add one second when speed exceeds 40 mph.

Railroad crossing procedure

Stop before the tracks if required, look both ways, select a low gear, keep moving without shifting, and never stop on the tracks.

Trailer skid recovery

Release the brakes and steer in the direction of the skid to pull the trailer back in line; avoid hard braking and oversteering.

Wide turns

Steer slightly wider at corners and watch the trailer wheels in the mirrors; the trailer cuts the corner and may strike curbs or vehicles.

Combination vehicle braking

Stopping distance is longer than for a single vehicle, so brake earlier and smoother; sudden braking can cause jackknifing or trailer swing.

Passing with a trailer

Allow extra time and distance because acceleration is slower and the trailer must clear the passed vehicle before returning to the lane.

Rollover threshold

The lateral acceleration, in g's, at which a vehicle tips; it equals track width divided by twice the center-of-gravity height.

Center of gravity

The point where a vehicle's weight is concentrated; a higher center of gravity increases rollover risk by moving outside the wheelbase sooner.

Load placement

Place heavy cargo low and centered, and secure it; a high or shifting load raises the center of gravity and increases rollover risk.

Curve speed

Slow down before entering a curve, not while in it; posted ramp or curve speeds are often too fast for a loaded combination vehicle.

Rearward amplification

The tendency of each trailer in a multi-trailer combination to swing wider and faster than the tractor during abrupt steering, increasing with speed and trailer count.

Sudden lane changes

Abrupt lane changes amplify trailer swing; steer smoothly and make gradual lane changes to reduce rearward amplification and rollover risk.

Multi-trailer handling

Doubles and triples have greater rearward amplification than a single trailer, so reduce speed and increase following distance in traffic.

Off-tracking

The difference between the path of the front wheels and the rear wheels; it increases with longer wheelbase, longer trailers, and sharper turns.

Off-tracking formula

Approximate off-tracking equals the square of the distance from the rear axle to the articulation point divided by twice the turn radius.

Jackknife cause

A jackknife happens when the tractor and trailer fold at the coupling, often from overbraking the drive wheels, sudden braking, or slippery roads.

Jackknife recovery

Release the brakes and steer in the direction of the skid; for a tractor jackknife, steer to pull the trailer back in line without hard braking.