Iowa CDL Combination Vehicles Test cheat sheet

Fifth wheel jaws must fully close around the kingpin; the locking lever is secured, mounting bolts are tight, and no gap exists between the fifth wheel and trailer apron.

Kingpin and apron: no cracks, bends, missing nuts, or loose fasteners; a worn kingpin can let the trailer separate from the tractor under load.

Trailer air lines: no cuts, cracks, or audible leaks; glad hands are sealed, and lines are supported and not tangled or rubbing.

Landing gear: fully raised, crank handle secured, legs straight, and no missing shoes or cracked welds.

Test trailer service brakes with the hand valve and emergency brakes by pulling the tractor protection valve; listen for leaks at each glad hand.

Tire tread minimum: 4/32 inch on front steer tires and 2/32 inch on all other tires; check inflation and sidewalls for damage.

Lights and reflectors: headlights, turn signals, brake lights, clearance lights, and reflective tape must work and be clean.

Rear impact guard and mud flaps: guard secure and undamaged; flaps present and not torn.

Before coupling, chock trailer wheels, check trailer height so the apron just touches the fifth wheel, and inspect the kingpin and fifth wheel.

Back slowly and squarely; the kingpin should enter the fifth wheel centered, and stop when the jaws lock and the lever snaps into place.

Tug test: after coupling, pull forward gently in low gear; if the trailer does not move, the jaws are locked, but still visually check the jaws.

Connect service (blue) to service and emergency (red) to emergency glad hands; crossed lines can release trailer brakes when they should apply.

After connecting air lines, charge the trailer system and check for leaks at the glad hands and along the hoses.

Raise landing gear fully and secure the crank before moving; low landing gear can strike the ground or railroad tracks.

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

Cap glad hands and close the tractor protection valve after uncoupling to keep dirt and moisture out of the air system.

Static leak test: fully charged system, engine off, release the service brake; pressure loss must not exceed 3 psi/min for a single vehicle or 4 psi/min for a combination.

Applied leak test: fully apply and hold the brakes; loss must not exceed 4 psi/min for a single vehicle or 6 psi/min for a combination.

Low air pressure warning must activate before pressure falls below 60 psi; stop and find the leak if the warning comes on.

The tractor protection valve should pop out and apply trailer emergency brakes at 20 to 45 psi; below that range, spring brakes apply.

Check slack adjusters by hand with brakes released: movement of more than 1 inch indicates adjustment or repair is needed.

Test trailer brakes separately with the hand valve; confirm the trailer holds and no air leaks are heard.

Drain air tanks daily to remove moisture; in cold weather, frozen moisture can cause brake failure.

On long downgrades, select a low gear before descending and use engine braking; do not ride the brakes.

Following distance: one second per 10 feet of vehicle length below 40 mph; add one second above 40 mph, so a 60-foot rig at 55 mph needs 7 seconds.

Turns: steer slightly wider and watch trailer wheels in the mirrors; the trailer cuts the corner and may strike curbs or vehicles.

Braking: combination stopping distance is longer, so brake early and smoothly; sudden hard braking can cause jackknifing or trailer swing.

Trailer skid: release the brakes and steer in the direction of the skid; avoid oversteering and hard braking.

Railroad crossings: stop if required, look both ways, use a low gear, do not shift while crossing, and never stop on the tracks.

Passing: allow extra time and distance because acceleration is slower and the trailer must clear the passed vehicle.

Keep both hands on the wheel and avoid abrupt lane changes; smooth steering reduces trailer swing.

Use mirrors and check blind spots before changing lanes; the trailer may hide vehicles alongside.

Rollover threshold in g's equals track width divided by twice the center-of-gravity height; a higher center of gravity lowers the threshold and increases risk.

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

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

Avoid sudden lane changes and sharp steering at highway speed; smooth inputs keep lateral acceleration low.

Tripping hazards such as curbs, soft shoulders, and potholes can initiate a rollover even at moderate speed.

A loaded trailer generally has a higher center of gravity than an empty one; reduce speed accordingly and secure the load.

Watch for strong crosswinds; they add lateral force and can tip a high-profile trailer.

Rearward amplification is the tendency of each trailer to swing wider and faster than the tractor during abrupt steering.

Amplification increases with speed, number of trailers, and sudden lane changes; doubles and triples are most affected.

Reduce speed and steer smoothly to limit rearward amplification; avoid abrupt lane changes and hard steering.

Increase following distance in multi-trailer combinations because trailer swing can encroach on adjacent lanes.

Shorter trailer wheelbases and uneven load distribution can increase rearward amplification.

Watch trailer tracking in the mirrors during lane changes; the last trailer swings the most.

Off-tracking: rear wheels follow a shorter path than front wheels; it increases with longer wheelbase, longer trailer, and sharper turn.

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

When turning, the trailer cuts the corner; watch trailer wheels in the mirror and allow enough room for the rear of the trailer.

Jackknife: the tractor and trailer fold at the coupling, often caused by overbraking the drive wheels, sudden braking, or slippery roads.

Prevent jackknife by braking smoothly, ensuring trailer brakes work, and reducing speed on slippery surfaces.

If a jackknife begins, release the brakes and steer in the direction of the skid; hard braking makes it worse.

Rearward amplification and off-tracking are different: amplification is trailer swing in a maneuver, while off-tracking is the rear path cutting inside.