Colorado CDL Combination Vehicles flashcards

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

Off-tracking

The tendency of the rear wheels of a combination vehicle to follow a path inside the path of the front wheels during a turn, requiring the driver to swing wide and check mirrors.

Total stopping distance

The sum of perception distance, reaction distance, brake lag distance, and braking distance; for a loaded combination it can exceed 300 feet at highway speed on dry pavement.

Following distance rule

One second for each 10 feet of vehicle length at speeds under 40 mph, plus one extra second above 40 mph; a 60-foot rig at 55 mph needs at least 7 seconds.

Crack-the-whip effect

The rearward amplification of movement in doubles and triples, where the last trailer swings farthest and fastest during a turn or lane change, requiring gradual steering.

Dual air brake system

Two separate air circuits in a combination vehicle; a failure in one circuit leaves a reserve in the other, and the low-air warning must activate before pressure drops below 60 psi.

Tractor protection valve

A valve that keeps air in the tractor and closes the trailer air supply when tractor pressure falls to about 20-45 psi or when the emergency line is disconnected.

Trailer hand valve

The trolley valve that applies only the trailer brakes; it is used only to test trailer brakes and must never be used to stop the vehicle because it can cause a jackknife.

Glad hand color coding

Red identifies the emergency or supply line that charges the trailer tanks, and blue identifies the service line that carries the brake application signal.

Air brake leakage check

With the engine off and pressure at governor cut-out, leakage must be less than 3 psi per minute for a single vehicle and less than 4 psi per minute for a combination.

Spring brakes

Brakes that apply automatically when air pressure is lost; they cannot be released until pressure builds to about 45-60 psi, so low pressure keeps the vehicle from rolling.

ABS purpose

Antilock braking modulates pressure at each wheel to prevent lock-up, helping the driver keep steering control and reducing the risk of a jackknife during hard braking.

ABS on loose surfaces

On gravel or snow, ABS may lengthen stopping distance because locked wheels build a wedge of material, but the driver retains steering control.

ABS pedal technique

With ABS, apply firm steady pressure and do not pump the brakes; pumping defeats the system and can reduce braking effectiveness.

Trailer ABS lamp

A yellow lamp on the trailer that should come on at start-up and then go out; if it stays on, the trailer ABS is not working and needs repair.

Tug test

After coupling, pull forward gently with the trailer brakes set to confirm the fifth wheel jaws are locked around the kingpin before continuing.

Fifth wheel release handle

The handle must be in the locked position with the safety latch secured after coupling; an unlocked fifth wheel can let the trailer separate from the tractor.

Uncoupling sequence

Lower the landing gear, chock the trailer wheels, disconnect the air lines and electrical cord, pull the release handle, and pull the tractor slowly forward.

Trailer air supply valve

The red knob pushed in charges the trailer air tanks; pulled out, it sets the trailer spring brakes and shuts off the air supply to the trailer.

Fifth wheel inspection

Check for cracks, bends, and worn parts; the jaws must be closed around the kingpin with no gap, and the mounting bolts must be tight.

Air line inspection

Check the lines for cracks, cuts, and wear; the glad hands must have good seals, and the lines must not drag on the ground or be tangled.

Tire inspection

Check tread depth, cuts, bulges, and inflation; dual tires must not touch, and tread depth must meet the minimum for the tire position.

Brake test at inspection

Test the tractor and trailer brakes separately and together; the trailer brakes should hold the trailer when the hand valve is applied.

Tiedown working load limit

The total tiedown working load limit must be at least 50 percent of the cargo weight for most loads, with different rules for aggregate loads.

Center of gravity

The point where the load's weight is concentrated; it should be as low as possible and centered over the axles to reduce rollover risk and maintain control.

Steering axle weight

The weight on the steering axle must be sufficient for steering control; too little weight makes steering difficult, and too much overloads the axle.

Cargo recheck interval

Loads must be checked after the first 50 miles and every 150 miles or 3 hours thereafter, and after any change in driving conditions.

Jackknife

A condition where the tractor's rear wheels lock and the trailer pushes the tractor, folding the combination; release the brakes and steer in the direction of the skid.

Drive-wheel skid correction

Release the accelerator and steer in the direction the rear of the vehicle is sliding; do not brake hard until the wheels regain traction.

Front-wheel skid correction

Release the accelerator and steer in the direction you want to go; braking hard will make the skid worse.

Brake failure response

Downshift to a lower gear, use the emergency brake gradually, and look for an escape route such as a runaway ramp or an uphill grade.

Doubles and triples endorsement

A combination with more than one trailer requires a special endorsement on the CDL; the rear trailer is the most likely to swing out.

Class A CDL threshold

A tractor-trailer combination with a gross vehicle weight rating over 26,000 pounds requires a Class A CDL; lower-weight combinations may require Class B or C.

Converter dolly

A device used to connect a second trailer; its air lines and electrical connections must be checked, and the dolly must be secured to the first trailer.