Colorado CDL Combination Vehicles cheat sheet

A combination vehicle is longer, heavier, and off-tracks more than a single unit; the rear wheels of the trailer follow a tighter path than the tractor's front wheels, so swing wide before turning and check the trailer's path in the mirrors.

Total stopping distance equals perception distance plus reaction distance plus brake lag distance plus braking distance; at 55 mph a loaded combination can need more than 300 feet on dry pavement.

The rule for safe following distance is one second for each 10 feet of vehicle length at speeds under 40 mph, plus one additional second for speeds above 40 mph; a 60-foot rig at 55 mph needs at least 7 seconds.

Speed on curves must be reduced before entering the curve; braking hard while turning a combination vehicle is a leading cause of rollover and trailer swing-out.

The 'crack-the-whip' effect means the rear trailer of a set of doubles or triples swings the farthest and fastest during a turn or lane change, so watch the rear unit in the mirrors.

Downshift before starting down a long grade; use the same gear going down that you would need to climb the grade, and use engine braking with light, steady brake applications.

When driving a combination vehicle in high wind or on a narrow road, keep the tractor-trailer in a straight line and reduce speed; the trailer's large side area makes it vulnerable to being pushed or tipped.

Rearward amplification is greatest for the last trailer in a multi-trailer combination; lane changes must be gradual so the rear units do not swing into adjacent lanes.

A combination vehicle uses a dual air brake system with two separate circuits; a loss of pressure in one circuit still leaves a reserve in the other, and the low-air warning must activate before pressure drops below 60 psi.

The tractor protection valve keeps air in the tractor when the trailer breaks away or the emergency line is disconnected, and it closes the trailer air supply if tractor pressure falls to about 20-45 psi.

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

The emergency (supply) line charges the trailer air tanks, while the service line carries the brake application signal; the glad hands are color-coded: red for emergency, blue for service.

Spring brakes apply automatically when air pressure is lost; they cannot be released until pressure builds to about 45-60 psi, so a vehicle with low air pressure will not roll away.

Before driving, perform the air brake check: build pressure to governor cut-out, shut off the engine, and confirm leakage is less than 3 psi per minute for a single vehicle and less than 4 psi per minute for a combination.

The tractor protection valve and the trailer spring brakes will set if the trailer separates from the tractor, which is why the emergency line must be connected and the trailer air supply valve open before moving.

A 'brake fade' on long downgrades is caused by overheating the drums and shoes; use a lower gear and short, firm applications rather than continuous light pressure.

ABS prevents wheel lock-up during hard braking by modulating brake pressure at each wheel or axle, which helps the driver maintain steering control and reduces the risk of a jackknife.

ABS does not shorten stopping distance on all surfaces; on loose gravel or snow it may lengthen stopping distance because locked wheels build up a wedge of material, but steering control is retained.

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

A trailer with ABS has a yellow ABS malfunction lamp on the trailer; the tractor has a yellow ABS lamp on the dash, and both should come on at start-up and then go out.

If the ABS lamp stays on, the ABS is not working and the vehicle has normal, non-ABS brakes; the vehicle may still be driven but should be checked and repaired.

For a combination vehicle, the tractor and trailer ABS systems operate independently; a trailer ABS fault does not disable the tractor ABS, but the driver should note the fault and have it repaired.

ABS is not a substitute for safe following distance; the system cannot overcome insufficient space, excessive speed, or poor road conditions.

Before coupling, check that the trailer kingpin is not damaged, the fifth wheel is open and greased, the trailer is at the correct height, and the wheels are chocked.

Back the tractor slowly and straight into the trailer so the fifth wheel jaws close around the kingpin; after coupling, tug-test by pulling forward gently with the trailer brakes set to confirm the coupling is locked.

After coupling, connect the air lines (red emergency to red, blue service to blue), the electrical cord, and the safety latch; then raise the landing gear and check that the trailer air supply valve is open.

The fifth wheel release handle must be in the locked position, and the safety latch must be secured; an unlocked fifth wheel can let the trailer separate from the tractor.

When uncoupling, lower the landing gear to the ground, chock the trailer wheels, disconnect the air lines and electrical cord, pull the fifth wheel release handle, and pull the tractor slowly forward.

The trailer air supply valve (red knob) must be pushed in to charge the trailer and pulled out to set the trailer spring brakes; the trailer hand valve should be used only to test trailer brakes.

After coupling, check that the trailer is level and that the landing gear is fully raised; a trailer left on its landing gear while being pulled can damage the gear and cause loss of control.

Inspect the coupling after the first few miles of driving; vibration can loosen connections, and a visual check of the fifth wheel jaws and air lines is required before continuing.

The pre-trip inspection for a combination vehicle covers the tractor engine compartment, cab, lights, and the full trailer, including the coupling system, air lines, and landing gear.

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

Check the air 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.

Check the trailer frame, sliding tandem locking pins, mud flaps, and lights; the rear doors or cargo must be secured, and the trailer must be level.

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

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

Check the landing gear for damage and proper operation, and confirm the crank handle is secured and the gear is fully raised before driving.

The inspection must include the coupling system, the air brake system, and the cargo securement; a defect in any of these can cause a crash, so the inspection must be completed before every trip.

Cargo must be secured so it cannot shift, and the total tiedown working load limit must be at least 50 percent of the cargo weight for most loads; for aggregate loads, the requirements are based on the commodity.

The center of gravity should be as low as possible and centered over the axles; a high or off-center load increases the risk of rollover and reduces steering and braking control.

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

Axle weight limits and gross vehicle weight limits must not be exceeded; load distribution must keep each axle within its rating, and the load must be checked before and during the trip.

Cargo must not extend beyond the legal limits without proper flags, lights, and permits; overhanging loads require warning devices and may require special escort.

Tiedowns must be attached to the vehicle's anchor points and to the cargo, and they must be tight; loose or frayed tiedowns must be replaced.

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

A jackknife occurs when the tractor's rear wheels lock and the trailer pushes the tractor, folding the combination; the cure is to release the brakes and steer in the direction of the skid.

A drive-wheel skid is corrected by releasing the accelerator and steering in the direction the rear of the vehicle is sliding; do not apply the brakes hard until the wheels regain traction.

A front-wheel skid is corrected by releasing the accelerator and steering in the direction you want to go; braking hard will make the skid worse.

To prevent a jackknife, brake gently and avoid locking the drive wheels; use the trailer hand valve only for testing, never to stop the vehicle.

If a trailer starts to swing, release the brakes and steer to keep the tractor in line with the trailer; do not counter-steer sharply, which can worsen the swing.

If the brakes fail, 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.

If a tire blows out, hold the steering wheel firmly, release the accelerator, and brake gently; do not brake hard, which can cause loss of control.

In a rollover situation, the combination vehicle's high center of gravity means speed must be reduced before curves; if a rollover is unavoidable, stay in the cab with the seat belt fastened.

Doubles and triples have more than one trailer; the rear trailer is the most likely to swing out, and the combination requires a special endorsement on the CDL.

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

Colorado requires special permits for oversize or overweight combinations; the permit specifies routes, escort requirements, and axle weight limits.

Long combination vehicles have longer stopping distances and greater off-tracking; the driver must allow more space and reduce speed on curves and in traffic.

A converter dolly is used to connect a second trailer; the dolly's air lines and electrical connections must be checked, and the dolly must be secured to the first trailer.

The maximum legal length and weight limits for combination vehicles vary by state and route; the driver must know the limits for the route being traveled and must not exceed them.

A combination vehicle carrying hazardous materials may require a hazardous materials endorsement and must follow routing and placarding rules; tunnels and certain roads may be restricted.

When towing multiple trailers, the driver must check the rear trailer frequently in the mirrors and must allow extra time for lane changes and turns.