This section has information you need to pass the CDL knowledge test for driving safely with double and triple trailers. It tells about how important it is to be very careful when driving with more than 1 trailer, how to couple and uncouple correctly, and about inspecting doubles and triples carefully. (You should also study Sections 2, 5, and 6.) Show
Triple combinations are not legal in California. Triples are discussed in this section because they are legal in many other states. The endorsement is given by knowledge test only. Do not bring in a set of doubles for a Class A skills test. 7.1 – Pulling Double/Triple TrailersTake special care when pulling 2 and 3 trailers. There are more things that can go wrong, and doubles/triples are less stable than other CMVs. Some areas of concern are discussed below. 7.1.1 – Prevent the Trailer From Rolling OverTo prevent trailers from rolling over, you must steer gently and go slowly around corners, on ramps, off ramps, and curves. A safe speed on a curve for a straight truck or a single trailer combination vehicle may be too fast for a set of doubles or triples. 7.1.2 – Beware of the Crack-the-Whip EffectDoubles and triples are more likely to turn over than other combination vehicles because of the “crack-the-whip” effect. You must steer gently when pulling trailers. The last trailer in a combination is most likely to turn over. If you do not understand the crack-the-whip effect, study Subsection 6.1.2 of this handbook. 7.1.3 – Inspect CompletelyThere are more critical parts to check when you have 2 or 3 trailers. Check them all. Follow the procedures described later in this section. 7.1.4 – Look Far AheadDoubles and triples must be driven very smoothly to avoid rollover or jackknife. Therefore, look far ahead so you can slow down or change lanes gradually when necessary. 7.1.5 – Manage SpaceDoubles and triples take up more space than other CMVs. They are not only longer, but also need more space because they cannot be turned or stopped suddenly. Allow more following distance. Make sure you have large enough gaps before entering or crossing traffic. Be certain you are clear at the sides before changing lanes. 7.1.6 – Adverse ConditionsBe more careful in adverse conditions. In bad weather, slippery conditions, and mountain driving, you must be especially careful if you drive double and triple bottoms. You will have greater length and more dead axles to pull with your drive axles than other drivers. There is more chance for skids and loss of traction. 7.1.7 – Parking the VehicleMake sure you do not get in a spot you cannot pull straight through. You need to be aware of how parking lots are arranged in order to avoid a long and difficult escape. 7.1.8 – Anti-lock Braking Systems on Converter DolliesConverter dollies built on or after March 1, 1998, are required to have anti-lock brakes. These dollies have a yellow lamp on the left side of the dolly. 7.2 – Coupling and UncouplingKnowing how to couple and uncouple correctly is basic to safe operation of doubles and triples. Wrong coupling and uncoupling can be very dangerous. Coupling and uncoupling steps for doubles and triples are listed below. 7.2.1 – Coupling Double TrailersSecure the Second (Rear) TrailerIf the second trailer does not have spring brakes, drive the tractor close to the trailer, connect the emergency line, charge the trailer air tank, and disconnect the emergency line. This will set the trailer emergency brakes (if the slack adjusters are correctly adjusted). Chock the wheels if you have any doubt about the brakes. For the safest handling on the road, the more heavily loaded semitrailer should be in first position behind the tractor. The lighter trailer should be in the rear. A converter gear on a dolly is a coupling device of 1 or 2 axles and a fifth-wheel by which a semitrailer can be coupled to the rear of a tractor-trailer combination forming a double bottom rig. See Figure 7.1. Figure 7.1 Position the Converter Dolly in Front of the Second (Rear) Trailer
Connect the Converter Dolly to the Front Trailer
Connect the Converter Dolly to the Rear Trailer
7.2.2 – Uncoupling the Double TrailersUncouple the Rear Trailer
Uncouple the Converter Dolly
Never unlock the pintle hook with the dolly still under the rear trailer. The dolly tow bar may fly up, possibly causing injury, and make it very difficult to recouple. 7.2.3 – Coupling and Uncoupling Triple TrailersCouple the Tractor/First Semitrailer to the Second/ Third Trailers
Uncouple the Triple-Trailer Rig
Remember: Operating triples is not allowed in California. 7.2.4 – Coupling and Uncoupling Other CombinationsThe methods described so far apply to the more common tractor-trailer combinations. However, there are other ways of coupling and uncoupling the many types of truck-trailer and tractor-trailer combinations that are in use. There are too many to cover in this handbook. You will need to learn the correct way to couple and uncouple the vehicle(s). Drive according to the manufacturer and/or owner specifications. 7.3 – Inspecting Doubles and TriplesUse the 7-step inspection procedure described in Section 2 to inspect your combination vehicle. There are more things to inspect on a combination vehicle than on a single vehicle. Many of these items are simply more of what you would find on a single vehicle. (For example, tires, wheels, lights, reflectors, etc.) There are also some new things to check. These are discussed below. 7.3.1 – Additional ChecksDo these checks in addition to those already listed in Section 2, Step 5: Do Walk Around Inspection. Coupling System Areas
Landing Gear
Double and Triple Trailers
7.3.2 – Additional Things to Check During a Walk Around InspectionDo these checks in addition to Section 5, Inspecting Air Brake Systems. 7.4 – Doubles/Triples Air Brake CheckCheck the brakes on a double or triple trailer as you would any combination vehicle. Section 6 explains how to check air brakes on combination vehicles. You must also make the following checks on your double or triple trailers. 7.4.1 – Additional Air Brake ChecksCheck That Air Flows to all the Trailers (Double and Triple Trailers). Use the tractor parking brake and/or chock the wheels to hold the vehicle. Wait for the air pressure to reach normal, and then push in the red “trailer air supply” knob. This will supply air to the emergency (supply) lines. Use the trailer handbrake to provide air to the service line. Go to the rear of the rig. Open the emergency line shut-off valve at the rear of the last trailer. You should hear air escaping, showing the entire system is charged. Close the emergency line valve. Open the service line valve to check that service pressure goes through all the trailers (this test assumes that the trailer handbrake or service brake pedal is on), and then close the valve. If you do not hear air escaping from both lines, check that the shut-off valves on the trailer(s) and dolly(ies) are in the OPEN position. You must have air all the way to the back for all the brakes to work. Test the Tractor Protection Valve. Charge the trailer air brake system (build up normal air pressure and push the “air supply” knob in). Shut the engine off. Step on and off the brake pedal several times to reduce the air pressure in the tanks. The trailer air supply control (also called the tractor protection valve control) should pop out (or go from the “normal” to “emergency” position) when the air pressure falls into the pressure range specified by the manufacturer (usually within the range of 20 to 45 psi). If the tractor protection valve does not work properly, an air hose or trailer brake leak could drain all the air from the tractor. This would cause the emergency brakes to come on, with possible loss of control. Test the Trailer Emergency Brakes. Charge the trailer air brake system and check that the trailer rolls freely. Then stop and pull out the trailer air supply control (also called tractor protection valve control or trailer emergency valve) or place it in the “emergency” position. Pull gently on the trailer with the tractor to check that the trailer emergency brakes are on. Test the Trailer Service Brakes. Check for normal air pressure, release the parking brakes, move the vehicle forward slowly, and apply trailer brakes with the hand control (trolley valve), if so equipped. You should feel the brakes come on. This tells you the trailer brakes are connected and working. (The trailer brakes should be tested with the hand valve, but controlled in normal operation with the foot pedal, which applies air to the service brakes at all wheels.) Section 7Test Your Knowledge
These questions may be on your test. If you cannot answer them all, reread Section 7. How do you get a doubles endorsement in Texas?This endorsement is required if the driver will pull two or three trailers. To receive this endorsement, applicants must pass a test The test consists of 20 multiple choice questions. Each question has four answer choices.
How do I get a tanker endorsement in Texas?The tanker endorsement applies to drivers who wish to drive a tank in Class A, B, or C CDL. To add this endorsement to your CLP/CDL, you must pass a knowledge test on the problems posed by large volume liquid cargos. The Texas CDL tank vehicles test consists of 20 questions.
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