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A “special rig” is any combination vehicle that differs from the standard tractor and dry freight trailer with five axles and up to 18 wheels. This definition includes vehicles:


  • That have more than one point of articulation (joints);

  • That have a high center of gravity when loaded;

  • That have load stability problems;

  • That require special handling and permits,

  • That are over length, over height, over width, or over weight;

  • That have very low ground clearance;

  • That are used for certain special cargos.


Although the majority of the information in this handbook is appropriate to all tractor-trailer combinations, certain rigs may need particular procedures. The most popular special rigs, their handling qualities, and the specialized abilities needed to operate them are all covered in this chapter.


Multiple Articulation Vehicles


Vehicles having several joints, or locations where two elements of the vehicle are linked, are sometimes referred to as lengthy combination vehicles. A normal tractor-trailer unit only has one joint, which is where the fifth wheel connects to the kingpin. As the name suggests, multiple articulation vehicles have two or more joints and come in different double and triple configurations.


Doubles — A double consists of a tractor and two semi-trailers, with the second trailer having a converter gear that turns it into a complete trailer (a set of wheels with a fifth wheel, called a converter dolly). One or more drawbars with eyes attached to pintle hook(s) on the rear of the first trailer are used to attach the converter dolly to the trailer.


The most popular form of dolly is called an A-dolly, and a rig outfitted with an A-dolly is called an A-train. A rig outfitted with a B-dolly is known as a C-train, and a dolly with two parallel drawbars is known as a B-dolly (B-trains will be discussed later). A C-train has two drawbars, which prevents the rear trailer from rotating at the hitch point.


While a standard tractor-trailer unit has one point of articulation or joint, doubles have three:


  • Front trailer kingpin and fifth wheel.

  • Pintle hook and eye.

  • Converter gear fifth wheel and kingpin of the second trailer. Following are some defining characteristics and handling requirements of several types of doubles.


Western doubles — A western double is an A-train that is normally 60–75 feet long with two equal-length trailers that are typically 24–31 feet long.


The trailers may be dry vans, tankers, flatbeds, or dumps.


There are several special handling concerns when pulling western doubles:

  • Avoid backing. This typically difficult maneuver is made even more difficult with a double trailer unit not designed to be backed.

  • Smooth steering is required. Jerking or aggressively moving the steer- ing wheel is greatly magnified in the second trailer. (To create a picture in your own mind, think of a group of ice skaters playing “crack the whip.”)

  • The heaviest trailer must always be ahead of the lighter one. This adds stability for turning and braking.

  • Don’t apply brakes in a curve, because this will cause the second trailer to dip.

  • The greater length of the rig requires the driver to plan further ahead. Additional time is needed for passing, changing lanes, and crossing intersections.

  • It’s very difficult to make tight turns with close coupled rigs.

  • Bumps and potholes may cause the tops of the trailers to strike each other.

  • On curves, be aware of the vehicle’s tracking so you stay within your lane while going around the curve.


The standard connection and inspection processes are extended with doubles. It is necessary to correctly position and hooks the drawbar and pintle hook. All airlines must be correctly connected, safety chains must be in place, electrical connections must be linked, and valves must be opened or closed as needed. These things also need to be thoroughly examined during pre-trip and in-flight inspections.


Finally, you must get a Double/Triple Trailers endorsement for your commercial driver's license (CDL) and become familiar with traffic laws before driving a western double.


Depending on the weight and length involved, several states have restrictions on or outright bans on the use of doubles. Before driving in certain places, you may need to get a state permit.


Turnpike doubles — A turnpike double has a greater total vehicle length than a western double, often 100 feet or more, and trailers that are typically 35 to 48 feet long. In most cases, they have nine axles. These rigs are often seen on the Eastern United States turnpikes.


Typically, a high-output engine and multiple gear range transmission are required for turnpike doubles.


Despite being somewhat exaggerated by the additional length, other important qualities, handling, and specific needs are essentially the same as with western doubles. However, a turnpike double driver must be aware of any zones where their usage is prohibited and any regions where special licenses are needed. A corporation that often utilizes turnpike doubles has to be informed of the latest federal and state rules that apply to their business. Operating a turnpike double requires a Double/Triple Trailers authorization.


Rocky mountain doubles — As suggested by the name, a rocky mountain double is a kind of trailer that has a longer trailer up front and a shorter trailer at the back. These trailers are often utilized in the Western United States. The lengths of the front (semi) and rear (full) trailers are typically 40–53 feet and 26–29 feet, respectively. An average length is between 80 and 100 feet.


Other essential features, handling, and special needs are the same as for western doubles, notwithstanding the long front trailer's need for more space while moving. A Double/Triple Trailers endorsement is necessary, just as with other doubles.


B-trains — Two semi-trailers make up a B-train. The second axle lies beneath the nose of the second semitrailer because the tandem rear axles on the first trailer are located at the rear of the trailer body. The need for a converter dolly is removed by a fifth wheel located above the second axle. Additionally, this gets rid of one articulation point that is typically on a double rig. Instead of three articulation points, B-trains only have two.


Trailer and overall length will vary depending on the geographical locations of B-train operations. Generally, there will be two 40-foot trailers or one

40-foot and one 27-foot trailer. B-trains are more common in Canada than in

the United States. You also may see a lot of them in Michigan.


Again, other major characteristics, handling, and special requirements are essentially the same as for western doubles, with adjustments for trailer length. Backing is easier with B-trains, though it should still be avoided. A Double/Triple Trailers endorsement is required.


Requirements for Double/Triple Trailers Endorsement


Applicants for a Double/Triple Trailers endorsement will be expected to know:

  • Procedures for assembly and hookup of the units.

  • Proper placement of the heaviest trailer.

  • Handling and stability characteristics, including off tracking, response to steering, sensory feedback, braking, oscillatory sway, rollover in steady turns, and yaw stability in steady turns.

  • Potential problems in traffic operations, including problems the motor vehicle creates for other motorists due to slower speeds on steep grades, longer passing times, possibility for blocking entry of other motor vehicles on freeways, splash and spray impacts, aerodynamic buffeting, view block- ages, and lateral placement.


Triples — Triple trailers, also known as triples, are becoming a much more common sight on America’s highways, especially in the West. Triples are made up of three semi-trailers, with the second and third converted to full trailers using converter gear and connected, as with doubles, by drawbar and pintle hook.


There are three kingpin/fifth wheel connections and two eye/pintle hook connections. Trailers are typically 27-28 feet long, with overall lengths of up to 100 feet.


Drivers of triple trailers need to possess sophisticated operational abilities. Their length necessitates more room and time for turning, halting, and other actions. The backing is one technique that should never be tried. The use of triple trailers is restricted to certain areas and on certain roadways, and a double/triple trailers endorsement is necessary. Any business that uses triples in its operations should keep this and other regulatory information up to date.


Oversize Vehicles


Lowboys, drop frames, flatbeds, open-top vans, and extensible trailers are some examples of large-vehicle trailers. Various wheel and axle combinations may be used, depending on the load's weight, size, and state laws. To support huge loads, many big trucks have outriggers installed. To distribute weight across multiple axles and sustain longer loads, converter dollies may be fastened to the trailer or tractor using a fifth wheel and kingpin or directly to the cargo.


Typically, overweight or over-dimensional freight necessitates the use of an enormous truck. This can include large construction equipment, industrial dryers, or equipment used in power plants. Larger vehicles are often only driven on roads that have been allocated for them and with specific licenses. Depending on the kind of cargo, size, weight, and state legislation, requirements will change.


Driving an oversize vehicle generally requires special knowledge and skills, and may require a Double/Triple Trailers endorsement on your CDL.


Some examples of these types of trailers include:


  1. Double-drop low bed with two axles — Over the fifth wheel and the two rear axles, the deck of this semi-trailer is at a conventional flatbed height, but it descends almost to the ground in the middle. Large, hefty cargo is transported with these trailers. The sides of the trailer may be extended using outriggers to handle greater loads.

  2. Removable gooseneck low bed — These low-bed trailers, which often have 2, 3, or 4 wheels, feature removable goosenecks that enable the trailer to rest on the ground for simpler loading and unloading of heavy machinery. Front loaders, bulldozers, backhoes, etc. are often employed. The sides of the trailer may be extended using outriggers.

  3. Folding gooseneck low bed — Similar to the above-mentioned detachable gooseneck trailer, the third option is a folding gooseneck low bed that collapses to the ground to facilitate loading and unloading.

  4. Hydraulic sliding axle trailer — As the name suggests, a sliding axle trailer features rear axles that can slide forward and a bed that slopes to the ground to make it simpler to load and unload heavy equipment.

  5. Hydraulic tail low bed — This kind of trailer features a fixed goose-neck and a hydraulic ramp that may be used to load and unload large machinery.

  6. Low bed with jeep dolly — A low-bed semi-trailer can be attached to a multi-axle jeep dolly to increase the trailer’s capacity. Part of the cargo rests on the dolly and part on the trailer.


Low-Clearance Vehicles


There are two basic kinds of low-ground-clearance vehicles:


  • Double-drop frame — Drops right behind the kingpin and right in front of the trailer axles, close to the ground.


  • Single-drop frame — Has a single drop located right behind the king- pin; only drops about half as much as a double-drop frame.


Low-clearance trucks are designed to carry bulky, heavy freight or carry loads with a higher cubic capacity. Depending on the kind of trailer, its size and weight, the cargo it carries, and the laws of each state, driving low-clearance trucks may sometimes need specific licenses and permits as well as limited road usage. Before operating these trailers, special training and expertise are needed.


As long as the unit is correctly set, both kinds of trailers are built with adequate space behind the kingpin to prevent the rear of the tractor from striking the lowered area of the trailer. The fifth wheel has to be backed up far enough for the driver to have ample clearance.


Double-drop frames are especially susceptible to bottom clearance problems at curbs, railroad crossings, and other places where the pavement is uneven. Special skills and knowledge are required to operate these vehicles. A double- drop low bed is also known as a lowboy, while double-drop vans are often called furniture, warehouse, or electronics vans. Furniture vans are com- monly used in the household goods moving industry, due to their larger load capacity. In addition, the dropped well allows for easier hand loading.

In comparison to traditional trailers, single-drop frames can support larger loads, and bottom clearance issues are not as severe. A drop-deck or step-deck trailer is another name for a single-drop low bed. A single-drop van is sometimes referred to as a warehouse, furniture, or electronics van, much like a double-drop van.


High Center-of-Gravity Vehicles


The bulk of the weight is carried high above the ground in high center-of-gravity vehicles, therefore the biggest risk comes from their propensity to tilt, particularly in bends. A lot more caution is needed while managing a vehicle due to the comparatively minimal legal constraints that do not apply to typical commercial motor vehicles. High-center-of-gravity vehicles need advanced planning from the driver to prevent sudden turning and braking. Dry bulk tankers, liquid tankers, livestock trailers, certain reefers (refrigerated units), and some big trucks are examples of vehicles with a high center of gravity.


Whenever feasible, this type of vehicle should be loaded with the heaviest material low in the trailer and forward (weight permitting) for improved handling.


Dry bulk products including cement, limestone, wheat, sugar, fly ash, etc. are transported by dry bulk tankers, which are typically cylindrical. Typically, they are loaded from the top and discharged from the bottom.

To operate properly, liquid tankers need a variety of specialized talents. It is the liquid rush that makes handling challenging. Drivers must accelerate and turn carefully, minimize stopping in curves, and prevent rollovers. On the driver's CDL, a tank vehicle endorsement is necessary. Depending on the weight, a Hazardous Materials endorsement may also be necessary. For further details, please refer to Unstable Loads.


Livestock trailers, Live animals such as cattle, sheep, pigs, and other livestock are transported in livestock trailers, also known as cattle liners, which feature a flat or double-drop construction. For further details, please refer to Unstable Loads.


Unstable loads — The two types of vehicles that carry unstable loads most often are liquid tankers and trailers used to transport animals or corpses. Due to their tendency to move about, both liquids and animals may cause a vehicle to become unstable.


Liquid tankers — Typically semi-trailers, liquid tankers may be square, oval, or even rectangular. There are three types of tankers: pressurized, heated, and cold. Hazardous products are often transported using tankers.


The length of liquid tankers varies. They might have one, two, or more compartments. They may or might not have baffles. Because of soaring loads that generate an unstable vehicle, handling may be challenging to learn.


Examples of liquid tankers include:


  • Petroleum or chemical tankers;
  • Acid tanks;
  • Liquefied gas tanks (high pressure);
  • Insulated tanks;
  • Food grade (milk, cooking oils, etc.).


Depending on the kind of trailer and payload, tanker drivers may need to undergo some specialized training. Depending on the kind of trailer and the load, access to the road may be limited and licenses may be needed.


All hoses, valves, fittings, the emergency valve release, and any other emergency equipment must be inspected by the driver for leakage. The usual inspection methods then take effect. Details on vehicle examination are provided in Chapter 4.


Livestock transport — Transport of livestock refers to the movement of live cattle, pigs, horses, sheep, and other less common living cargo. It is carried in a semi-trailer with a flat floor or a double-drop frame. Livestock can breathe thanks to side slots or holes. Side doors are often used in place of or in addition to the rear doors in livestock trailers.


A fixed tandem or spread axle and varying lengths are characteristics of trailers. Smaller animals like pigs or lambs are often transported on trailers that have been modified to fit two or three decks.


Live cargo may cause issues. Animals just shift. You must maintain a speed that will allow you to safely operate the car. When coming to a halt, briefly touch the brakes to start the animals, then gradually use the brakes. Never try to move cattle without the necessary training. Because the driver is in charge of the animal's welfare and safety while they are being transported, livestock hauling is special.


Refrigerated trailers — Refrigerated trailers, or reefers, come in basically two types, nose mounts and belly mounts. Nose mounts have the refrigeration unit mounted on the upper front of the trailer. Belly mounts, as the name implies, have the refrigeration unit mounted under the trailer.


Trailers of the van or box design are often reefers. Some are susceptible to instability because animal corpses are hung on racks or rails that are hanging from the ceiling. The same level of care must be used while handling liquid tankers and moving cattle.


Others have different chambers where they can keep some cargo chilled and other goods are frozen. Slotted flooring enables the flow of gas or air. It has insulated floors, walls, and roofs.


Reefer cargo often includes fruits, vegetables, meat, and other goods, as well as certain chemicals. Meat that is hung is a particularly challenging load because it might swing and interfere with stability.


Independent engines that use liquefied petroleum gas or diesel power the refrigeration trailers. They also have their fuel tank.


Checking the floor and ceiling ducts, checking the doors and door gaskets, and looking for holes in the walls are some of the special reefer inspection processes. Regular checks of the levels of fuel, coolant, oil, and refrigerant are also necessary. To prevent cargo damage, the trailer temperature must be maintained.

Note: Some states require reefers to stop at agricultural inspection stations.



Special Cargo Vehicles



Any vehicle built specifically to transport one specific kind of goods is considered a special cargo vehicle. Pole trailers and auto carriers are two of the most popular kinds of special cargo vehicles.


Cargo securement requirements for these and other special cargo vehicles are addressed in Part 393, Subpart I of the Federal Motor Carrier Safety Regulations (FMCSRs).


Pole trailers — Poles, steel girders, logs, and other long, thin items may all be transported on pole trailers. To accommodate the load, these trailers may be stretched or shortened as necessary. A pole trailer often has two bunks, or U-shaped cradles, to keep the goods in place. Sometimes the cargo itself serves as the trailer's body, and other times a steel beam does.


Auto transporters — On average, car transporters can fit 6–10 vehicles. On certain tractors, a rack is used to support an automobile. There are ramps at the back of these trailers that may be utilized for loading and unloading. Vehicle carriers have extremely little ground clearance. Each load of automobiles being carried may have a different amount of overhead clearance.





It takes more than the ability to recognize a particular gear to be able to use one. The operation of the special rigs covered in this chapter requires operators to have received adequate driving training. Each has a different set of operating characteristics and risks, and each may need a different set of licenses, endorsements, and training to operate.

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