A History and Overview of the Trucking Industry Part 2
Fifth-wheels come in three different designs: fixed, manual slide and air slide. A fixed fifth-wheel is bolted to the tractor frame and is not adjustable. A manual has a pull lever mounted on the sliding plate that unlocks the fifth-wheel from the frame and allows it to be repositioned. The air slide design is more convenient because it allows it to be unlocked from the frame by pressing a switch on the dashboard.
The procedure of moving the fifth-wheel requires the driver to first back the tractor up and couple the fifth-wheel to the kingpin of the trailer. The driver will unlock the fifth-wheel slide mechanism and slowly move the tractor forward or backward while the semi-trailer brakes are still activated.
With the kingpin locked into the fifth-wheel, as the tractor moves, the fifth-wheel will slide on the tractor frame and after correct positioning of the fifth-wheel is completed, the driver will lock it into place. A movable fifth-wheel is advantageous in that it allows the driver to change the weight distribution of the semi-trailer on his tractor by moving the fifth-wheel fore or aft. This allows for proper weight distribution, even tire wear and an adjustable wheel base to meet length restrictions and help maneuverability in congested city traffic.
Dollies or converter dollies allow multiple trailers to be hooked together. A converter dolly is a removable single axle with a fifth-wheel mounted on top. A dolly has a drawbar and a tow bar eye or ring for connecting to the pintle hook at the rear of a leading trailer or straight truck. When a dolly is attached to a semi trailer, it becomes a full trailer. The dolly is hooked to the rear of another trailer. Any semi-trailer can be attached to a dolly and is converted into a full trailer with ease.
Jeep wheels are gooseneck dollies that are used with very heavy or long loads. They are like converter dollies, but have a larger, removable suspension system that hooks onto the tractor’s fifth-wheel and has a fifth-wheel of its own to which the semi-trailer can attach. It also adds to the weight-carrying capacity of the semi-trailer and tractor and adds another articulation point.
Trailers come in a variety of styles and designs depending on the type of cargo they are to haul. Semi-trailers vary in length, but the most common trailers are between 26 and 53 feet long. A semi-trailer that is enclosed is commonly known as a box trailer, or van shaped like a rectangle with a rear door. The door can either be a roll-up door or be swing doors. A van-type trailer with an open roof is called an open top. A van trailer with a larger dimension from floor to roof is termed a high cube van and is designed for maximum loading space. Tire and wheel sizes on these types of trailers are smaller to keep the trailer below the 13’ 6” height limit.
A reefer is a refrigerated trailer. It has a cooling unit attached to the front or underside of the trailer and circulates cold or hot air to keep the cargo at a proper temperature. Reefers also have a fuel tank under the frame rearward of the landing gear and usually have a small inspection door within the longer door at the rear of the trailer to check the temperature or inspect the cargo.
Flatbed trailers are platforms on wheels and are used to haul items that are uniquely shaped, need little protection from the elements or that can be protected with tarps. A step deck trailer is different from a conventional flatbed in that the platform of the trailer is lower than the height of the fifth wheel on the tractor. A double drop deck trailer is a trailer bed that is lower than the fifth-wheel plate and even lower than the trailer axles. The floor of the trailer is within a few inches of the pavement. A gooseneck trailer is a drop deck trailer with a long narrow neck. It usually has a ball hitch and is pulled by either a straight truck or a pickup truck. It allows for the trailer to pivot and not strike the truck.
Tanker trailers are cylindrical-shaped trailers that can haul liquid or bulk dry commodities. Tankers that are used to haul fuel or liquid are equipped with baffles in the tank which restrict the fore-aft fluid movement when the vehicle is braking and/or accelerating. This reduces the tendency of the trailer to push and pull on the tractor and cause excess wear on the fifth wheel. Tankers that are designed to haul food products do not have baffles because of sanitary issues regarding cleaning.
Livestock trailers are used to haul all types of livestock including pigs, sheep and cows. They have adjustable decks and compartments to accommodate all kinds of sizes of animals. Livestock trailers come in three styles: single-deck, double-deck or triple-deck. Double and triple-decks are often called pot belly trailers. Moving floor trailers are designed to help make unloading cargo easier. They have moving floors such as belts or linked sections to a moving belt and can walk the cargo from the front of the trailer to the rear and vice versa.
A hopper trailer has V-shaped openings on the bottom of it. This allows for product to be dumped out of the bottom of the trailer and makes it easier to fill it up on the top. They are used to haul grain and other bulk products. A dump trailer usually transports granular construction materials and coal. These trailers have two styles: framed or frameless. A hydraulic cylinder raises the front end of the trailer and dumps the cargo out of the rear of it. All modern dump trailers are hydraulically operated.
Frame dump trailers have a frame between the kingpin and the axles; the frame carries the load and provides strength and stability. A frameless dump trailer supports its weight and cargo by using a platform instead of a frame rail, which makes it lighter. When a load is dumped, a hydraulic cylinder on the front of the trailer body and attached to the fifth-wheel is raised. The tractor and trailer wheels will be brought closer together, as either the trailer wheels are moved forward or the tractor is pulled backward. When a trailer is added to a tractor or straight truck, it is called a combination vehicle or rig.
GAWR is an acronym for gross axle weight rating and is the maximum weight an axle is designed to carry. In order to meet the GAWR required, truck component manufacturers design their products accordingly. The type of suspension, number of wheels per axle and tire ratings all affect the GAWR. The GAW, or gross axle weight is the total load that an axle is supporting and should never exceed the GAWR for that axle.
The GVWR, or gross vehicle weight rating is the value specified by the manufacturer as the loaded weight of a single vehicle. This is the maximum total cargo weight plus vehicle weight for which it is designed to carry. It is determined by the number and placement of axles. When a vehicle has more axles, it can carry more weight and other components, such as frame rails and transmissions must also be designed to handle the specified loads. The gross vehicle weight (GVW) is how much the vehicle weighs including the cargo, driver, fuel, etc. The GVW must never exceed the GVWR.
Gross combined weight rating (GCWR) is the maximum combined weight of that vehicle including any towed or attached vehicles. For example, a tractor labeled with a GVWR of 45,000 lbs and a GCWR of 75,000 lbs will only allow for the tractor alone, carrying equipment, fuel, passengers, etc. to haul up to 45,000 lbs. According to the GCWR, the tractor can haul a total of 75,000 lbs, including the weight of the tractor, fuel, passengers, equipment, any attached trailers, and any payload within the trailers. Trailers are also stamped with GVWR and GCWR specifications.
It is a mistaken belief that adding the GVWR of a tractor and the towed unit yields the GVWR of the combined unit, but that is not the case. If a tractor is labeled with a GVWR of 50,000 lbs and a GCWR of 80,000 lbs and it hooks up to a trailer with a GVWR of 60,000 lbs and a GCWR of 80,000 lbs, the GVWR of the combination is not 50,000 + 60,000, or 110,000 lbs. The GCWR supersedes the GVWR as soon as the two vehicles are linked together, therefore, the maximum weight of the combined vehicle is 80,000 lbs. The maximum legal gross weight for a vehicle is 80,000 lbs, except where a lower GVW is dictated by the bridge formula.
The GVWR and GCWR are not only the manufacturer’s specifications and load limits, but they are also the legal load limits for that vehicle. When a truck is loaded greater than the GVWR or GCWR, it is against the law as well. There are eight categories that all trucks fall into based on their GVWR, light, medium, heavy and extra heavy. Heavy and extra heavy trucks fall into Classes 6, 7 and 8. Class 6 trucks have a GVWR of 19,501 to 26,000 lbs. Class 7 trucks have a GVWR of 26,001 to 33,000 lbs. Any vehicle combination with a GVWR over 33,000 is considered a Class 8 vehicle.