Trucks are divided into light-duty, medium-duty, and heavy-duty classifications depending on their weight. Heavy-duty trucks have a gross vehicle weight of 33,000 lb (15,000 kg) or more (i.e. the weight of the vehicle plus the weight of the payload is 33,000 pounds or more). When a heavy-duty truck is pulling a trailer, it may have a gross combination weight of 80,000 lb (36,360 kg) or more.
Technically, a vehicle that carries the load by itself, without a trailer, is known as a truck, or a straight truck. Examples include certain dump trucks, concrete mixers, and garbage trucks. A vehicle that pulls the load in a trailer is known as a tractor. The tractor is coupled to the trailer through a pivot point, known as the fifth wheel, which is mounted on top of the tractor frame. Most of the big rigs on highways are tractors pulling trailers.
The first gasoline-engine trucks were developed in the United States in the 1890s. During World War I, trucks played an important role moving supplies at home and overseas. With the development of a system of paved roads in the United States during the 1920s, the number of truck manufacturers grew. By 1925, there were more than 300 brands of trucks on the road. Some manufacturers came and went quickly. The Great Depression of the 1930s finished many more. By the 1990s, there were only nine heavy-duty truck manufacturers left in the United States. Together they build about 150,000-200,000 trucks a year.
Trucks use steel for strength and durability, aluminum for light weight and corrosion resistance, polished stainless steel for bright finishes, and molded plastics for complex shapes.
Frame rails and crossmembers are usually formed from high-tensile steel. Suspension components, axles, and engine mounts are also made from steel. Some are cast and some are fabricated and welded.
The cab structure and outer skin may be made from steel or aluminum. If steel is used, the metal is coated with one or more layers of corrosion barriers such as zinc. On some cabs the roof may be made of fiber-glass to form the complex curves required at the corners.
The hood and front fenders are usually molded in plastic or fiberglass because of the complex aerodynamic shapes. The front bumper may be stamped and drawn from steel or aluminum, or it may be molded in plastic and backed with a steel substructure.
Bright trim pieces—such as outside mirrors, sun visors, radiator grilles, and grab handles—are often made from polished stainless steel to give a long-lasting bright finish that will not crack or corrode.
The cab interior is finished with vinyl or cloth upholstery. The floors are covered with synthetic fiber carpeting or rubber mats. The dashboard and interior trim pieces are molded from plastic. The windows are made of laminated safety glass.
Fluids used in heavy-duty trucks include diesel fuel, petroleum-based or synthetic lubricants, antifreeze, power steering fluid, and an environmentally safe, non-fluorocarbon gas known as R134A, which replaces freon in the air conditioning system.
Truck manufacturers usually design a new model about every five to seven years. The new design incorporates advances in technology and materials, as well as changes desired by the customers. The design team will use a clay model to determine the overall styling, then build a prototype cab and hood for review and durability testing. As the design progresses, they will build an entire prototype vehicle for road testing. Just before the new truck goes into production, they will build one or more pilot models using actual production parts to spot any last-minute assembly problems.
In addition to the basic model, the engineers must also design all the options required by customers for different truck applications. Some manufacturers have as many as 12,000 options for their line of heavy-duty truck models.
Heavy-duty trucks are assembled from component parts. Each truck manufacturer usually builds its own cabs, and a few also build their own engines, transmissions, axles, and other major components. In most cases, however, the major components (and many of the other components) are built by other companies and are shipped to the truck assembly plant.
In most plants, the trucks move along an assembly line as components are added by different groups of workers at successive workstations. The truck starts with a frame assembly that acts as the "backbone" of the truck and finishes with the completed, fully operational vehicle being driven off the end of the assembly line under its own power.
Today's most recognized form of heavy-duty truck, the tractor-trailer, or semi truck, was commercially developed in the 1910s. Some truck designers believed tractors, motor trucks designed only to pull separable trailers, could make truck operation profitable. If tractors easily connected to trailers, the more costly motorized tractors could remain busy hauling full trailers, while leaving less expensive trailers idle during loading or unloading.
In 1911, truck designer Charles Martin built a gasoline powered tractor to pull modified horse-drawn commercial wagons. His most significant innovation, however, was the fifth wheel coupler. A round plate with a central hole, it attached to the top of tractor frames to connect and support trailers. Buyers converted wagons into semi-trailers by raising them with jacks and removing their front axles. Lowering and locking a trailer's bottom mounted kingpin into a tractor's fifth wheel coupled the vehicles. Martin's Rocking Fifth Wheel handled the period's rough roads. It allowed tractor-trailers to bend when turning, but also accommodated the ups and downs of uneven surfaces. Nearly every truck manufacturer purchased Martin's popular device.
August Fruehauf, a Detroit blacksmith, launched an early trailer manufacturing company in 1914, by building a boat trailer for local lumberman Frederick Sibley. Sibley pulled it with a Model-T Ford car that he turned into a one ton truck with a Smith Form-A Truck conversion kit. Impressed that pivoting tractor-trailers maneuvered long, heavy loads through tight quarters, Sibley ordered more trailers for his business. By 1916, Fruehauf was a noted trailer manufacturer.
Erik R. Manthey
Here is a typical sequence of operation for the assembly of a heavy-duty truck:
[Steps 15-23 are performed in a separate area off the assembly line]
18 The cab, hood, and sleeper for each truck are painted at the same time. The surfaces are cleaned and the areas that are not to be painted are masked off with paper or tape. If a paint design such as a different color stripe is specified, the stripe area is painted first, then the stripe is masked off and the main body color is applied on a second pass through the paint booth. After each pass, the cab, hood, and sleeper go through a drying oven. After the final pass, the masking is removed and the paint is visually inspected.
In addition to testing the completed truck, each component part and assembly operation is inspected. Parts are checked for correct dimensions before they reach the assembly line. Assembly operations are checked by the production workers themselves and are double-checked by quality control inspectors. The instrument panel is tested to make sure all the gauges and switches are working before it is installed in the truck. Even the thickness of the paint is checked with an electronic meter to ensure it meets the standard.
Heavy-duty trucks have evolved slowly over the last 100 years and will probably continue a slow evolution in the future. An increased concern about fuel efficiency has led to more aerodynamic designs. Likewise an increased concern about exhaust emissions has led to cleaner combustion engines. Heavy-duty trucks are still one of the most economical ways to ship the wide variety of raw materials and finished goods needed in our complex society, and they will probably remain one of our principal forms of transportation for many decades to come.
Karolevitz, Robert F. This Was Trucking. Superior Publishing Company, 1966.
Rasmussen, Henry. Mack: Bulldog of American highways. Motorbooks International, 1987.
Rasmussen, Henry. Peterbilt: The class of the industry. Motorbooks International, 1989.
Freightliner home page. 1996. http://www.freightlinertrucks.com (July 9, 1997).
Kenworth home page. 1996. http://www.paccar.com (July 9,1997).
Peterbilt home page. 1996. http://www.peterbiltmotors.com (July 9, 1997).
Volvo GM Heavy Trucks home page. March 18,1997. http://www.volvotrucks.volvo.com (July 9,1997).
— Chris Cavette