The term “tandem truck” covers a wide range of three-axled vehicles -- one axle in front, two in the rear -- from a dump truck to the tractor that pulls a semi-trailer. Tandem trucks have been around since 1926, when they were developed by the Hendrickson Motor Co. to both increase a truck’s load-bearing capacity and to smooth out the ride on bumpy roads and at construction sites.
Any truck with a double axle in the back is considered a tandem truck. Both axles are usually drive axles -- they propel the vehicle. Many have two wheels on each end of each axle. The tandem axles allow the truck to carry much more weight, they provide stability and a smoother ride, and they provide better traction on slippery surfaces. Many dump trucks are tandem trucks, as are many firetrucks, fuel and water trucks and trucks equipped with cranes.
A semi-trailer has either one or two axles at the rear -- those with two are called tandem trailers -- and a kingpin at the front that connects to the fifth wheel, or hitch, on the back of the tractor that pulls it. If the tractor has two rear axles, it is also a tandem truck.
Though sometimes confused with “tandem truck,” the term “double bottom truck” refers to a tractor that is pulling both a semi-trailer and a full trailer -- one with axles and wheels on both front and back.
Truck designer Magnus Hendrickson and his sons Robert and George designed the first tandem truck suspension in Chicago in 1926, just four years after inflatable tires first replaced solid rubber tires on trucks. The Hendricksons used a metal beam connected to each axle by a pivot, a design that distributed the load evenly and reduced the effects of uneven terrain. In 1933, the International Harvester Co. signed a contract with the Hendrickson Motor Co. granting IH exclusive use of the tandem suspension. IH held the exclusive right to the tandem design until 1948, when it agreed to let Hendrickson offer the suspensions to all truck manufacturers. Hendrickson continues to manufacture the tandem suspensions as of 2011.