Studebaker US6 2½-ton 6×6 truck

In 1939–1940 the US Army Ordnance Corps was developing 2+1⁄2-short-ton (2,300 kg) tactical 6×6 trucks that could operate off-road in all weather. Studebaker, Yellow Coach (a GM company) and International Harvester all submitted designs that were accepted and went into production in 1941.

A total of 219,882 2+1⁄2-ton 6×6 trucks and similar 5-short-ton (4,500 kg) 6×4 versions in thirteen variations were built. Studebaker was the primary manufacturer, which built 197,678 of them at its South Bend IN plant, while Reo produced 22,204 more at its Lansing, Michigan plant from 1944 under a sub-contract. Reo trucks are identical to Studebakers, but Reo built only cargo-model trucks with the long wheelbase and without the front-mounted winch, more specifically referred to as the US6 U9. All production by both manufacturers ended in 1945.[2][3]

The US6 was manufactured primarily for export under Lend-Lease. The Soviet Union would become the largest foreign operator. The first Studebaker US6 trucks arrived in the USSR in the autumn of 1941. The Red Army organized a test of eleven 6×6 "Studebekkers" (as they become referred to in the USSR) which took place between July 1942 and May 1943. The results were used to direct the enlargement of the payload from 2+1⁄2 to 4 short tons (2,300 to 3,600 kg).[4] In 1945, it was lowered to 3+1⁄2 tons (3,200 kg), although on improved roads they could carry up to a maximum of 5 tons (4,500 kg).

Large numbers of Studebaker US6 trucks were supplied to the Soviet Union via the Persian Corridor in Iran under the USA's Lend-Lease program. The truck fulfilled many important roles in service with Soviet military forces during the war, such as towing artillery pieces and anti-tank guns and transporting troops over long distances. It was renowned for its overall ruggedness and reliability, including its ability to run on poor-quality fuel. The Soviet Red Army also found them to be a suitable platform for conversion into Katyusha rocket launchers, although this was not their main purpose. The truck became affectionately known as the Studer by Soviet troops and was even recognised of its importance (to the Soviet war effort) by Joseph Stalin, who sent a personal letter of appreciation to Studebaker, in which he thanked it for the superb quality of the US6 for Soviet service.

Studebaker US6 trucks were also used by the US military in the construction of the Ledo Road in Burma, and the Alcan Highway in North America, during WWII.

Hercules JXD engine

Dump truck w/winch shift patterns

Long wheelbase frame

The US6 used a Hercules JXD engine, with an 320 cu in (5.2 L) L-head inline 6 cylinder gasoline engine developing 86 hp (64 kW) at 2800 rpm and 200 lbf⋅ft (271 N⋅m) of torque at 1150 rpm. A conservative-type and highly-reliable engine with a compression ratio of only 5.82:1, it could use 68-octane gasoline. This same engine was also used in the M3 Scout Car and, later, M8 Greyhound and M20 armoured cars (the latter was a variant (lacking the gun turret) of the M8 Greyhound).[5][2][3][6]

The Warner T 93 5 speed transmission had a very low first, a direct fourth and an overdrive fifth gear. A power take-off could be fitted to operate a winch (mounted just below in front of the radiator) and/or the hydraulic hoist on dump trucks (the U10/U11 and U12/U13 dump truck models).[7]

The Timken T-79 transfer case had high and low ranges, a neutral position and could either engage or disengage the front axle. There was one output shaft mounted forward to the front axle (not used in 6×4 trucks) and two to the rear, with one for each rear axle.[8]

Both front and rear axles were of the Timken split-type with a ratio of 6.6:1. The front axle had ball-type constant-velocity joints while the two at the rear were full-floating.[9]

The US6 had a ladder frame with three beam axles, the front on semi elliptical leaf springs, the rear tandem on quarter elliptical leaf springs with locating arms.[10][11]

There were two wheelbases, the short 148 inches (3.76 m), used in semi tractors, dump trucks, and short cargo models, and the long 162 inches (4.11 m), used in tankers, long cargo models, and the U9 chassis cab (measurements are from the centerline of the front axle to the centerline of rear bogie). All models had 7.50-20" tires and dual rear tires. 6×4 models, intended for on road use only, were rated at 5 short tons (4,500 kg), twice the 6×6's off-road rating.[3][12]

The US6 carried the design of Studebaker's civilian truck cab, although it was modified for military use. Studebaker trucks were different from other 2+1⁄2 6×6 trucks built for the war effort of the USA because vent windows were included in each door. These vent windows were separate from the main window that rolled down into the door-frame and could be swung out to help with the truck cab's ventilation.

Studebaker also designed the open-type military truck cab which was featured on the GMC CCKW (later models), but their major customer, the USSR, preferred the closed cab for their generally harsh (cold-weather) climate. While Studebaker's open-type truck cab became the American standard, production of the US6 with the closed-type truck cab was restarted after only 10,000 units of the former.[3]