The Unimog 406 is a vehicle of the Unimog-series by Mercedes-Benz. A total of 37,069 units were manufactured by the Daimler-Benz AG in the Unimog plant in Gaggenau from 1963 to 1989. The 406 was the first medium duty Unimog, having a larger wheelbase of 2380 mm and more than twice the engine power of the Unimog 401. Unlike the initial Unimog, the 406 does not have a car engine but a heavy duty truck engine instead. Several following Unimog versions were based on the 406. There were eleven different types made of the Unimog 406, which were available in four models (U 65 – U 84) with a closed two-door or four-door cab, as Cabrio and as an OEM part (a "half" Unimog lacking the rear part, as a basis for third party vehicle manufacturers). During its long production period, the 406 received several technical refinements. In 1964, the precombustion chamber diesel engine OM 312 was replaced with the direct injected OM 352. Disc brakes followed in 1973. For many enthusiasts, the Unimog 406 represents the classical Unimog, having agricultural and silvicultural applications. It was successful and the best embodiment of the word Universal-Motor-Gerät considering all prior Unimogs.
History
Conception and development
The initial Unimog concept was favoured by customers and accessory manufacturers, however, starting in the early 1960s, they desired a "heavy duty Unimog", following the trend of more powerful agricultural tractors beginning in the late 1950s which was due to German agriculture's shift to heavy mechanisation requiring less personnel. It led to the end of the tractor boom in Germany in the mid-1960s, a demand for agricultural tractors with low power output. The most powerful Unimog 411 model at that time was offered with a 25-kW-engine which was considered too weak for several applications. Analysts warned that the demand for the Unimog 411 would pass the zenith after 1960 and fall below 3000 produced units per year. This point was reached in 1964. Also, a decline of the military Unimog 404 production was foreseeable since the Bundeswehr had reached their required number of vehicles for the majority of their regiments and battalions. Therefore, Daimler-Benz decided to create a more powerful Unimog, the 406.
In 1960, the performance specifications were completed. The 406 was still supposed to be an agricultural and forestry vehicle and tractor but have a greater wheelbase, a higher top speed, the downbent frame of the Unimog 404 and a stronger engine. Three cabs were planned: The Cabrio cab, a closed cab and a closed double cab. The initial engine concept favoured the direct-injected four-cylinder, diesel engine OM 314 with 54 PS (40 kW). Since Heinrich Rößler, the leader of the Unimog development did not want this engine, it was decided to use the, also direct injected, six-cylinder OM 352 instead. New tyres had to be developed by Dunlop and Continental, also, the Unimog 406 needed a new hydraulics system for auxiliary devices and a new drivetrain with new axles and a new gearbox for the increased engine power. Several new sheet panel presses had to be installed in the plant in Gaggenau for producing the closed cabs.
The first prototypes were tested in 1961, prototype 1 was a disguised prototype vehicle lacking the Mercedes-Benz and Unimog emblems with a slightly different cab and the bumper of the Unimog 404, prototype 2 already had the cab of the series production model with emblems but still had the Unimog 404 bumper. The final presentation of the Unimog 406 was at the German Agricultural Society (DLG)-exhibition in Munich 1962, led by the Daimler-Benz board of directors. In the run-up to the presentation on 20 May 1962 a lot of minor Unimog changes and improvements as well as improvisations were made. Daimler-Benz split the truck production in Gaggenau in 1963; while the truck and lorry production was moved to the new Daimler-Benz plant in Wörth, the Unimog production stayed in Gaggenau. More production capacity could be used for the new Unimog 406. In 1963, 800 Unimog 406 were produced, the first 100 of them being pre-series production models.