Unimog 406
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 with a closed two-door or four-door cab, as Cabrio and as an OEM part. 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 now highly collectible Unimog 406 represents the classical Unimog, having agricultural and silvicultural applications with the last of the drum braked 1973 406BT being the most desirable for collectors. 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. 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 -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.
During the late 1960s, the Unimog 406 was a popular type though the less powerful but cheaper types were introduced. Yearly improvements increased the quality of the Unimog, the highest production figures were reached in the first half of the 1970s. With the start of the series production of the heavy duty Unimog types 425 in 1974 and 435 in 1975, the demand for the 406 was declining. The yearly improvements were reduced and since 1979, the 406 had not been changed anymore. Soon it became very unpopular with an average production number of only 380 Unimogs per year during the 1980s. Series production was stopped after 27 years in 1989.
- Also see → ''Production figures''
Model changes
Due to a shortage of OM 352 diesel engines, the first 1766 series production Unimogs as well as the prototypes were fitted with the precombustion chamber injected OM 312 diesel engine, detuned to 65 PS. In 1964, OM 352 engines were available for the Unimog 406 and they were used instead, also detuned to 65 PS. This power output turned out being insufficient for many applications the Unimog was used for, later Daimler-Benz decided to offer more powerful variants of the OM 352 engine for the 406. Further changes in 1964 included a Rockinger trailer hitch, an indentation in the front bumper for the front PTO, a compressed-air tank with a capacity of 27 litres instead of 20, uprated springs, new portal axles and new rims without the four apertures at an angle of 90°.
;1965
In this year, the Unimog 416 production was started as well as the Unimog 406.130 / 406.131 OEM part production. The 406 was fitted with new mudwings, a new clutch and rectangular tail lights. Due to DIN and ISO standards which intended that the PTO shaft had to be underneath the trailer hitch, Daimler-Benz offered a PTO-relocating-gearbox for the rear PTO shaft.
;1966
For the Unimog 406 production, 1966 was an important year. A tilting mechanism was added to the cab. The cab was also fitted with a new removable bonnet and a new windscreen, the air louvres for the heating and ventilation system next to the headlights were removed and replaced with smaller louvres near the indicators. The production of the U 65 came to a close, it was replaced by the more powerful U 70, now offering 70 PS. Starting with the U 70, the four door double cab was offered for the first time. The drivetrain was improved with ruggedised reduction gears, new portal axles and axle stubs as well as optimised torque tube welding. Modifications of the electrical system included an electrical thermostat and a new alternator with an external governor. Also, the main brake cylinder was replaced. Some parts of the interior were changed as well.
;1967
1967 brought only minor changes, the driver and passenger seats were upgraded, the rear-view mirror was modified. The portal axles were improved slightly.
;1968
In 1968, the F-Gearbox was replaced with the G-Gearbox. Both are basically the same sliding gearboxes, however, the G-Gearbox has an additional intermediate gearbox. Furthermore, the 406 received a new governor for the compressed-air system and a new oil tank for the steering system as well as new outside mirrors.
;1969
With the 1969 model changes, Daimler-Benz introduced the hinged windscreen for the Cabrio and a new closed cab with a bigger ventilation flap. The model U 80 was added to the model range in addition to the U 70. Also, the doors were made sturdier.
;1970
The steering system was replaced with a hydraulically assisted ball-and-nut steering in 1970, the mechanical steering system was still offered as an option. New safety regulations required a roll bar for the 406, also, the closed cab models were equipped with two additional rear windows. The brake cylinders were upgraded again.
;1971
In March 1971, the U 80 was replaced with the U 84. It was fitted with bigger, rectangular indicators, new tail lights, upgraded portal axles and a new aluminium bed.
;1972
The 1972 changes were a removal of the ventilation louvres above the windscreen and the introduction of a plastic coolant reservoir.
;1973
The last major changes were made on 2 July 1973 when the drum brakes were replaced with disc brakes. With the introduction of the disc brakes, a dual-circuit brake system was installed and the rim colour was changed from red to black. Also, the steering system was upgraded and the radius of the exhaust pipe was increased to 35 mm.
;1974–1989
The most important improvements for the Unimog 406 were completed by 1974. In 1974, the exhaust silencer was modified, followed by a new step for the cab in 1975 and a constant-mesh version of the gearbox in 1976. The all-wheel-drive switching mechanism was upgraded to a pneumatic version in 1977, the chromed moldings on the front grille were removed in 1979. From 1979 to 1989, the 406 was produced without any major changes.
Unimog 406 types
Eleven different Unimog 406 types were offered with the standard cabrio and closed cab types being the most important, 96% of all Unimog 406 ever produced are either one of these two. The OEM part models were not given a type number until 1965 when they received the type numbers 406.130, 406.131 and 406.133. In 1968 they were merged into the Unimog 416 series. Each Unimog type was offered in different models, the four models were; U 65, U 70, U 80 and U 84.Further Unimog 406 models
The 406 was used as a basis for road-rail-vehicles made by external companies, the rated towing capacity is up to 300 t.Initially, the Unimog 406 was offered with a 65 PS engine, this figure was later increased to 84 PS and series production models never exceeded it. Some prototypes and special order models were fitted with more powerful engines though. The first prototype of a road legal tractor for local hauling and internal works transport in the Unimog family was the 406.200 A in September 1969. It had the long wheelbase of the Unimog 416 and an OM 352 engine with 100 PS. The 1971 series production model of this particular tractor was integrated into the Unimog 416 series and had the type numbers 416.140 and 416.141. The latter was produced 3496 times.
For industrial internal works transport, the types 406.142 and 406.143 were made. Daimler-Benz decided to offer the default 84 PS engine for these types, though customers could also purchase more powerful variants of these types with either 100 PS or 110 PS. None of these was produced more than approximately 300 times which makes any Unimog 406 with more than 84 PS very rare.