The Unimog 406 is a multi-purpose off-road capable vehicle with four wheels of the same size. Like the Unimog 404, it has a downbent ladder frame, two portal axles with torque tubes, reduction gears and coil springs with hydraulic shock absorbers for the rear and front axles. For powering mountable equipment, two standard switchable 540/1000 rpm PTOs are built in; both can be switched on and off independently. The 406 is a rear-wheel-drive vehicle with switchable all-wheel-drive and three differential locks. A bed is mounted on the rear part of the Unimog frame. The 406, like the initial Unimog, has a 12V electric system by default though a 24 V electric system with two 12 V lead acid batteries was available for military and other customers who preferred it - often because of trailers shared with trucks.
Cab
Daimler-Benz made the Unimog 406 with a cabrio cab and with a closed single or double cab; while the cabrio and closed single cab were built by Daimler-Benz, the double cab was produced by Wackenhut. All cabs have a three-point-mount. Both cabrio and closed single-cab versions have two seats, the double cab version has three additional seats. Gauges, steering wheel, levers, the sliding seats and further interior parts such as the ventilation and heating system resemble the parts also used in Mercedes-Benz trucks of the 1960s and were considered ergonomical and comfortable at that time. Cabrio models from 1969 onwards have a fold-down windscreen. Starting in 1970, the cabrio models were fitted with a roll bar that protects the driver in case of an accident.[35] The air-inlet is on the left hand side of the Unimog 406, a snorkel was a factory option. The snorkel comes with a built-in cyclone dust separator.[36]
Bed
Series production Unimogs have a bed with the dimensions 1950 × 1890 mm and approximately 3.7 m2. A hydraulic dumping ram makes it possible to dump the bed. For several applications, Daimler-Benz also offered a smaller bed (1475 × 1890 mm; 2.8 m2), furthermore, it is possible to demount the bed completely.[37]
Engine
Initially, the water-cooled straight-six precombustion chamber diesel engine type OM 312 powered the Unimog 406. It has a displacement of 4.580 litre and produces roughly 48 kW. In 1964, this engine was replaced with the direct-injected straight-six diesel OM 352[38] that was produced until 1989. The OM 352 displaces 5.675 litre and is naturally aspirated in the Unimog version, its power output figure was changed over the production period several times, however, the standard power output never exceeded 62 kW. Some special order Unimog versions of the 406 series were also offered with higher engine power outputs, these special versions are very rare though.
Drivetrain and suspension
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Front and rear part of the Unimog frame allow torsional flexing while the centre is rigid.]]
Like the Unimog 404, the 406 has a downbent ladder frame and rigid portal axles. The ladder frame allows torsional flexing at the front and rear part but is rather rigid at its centre. This is achieved by the position of the transverse rails with tulip shaped weld seams.[39] The portal axles have reduction gears so that the Unimog would have a good ground clearance even with small wheels.[40] Most important part of the drivetrain are the two torque tubes that transmit the torque from the gearbox to the portal axles. Since the torque tubes hold the portal axles in place, the Unimog does not need longitudinal control arms. A torque ball connects the torque tube to the gearbox and allows some flexion. The portal axles have coil springs and telescopic shock absorbers. This construction allows very long suspension travel.[41] The standard tyres have the size 10.5–20", Daimler-Benz also offered 12.5–20", 14.5-20" and 22-20"-tyres as a factory option.[42]
Gearbox
The 406 has the fully synchronised four-speed UG-2/27 group sliding gearbox that is designed for a maximum input torque of 27 kp·m (264.8 N·m). It was available with four different gearbox layouts:
The default gearbox layout (F-Gearbox) was used for the models U 65 and U70. It has four gears and two groups. The first group however only allows the first and second gear to be engaged so that the total number of gears is six; the gearbox lever has a six-speed H-layout. When shifting from second into third gear, the gearbox automatically switches from the first into the second group. The gearbox lacks a reverse gear, instead, the Unimog has a second gear lever for switching the driving direction. This lever is part of an additional reverse gear unit. When in reverse, due to the gearbox construction, only the first group of the gearbox, that can make use of the first two gears only, can be used. This results in two reverse gears.
The standard gearbox was available with an additional intermediate gearbox for the model U 80 and U 84 (G-Gearbox). The intermediate gearbox simply doubles the number of gears, resulting in 2 × 6 forward and 2 × 2 reverse gears. Its gear lever has the three positions ″main gearbox″, ″intermediate gearbox″ and neutral. In addition to the additional intermediate gearbox, Daimler-Benz also offered a crawler-gearbox. The crawler-gearbox can be used with the first and second gear of each group of the main gearbox, it has two crawler gears, ″crawler″ and ″super crawler″, a neutral position and a ″main gearbox″ position. When in main gearbox position, all main gearbox gears can be used. This results in 2 × 6 + 2 × 4 forward and 2 × 2 + 2 × 2 reverse gears.
In 1976, the default gearbox was upgraded to a constant mesh version and made available with a fully usable first gear group, resulting in eight forward gears and four reverse gears. The eight-speed-gearbox-lever has a single-H-layout with an additional lever for switching the groups, rather than the six-speed-H-layout.
Depending on the gearbox, the Unimog 406 was equipped either with a single-disc dry clutch (usually type G 280 KR) or a dual clutch.
Hydraulic system
Daimler-Benz installed a hydraulic system made by Westinghouse in Hannover. It is sturdier than the Unimog 411 hydraulics system[43] and consists of six main components: one pump, one oil reservoir, two hydraulic cylinders and two control units. The hydraulic pump is a gear-driven Westinghouse Typ 5 P 41-13 and initially had an operating pressure of 150 bar and a delivery rate of 32 litre per minute, later these figures were increased to 40 litre per minute and 180 bar. In the front part of the Unimog, the hydraulic oil reservoir holds the operating fluid, the filling quantity of the entire hydraulic system is 18 L. The two control units are located behind the engine, they have one lever each. Both levers are mounted on the steering column underneath the steering wheel. With the first lever, the driver can control either the hydraulic dumping cylinder for the bed or the hydraulic cylinder for the rear three-point linkage. The second lever allows the control of auxiliary devices.[44]
Initially, the hydraulic steering system was attached to the main hydraulic system. Later, it received a separate pump and finally a separate hydraulic circuit.[44] The standard hydraulic power-steering system is a worm-and-nut steering type ZF 19/74, a hydraulically assisted ball-and-nut steering was offered optionally and later became standard, replacing the worm-and-nut steering system in 1973.
Pneumatic system
Like earlier Unimogs, the Unimog 406 has a pneumatic system, however, it is mainly used for the brake system. Also, it is used for engaging the differential lock. It consists of several parts: An air compressor driven by the engine that generates the pressure for the pneumatic system, a control valve in front of the rear axle, the compressed-air tank in front of the left rear tyre, the valve for the differential lock, the pneumatic brake booster and the master brake cylinder for activating the hydraulic brake and the trailer brake system. The pneumatic trailer brake systems connects to the Unimog using two quick connectors, one for the compressed-air for the trailers compressed-air tank and one for activating the brake itself. An additional air compressor was available that reduces the amount of time required to fill the compressed-air tanks.[43] Initially, the compressed-air tank had a capacity of 20 L, it was increased to 27 L in 1966.[3]
[43]
