A history with pulling power: Mercedes-Benz all-wheel-drive vehicles
Stuttgart
Aug 02, 2011
The Unimog: a real all-rounder
  • The Unimog has an unparalleled history of success
  • Equal to a vast number of tasks
  • Its versatility makes it an extraordinarily useful all-wheel-drive workhorse
Germany was in a parlous state after the Second World War, and for a time there was even a serious possibility that the Morgenthau Plan might actually be implemented to turn the country into an agrarian state.
This background lent a very special importance to the Unimog project, which was launched in the immediate post-war period. This new vehicle class was conceived as a general workhorse that would be superior to conventional tractors and would ease the workload of farmers as far as possible. This was the approach taken by the former head of aircraft engine design at
Daimler-Benz, Albert Friedrich – who found himself unemployed after the war, but whose hands were by no means idle.
The name Unimog already suggested that a very special type of vehicle was being created. In the case of the Unimog, the dream of marketing strategists was realised of its own accord, when almost immediately the product name came to stand for an entire vehicle category.
Enthusiastic reception from the start
Originally coined as a convenient abbreviation for the long-winded description ‘Universalmotorgerät’ (‘Universal Motor Vehicle’), the name ‘Unimog’ is today a generic term whose origin sometimes needs to be spelled out before it is understood. No wonder that visitors to the 1948 DLG Show, the principal trade fair of the German Agricultural Association, gave the new vehicle an enthusiastic reception: apparently the team on the Unimog stand received no fewer than 150 orders there and then.
The magnitude of this trailblazing achievement by the fathers of the Unimog in autumn 1948 was shown by the fact that many of the original features of the first Unimog have been retained to this day: four wheels of equal size, four-wheel drive with front and rear differential locks, all-terrain portal axles, front and rear power take-offs and a small platform for carrying loads and equipment.
Albert Friedrich had already begun to work on ideas for a compact agricultural working machine during the war. Particularly with the prospect of the Morgenthau Plan in mind, these thoughts crystallised into the idea of a universal workhorse with a 25 hp (18 kW) engine which could double as a tractor, equipment carrier and transport vehicle for agriculture. As one of the development partners Friedrich engaged the services of his former colleague Heinrich Rössler, who was able to contribute valuable practical experience: Rössler had been employed as an agricultural labourer since the end of the war and knew exactly what was required.
Moreover, Friedrich had been able to secure one of the rare ‘Production Orders’ from the American occupation forces as early as October 1945, i.e. a manufacturing permit. At the time the allies were still suspiciously monitoring all industrial activity in Germany – 150 hp (110 kW) was the maximum output allowed for trucks, for example.
Prototype with a petrol engine
Friedrich chose a company named Erhard & Söhne based in Schwäbisch Gmünd as a development and production partner for the Unimog. In January 1946, Rössler was installed there as technical manager for the Unimog project, and he was keen to build a prototype as soon as possible. The engineering company Boehringer in Göppingen supplied the gear wheels and shafts for the transfer case, while Renk in Augsburg produced the axles. The vehicle was powered by a 1.7-litre petrol engine from Daimler-Benz (M 136 from the pre-war Mercedes-Benz 170 V passenger car), and on October 9, 1946 the first roadworthy chassis was ready for its initial trials. The vehicle differed from a conventional agricultural tractor in ten major respects:
  • 1. A speed range of three to 50 km/h.
  • 2. Dampened, sprung axles to keep the maximum speed controllable during on-road use.
  • 3. Four-wheel drive with front and rear differential locks.
  • 4. Braked front and rear axles (agricultural tractors only had rear brakes).
  • 5. A robust frame construction adopted from passenger car and truck engineering.
  • 6. A two-seater cab with a folding roof, fold-down windscreen, padded seats, and heating.
  • 7. A load platform measuring around 1.5 square metres, with a carrying capacity of at least one tonne.
  • 8. An advantageous weight distribution for difficult terrain: two thirds on the front axle, one third on the rear.
  • 9. Equipment mounting facilities at the sides, front, rear, and top.
  • 10. Front and rear power-take-offs, with a belt pulley in the middle.
But as what should this compact all-rounder be classified? It was not an agricultural tractor in the classic sense, nor a truck or an implement carrier. Nonetheless it was important to have the new vehicle accepted as an agricultural tractor, and it was the engineer Hans Zabel who came up with the brainwave of abbreviating the term ‘Universalmotorgerät’ to the more memorable ‘Unimog’. His reward was a bottle of wine – a very rare treat in those days.
A brand-new diesel provides the answer
By 1946 it had become clear that the petrol engine was not a viable proposition, when low-cost diesel fuel was officially approved for agricultural purposes. There was no avoiding the diesel engine if the vehicle was to remain competitive. So it was fortunate that at 
Daimler-Benz at the same time, the motorboat engine designer Julius Witzky, who was on the lookout for new projects, developed for the plant a high-speed diesel engine with 1.7-litre displacement, the OM 636, based on the engine M 136 of the Mercedes-Benz 170 V passenger car. The first test units were delivered to Erhard & Söhne on March 22, 1947.
Nor was the initially installed four-speed transmission supplied by ZF in Friedrichshafen the ideal solution. Rössler developed a new, six-speed constant mesh transmission with the idea of adding synchromesh at a later stage. Erhard & Söhne did not have the capacity for series production, however, and outsourcing its manufacture would have been too expensive.
The day was saved by Boehringer in Göppingen, which was already supplying cast components for the Unimog. The company was very interested in producing the entire vehicle (not least to avoid it being dismantled), and met with success: from February 1948 it became responsible for producing almost all of the Unimog.
As early as August 1948, the company sent two Unimog vehicles to the DLG fair in Frankfurt am Main, where they were given an enthusiastic reception – even though the engine compartments had been sealed, and it was not possible to see the engines: the patents for the OM 636 had not yet been registered.
On November 21, 1948, the Unimog was granted its own patent as a multi-axle motor vehicle for agricultural businesses. This meant that it benefited from official tax exemption for agricultural vehicles, could be operated on cheap diesel fuel and was placed in a more favourable insurance category. The chassis design, however, was only granted a patent on February 26, 1950.
The success of the Unimog outgrows its inventors
Boehringer was able to sell a respectable 600 units in the two years between autumn 1948 and autumn 1950, but neither Erhard & Söhne nor Boehringer were able to afford the increase in production capacity which this sales success made necessary. Negotiations with Daimler-Benz bore fruit, for on 5 September 1950, the company agreed to take over the entire Unimog sector.
Production was relocated from Göppingen to Gaggenau. The Gaggenau plant, which had always specialized in commercial vehicles, set itself a production target of 300 Unimog units per month. Indeed the first Gaggenau-produced Unimog left the plant for customer delivery on 3 June 1951. As of October 1953, the vehicle also became available with an enclosed, all-steel cab, for example.
The glorious career of the Unimog S
By as early as March 1953, Gaggenau had also developed a boxy prototype of what was to become the Unimog S. There were plenty of arguments for putting the Unimog into uniform for military use, turning it into an all-terrain light truck with a petrol engine. In fact specialists from the US military occupation forces had already shown themselves impressed with the clever concept of the original Unimog when it was demonstrated to them in 1947.
The plant took the first steps towards the Unimog S in 1953. It tentatively developed a prototype with a track width of 1400 millimetres rather than the previous 1284 millimetres, as well as increasing the wheelbase to 2120 millimetres (which meant that the clutch, transmission, and axles could remain unchanged), and finally ended up with a track width of 1600 and a wheelbase of 2670 millimetres for the first demonstration vehicle produced in 1953. The power unit chosen was the 2.2-litre petrol engine from the model 220 saloon.
Immediate order from the French army
During the first demonstration for specialists from the European Defence Commission in summer 1953, this vehicle made such a good impression that the French occupying forces immediately placed a request for a prototype, and two units were delivered in June 1954. This soon led the French army to place a large order totalling 1,100 units, which Gaggenau began to deliver from May 1955.
From then on, military authorities all over the world showed a lively interest in the Unimog S. It also suited the new Unimog that rearmament commenced in the Federal Republic of Germany in 1956. Formed in that year, the federal defence forces were to account for no less than around 36,000 of the 64,242 units of the new Unimog S, alias the Unimog 404 (production continued until 1980).
A fully-fledged load platform makes the difference
The Unimog S was considerably different from its agricultural counterpart with a rudimentary load platform. It featured a fully-fledged platform with a length of 2700 and a width of 2000 millimetres, mounted on a chassis with a track width of 1630 millimetres and a wheelbase of initially 2670, then 2900 millimetres from 1956.
The pedestrian, 25 hp (18 kW) pre-chamber diesel OM 636 with its awkward glow-plug ignition system was replaced with the 82 hp (60 kW) six-cylinder petrol unit M 180 II from the saloon cars, and this enabled the Unimog S to travel at twice the speed of its diesel-powered colleagues, namely 95 km/h. Optionally available from 1972 was also the 2.8-litre M 130 unit from the passenger car, although here with output derated to 110 hp (81 kW). A slick, synchromesh transmission rather than the constant mesh unit, power-assisted rather than hydraulic drum brakes and a 1.5-tonne payload were other features that distinguished the Unimog S from its civilian brothers.
Suitable for any type of body
With these genes and a willingness to accept any type of body, the Unimog 404 military version enjoyed a no less varied career than previously in the civilian sector: a material and troop carrier, a tractor for equipment and guns, a mobile weather station, workshop vehicle, military ambulance, and mobile office were just some of the functions performed by the Unimog S. Even the airborne troops had their own Unimog, which they could dispatch back to mother earth by parachute.
On request the plant would not only supply the Unimog with its standard folding roof (which could be stowed behind the seats together with the side windows), but also with an enclosed steel cab and a 3000-millimetre long platform. It was particularly this version that soon attracted the attention of the civilian market, as the 82 hp (60 kW) petrol model was far better able to keep up with road traffic than the diesel-powered agricultural version, which was limited to a speed of 52 km/h. The Unimog S was happy to tow a trailer weighing up to 4.4 tonnes.
The fast Unimog S also enjoyed enormous sales to fire services all over the world, since these placed a premium on speed in emergencies, and often the need to travel over rough terrain. Whether for forest fires, as an equipment carrier, pump water tender or dry powder tender, the fire-fighting talents of the Unimog S are still highly appreciated in many regions today.
An enduring concept for a quarter of a century
Like the original Unimog of 1948, the 1955 Unimog S was an extremely enduring concept which was subjected to hardly any major modifications in its production time extending over a quarter of a century.
Strictly speaking, any model updates were confined to a few important additions to the range: from 1971 some variants were also produced with the cab of the 406 series (in production from 1963), with the option of a more powerful engine. This particularly muscular unit beneath the short bonnet was the 2.8-litre six-cylinder M 130 engine, which developed 110 hp (81 kW) and gave the Unimog S a maximum speed of 100 km/h.
Civilian all-rounder
Over time the range of applications for what was originally an agricultural vehicle was naturally widened as well. The 25 hp (18 kW) first-born of the post-war period possessed many hidden talents, and these only needed to be discovered. Whether in forestry or for municipal duties, fire-fighting, the military, the construction and energy sectors or oil exploration teams in the desert, the more specialized the application, the fewer people were willing to do without the Unimog.
From 1956 onward, the engine output was increased from 25 to 30 hp (18 to 22 kW), and the model designation was changed from Unimog 401/402 to Unimog 411. One year later a synchromesh transmission became an option, and was standard equipment from 1959. A new, enclosed cab also became available for the long-wheelbase Unimog from 1957.
It was not until the early 1960s that the compact, basic Unimog began to reach its limits, with the Unimog S also losing ground for more specialized applications. Accordingly Daimler-Benz supplemented these with the ‘406 series’, whose short bonnet concealed the powerful 65 hp (48 kW) OM 312 diesel engine. This also enabled the Unimog to perform convincingly as a tractor unit.
Increasingly diversified range
In the mid-60s, Daimler-Benz completely restructured the Unimog range, inserting the 421 and 403 series between the small models with a choice of 34 or 36 hp (25 or 26 kW) and the medium models with 65 hp (48 kW). The four-cylinder OM 621 diesel engine of the 421 series was adopted from the passenger cars, and covered an output range from 45 to 60 hp (33 to 44 kW). The wheelbase was a standard 2250 millimetres, and the permissible gross vehicle weight ranged from 3700 to 4100 kilograms.
The diesel engine of the 403 series was adopted from the commercial vehicle. The OM 314 had a displacement of just under four litres, and was available with outputs of 54, 66, and 72 hp (40, 49, and 53 kW). This series had a standard wheelbase of 2380 millimetres, and the permissible gross vehicle weight ranged from 4800 to 5800 kilograms.
Entry into the heavy-duty class
In 1974, the Unimog entered the heavy-duty class with a gross vehicle weight of around ten tonnes. It was not only the high permissible gross vehicle weight of 9000 kilograms and a 88 kW (120 hp) six-cylinder diesel engine OM 352 that was new about the U 120. The 425 series also featured the angular cab typical of the time, and a large bonnet with only a slight downward slope. The basic design of this cab was to remain unchanged for almost a quarter of a century. From 1979 the engine also became available in a turbocharged version OM 352 A developing 110 kW (150 hp).
This Unimog was renamed the U 1500 as part of the reorganization of model designations which Daimler-Benz carried out during the 1970s: the basic models retained their rounded form and were renamed the U 600 L, U 800 L, U 900, and U 1100 L. Angular front ends adorned the models U 1100, U 1300 L, and the particularly powerful U 1500 and U 1700 L (124 kW/168 hp). The letter ‘L’ stood for ‘long wheelbase’. Two different wheelbases had meanwhile become available for most Unimog models.
The Unimog was not only a technological leader in the commercial vehicle sector because it made use of turbocharged engines for a high output at an early stage. It also belonged to the avant-garde because with the exception of the entry-level models, it already featured dual-circuit disc brakes all-round in the 1970s.
Completely new vehicles introduced in the 1980s
The Unimog line-up remained substantially unchanged until the second half of the 1980s, but then the angular cab of the medium and heavy series was also introduced in the light models. The Unimog was subjected to a complete redesign by the plant, and in practice different dimensions, wheelbases, drive trains, gross vehicle weights, and engines resulted in completely new chassis.
The 407, 417, 427, and 437 series offered customers a wider model range than ever before. This extended from the compact U 600 with 44 kW (60 hp) and a permissible gross vehicle weight of 4.5 tonnes right up to the U 2400, whose 176 kW engine (OM 366 LA, 240 hp) could easily cope with a gross vehicle weight of 12.5 tonnes.
From 1993, the range was even headed by an all-wheel-drive, three-axle Unimog built in Gaggenau; however the Iron Curtain between east and west had by now fallen – and demand by the military declined. Tighter budgets also meant that sales to municipal authorities, who had traditionally been good customers, fell to some extent.
New cabs with a sloping front end
Daimler-Benz responded by thoroughly restructuring the Unimog range. As early as 1992, the light 408 (U 90) and 418 (U 110 and U 140) series replaced their still youthful predecessors. Typical features of the new models included a redesigned cab with a heavily sloping front end, providing particularly good forward visibility.
On request the new models were also available with an asymmetrical bonnet cutaway on the driver’s side, which provided a good view of mounted implements and allowed rapid coupling and uncoupling as a one-man operation. The driving characteristics were improved with a new frame and progressively acting coil springs, and the completely redesigned cab offered significantly more space and comfort than before.
A little later the Unimog was even joined by a little brother: this scaled-down implement carrier was named the UX 100, but despite technical refinements such as transferable steering it was never able to carve a niche for itself. The rights to this concept were sold to the Hako group after 790 units had been built.
Form follows function
Meanwhile the 408 and 418 were already harbingers of what was to come in 2000 and beyond: the model range was split into vehicles mainly designed as off-road implement carriers and those designed for all-terrain transport operations. The former category was represented by the U 300 to U 500 models (405 series) introduced in 2000. The target groups were municipal authorities, government departments, service providers and the transport sector. In 2003, DaimlerChrysler’s North American subsidiary Freightliner even started marketing the U 500 in the USA under its own brand name. However, Freightliner discontinues sales in 2007, when emission standard EPA 07 came into force, and having sold only around 200 units in five years.
The typical feature of the 405 series was a new short-nosed cab with a high level of practicality and ergonomics. The cab was of composite fibre construction, and provided outstanding visibility by virtue of an extremely deep windscreen and side windows.
The ‘VarioPilot’ feature enabled the steering wheel, instrument panel and pedal cluster to be moved to the other side of the vehicle in a matter of seconds. This was because 75 per cent of all Unimog vehicles were used on an all-year round basis, with a variety of mounted implements which needed to be operated from either side of the cab. Moreover, in many cases one-man operation was fast becoming a necessity to ensure overall cost-effectiveness.
The UG 100 transmission was another new feature of the 405 series. The basic version had eight gears, semi-automatic Telligent gearshift (fully automatic Telligent gearshift also became available from 2003), and cruise control. Other refinements such as additional crawler or working gears, a torque converter and hydrostatic drive for infinitely variable speeds up to 25 km/h were available on request.
The second category included the all-terrain models in the 437.4 series (U 3000 to U 5000) presented in 2002. These were intended for fire services, the energy sector and all those who needed to transport materials and equipment on extremely rough terrain. Major design priorities included using as many parts as possible in common with the ‘implement carrier’ range, while clearly differentiating according to the respective target groups where necessary.
Model range rationalised with more focus
Accordingly both categories were equipped with the engines of the OM 904/906 LA and OM 924 LA series (110 to 205 kW/150 to 279 hp), the UG 100 transmission and identical instruments, steering and brakes. The all-terrain models went their own way with respect to the cab, however, which remained a tilting all-steel design.
Rather than permanent four-wheel drive like their colleagues in the 405 series, they were equipped with selectable four-wheel drive. A particularly high ground clearance was provided by portal axles with spur gear hub drives. The drive shafts were also enclosed within the torque tube for best possible protection under hard off-road conditions. The engine, transmission, cab and body were on three-point mountings for optimum torsional flexibility.
This reorientation, which was incidentally accompanied by the relocation of production from Gaggenau to Wörth, effected a significant reduction in the number of model variants and parts, thereby bringing a considerable improvement in cost-effectiveness. The new model series reduced the number of all-terrain model variants from 36 to 4, for example, while 46 cab variants were reduced to just 6. At the same time the number of frame variants tumbled from no fewer than 135 to just 4.
A new face: the favourably priced compact Unimog
Suddenly there was room for something completely new, and at the International Motor Show in 2006 Mercedes-Benz began a new chapter in the almost 60-year history of the Unimog. This was the launch of a new, light Unimog implement carrier with a gross vehicle weight of 7.5 to 8.5 tonnes, whose major characteristics were compact dimensions and a similarly compact cab-over-engine design – unprecedented in Europe.
The cab was adopted from the Acelo light truck, which was produced in Brazil and specialized in urban operations. The new, compact Unimog had a shortened 2700-millimetre wheelbase, and was therefore highly manoeuvrable (turning circle just under 12.8 metres). With a height of around 2700 millimetres it also passed through low entrances with ease.
The new model came with typical Unimog technology in the form of portal axles, single tyres, three differential locks, permanent four-wheel drive, and the UG 100 transmission. The mounting points for implements of all kinds were identical to those of the two classic Unimog trucks. The engine was the 115 kW (156 hp) variant of the four-cylinder OM 904. However, the new, compact Unimog dispensed with refinements such as transferable steering or extreme slow-speed characteristics, which made it considerably less expensive than the classic 405-series implement carrier.
It was not only thanks to its tilting cab-over-engine design that this compact Unimog became more truck-like, as the new target groups cited as new potential customers during its presentation showed: the new arrival was to be aimed at the building trades and the landscape gardening sector as new potential customer groups (in addition to municipalities and the energy supply industry), on the basis that ‘less is more’.
Today the extensive Unimog family is divided into a total of seven model series with three basic operating profiles. Introduced in 2002, the U 3000, U 4000, and U 5000 series with a gross vehicle weight from 7.5 to 12.5 tonnes have outstanding all-terrain capabilities and are mainly intended for the fire services (and especially forest fires), disaster relief, as basic chassis for expedition vehicles, and for servicing and maintenance work on extremely difficult terrain.
Accordingly they ideally complement the Unimog series U 300, U 400 and U 500 introduced in spring 2000, which are designed as typical implement carriers with gross vehicle weights from 7.5 to 16.0 tonnes and mainly perform their duties with public service organizations and municipal contractors, in the construction and energy sectors, and for in-house industrial transport. As a favourably priced light version with a maximum gross vehicle weight of 8.5 tonnes and a particularly good motorway performance, this remarkably versatile family has recently been enriched by the new compact Unimog.
The U 20 first demonstrates its ability in 2007
Customers had to wait until the first quarter of 2008 for delivery of the U 20. It gave a first public demonstration of its capabilities carrying summer utility implements at the demopak trade fair in Eisenach in June 2007. It later proved its abilities on snow and ice during a tour of German winter sports resorts.
The U 20 came at just the right time, for in 2008 Germany alone boasted around 12,000 Unimog units from the 424 and 427 series, vehicles built from 1982 to 2000, that were gradually approaching retirement age. With the first cab-over-engine Unimog the plant extended the weight range for the 2008 IAA, introducing a variant of 9.5-tonne gross vehicle weight.
Additional functions for Isobus, heat protection ex factory
Also at the 2008 IAA, Mercedes-Benz presented the Isobus control system introduced into the implement carrier in 2002 with an extra memory function for recording operating data. Fitted to the Unimog, the Isobus system provided a universal joystick control that allowed implements by different manufacturers to be operated using a single cockpit terminal.
In the case of the extreme off-road Unimog models from the U 3000 to U 5000 series, Mercedes-Benz presented both heat protection and mechanical protection equipment as ex factory solutions at the 2008 IAA.
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