A history with pulling power: Mercedes-Benz all-wheel-drive vehicles
Stuttgart
Aug 02, 2011
Transport with traction: all-wheel-drive trucks since 1945
  • Beginning with the LA 3500 in 1950
  • Extreme off-road capability even for the heavy classes
  • An all-wheel-drive range without equal
Anything that might be suitable for transporting very heavy freight, especially if it had an off-road capability as well, was banned in Germany after the Second World War. The occupying powers were only too aware how suitable correspondingly designed heavy trucks were for transporting tanks. Accordingly the Allies limited the engine output of trucks to 150 hp (110 kW), while semitrailer tractors, three-axle trucks, and all-wheel-drive vehicles were likewise taboo.
These restrictions were however relaxed relatively soon, and in 1950 Mercedes-Benz was able to introduce an all-wheel-drive truck, which was not an isolated solution for the military as with previous trucks of this kind, but was basically intended to run in parallel with its less off-road capable brothers featuring conventional rear-wheel drive (drive configuration 4x2: four wheels, of which two are driven; all-wheel drive: 4x4).
A dramatic entrance by the LA 3500
This trend in all-wheel-drive trucks was started by the LA 3500 in 1950. The conventional rear-wheel drive L 3500 (drive configuration 4x2) was a major success for Daimler-Benz: this medium truck with the new 90 hp (66 kW) six-cylinder OM 312 diesel engine immediately became a bestseller, with many thousands finding their way to export countries as well. Nothing could therefore have been more obvious than to produce an all-wheel-drive version of the same.
To keep the price acceptable, the 4x4 was to adopt as many components as possible from the 4x2 version of the L 3500, and the engine, radiator, clutch, range-change transmission, rear axle, brakes, and cab, for example, were indeed common to both. This not only saved additional cost and effort in production, but also made life easier for customers with respect to replacement parts and repairs.
Like the 4x2 on-road version of the L 3500, the maximum speed of the LA 3500 (the ‘A’ stood for ‘all-wheel drive’) was 80 km/h. It is however remarkable that there was also hardly any difference between these on-road and off-road models where payload was concerned, as the 4x4 was only around 75 kilograms heavier than the tipper chassis of the 4x2. And in fact the all-wheel-drive version of the semitrailer tractor weighed 65 kilograms less than the 4x2 version.
Extremely low kerb weight has many benefits
The developers paid particular attention to the lowest possible kerb weight for their new all-wheel-drive truck, and for good reasons. The plant described the advantages in the following terms at the time: ‘Developing each individual component in the best possible form for the power flow, as well as using high-quality materials, has resulted in the extremely low weight of 2585 to 2680 kilograms for the entire chassis, depending on the wheelbase and specifications. This means a lower ground pressure, less tendency to sink in and better progress over difficult terrain than with other vehicles of the same size fitted with the same tyres.
The chassis of the LA 3500 was a very special design. The frame was made particularly flexible, as the LA 3500 needed a great deal of torsional elasticity in order to keep its wheels on the ground and provide traction at all times. Particularly long front axle springs mounted outside the frame also made sure that the LA 3500 had remarkable torsional flexibility.
It was not acceptable for the cab to be compromised in its function during the process, however, and rubber mountings at three points gave it suitably stoical characteristics. As one highly impressed truck tester reported at the time: ‘Even when the vehicle was completely distorted, it was possible to open and close both the doors of the cab without difficulty.
Two decades of all-wheel-drive know-how come to the fore
A correspondent from the specialist commercial vehicle magazine Das Nutzfahrzeug was even more impressed with the off-road performance of the new all-wheel-drive truck during a test in 1953: ‘Quite amazingly, it was possible to move off from standstill with a load of gravel weighing a good three tonnes on an uphill gradient exceeding 50 per cent [26,57°]. This was based on the plant’s two decades of all-wheel-drive know-how, which came to the fore in the LA 3500 as follows: a conventional range-change transmission transferred the engine torque (a maximum of 27 mkg at 1600 rpm, corresponding to around 265 newton metres) to the unsynchronised, five-speed transmission usual at the time.
A two-speed transfer case and a differential at both front and rear made it possible to drive all the wheels or only the rear wheels. Daimler-Benz omitted a third, inter-axle differential for a very good reason: it saved the cost of this as well as that of an additional lock. ‘The benefit of dispensing with this, the plant wrote, ‘is that if one of the axles begins to slip, the full available torque is still transmitted to the ground via the second axle.
Differential locks were simply not part of the repertoire at that time. Nevertheless, the off-road ratio selectable via the transfer case still increased the vehicle’s climbing ability considerably. While this was 27 per cent (15.11°) for the fully loaded solo vehicle in first gear, engaging the off-road gear increased it to a maximum of 43 per cent (23.27°).
Just under 3000 units of the LA 3500 produced
There was certainly a market for such vehicles. Whether because of the catastrophic condition of Germany’s roads after the war, a poor road infrastructure in the export countries or quite simply the upsurge in construction activity during the years of the German economic miracle, the plant produced just under 3,000 all-wheel-drive units of the LA 3500 between 1950 and discontinuation of the series in 1961, and just under 7,700 units of its close relative, the LA 4500. By way of comparison, almost 52,000 of the basic L 3500 (including the preceding L 3250, which was only produced for a short time) left the production lines between 1949 and 1961.
It was therefore no wonder that Daimler-Benz did not hesitate to extend the all-wheel-drive concept to other models. 1953 saw the introduction of the 4.5-tonne LA 4500, followed in 1954 by the 145 hp (107 kW) model LA 315 with a payload of around 6 tonnes and a gross vehicle weight of 13.8 tonnes, which also proved its worth performing tough military service as the LG 315. This was then joined in 1957 by the short-nosed LA 321 designed for a gross vehicle weight of 9.25 tonnes and the LA 331 export model (15 tonnes GVW). The heavy two-axle models LA 329 and LA 332 built from 1958 were mainly intended for Brazil.
Meanwhile these classic cab-behind-engine trucks were becoming less and less suitable for the European market. In Germany, for example, extremely restrictive regulations covering dimensions and weights came into force during the second half of the 1950s, giving cab-over-engine designs a strong tailwind. At the time Daimler-Benz considered the cab-over-engine concept to be a passing fad, and although it offered such vehicles in the form of e.g. the LP 315 as early as 1955, it devoted much more effort to new short-nosed designs.
Short-nosed models continue the all-wheel-drive tradition in style
First presented in March 1959, short-nosed trucks were a compromise on wheels. This is because with limited exterior dimensions, the designers were obliged to create as much space as possible for the load platform and keep weight as low as possible to ensure the maximum payload for a likewise strictly limited gross vehicle weight.
The short-nosed truck had two advantages over the cab-over-engine design, however. Firstly, many drivers felt safer behind even a short bonnet than in a cab with no crumple zone. And secondly, the engine intruded only slightly into a short-nosed cab, thus still permitting reasonable cross-cab access. This also allowed enough space for a third seat between the driver and co-driver (very popular in those days), and the engine gave off less heat and noise than in a cab-over-engine design where the cab was mounted directly above it.
A short-nosed design also had a further, not insignificant advantage for all-wheel-drive trucks used as construction site vehicles: the more forward location of the engine provided better traction, especially on steep ramps, because the front axle was less likely to lift under critical handling conditions.
A first all-wheel-drive version of the new short-nosed trucks already appeared in 1959, in the form of the medium-weight LA 1113 (also supplied to Brazil in CKD form as the LAP 1113). Two years later this was followed by the 7.4-tonne LA 710, which could still be driven with a car driving license in Germany. In the heavy short-nosed truck segment, the LA 1518, LA 1620 and LA 1920 trio of all-wheel-drive models celebrated their debut in 1964.
However, it was above all two derivatives of these, the three-axle LA 2220 and the LA 2620, that would become the epitome of the heavy all-wheel-drive construction vehicle for many years to come. A gross vehicle weight of 22 tonnes was initially permitted in Germany, and in off-road use they were capable of up to 26 tonnes.
Gigantic three-axle trucks set new standards
All-wheel-drive technology had made enormous progress. To avoid excessive tyre wear, a differential was installed in the transfer case between the two rear axles, though this was automatically locked when the low-range gearbox was selected. Pneumatically selectable differential locks for the rear axles were available on request. Selecting the low-range gearbox also activated the front-wheel-drive system. This form of drive selection ensured particularly easy operation, making errors less likely.
The drive axles used spur gear hub drives to transfer the torque. The driven front axle used by Daimler-Benz was a live axle with encapsulated constant velocity joints at the wheels. The rear axles were the ten-tonner units with a separate carrier and drive shafts usual at the time. These separate drive shafts were an unusual drive principle derived from the newly developed triple-shaft transfer case with differentials for the two rear axle drive shafts.
Climbing ability of 56 per cent
The 220 SAE hp (202 DIN hp/149 kW) OM 346 direct-injection engine was connected to the transfer case via a six-speed gearbox named the AK 6/80, which gave the vehicle a final-drive ratio of 1:7.35 and a maximum road speed of just under 70 km/h. In first gear the maximum speed was 8.5 km/h, and with the low ratio engaged (1.82) this was reduced to 4.7 km/h, however the climbing ability of the solo truck increased from 28.9 (16.12°) to 56.2 per cent (29.34°).
A number of other refinements rendered this new three-axle truck entirely suitable for any kind of terrain. The backbone of the vehicle was an extremely strong fish-belly frame, whose cross-members were riveted and whose front section was wider than the rear. Two lower and one upper, maintenance-free control arms on each side bore the drive and braking forces of the rear axles, which meant that the leaf springs merely had to cope with the transverse and load forces.
Axle suspension with extreme off-road capabilities
In fact the engineers took particular pains over the axle suspension: the twin rear axle had a centre bearing – both rear axles were supported by a robust package of leaf springs whose free ends moved in slides on the axles. This design allowed a previously unachieved upper and lower deflection angle of 13 degrees, providing full ground contact for the wheels and optimal traction even under extreme conditions. Telescopic dampers at the front axle ensured that the drive axle could be relatively softly sprung in the interests of good off-road performance, without the springs and their mountings being subjected to excessive wear and tear.
There was also more comfort than ever for the driver. The days of silent blocks as a Spartan cab suspension had long gone: these were now replaced with a transverse leaf spring positioned in the middle of the cab rear, and in conjunction with additional shock absorbers on the right and left this ‘absorbs impacts perfectly, as a truck tester succinctly put it at the time. His conclusion after a journey of several hours: ‘One cannot complain of noticeable fatigue.
The particularly high level of comfort for those days in the three-axle LA 2220 all-wheel-drive tipper was helped by other features, for example fan-assisted ventilation and heating which could be separately adjusted for the driver and co-driver. While the driver’s seat was unable to match today’s suspension seats, it had three adjustment functions and was extremely well padded. To quote the same truck tester: ‘This seat has clearly been very carefully designed and holds the driver in a fatigue-free, alert posture at all times.
Cab with an unprecedented level of comfort
Other features that helped make life pleasant in the cab included a padded sun visor, two stowage compartments in the doors, a lidded glove compartment and clothes hooks on the wall. The transmission tunnel and door linings were of noise-absorbent material, while the roof liner was of perforated plastic. A cranked steering column increased the space available for the driver’s feet, the flat steering wheel ensured good visibility and even tall drivers were in no danger of hitting their heads on the roof thanks to a generous interior height.
The era of single-line braking systems was also over. The new dual-circuit system of the LAK 2220 was not only designed to cope with the high gross vehicle weight of the three-axle truck, but even anticipated the expected new regulations of the then European Economic Community (EEC). The solution at the time: one circuit braked the front axle and the second rear axle, while the second circuit was responsible for the first rear axle and the trailer. If one brake circuit should fail owing to damage, the other circuit remained fully effective. Where trailer operation was concerned, the compressed air reservoir of the towed trailer could at last also be refilled when the trailer brake was operated. Until then, many accidents had been caused during prolonged downhill braking because the compressed air supply of the trailer became exhausted at some point, which meant that the brakes of the tractor unit could not cope with the unbraked load of the trailer for long.
Although Mercedes-Benz continuously expanded the range of all-wheel-drive short-nosed trucks (soon adding nine, 13- and 15-tonners to the heavy 16- and 22-tonne models), the writing was on the wall for short-nosed designs, at least in Europe. While short-nosed cabs still remained attractive for decades in export markets such as the Middle East and the Third World – it was only in 1995 that the last short-nosed truck left the production line in Wörth – the European markets were increasingly demanding full cab-over-engine designs from the 1960s onward.
Clever interim solution for all-wheel drive and full forward control
While there were a few variants of the 1963 cab-over-engine trucks with the so-called cubic cab on offer for on-road and medium-heavy construction site operations, presentation of the succeeding ‘New Generation’ was planned for as early as 1973, and in-house development of all-wheel-drive variants of these trucks was not worthwhile. The plant therefore came up with an interim solution: from 1970 it simply equipped the already available cab-over-engine models produced by Henschel, in which Daimler-Benz had held a 51-per-cent interest since 1968, with new V-engines from the modular OM 400 series and the newly developed planetary axles. In this way it was still able to offer all-wheel-drive trucks with full forward control.
The new V-engines were only available for all-wheel-drive trucks as V10 units with 235 kW (320 hp). This high output was transferred by an eight-speed transmission with an additional crawler gear. The frame was mounted on U-section longitudinal members offset outwards at the front. The small differential made possible by the new planetary axles increased the ground clearance to a level never before seen in a tipper.
Henschel-Mercedes all-wheel-drive tippers designated as the LAPK series was a banjo axle with a pinion and crown wheel drive and external planetary gears. The rear axle, e.g. of the two-axle LAPK 1632, was constructed on the same principle but also had a pneumatically operated inter-wheel lock. The transfer case was the familiar triple-shaft unit which prevented drive train distortion. The cab was tiltable, which allowed easy, rapid access to the components beneath it.
These all-wheel-drive tippers with a Henschel cab and Mercedes components only represented a brief intermezzo, which began in 1970 and came to an abrupt end in 1973, for that is when Mercedes-Benz presented the vehicles of its legendary ‘New Generation’. And it was not the on-road variants but the construction vehicles that ushered in the changeover.
New Generation headed by the construction vehicles
The plant presented the new construction vehicles as a ‘rational concept’ that was based on a sophisticated modular system and thereby made two things possible: ‘Flexible adaptation to the markets, according to Board of Management member Rolf Staelin, and ‘market-related prices, to cite head of development Arthur Mischke. Mischke: ‘The modular system was applied so systematically that the minimum number of assemblies and parts made the maximum number of models available for all transport requirements.
All the engines of the 400 series, for instance, had a bore of 125 and a stroke of 130 millimetres in common. This resulted in a 9.6-litre displacement for the new V6 unit known as the OM 401, the smallest of the V-engines with an output of 141 kW (192 hp). The output class of 191 kW (260 hp) was catered for by the 12.8-litre V8 already well established in the LP series, as well as by the mighty V10 with 235 kW (320 hp) from a displacement of 15.9 litres. This commonality of parts also made for a considerable reduction in the number of different components, which not only benefited production but also made life easier for customers. Instead of the 1600 parts required by the previous engine range, the 400 series only needed 650, for example.
The same particularly applies to the new planetary axles of the New Generation, with which not only the construction variants but also the road-going vehicles introduced in 1974 were equipped, and which are still performing admirably in today’s heavy Mercedes-Benz construction trucks (the Actros, Atego and Axor series). Compared to the two conventional axle series they replaced, the new planetary axles made do with just 220 parts rather than 480.
This rationalized approach had two advantages: more standardized parts meant larger production volumes and cost-reducing automation of production. This in turn made it possible to increase component dimensions for greater durability. This is why the new planetary axles introduced in 1973 are not only an indispensable feature of today’s construction vehicles because of their high ground clearance – they are also highly appreciated for their great robustness and reliability.
Extensive all-wheel-drive range right from the start
The New Generation was ready with all-wheel-drive variants right from the start. Two-axle vehicles were available with 125, 181 or 235 kW (170, 260 or 320 hp), and Mercedes-Benz offered three-axle variants with the now permissible gross vehicle weight of 26 tonnes and outputs of 181 and 235 kW (260 and 320 hp). All-wheel-drive medium trucks with gross vehicle weights of 10, 12 or 14 tonnes and an in-line six-cylinder engine generating 96 or 125 kW (130 or 170 hp) followed in 1975. The 16.5-tonne 1719 still featured a selectable front axle drive, while all the other all-wheel-drive variants now had permanent front axle drive.
The all-wheel-drive variants retained the tried and tested fish-belly frame and a 13-degree rear axle swing angle. The rear axles were now equipped with two-stage leaf springs and in the three-axle variants the spring bracket and connection to the frame were more robust. This improved swing axle ensured a particularly smooth power flow and favourable power transfer. The all-wheel-drive front axle not only allowed a particularly high ground clearance, but also a previously unprecedented steering angle of 42 degrees.
Whereas the engines of the exclusively rear-wheel-drive trucks were recessed into the frame, the designers generally suspended the engines and transmission blocks of the all-wheel-drive variants separately in the frame. This enabled them to achieve the smallest possible prop shaft angle. The engine mountings were however identical for all the vehicles of the New Generation: at the front Mercedes-Benz used two large, inclined mounts with limited travel, and at the rear two wedge-type mounts anchored fore-and-aft with limitation of lift.
Synchromesh transmissions for easier gear shifting
The all-wheel-drive variants were now also equipped with synchromesh transmissions as standard. ZF 5S–1106 PA was the name of the eight-speed transmission in the 2626 AK and 2632 AK, for example. To prevent shifting errors this transmission featured an electronic/pneumatic lock. It was connected to a transfer case with a lockable differential, which distributed one third of the torque to the front and two thirds to the rear. A second transfer case was connected to the first rear axle (direct-drive axle) and distributed the drive torque to the two rear axles. A differential lock between the rear axles was standard, with an inter-wheel lock available on request.
Customers had a choice of three final-drive ratios for top speeds of 75, 85, and 95 km/h. The vehicles of the New Generation were braked by pivoted brake shoes with a drum diameter of 410 millimetres, which were used on all the axles. For the all-wheel-drive variants Mercedes-Benz also threw in a further development of ALB (automatic load-dependent brake force control), which now also acted on the front axle brakes rather than just the brakes on the rear axle(s).
While the swing axle principle of the New Generation all-wheel-drive models was already known, Mercedes-Benz introduced a completely new cab suspension system: at the front the cab was mounted on two pivot bearings with flexible rubber bushings. At the rear the cab was softly sprung on dampened spring struts for low vibration. The particularly high tilt angle was a welcome addition for workshop personnel, as it afforded very easy access to the engine and other units.
The driver was easily able to carry out daily checks via flaps in the front end. Peace and quiet in the cab was ensured by fixed installation of the gearshift: when the cab was tilted, the steering and gearshift linkages extended telescopically. This enabled the shift lever to have a fixed place in the cab, which was very effectively insulated against noise, heat and cold. The cabs of the New Generation also offered more passive safety than ever before.
The naturally aspirated engines show plenty of staying power
While particularly powerful engines with turbocharging and intercooling began to be introduced into the New Generation 80, this technology was not granted to the construction vehicles for the time being. Nonetheless the V10, which had now been brought to a displacement of around 18 litres, at least gave the 1936 A – built from 1980 – an output of around 265 kW (360 hp). By way of comparison, the turbocharged eight-cylinder OM 422 A generated no less than 243 kW (330 hp) with peak torque of 1,402 newton metres in the road-going truck, and the technicians were already obtaining 276 kW (375 hp) and peak torque of 1,550 newton metres from the intercooled OM 422 LA V8-engine. From 1980, however, the 1938 AS arrived as an all-wheel-drive semitrailer tractor equipped with this, the most powerful of all Mercedes-Benz V8s.
After the major model facelift in 1980 (New Generation 80), the new, standard engine for the always conservative construction sector was the naturally aspirated OM 422, which had been uprated to 206 kW (280 hp). It was with this engine that Mercedes-Benz offered all-wheel-drive tippers, semitrailer tractors, and chassis for special-purpose bodies.
The legendary 3850 AS heavy-duty tractor unit
Equipped with a turbocharger and intercooler, the V10, which was now mainly used on construction sites and had found its niche as a naturally-aspirated heavyweight, was awarded particularly honours from 1984 onwards. It now became the potent 368 kW (500 hp) driving force for the now legendary, all-wheel-drive, three-axle 3850 AS heavy-duty tractor unit, which offered spectacular peak torque of 2,000 newton metres.
This vehicle was easily capable of coping with a 220-tonne gross combination weight, and was only available with a torque converter clutch and the new wide-body cab, which the plant laconically described as follows: ‘Even the external appearance confirms the progress made: 16.4 centimetres wider, eight centimetres higher, headroom 168 centimetres.
Spectacular 8x8 four-axle units
As a similarly spectacular highlight, all-wheel-drive construction vehicles with an 8x8 axle configuration appeared in 1987: these four-axle behemoths were even capable of following a Leopard tank over rough terrain if necessary. As the model 3528 AK or 3535 AK for a 35-tonne gross vehicle weight, they were available with either a naturally aspirated or turbocharged 14.6-litre V8 and were the crowning glory of the relatively recent four-axle truck species in Germany.
Such vehicles required special operating permission, as the road transport regulations categorized four-axle trucks as vehicles with ‘excessive road use’. Nonetheless, construction trucks with four axles were soon nothing unusual, as a loophole in the law was discovered: it was perfectly legal to add a further axle to a three-axle truck and call it a trailer – without a drawbar, but with its own registration plate.
In the late summer of 1981, the trailer manufacturer Theodor Meierling had presented one of the first of these so-called coupled combinations as a loophole-exploiting four-axle unit with a gross vehicle weight of 32 tonnes, and these quickly found favour with customers. In 1984, the lawmakers responded by allowing genuine four-axle units (with two steered front axles) with a gross vehicle weight of only 30 tonnes, and the 32-tonne coupled combinations were no more.
The four-axle truck only became really interesting for most customers in 1986, when the law also allowed genuine four-axle units a gross vehicle weight of 32 tonnes, which meant that payloads of 17 to 18 tonnes became possible for tippers. Just one year later Mercedes-Benz was even able to offer an all-wheel-drive version of the four-axle unit, though like the other four-axle trucks it was initially built by Arbon & Wetzikon (NAW) in Switzerland. It was only in the 1990s that four-axle production was relocated to Wörth.
A 324 kW (440 hp) engine heads the SK all-wheel-drive range
By this time, the heavy-duty all-wheel-drive trucks were members of the heavy class (SK), which had begun to replace the New Generation 80 from 1988. All-wheel-drive technology underwent few changes, although the engines became increasingly powerful. The maximum output available with the 14.6-litre OM 422 LA was 324 kW (440 hp) and peak torque of 1,900 newton metres for the 8x8 and 6x6, and for the 4x4 construction vehicle a new 12.8-litre V8 topped the range with 279 kW (380 hp) and peak torque of 1,775 newton metres.
In 1997, Mercedes-Benz presented the Actros series of construction vehicles as successors to the SK, and once again there was a full range of all-wheel-drive variants from 4x4 to 8x8. The new series continued to use the model VG 1700 and VG 2400 transfer cases, however the Actros construction vehicles were now equipped with a hydraulic/pneumatic rather than a mechanical gearshift as standard, which reduced the number of variants. Four standardised connecting lines from the cab to the shift cylinders in the transmission replaced the previous, large number of conventional, mechanical linkages.
Off-road EPS in the Actros construction vehicles
As an alternative, Mercedes-Benz also offered the new Actros construction vehicles with a version of the semi-automatic EPS system (Electronic Power Shift) specially modified for construction site use. Its major attribute was considerably faster gearshifts, for when the driver engaged the inter-axle lock; the EPS performed much more rapid gearshifts at least in the lower range group. In conjunction with differential lock activation, this also increased the possible EPS preselection time from ten to 30 seconds.
As a titbit for the construction sector, the new Actros cab was available with an optional ‘construction package’ consisting of oilskins, a helmet and hammer bracket, and a robust step mounted outside the vehicle which allowed much more convenient inspection of the tipper mechanism.
The underbody had also been greatly modernized: instead of the heavy, relatively hard trapezoidal springs, Mercedes-Benz equipped the new construction vehicles with maintenance-free parabolic springs on all the axles. Hydraulic shock absorbers all round did the rest to ensure a high level of comfort.
Clever front axle load compensation for the Actros four-axle range
The developers were particularly proud of their unique new front axle load compensating system for four-axle units, which allowed full axle load compensation between the two steered front axles up to an obstacle height of 100 millimetres. When driving over an obstacle, a combination of swivelling levers and pull rods initiated a vertical shift of axle position, thereby obviating any undesirable change in axle load. This improved off-road ride comfort, provided better traction and prevented over braking of an individual axle. It also reduced wear and tear to the components, as well as tyre wear.
It was specifically the all-wheel-drive variants that were denied disc brakes, however. They had to make do with drum brakes all-round, as Mercedes-Benz considered the risk of damage during hard off-road operations to be too great. Nonetheless the drum-braked all-wheel-drive Actros models were equipped with an electronic brake control system.
The new all-wheel-drive trucks were also characterized by the latest technology and maximum customer benefit in other respects: weight was optimized by the use of high-strength steel for the side and cross-members of the frame, whose design provided extreme flexural rigidity despite great torsional flexibility. Additional strength was added by load distribution plates which specifically strengthened the frame in highly stressed areas. The new construction Actros also featured a wealth of low-maintenance and maintenance-free components. One of many examples was the use of molecular rubber bearings for the springs. This is one of the reasons why it was possible to dispense with a central lubrication system.
An all-wheel-drive range second to none
Following the Actros model facelift in 2002, whose major feature was an extremely luxurious and uncompromisingly driver-friendly cab; the heavy trucks with all-wheel drive were in a stronger position than ever before. The 8x8 flagship of the Actros all-wheel-drive fleet was available with a gross vehicle weight of up to 41 tonnes and 375 kW (510 hp) from a 16-litre V8 engine (complying with the Euro 4 or 5 emission standard), for example, and the 6x6 Actros models were also to be had with a maximum output of 375 kW (510 hp). Up to 353 kW (480 hp) was even available for the 4x4 variants.
Yet the range of Mercedes-Benz trucks with all-wheel drive is by no means restricted to the Actros series. Customers needing a little less are well served by the weight-optimized Axor series of heavy trucks in the form of the all-wheel-drive, two-axle models 1823, 1828, and 1833. These are powered by 169, 206, and 243 kW (230, 280, and 330 hp) in-line six-cylinder engines from the 900 series. The medium-size Atego is available as an all-wheel-drive two-axle truck with gross vehicle weights from ten to 15 tonnes. These two-axle units are likewise powered by four and six-cylinder engines from the 900 series, with outputs from 132 to 206 kW (180 to 280 hp).
Prioritising customer benefits: construction variants of the Actros 3
The third-generation Actros construction variants were presented at the 2008 International Commercial Vehicles Show (IAA) and were better equipped than ever for the harsh environment of the construction site. The all-wheeler retained the familiar configurations of 4x4 to 8x8. But the new construction Actros featured, for example, a new protective plate to protect the oil sump, engine and radiator. Made of four-millimetre thick, high-strength stainless steel, this protective plate protruded a good hand-width from the substructure like a jutting chin and curved upwards into the front end of the new construction Actros almost like a front spoiler.
Then there was the pivoting step bar, which attractively wrapped around this front-end ensemble like a kind of mini cowcatcher. Triple beading in the protective plate itself provided visual relief – coordinating perfectly with the three ventilation slots in the radiator grille a level above. The vehicle’s overall appearance was a highly muscular one – as was only apt for a construction vehicle.
Other new features included the highly robust headlamp grilles, which now came in black steel rather than plastic as before. However, a quick-release mechanism on the grilles ensured that a sponge could find access without difficulty. For potential threats from the rear, the Actros 3 construction versions were also well equipped with steel-grilled tail lights. And adversity from above, such as falling concrete, was kept at bay by protective steel covers over the tail lights.
Extra protection for the tank
An additional optional protective plate – simple to remove, clean and refit – served to repel attacks on the ever vulnerable fuel tank. The Actros 3 construction variants also benefited from a new one-piece handrail routed across the cab roof, which complemented the optional entry step on the driver’s side of the vehicle more fully than was previously the case, the hand now knowing exactly where to find a rearward hold. Of entirely new design, too was the first rung of the entry step, which gave way not only to longitudinal but also to lateral pressure.
‘Pivoting entry step’ was the name given to this new part, which was designed for retrofitting even to older designs of Actros trucks. One practical feature was that in the event of damage occurring to the horizontal step plate, only the plate itself required changing rather than the entire unit.
Reinforced mirror housing, further upgraded interior
The new mirrors of the Actros 3 construction version were also particularly well equipped against damage. The housings were shrouded in coarse ribbed and impact-resistant plastic to reduce the likelihood of unsightly scratches left by the whip of branches or other such hardships.
As far as the interior was concerned, the construction Actros kept abreast of its road-going brothers – although carpeting was generally a little more robust. This could be seen, for instance, in the new instrument panel, which now also featured four elegant chrome-surround dials, and in the introduction of sun blinds both for the front windscreen and for the first time the side windows.
Another feature was the new, special and absolutely standard Actros rubber mat such as was produced for Actros 3 road vehicles. The construction vehicles also provided an optional folding table on the passenger side, ideal for serving up a quick lunch or dinner.
Innovative features for the all-wheel-drive truck: rain sensor and automatic transmission
Nor was there any shortage of detailed refinements, such as the new compressed-air gun for rapid cab cleaning, or the light and rain sensors, although these were only available in combination with tinted windows. As soon as circumstances required, the sensors automatically activated dipped lights or windscreen wipers.
And the new construction Actros made use of the automated
12-speed transmission from the PowerShift family – not as standard, as with the road-going vehicles, but with special software and hardware for construction site use. The highly robust and durable, forged-steel linkages, for example, were specially designed to cope with tough conditions off-road.
Power-off-road mode made its debut in the vehicle software, designed to guarantee ultra-fast shift times and exactly the right bite for difficult surfaces. Two rapid reverse gears provided the wherewithal to speed up operations where turning manoeuvres were not possible (for example, in tunnels). And Rocking mode would be of particular benefit on construction sites, where loads often literally become bogged down.
Vertical exhaust pipe with height and direction adjustment
And that was not all. Tractor units and pickups were now equipped on the rear wall with a new optional floodlight, which was mounted centrally and provided strong, bright illumination for an area measuring ten metres by five metres. Also positioned directly behind the cab was the new vertical exhaust pipe concept: two- and three-axle vehicles now came with a stainless steel exhaust, which was fully adjustable in terms of height and direction of exhaust flow.
Mercedes-Benz also supplied the new tipper trucks in an additional version specially designed for road construction use. The so-called road construction packet included, for example, a high-mounted tail light, side-mounted rear floodlights, and shortened wings with folding splash guards.
Heavy-duty truck for demanding off-road use: the Mercedes Zetros
In addition to the construction variants of the Actros 3, a further highlight of the 2008 IAA was an entirely new representative of the off-road family, the new all-wheel-drive Mercedes-Benz Zetros. This not only added a very special off-road variant to the all-wheel-drive truck portfolio, but was also seen as a continuation of the Unimog, with a focus on transporting particularly heavy loads.
The Zetros came exclusively with all-wheel drive and was available with both two and three axles. The payload of the two-axle version (1833 A 4x4) ranged from four to six tonnes, and from seven to ten tonnes in the case of the three-axle version (2733 A 6x6). The all-wheel-drive basis was borrowed from the Zetros’ civilian colleagues, the Actros and Axor.
Made-to-measure cabs: layout conceived for air transport
The low height of the specially designed cab, which made only partial use of existing designs from the corporate construction kit, provides an immediate clue as to the purpose for which the Zetros was conceived.
The interior was taken from the global cockpit of the Axor, the doors came from the Unimog. But otherwise the layout was all-new and specifically designed for various modes of transportation by rail and air. The vehicle fit without any problem into transport aircraft such as the Hercules C 130 or the Transall C 160. In addition, dimensions complied with measurements internationally prescribed for rail transport.
None of this was by chance. After all, the vehicle was designed not only for use in disaster relief, rapid response with forest fires or the energy sector, but also for deployment by international peacekeeping missions, for example.
Form follows function: Renaissance of the cab-behind-engine design
This was the reason behind the renaissance of the cab-behind-engine design, unique nowadays among European trucks. This design not only created extra space in the cab for passengers, it also lent itself in particular – and without any great risk of overloading the front axle – to armouring the vehicle with a view to protection from sudden mine detonations or ballistic attacks. Circumstances in Iraq or Afghanistan, for example, have meant that the demand for vehicles like the Zetros has continued to rise in recent years.
Moreover, when it comes to repairs and maintenance, the bonnet of a cab-behind-engine design can be opened in less time than it takes to tip a traditional COE cab. One important consideration in extremely cold climes, where temperatures of minus 50 degrees Celsius are not unknown, is that the crew can remain in the cab while the maintenance team get to work on the engine.
Typical design, practical technology
The clever thing about the Zetros, whose distinctive radiator grille shows clear similarities with the current range of off-road vehicles with the three-pointed star, is that the corresponding cab-over-engine design could be built using series-produced items. Not only did the cab interior come from the global cockpit of the Axor, for example, so too did the steering. And the Axor also supplied the engine, the 243 kW (330 hp) six-cylinder from the 900 series.
The axles of the Zetros made use of the tried-and-tested HL 7 planetary axles dating back to the New Generation, and which even today remain a key feature of heavy-duty construction trucks. Brakes and suspension were as rugged as the conditions for which the trucks were intended: leaf springs on all axles and drum brakes stood for technology that was as uncomplicated as it was robust.
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