Mercedes Benz

OFFICIAL PRESS RELEASE

Stuttgart/Worth, Germany, Apr 01, 2009

* Clear commitment to the cab-over-engine principle

* Uncompromising modular design

* All-new components

Though it was a momentous event, nobody was too surprised when Daimler-Benz followed the construction-site trucks with its new generation of road vehicles. But in fact this firmly established those particular contours that gave this vehicle generation its extraordinary format.


Customers had already become acquainted with the first V-engines of the new 400 series (256 and 320 hp) in the LP truck series at the beginning of the 1970s. In 1973 an initial range of 15 dump trucks with gross vehicle weight ratings between 16 and 26 tons formed the precursor to a wide portfolio of road vehicles which increased the number of available variants based on a sophisticated modular system to an impressive 76 different basic configurations from May 1974.

A radical new departure

This enormously wide range could only ever be introduced in stages. The cab, engines, axles and transmissions – compared to the LP series of 1963, almost everything about the Mercedes-Benz COE trucks was new. Any revitalization attempt using a refined mixture of the old concepts would not have led to very much.
Major changes were taking place in the transport sector. The previous 24-ton combination weight limit had first been raised to 32 tons, and then to 38 tons, and even higher weight limits were under discussion. It was therefore clear that 320 hp could not be the final word where engine output was concerned. And anyway, the spur gear axles used hitherto were fast approaching their technical limits in this respect. Moreover, the square cab of the LP series featured a slightly baroque architecture which gave rise to uncompetitive production costs. In addition, the poor aerodynamic showing of the four-square cab increasingly proved to be a wasted potential in terms of fuel economy. Not to mention the fact that the side windows tended to become soiled as a result.

In short, the expectations which particularly affected the heavy-duty class – where the COE principle was clearly establishing itself – were increasing in leaps and bounds. Radical action was therefore needed, especially since there was a boom in heavy-duty truck sales: the statistics show an increase in Daimler-Benz production of 153 percent in this segment between 1965 and 1973, while overall commercial vehicle sales (from 4 tons gross vehicle weight) only increased by 65 percent. 35,000 units were planned for heavy-duty truck production in 1974, which meant that in this class too, Daimler-Benz managed to become one of the major players.

Development of an international strategy

One of the means by which this peak position in an increasingly international business could be held and possibly even improved was the new generation. "The modular construction principle has substantially enabled us to adapt to the markets," is how board of management member Dr. Rolf Staelin brought the strategy into focus. The industry in general was anyway facing a fairly severe headwind which made strict rationalization indispensable. Floating exchange rates made things more complicated, and inflation reached its highest level since the Korean crisis in 1950/51. Raw material price levels soared, and the oil crisis was just around the corner.

Despite the strong demand there were clearly difficult times ahead. "To ensure that we are able to offer market-related prices while diversifying as required, the modular principle has been applied so systematically that the maximum range of models for all transport requirements has become possible with the minimum of major assemblies and components," said Chief Engineer Arthur Mischke during the presentation of the new road vehicles in Wörth on the River Rhine.

Intelligent use of shared components

All the engines in the 400 series had a bore of 125 mm and a stroke of 130 mm in common, for example. This produced an overall displacement of 9.6 liters for the new V6 unit known as the OM 401, which had an output of 192 hp. The V8 designated the OM 402 (displacement 12.8 liters) for the output class around 260 hp and the mighty 320 hp V10 with a displacement of 15.9 liters were already proving highly successful. This shared component concept allowed a significant reduction in the number of parts, which not only benefited the production process, but also made life much easier for the customer. Rather than the 1,600 components of the previous engine range, the 400 series made do with only 650.

The situation was similar for the new-generation planetary axles. Compared to the two previous axle series they replaced, these had only 220 components instead of 480. This form of rationalization had two-fold benefits: more standardized components meant larger volumes and allowed cost-cutting automation of production processes. This in turn allowed major assemblies and components to have larger dimensions with a longer operating life, as the costs involved are more easily borne thanks to rationalized production.

Planetary axles have proved their worth to the present day

As enormous as the leap from the LP series to the new generation may have been, nobody could have imagined at the time that in its basic lines this concept would only be discontinued some 23 years later – in 1996, with the introduction of the
Mercedes-Benz Actros. Even today the planetary axles newly introduced in 1973 are indispensable in the Actros construction-site vehicles by virtue of their high ground clearance, and remain highly appreciated thanks to their robustness and outstanding reliability.

The 400 engine series was only destined to enjoy such a long life (likewise until 1996) because it was given the right characteristics right from the start: "All the engine variants have thermal and mechanical reserves," Mischke summarized in 1974, "and can therefore be readily turbocharged."

This did not happen for some time, at least on a large scale. The engineers first tentatively tried turbocharging in the small OM 352, which was installed in the medium-duty variants of the new-generation trucks in 1975 and had an output of 130 hp in naturally-aspirated form, but 168 hp when turbocharged. The 400 series was only given this technology in 1980 when Mercedes-Benz carried out a first model facelift and launched the "new generation 80" (NG 80).

More muscle

Then the turbocharged OM 422 A V8 engine arrived. Its output had now risen to 330 hp, though this was based on a displacement of 14.3 liters with a bore of 128 mm and a stroke of 142 mm. In addition to turbocharging, the even more powerful variant of this V8 (OM 422 LA) made use of intercooling for the first time in a Mercedes-Benz truck, generating the formidable rated output of 375 hp. Many customers initially remained skeptical about this new technology, however, and opted for the naturally-aspirated OM 422 already introduced in 1979, which had an output of 280 hp and joined the popular 256 hp power unit.

But even with 375 hp this muscular V8 had by no means finished yet. In the mid-90s it even reached the 530 hp mark in the SK and met the EURO II standard before being replaced by the new engines of the 500 series. It did not take so long for the 320 hp V10 designated the OM 403 to meet its fate, as it was subject to severe competition from precisely this V8. From 1985 it remained in the range as the OM 423, a naturally-aspirated unit with a displacement of 18.3 instead of 15.9 liters, and it continued to have loyal friends mainly in the construction sector. For special-purpose applications – but not in standard trucks – it was also available as the OM 423 LA with a beefy 500 hp from 1984. Things then went quiet around this giant engine, which once ushered in the 320 hp era at Mercedes-Benz.

Lower exhaust emissions and fuel consumption

Four-hole injection nozzles and an air swirl in the intake duct were further characteristics of the 400 series at a time when thoughts were also beginning to turn to environmental protection. "We are already able to meet the most stringent standards in existence according to the 13-stage California Test," is how Mischke praised the exhaust emissions of these power units, whose fuel consumption was also "very good in state-of-the-art terms, and in our view ahead of the field."

This was assisted by the improved aerodynamic efficiency of the new cab with its slightly angled windshield and rounded edges, which also significantly reduced the tendency of the side windows to become soiled. And not least, the introduction of the new generation coincided with a new powertrain philosophy based initially on the familiar six-, eight- and twelve-speed transmissions but soon adopting new 16-speed transmissions and, most importantly, increasingly high final drive ratios. Whereas the previous LP had a theoretical top speed of around 85 km/h, the overall ratio of a 1632 from the year 1974 permitted a top speed of close on 100 km/h, and from 1980, ratios became another 20 percent “longer”. Transmissions with a wide ratio range killed two birds with one stone: slow maneuvering with low clutch wear in the lower gears became just as possible as convoy driving on the motorway in the highest gear, with a low engine speed and maximum fuel economy.

Completely reconfigured powertrain

Generally speaking there was hardly any limit to the variations possible with this combination of the new planetary axles and 16-speed transmissions. Theoretically the range of achievable final speeds extended from 50 to 130 km/h. In 1974 the capacity of the single axles was already a full 500 hp, and even 700 hp for the tandem axles. This was man enough for combination weights up to 100 tons.

In the case of the cab, a clear and easily produced concept replaced the previous, historically based variety in the new generation. While there were previously four different cabs for the range between 10 and 19 tons gross vehicle weight (both a COE and short-nose version for 10 to 15 tons, as well as one each for 14 to 19 tons), they were replaced by a single new, hydraulically tilting cab which could be produced with one set of stamping tools.

The short cab had been introduced with the dump trucks in 1973, and when
Daimler-Benz launched the road vehicles it was joined by the 600 mm longer long-distance cab. These did not remain alone for long, however, as there was still potential which could gradually be exploited here too: a medium-length variant followed in 1977, and in 1979 the so-called extra-large cab with 164 mm more width and 148 mm more height was added. And this was not all: in 1992 this quartet was joined by a high-roof variant which increased the standing room in the heavy-duty truck, now known as the SK, by another 640 millimeters.

Ride comfort and ergonomics as a major priority

Like the short cab, the long-distance version with two berths featured a fully paneled floor and a cab-mounted gearshift lever – when the cab was tilted, the gearshift linkage simply extended telescopically. The developers were particularly proud of this noise insulation: "We obtained measured values which were three to four dB (A) lower than those of most competitors," said Chief Engineer Arthur Mischke.

Meanwhile comfort was improved in every respect. Even though the new cab looked less imposing than the four-square truck cab, the driver benefited from a more spacious layout in the interior. The headroom increased by 80 millimeters, for example. All-round visibility was improved by a low windshield and low quarterlights in the side windows. Rounded contours at the front edges ensured that the side windows were less prone to soiling than those of the LP. The seats for both the driver and co-driver were fully adjustable, and as a new feature the steering column was variably adjustable by 40 millimeters for height and by ten degrees to the front or rear for reach.

The instrument cluster consisting of three dial instruments was in the driver’s central field of vision: on the left the tachograph, in the center the rev counter (which already had something akin to a green sector) and on the right the five gauges for fuel level, coolant temperature, oil pressure and the pressure in the newly designed dual-circuit braking system with warning light, which was designed to meet impending EC regulations. As a further innovation which was to prove seminal, the so-called combination switch extended from the steering column on a stalk: operated with the left hand, it controlled the indicators, wipers, high beams and headlamp flashers.

Passive safety greatly improved

To improve passive safety both the instrument panel and the cowling were given a foam safety lining. All handles and operating levers were made from flexible material, and the doors, door pillars, roof frame members and rear wall were likewise lined with deformable foam plastic. This soft core was surrounded by a hard shell, however: large, easily deformable stampings and a double-skinned front end gave rigidity to the interior cell. In 1974 seat belts were by no means a must in trucks, however anchorage points for the optional three-point inertia-reel belts were already included in the new generation as standard.

Immense improvement in suspension comfort

The designers took unprecedented pains with the suspension system for the long-distance cab. At the front two hairpin springs with vibration dampers and a transverse stabilizer took the sting out of poor road surfaces, while a parabolic spring and two vibration dampers did sterling service at the rear. Two pivot bearings with flexible rubber mountings at the front and damped spring struts at the rear proved sufficient for the short cab.

The two-axle roadgoing trucks of the new generation were equipped with two-stage leaf springs assisted by shock absorbers at the rear axle. The three-axle variants were even equipped with a new swing axle suspension consisting of a very soft and extremely long bogie spring plus shock absorbers and stabilizers. The first air-sprung rear axles were introduced in the new generation in 1975, for use with demountable-platform chassis.

Up-to-date for more than two decades

The increasingly numerous new generation family underwent countless modifications during the course of its long life: from 1980 it continued its successful course under the "new generation 80" (NG 80) flag. In 1988 Daimler-Benz thoroughly revamped this armada and renamed it the "heavy-duty class" (SK), until it was finally replaced by the new Mercedes-Benz Actros in 1996.

Nonetheless, at least some of the new generation’s children and grandchildren have still not disappeared entirely. The basic cab of the NG 80 series and the now more than 30 year-old engine concept of the OM 352 (130 hp) and the V10 OM 403 with 320 hp are still doing sterling service for the Serbian manufacturer FAP in Priboj, for example. At SsangYong in Korea it is unmistakably the SK cab with its low, oblique side windows that characterizes the range of heavy-duty dump trucks. And growling away beneath them is the beefy OM 442 LA V8 engine.