The long road to the rear-mounted engine
17.11.2011 - 01:00

German version

The long road to the rear-mounted engine
  • Bodies made of steel enhance safety
  • Semitrailer-type buses for maximum 170 passengers
  • The first bus with a rear-mounted engine debuts in 1951

Mercedes-Benz N1 from 1927 with construction of Kässbohrer

After the merger that created Daimler-Benz AG, the letter “N” stood for vehicles with low frames, which in almost all cases were buses. But at the same time there were also trucks with low frames, for instance the N 5 model, a five-ton truck launched in 1928.

Three basic models made up the first post-merger range of buses: N1 stood for the 16-passenger bus with four-cylinder M 14 engine. The N2, which used the six-cylinder M 26, was designed for 26 passengers. N5 in turn referred to the big flagship of the period, which offered space for a maximum of 60 passengers and was powered by the four-cylinder M 5 engine.

Of course, this wide-meshed range did not suffice in times of decreasing business activity to secure an adequate volume of orders for the factory. As early as 1928 a number of new models were added and the existing range was modernized.

Wood gives way to strong steel in the bus

One aspect of this modernization was that steel soon made itself useful in bus bodies in place of wood. For passenger transportation, in 1930 Daimler-Benz already was offering a new all-steel body that made the vehicles sturdier, safer and yet lighter. Until then, wood had been the preferred material of bodybuilders. Step by step, from 1930 on Daimler-Benz introduced a framed steel structure to supplant wood, and in doing so anticipated today’s designs: on a chassis with a low frame the bodybuilders placed a delicate-looking steel framework consisting of channel-shaped pressed steel ribs, which in combination with cross members and longitudinal members created a kind of cage. To strengthen the connections the designers used so-called gusset plates. Rivets durably joined this meshwork, and rivets served to attach the panels to the body afterwards.

Mercedes-Benz O 4000 with diesel engine and steel construction, 1930.

The new design combined numerous advantages. A body like this weighs less than the wooden frameworks with metal paneling used up until then; the stability also profited from the new architecture. Spectacular photos drastically underscored the high load-bearing capacity of the new body, as documented by one showing a considerable number of employees of the Sindelfingen body plant gathered on the roof of the new bus. As steel, unlike wood, does not splinter, the all-steel construction went hand in hand with higher passive safety: “Since in collisions injuries caused by splinters of wood are impossible,” one contemporary brochure summed it up.

Roof loading of the Mercedes-Benz Lo 2000’s steel body, 1932.

Additional badges on the buses promoted the new technology. They read: “Steel Body Daimler-Benz AG Sindelfingen.” Though the chassis of the buses came from the Gaggenau plant at the time, Sindelfingen was responsible for the body – since 1928 Sindelfingen served as bus plant of Daimler-Benz. The large buses took the lead in the transition to all-steel bodies; the smaller models followed during the next few years.

In 1935 Daimler-Benz summed up the results: “For the smallest and the biggest buses and for all-weather vehicles, all-steel bodies are the choice because this design has shown itself to be superior to wood structures in every respect.” In practically no time at all the factory had changed over its entire program.

Semitrailer-type buses come in fashion in the 1930s

The LZ 4000, LZ 6000 and LZ 8000 tractor trucks built starting in 1933 were very popular not only for transporting freight, but also passengers. The advantages of the tractor/semitrailer combination in freight transportation resulted (and still do today) from the decoupling of tractor and load-carrying unit. On the one hand, this makes a very favorable ratio of combination curb weight to payload possible. On the other hand, the distributing of the load among three axles permitted advancing into payload regions which had appeared impossible until then in view of the low weight of the means of transportation itself.

In particular, the tractor/semitrailer combination also cost little in taxes. To calculate tax, the authorities only included the weight of the (unladen) semitrailer which rests on the axles of the tractor unit. What the axles of the semitrailer of the empty combination have to carry is simply disregarded. On top of that, the tractor/semitrailer combination permitted dispensing with staff which was absolutely required for many a drawbar trailer combination: “We should especially emphasize,” the advertising of that period stated in a somewhat laconic style “that operation of the tractor is handled by the driver alone, i.e., ONE-MAN OPERATION.”

To top it all, compared with conventional drawbar trailer combinations the tractor/semitrailer combination afforded much greater flexibility. After all, demountable platforms as are common mainly in dual-mode transport today were not yet a topic in those days. So anyone in those days who planned his transport operations in the form of a shuttle service or wanted to make use of different bodies could not get around the tractor/semitrailer combination: it alone enabled switching between different body concepts or simply parking the load carrier for loading or unloading and continuing to make productive use of the tractor during that time – even for passenger transportation, if need arose.

Tatzelwurm: space for 170 passengers

The smart practice of alternating between freight and passenger transportation with one’s tractor quickly took root in those days. During the economic boom of the 1930s the population acquired a mobility which the manufacturers of mass conveyances no longer could satisfy with their old concepts. Tractor/semitrailer combinations designed as gigantic large-capacity buses and called Tatzelwurm (a mythical reptile of Alpine provenience) stepped into the breach. The biggest of its kind reached the impressive length of 18.7 meters and could carry no less than 170 passengers including standees.

tractor/semitrailer combination of Kässbohrer with L 6500 tractor, 1936.

As time went on, the three-member basic range of 1933 was joined by an additional model which can be viewed as a consistent development towards higher payloads. The new tractor LZ 10000 shown by Daimler-Benz at the 1938 Berlin auto show was designed for a total payload of ten tons – ten tons, with a combination weight of only 17 tons and a chassis weighing easily 3800 kilograms. Compare this: the three-axle flat-bed truck L 10000 without body already weighed 7400 kilograms. Under its hood the LZ 10000 still had the lean six-cylinder from the Lo 3500, which had meanwhile been upgraded to 100 hp and 7.3 liters displacement with a ten millimeter bigger stroke and a five millimeter smaller bore.

As a side note, in response to the quickly progressing expansion of the Reichsautobahn system, from 1935 on stylish, futuristic design studies for new touring coaches of unprecedented dimensions quickly were produced. Daimler-Benz, Büssing-NAG, Henschel, MAN and Vomag approached the public with impressive plans for gigantic large-capacity buses. Daimler-Benz presented a model of one such vehicle at the 1935 Berlin auto show. The MB 805 V12 diesel previously used in locomotives was supposed to give this vehicle a particularly strong heart; a proper engine room was provided for it at the back of the bus. Under discussion were outputs of 350 and 450 hp. However, the Second World War put an abrupt end to these pipedreams.

Mercedes O 3200, 1937

First postwar bus arrives as early as 1948

After war’s end the want and need were great and no one could waste a thought on futuristic vehicles such as this. But in March 1948 Daimler-Benz already managed to introduce the new O 4500 bus. This vehicle took up the traditional cab-behind-engine design, but integrated it into an aesthetically extremely pleasing overall design.

Mercedes-Benz O 4500 bus at Stuttgart main station, 1948.

As if they knew their time would soon be up and the forward-control or cab-over-engine vehicles couldn’t be stopped, the long-nosed buses of the postwar days flaunted especially elegant designs. More akin to a coquettish snub nose than a powerful hood, the conventional front end of the vehicle fitted into an overall picture which seemed to express mainly one thing: verve. The stylistic elements serving to create this impression were an arched roof, a rounded rear end, and a lateral line which lightly and elegantly rose like a treble clef from hood to window area and then seemed to bow down in an agreeable way at the rear of the bus.

But design as an end in itself is something no one could have afforded in the hard times after the war, when the car played practically no role and bus and rail were responsible for mobility. In part a certain economical calculation was at the root of the novel, light design of the O 4500, because key raw materials still were rationed.

True, as early as 1949 General Director Wilhelm Haspel was able to state it was clearly evident “that the problem of the available iron” soon would belong to the past. But the builders of the O 4500 and the O 5000 which soon joined it made economical use of steel for good reason, practicing systematic lightweight design. The side walls, for example, were nowhere more than 50 millimeters thick. Nevertheless, the O 4500 and O 5000 impressed with “remarkably high vibration fatigue limits,” as a description from those days underlines.

“Rigorous efforts were made,” the same text continues, “to arrange and design all load-bearing elements so that clear loading ratios resulted and torsional strain has been almost entirely eliminated.” The arched roof had two different functions: One is that it provided a tremendous amount of standing room in the center aisle – the headroom was 2050 millimeters. But it also gave the vehicle added strength which could not be attained with a strictly cubical design.

Interior spaciousness as never before

Despite a relatively slim overall width of 2460 millimeters, which made the bus fit “to drive on narrow roads,” the engineers were able to design a great deal of spaciousness into the interior. A clear interior width of 2250 millimeters easily permitted placing five comfortable seats next to each other at the rear of the bus. And the center aisle also was worth an extra look: “Extra wide,” the factory pointed out, “on a scale you can never hope to find anywhere else, taking the usual twin seat configuration as a basis.” Not only lightness and verve, but a certain degree of spaciousness too was quite en vogue in the postwar years.

“Extra wide” was how the plant proudly described the generous centre aisle of the O 4500.

Demands for comfort also began to be heard again. The customers appreciated the painstaking care which Daimler-Benz applied to the heating and ventilation of the O 4500 and O 5000. A ventilation switch in the standard-fit fresh-air heater, for example, was one of the many small details which distinguished the O 4500 and its big brother, the O 5000. In addition, there was a special duct which sent warm air across the windshield and prevented it from becoming fogged or iced up.

Air conditioning for the warm time of the year was still out of the question in the late 1940s, instead this postwar model series featured an ingenious ventilation system. “Sensibly arranged flaps and ducts conduct fresh air, draft-free, into the interior of the vehicle,” a contemporary description praised the mode of operation of the system, which continuously circulated the air inside the bus. Two large air scoops on the roof permitted air to “wash around the inner ceiling without creating a draft.” These devices were supplemented by more rustic and certainly not entirely draft-free ventilating means such as a crank-operated window on the driver’s side plus three sliding windows and a ventilator window at the rear of the bus.

As touring coaches, the two variants were designed for 39 to 47 passengers. The urban bus variant could carry a maximum of 60 passengers.

The O 4500 provided urgently needed mobility in post-war Germany. - 1948

A proven foundation for frame and floor

For the chassis of models O 4500 and O 5000, on the other hand, Daimler-Benz relied on proven components of many years standing. The Gaggenau factory supplied the chassis-cum-cowl, which was closely related to the low-frame truck chassis, much in the tradition of prewar buses. The engines of these first postwar buses also were the equivalents of the engines in the corresponding truck models, L 4500 and L 5000, which both used the proven prechamber diesel engine OM 67/4.

This engine originated in the 1930s and, as six-cylinder variant, was a welcome supplement to the legendary 3.8-liter unit OM 59 introduced in 1932, which, as first production diesel engine in a light truck, really made the diesel popular in the commercial vehicle. The technical data for this engine list “120 hp maximum output” and “112 hp continuous output,” which propelled the O 4500 to a top speed of 62 km/h and the O 5000 usually to a speed of 65 km/h, but could even push it to 75 km/h with a special gear ratio.

However, this first little postwar bus family was not destined to live a long life. From spring 1948 to autumn 1950 a total of 649 units left the Sindelfingen plant as complete buses. Including the units supplied to customers as chassis, the production figure for the O 4500 and O 5000 buses was exactly 770 units. Even though their conventional cab-behind-engine design and low-frame construction marked them as members of a species which was gradually nearing extinction, and they would have to leave the field to the forward control vehicles and the self-supporting body design which soon came into fashion, they did have one thing to their credit: making a fresh start, they pointed the way forward in a difficult period and effectively provided urgently needed mobility in their simple, yet elegant way.

In the truck, the engine moves underneath the cab; in the bus, to the rear

As with the trucks of Mercedes-Benz, in the case of the buses it was mainly export customers at first who pressed for forward control variants. From the introduction in summer 1950 of the conventional hooded bus O 6600, derived from the L 6600 truck, it took exactly until spring 1951 to give the O 6600 a little flat-nosed brother. The O 6600 replaced the previous five-ton bus O 5000 in 1950 and had a 145 hp diesel engine from the OM 315 series. This last big cab-behind-engine model from Daimler-Benz was offered for sale until 1955 (from 1954 under the model designation O 304) and attained a production volume of 625 units.

The O 6600 long-nosed bus was launched in the summer of 1950.

The new forward control bus of 1951 was called the O 6600 H. It had a length of eleven meters and an OM 315 engine transversely mounted in its rear end. Further features of this highly up-to-date bus: electrically shifted ZF Media six-speed transmission, bolted-on lightweight steel body, rubber-mounted anti-roll bars. People were surprised.

The O 6600 H was the first ever Mercedes bus to feature a transverse-mounted rear engine.

Despite its high price, the O 6600 H met with a relatively good reception. But it remained an outsider of its class, as did the O 6600 T that soon was added, a trolleybus whose electric motor drew its juice from an overhead wire. During the 1950s it was very much the fashion in German and foreign cities to replace streetcars with trolleybuses. The O 6600 T (together with its successor, the O 320 T) managed a total production run of 364 units (including two chassis for out-of-house bodies). For comparison: the overall production figure for all other variants of the 6600 bus family together was 2560 units.

New Objectivity confronts Late Rococo: Mercedes built the cab-over-engine O 6600 H from 1951 on.

Mercedes O 6600 T, export model for Argentina 1953.

Photos and text:
Daimler AG


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