Resounding success: Mercedes-Benz O 321 H/HL bus (1954 to 1964)

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Daimler Press Release  /  December 5, 2019

• Premiere of the success story that started 65 years ago
• Comfort and driving safety on par with that of a passenger car
• Reliability, robust design and an unrivalled service network
• Pioneer of the company’s internationalisation strategy at the time

Stuttgart. Its impressive overall design concept made the Mercedes-Benz O 321 H/HL bus ─ which was unveiled 65 years ago ─ a resounding success. And the production figures provide solid proof of this: from October 1954 until December 1964, what was then Daimler-Benz AG manufactured a total of 18,083 buses, chassis and ckd parts kits for the O 321 H/HL (“ckd” stands for “completely knocked down”; parts kits for assembly in export countries). The competition ─ particularly in Europe ─ simply could not keep pace. The O 321 H/HL also ultimately turned Mercedes-Benz into a leading bus manufacturing company.

The innovative concept employed laid the foundation for the vehicle’s success. To this end, the O 321 H/HL brought to the table what municipal, regional and interregional carriers as well as private companies were looking for. This not only gave Mercedes-Benz a considerable edge in Europe, but also paved the way to entering large key foreign markets such as Latin America.

The O 321 H/HL became a very popular export item and an ambassador of the brand; furthermore it underscored the meaning of quality German workmanship all over the world. The exceptional starting point this generation of buses from Mercedes-Benz signified when it came to internationally positioning the company in the bus segment is also backed by the production figures of its even more successful successor model. From March 1965 until June 1976, no fewer than 32,281 O 302 buses, chassis and ckd parts kits were assembled.

First all-new commercial vehicle design of the post-war period

Taking a look back: in December 1954, when Daimler-Benz launched the O 321 H (the O 321 HL variant, which was 1,325 millimetres longer, entered the scene in mid-1957), the Second World War had not even been over for ten years yet. In the mid-1950s, Germany was still very much in the process of rebuilding its infrastructure. Although the company resumed manufacturing trucks as soon as 1945 and buses in 1948, the designs used were nothing more than pre-war specifications or modified vehicle layouts based on pre-war technology.

An unmistakable departure in the spring of 1951 was the unveiling of the representative Mercedes-Benz 300 (W 186) ─ and one year later, the 300 SL racing sports car (W 194): Mercedes-Benz had returned to the international scene. In September 1953, the brand introduced the “Ponton” saloon type 180 (W 120) featuring a unibody construction, which was the first genuinely new design in the passenger car segment. This model was then followed by the 220 (W 180) in 1954. The 190 SL (W 121) and 300 SL “Gullwing” (W 198) production sports cars also debuted in 1954.

If the company wanted to assert itself in the national and international playing fields, however, it not only needed to build on its pre-war success with passenger cars, but also with commercial vehicles. It also became generally accepted knowledge that different design principles apply to transporting people than to inanimate objects such as sand, steel and boxes. As such, the basic truck design did not represent the ideal platform for a bus.

Design principles borrowed from passenger cars

Daimler-Benz therefore adopted an innovative approach for the O 321 H. Designers devoted their attention to the design of the “Ponton” passenger cars to develop the new bus with an integrated floor assembly, a load-bearing body structure, a subframe front axle suspension with coil springs and recirculating-ball steering with dampers. So equipped, the new commercial vehicle easily met the requirements of high ride comfort and outstanding driving safety. Efficient production in the form of semi-integral construction and good product value rounded off the product offering.

The floor assembly replaced the conventional ladder-type frame construction typically used in truck building. It comprised a welded floor frame group with a robust, torsionally stiff centre carrier as the main design feature. The sidewalls were designed as tall longitudinal members. In conjunction with the steel profile framework construction and panelling, the floor assembly formed a self-supporting unit designed to integrate with the factory bodies. The closed centre carrier also doubled as a ventilation and heating duct as well as accommodated linkages, electrical cables and compressed-air lines. Moreover, this design made it possible for external body manufacturers to offer chassis assemblies together with the matching bodies.

The subframe supported the front axle construction and could be easily dismantled. Four wide rubber cushions served as connecting points to the body ─ a design feature that contributed significantly to a smooth ride and low vibration levels. The suspension fitted to the rigid front axle comprised coil springs pushed to the far corners of the vehicle, which were coupled with auxiliary springs and large telescoping shocks. Two transverse control arms aligned in parallel to the front axle beam absorbed cornering forces. A triangular strut channelled longitudinal and braking forces away from the front axle. The rear axle, which was fitted with leaf springs, integrated a robust torsion stabiliser bar.

Agility and comfort

The ball-and-nut steering system with damper assembly was “very smooth and almost entirely free of vibrations for such a large vehicle”, as commented by German trade magazine “Lastauto und Omnibus” in its 4/1955 issue. This, in conjunction with the large steering angle, gave the O 321 H an astounding level of agility. Its turning circle was a mere 16 metres! This was very beneficial during daily driving as the bus could easily navigate through tight city streets. And on long-distance runs, the bus was capable of traversing alpine roads with numerous hairpin bends ─ such as the Stelvio Pass ─ without having to back up.

An outward indicator of its modern design was the appearance of the O 321 H. By leveraging the look of the front end, Hermann Ahrens lent the bus an air of the 190 SL and 300 SL, thereby making it the trendsetter for the design language of commercial vehicles to come. Also, as the export delivery ratio of the bus was unusually high at around 50 percent, this design became a familiar “face” for many different people spread out over multiple continents. The bus was shipped to no fewer than 63 countries, with leading customers in Iran, the United Arab Republic, Turkey, Austria, Saudi Arabia and Argentina.

The O 321 H impressed on a global scale

The design concept of the O 321 H impressed professional fleet operators around the world. It was available not only as a city bus with folding doors at the front and back (DM 45,000), but also as an intercity bus with hinged doors (DM 43,700) and a long-distance bus (DM 44,800). On the long-distance bus variant, roof edge glazing really opened up the interior in terms of space and light, and a sunroof option was also available! The OM 321 diesel engine rated to 81 kW (110 hp), which had a displacement of 5.1 litres from a six-cylinder block, was good for a top speed of 95 km/h. By way of comparison, at the time, the Mercedes-Benz 180 passenger car model reached no more than 126 km/h and the 180 D just 112 km/h.

The O 321 H impressed experts right from the start with its appearance alone. “Lastauto und Omnibus” summed up in its 4/1955 issue: “We can summarise our assessment of the Mercedes-Benz O 321 H by stating that this all-new design represents one of the best (if not the best) production buses in this category that is currently on the market. The advertising slogan ‘handles like a passenger car’ is therefore not an exaggeration at all. The bus simply has a car-like feeling to it, which we experienced for ourselves in dense city traffic as well as on the icy roads in the Upper Black Forest.”

Trade magazine “Auto Motor und Sport” came to the following conclusion in its 8/1955 issue: “The groundbreaking new path taken by Daimler-Benz is perhaps most impressively demonstrated by citing the following figures: compared to the O 3500, the kerb weight per passenger seat has been reduced from 172.5 kilograms to 138.5 kilograms, while the engine output per tonne of gross weight has increased from 11.6 to 13.1 hp. Who would not call this progress?”

Successful production in Mannheim

On 26 April 1960, the ten thousandth 321 model series bus rolled off the assembly line at the Mannheim plant ─ a milestone marker for Daimler-Benz AG, which was able to achieve a bus production output in Europe that differed only slightly from that of passenger cars in a proportional comparison.

In December 1964, production of the O 321 H/HL at the Mannheim plant came to a halt after 18,083 units had been assembled. This marked the end of the professional career of the most successful bus to ever bear the star up to this point in time ─ a bus that also made a significant contribution to building and strengthening the global position of the Mercedes-Benz brand. The O 302 successor model continued this tradition with ease and even greater success.

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