Rumpler Tropfenwagen: the aerodynamic advance

Since its inception, engineers have sought to make automobiles faster and more economical. One of the most innovative concepts of the time was the application of aerodynamics to vehicles. The concept itself was already used in aeronautics; its novelty stemmed more from its adaptation than its development in production cars.

The first to adopt it was naturally the Rumpler Tropfenwagen thanks to its designer, the Austrian engineer Edmund Rumpler, who had previously worked in the design and production of aircraft, but who had to retrain because of the Treaty of Versailles of 1919 which prohibited him from continuing his aeronautical work, especially since he was working on a German combat aircraft: the "Taube" (Pigeon).

Rumpler also had experience in automobile design and manufacturing. In 1903, he patented a pivoting axle rear suspension system. Rumpler's efforts produced a car with a surprisingly low drag coefficient of just 0.28 (when tested in 1979). His original rear-engine layout combined with independent rear suspension heralded the future. Rumpler's concept was of historical importance, but most people didn't know what to make of this strange vehicle. One exception was Benz's Berlin representative, Willy Walb, a future racing team manager at Auto Union, who was intrigued by its possibilities. He convinced Benz to look into building cars with a similar design for both commercial and racing purposes. Karl Ludvigsen: "Edmund Rumpler and his patent lawyers gave Max Wagner sleepless nights." Without a Rumpler license agreement, Benz had to use a rear suspension design that would not infringe Rumpler's numerous patents. As a result, the HR had a much more practical system. Its conventional differential was fixed to the frame. It drove the axle half-shafts through universal joints in spherical housings similar to those then widely used for forward mounting of torque tube axles. The car competed in only one major race, the European Grand Prix at Monza on September 9, 1923.

In 1921, he created a car that caused a sensation at the Berlin Motor Show that year with a vehicle unlike anything ever seen before. This car was based on the principle of what he called the "drop car" (Tropfen wagen) and was about ten years ahead of the Tatra 77 and the Chrysler Airflow. He thus placed the aerodynamics of his car at the center of his concerns.

Built in Rumpler's small Berlin factory, the Tropfenwaagen (Teardrop Car) was completely unconventional. In plan, it resembled a fish, with a rounded front end and a long, tapered tail, for maximum air-penetrating efficiency. Horizontal mud flaps were mounted above the disc wheels to protect the sides of the body from spray without creating any forward resistance. Even the rigid steel chassis was streamlined and designed so that it did not impede airflow, and closed Tropfenwagens also had curved window glass molded to the body contour. The driver sat well forward, in a solitary center seat, for optimum visibility, and all the seats were carried between the axles for maximum ride comfort. Long cantilever springs carried a beam axle at the front, swing axles at the rear.

Its innovations are numerous: curved windows, independently suspended wheels, mid-mounted engine... The latter was initially a W6 (6 cylinders in W), ranging from 2310 to 2580 cm3, for a power of 35 horsepower. Too complicated, it was replaced by a Mercedes in-line 4 cylinder, of 2660 cm3 and 50 horsepower, allowing the Tropfenwagen to reach 115 km/h, in 1924.

Rumpler's patents were used in a number of automobile productions: for example, the transaxle patent was used on the Austin Mini, as well as on the Porsche family's productions (the Beetle for Ferdinand, the 356 and 911 for Ferry). Like the McLaren F1, another off-road vehicle designed by a brilliant engineer (Gordon Murray), the driver of the Tropfenwagen was in the middle of the chassis, while a two-seater bench seat was behind him.

It is above all its drag coefficient that remains the most impressive: measured only in 1979 by Volkswagen in a wind tunnel, it revealed a Cx of 0.28, a value that the German brand only reached in 1988 with its Passat! The water drop solution is more than rational, and comes directly from aeronautical production. But, too far ahead of its time, the Tropfenwagen was a commercial failure: only a hundred models left the factory.

Its problems were numerous and prohibitive: in addition to its surprising atypical aesthetics, the cooling of the Siemens W6 engine was poorly designed, while the steering confused drivers; low capacity, the car could only carry very little luggage. Its sole market then became the Berlin taxi company, where its high ceiling made it easy to get in. Production ceased in 1925.