Rumpler Tropfenwagen on display at Deutsches Technikmuseum Berlin.
|Body and chassis|
|Body style||Saloon car|
|Engine||2,580 cc (157 cu in) W6 OHV engine|
Rumpler, born in
Vienna, was known as a designer of aircraft when at the 1921
Berlin car show he introduced the Tropfenwagen. It was to be the first streamlined production car, before the
Chrysler Airflow and
Tatra T77. The Rumpler had a
drag coefficient of only 0.28, a measurement which astonished later engineers
 and would be competitive even today. For comparison: the top ten most aerodynamic production cars in 2014/2015 worked their way down from a value of 0.26.
Fiat Balilla of the mid-1930s, by contrast, was rated at 0.60.
To enable the car's aerodynamic shape, the Tropfenwagen also featured the world's first (single plane) curved windows. Both the windscreen and the side windows were significantly curved. 
The car featured a Siemens and Halske-built  2,580 cc (157 cu in) overhead valve W6 engine, with three banks of paired cylinders, all working on a common crankshaft.   Producing 36 hp (27 kW),  it was mounted just ahead of the rear axle.  The engine, transmission, and final drive were assembled together and installed as a unit. The Rumpler-invented rear swing axles were suspended by trailing leaf springs, while the front beam axle was suspended by leading leaf springs. 
Able to seat four or five,  all the passengers were carried between the axles for maximum comfort, while the driver was alone at the front, to maximize view.  With the 1923 model, two tip-up seats were added.
Weighing nearly 3,000 lb (1,361 kg),  the Tropfenwagen was nevertheless capable of 70 mph (110 km/h) on its mere 36 hp (27 kW).  This performance got the attention of Benz & Cie.'s chief engineer, Hans Nibel. Nibel conceived the Mercedes-Benz Tropfenwagen racers using the virtually unchanged Rumpler chassis.  Poor sales and increasing losses led Benz to abandon the project.  Later Auto Union racing cars resembled the Benz Tropfenwagen racers and were built in part by Rumpler engineers. 
Rumpler made another attempt in 1924, the 4A106,  which used a 50 hp (37 kW) 2,614 cc (159.5 cu in) inline 4-cylinder engine.  This compelled a growth in wheelbase, with a consequent increase in seating to six or seven. 
Although the car was very advanced for its time, it sold poorly – about 100 cars were built. Small problems at the start (cooling, steering), the appearance of the vehicle, and the absence of a luggage compartment hindered sales. Most were sold as taxis, where easy boarding and the high ceiling were advantages. The last cars were built in 1925.
- Tested at a Volkswagen windtunnel in 1979. Lyons, Pete. "10 Best Ahead-of-Their-Time Machines", in Car and Driver, 1/88, p.73.
- 12 of the most aerodynamic cars in production right now – Motorburn
- 10 of the Sleekest Cars on the Road — CheatSheet.com
- Setright, L. J. K. (1974). "Aerodynamics: Finding the Right Shape for the Car Body". In Tom Northey. World of Automobiles. 1. London: Orbis. p. 38.
- The Rumpler Tropfenwagen – AutoSpeed
- Wise, David Burgess (1974). "Rumpler: One Aeroplane which Never Flew". In Tom Northey. World of Automobiles. 17. London: Orbis. p. 1964.
- Rogliatti, Gianni (1973). Cyril Posthumus, ed. Period Cars. Feltham, Middlesex, UK: Hamlyn. pp. 108–109. ISBN 0-600-33401-5.
- Wise, p.1965 diagram.
- Wise, p.1966.
- Wise, p.1965.
- Lyons, p.74.
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