Inspiring to the present day: Caracciola’s 432.7 km/h on a public road on 28 January 1938.

On 28 January 1938, driving the Mercedes-Benz W 125 twelve-cylinder record-breaking car, racing driver Rudolf Caracciola established a world speed record on a public road: he reached 432.7 km/h over a flying kilometre on the autobahn between Frankfurt am Main and Darmstadt.

The record was only broken almost 80 years later, in November 2017. What remains unbroken is the enthusiasm for the aesthetic appeal and technical pioneering spirit of the brand. An exciting cooperation project between the Mercedes-Benz design department and the University of Applied Sciences in Munich has concerned itself with the design reinterpretation of such record-breaking cars – using innovative, cutting-edge electric drive systems.

Under the guidance of Prof. Othmar Wickenheiser, would-be automobile designers found particular inspiration in the body shapes of the historic Mercedes-Benz record-breaking cars of the 1930s, and gave free rein to their creativity. The result is outstanding design concepts for record-breaking cars capable of over 800 km/h. Almost literally electrifying insights into the project are provided by the recently published book “Electric High Speed”. Some of the design studies developed as part of the project have been implemented in the form of 1:4-scale models. The brand plans to present these and other impressive models of historical Mercedes-Benz record-breaking cars at the Techno Classica specialist fair in Essen (21 to 25 March 2018).

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The 1937 motor racing year was going outstandingly for Mercedes-Benz: the brand won the European Grand Prix as a major triumph. And it wanted to cap this with an equally stunning world record. An undertaking that was doomed to failure. During the record attempt week in Frankfurt am Main in October 1937, the W 125 with a 5.6-litre V12 engine was unable to better the competing cars of Auto Union. The Stuttgart-based brand withdrew the car from the competition – and decided that the car should be comprehensively developed further for the next record attempt. Within just eight weeks, Rudolf Uhlenhaut, technical manager of the racing department, and the Board of Management member for development, Max Sailer, had the necessary work under way. The next record-breaking attempt was to take place on 28 January 1938.

The instructions issued to the engineers were to modify the chassis and engine and develop a completely new vehicle body. The reason was above all the excessive front axle lift of the 1937 version, which at times led to loss of steerability.

Aerodynamics: Ideas from the aircraft industry

Mercedes-Benz received major impulses from the aircraft industry for its new vehicle body: among others, it was the development departments of aircraft manufacturers Ernst Heinkel and Willy Messerschmitt that made the recommendation to shorten the front overhang and give the car a rounder front end. In addition, the front end was lowered and more sharply tapered. This had the desired effect of reducing front axle lift.

A longer and more raised rear end reduced lift at the rear axle. The cross section of the car was also more heavily rounded off, which reduced susceptibility to crosswinds. And finally the cockpit glazing was modified to resemble the shape and cross section of a teardrop. Racing driver Rudolf Caracciola, the European Champion in 1935 and 1937, had previously rejected such a solution owing to the risk of distorted visibility. Then Mercedes-Benz, working with a supplier, found a visually and aerodynamically acceptable solution. The uncompromisingly streamlined form of the car was also accentuated by the covered wheel arches.

Engine cooling: Iced water rather than airflow

The diameter of the air inlets in the front of the streamlined body was usually small. This was made possible by the innovative ice-cooling system of the car for the record attempt. The radiator was housed in a container holding 48 litres of water and 5 kilograms of ice. If needed, an additional cooling effect could be achieved with dry ice. This eliminated a strong airflow through the radiator, which would have made itself felt with a higher drag coefficient.

The suggestion to use ice-cooling came from the German test institute for aviation in Berlin-Adlershof. Four decades later, measurements in the Mercedes-Benz wind tunnel showed how successful equipping the W 125 record attempt car with its new body was. In its race configuration, the Silver Arrow achieved an excellent drag coefficient of cd = 0.170.

The V12 with a displacement of 5577 cubic centimetres which had been used during the aborted record attempt in October 1937 was completely stripped down and optimised. Its reliability was to be increased, e.g. by means of a richer combustion mixture and reinforced piston pins. Other improvements included nickel-plated piston crowns to reduce the risk of charring. The piston clearance was also increased. And finally Georg Scheerer in the engine workshop of the racing department ensured a steady supply of fuel/air mixture to each cylinder.