Mercedes-Benz Grand-Prix racing car W 25, 1934
Notwithstanding the massive impact of the world economic crisis that had been raging since 1929, which was also felt by all car manufacturers, the supreme motorsport authority, the Association Internationale des Automobile Clubs Reconnus (AIACR), adopted a new formula for single-seater racing cars in the so-called Grandes Épreuves category in the autumn of 1932. The Grandes Épreuves - literally translated "Grand Tests" - were the international racing events with the highest importance, first and foremost the Grand Prix races of the traditional core countries represented in the AIACR - France, Italy, Belgium, Spain, Great Britain, Germany, and the USA - as well as Monaco and Switzerland.
The new formula was set to take effect at the start of the 1934 season. Those responsible tried to take account of the extremely tense sales situation across the entire automotive industry by creating technical regulations that provided for only two major restrictions - a weight limit of a maximum of 750 kg without fuel, oil, coolant and tyres, and a vehicle width of at least 850 mm. It was assumed that above all the low permissible weight would limit the technical upgrading of the vehicles both in terms of cost and achievable speeds - an assumption that was to prove illusory.
Despite all the economic problems of Daimler-Benz AG, which resulted in a considerable reduction in the workforce, Wilhelm Kissel, who had been Chairman of the Board of Management since 1930, initially informally, but then officially in 1937, took the view that involvement in the highest motorsport category was essential for the prominence and reputation of the company and its products at the international level. However, the last development stage of the Mercedes-Benz SSK - the SSKL, with which Caracciola had still won the Mille Miglia, the AVUS race and the European Hillclimb Championship for sports cars in 1931 - no longer corresponded to the current state of the art and was incapable of achieving much at the highest level. The standard was set by the Alfa Romeo P3 and the Bugatti models T 35 B, T 51 and T 54, lightweight as well as powerful racing cars, the technical concept of which had already been unconsciously oriented towards the philosophy of the new regulations before they came into force.
In order not only to be able to be part of this show, but if possible to take the leading role, Daimler-Benz AG had to develop a completely new racing vehicle that would be in accordance with the racing formula in effect from 1934. However, the financing of this project proved to be a problematical issue in the company's situation at the time. As early as spring 1932, Wilhelm Kissel and chief engineer Dr Hans Nibel, who was also a member of the Daimler-Benz Board of Management, therefore approached the Reich Ministry of Transport in Berlin with a request for subsidies for the construction of racing cars. However, due to the tense financial situation in the German Reich at the time, this wish could not initially be fulfilled. This only changed under the National Socialist government, which came to power in January 1933 and sought to exploit the propaganda value of possible German successes in international motorsport: not only Daimler-Benz, but also Auto Union, founded at the end of 1931 as a new player on the racing scene, subsequently enjoyed regular support payments from the state until 1939. However, the amount remained significantly below the sums that had to be raised by the companies themselves for the development of the racing cars.
Although Daimler-Benz chief engineer Nibel had already been thinking about a new racing car design since 1931, it was not until the announcement of the new racing formula for Grand Prix cars in the autumn of 1932 and the emergence of a concrete prospect for solving the financing problems that development work on the new racing car with the internal designation W 25 began, in the spring of 1933, and the company could announce its re-entry into factory-sponsored motorsport.
In order to gain a first-hand impression of the technical level of the leading competitors, they took the opportunity to acquire the badly damaged Alfa Romeo 8C 2300 in which Rudolf Caracciola had suffered a serious accident during practice for the Monaco Grand Prix at the beginning of April 1933. Although this was not a thoroughbred racing car like the Type P3 - this was initially no longer used after the factory's withdrawal from Grand Prix racing announced at the end of 1932 - but a modified sports car, both vehicles were very similar in their design.
After Hans Nibel's development team had spent the remaining months of 1933 on design work, extensive test drives with the first completed W 25 began in Italy as early as January 1934. The location chosen was the well-developed autostrada between Milan and Varese.
The basic architecture of the new Grand Prix racing car followed proven patterns and corresponded to the design of competing racing cars: a supercharged inline eight-cylinder positioned as a front-mid engine was mounted in a lightweight ladder-frame chassis. Numerous technical solutions, for example in the areas of wheel suspensions and brake systems, demonstrate how advanced the W 25 was in its design details.
When it came to the power unit, Mercedes-Benz, like its competitors, relied on the formula "increase in the number of cylinders equals increase in power". Since the 750 kg Formula did not provide for a displacement limit, the first version of the eight-cylinder inline engine in the W 25, called the M 25 A, opted for bore x stroke dimensions of 78 mm x 88 mm and a total displacement of 3364 cc. This was well above the previous Bugatti engine, with a displacement of just under 2.3 litres, and also above the inline eight-cylinder of the Alfa Romeo P3, which had a displacement of 2.6 litres. In the T 59 model, however, the French launched an inline eight-cylinder engine that had also been enlarged to 3.3 litres for the 1934 season. Alfa Romeo also followed suit and bored out the P3 engine to 2.9 litres displacement.
The M 25 A drew on a characteristic design feature of Mercedes-Benz racing engines: the aluminium crankcase, consisting of two four-cylinder blocks, was fitted with forged steel cylinders, which in turn had welded-on cooling jackets, also made of steel. Since they were welded together as a single unit, the cylinder and cylinder head did not require a cylinder head gasket. The two intake and exhaust valves per cylinder were actuated by fork rocker arms via two overhead camshafts driven at the rear end of the engine by a cascade of gears. All valves had 34 mm valve disks, the exhaust valves were sodium-cooled. The long crankshaft was mounted on five bearings; a dry sump lubrication system ensured the oil circulation of the high-performance engine. At the front end of the engine were the constantly engaged Roots blower and the two hermetically sealed updraft carburettors. The carburettors were designed as so-called pressure carburettors, which were supplied with compressed air from the compressor. This form of forced induction is also called dry forced induction because the compressor compresses the intake air and not the ignitable mixture.
The output volume of the eight-cylinder was enormous: right from the start, test stand measurements showed a peak output of between 314 hp/231 kW and 354 hp/260 kW at 5800 rpm in each case, depending on the fuel composition. The established competition, meanwhile, with their engines revised and enlarged for the 1934 season, ranged from 250 hp/184 kW to 270 hp/199 kW. The first version of the Auto Union mid-engine racing car (Type A) with a 4.4-litre V16 engine, also ready for use in 1934, produced 295 hp/217 kW at 4500 rpm.
The M 25 A did not remain the only engine version during the 1934 racing season. In the permanent quest for a further increase in engine performance, the developers launched two evolutionary stages of the eight-cylinder engine. The first was designated M 25 AB and had a bore enlarged by 4 mm to 82 mm, resulting in a displacement of 3718 cc. In conjunction with a 1.5 mm increase in the diameter of the valve discs to 35.5 mm and the use of a higher-throughput compressor, this led to an increase in power to between 348 hp/256 kW and 398 hp/293 kW at 5800 rpm in each case, depending on the fuel composition. To this day, however, there is no evidence in the factory documents that such a specified engine was ever used in a W 25 for racing purposes.
The second evolutionary stage of the inline eight-cylinder engine was ignited as soon as in August 1934 - just two months after the victorious premiere of the W 25 driven by Manfred von Brauchitsch in the Eifel Race on the Nürburgring. Under the designation M 25 B, by comparison with the M 25 AB version, the stroke had been lengthened from the previous 88 mm to 94.5 mm. The displacement thus grew to a total of 3992 cc. In combination with an even more powerful Roots blower with rotors 240 mm long, the peak performance reached an unprecedented level, depending on the fuel used: it ranged between 370 hp/272 kW and 430 hp/316 kW at an unchanged rated speed of 5800 rpm. Above all, the torque maximum had increased and was now 500 Nm.
With a view to keeping the centre of gravity as low as possible, Hans Nibel's development team decided to reconfigure the power transmission from the crankshaft to the gearbox positioned on the rear axle and on to the differential in an unusual way. The power was transmitted to the 4-speed gearbox via the propeller shaft running at engine speed, and from there to the differential from below by means of a bevel gear. This allowed the propeller shaft to be positioned 9 cm lower, which resulted in a correspondingly low seating position for the driver.
The chassis frame of the W 25 with its two multiple cross-braced U-profile longitudinal members made of pressed steel still largely corresponded in conceptual terms to that of the supercharged racing sports cars of the late 1920s and early 1930s and - like the frame of the SSKL at the time - featured hundreds of weight-reducing holes of varying diameter drilled through it.
Progressive and one step ahead of the competition was the use of individually suspended wheels all around. However, Untertürkheim was still struggling to find the most promising technical solution: during development and in the first racing season, a total of seven different variants were used on the front and rear axles. Two T-shaped transverse control arms served as wheel control elements at the front; springing and damping were provided by a helical spring housed in the front axle tube and actuated by a toggle lever from the right and left lower transverse control arms, as well as a friction shock absorber of contemporary design for each wheel. At the rear, a swing axle was used, the natural movements of which were kept under control by means of a quarter elliptical spring and a friction shock absorber.
The hydraulic actuation of the internal-expanding shoe brakes, which were fitted all round with brake drums that were heavily ribbed for the purpose of optimised cooling, was also consistently modern. Meanwhile, the competitors still relied, at least in part, on mechanical, mostly cable-operated brake systems.
The light-alloy body of the W 25 was smooth and harmoniously proportioned. Streamlined, rounded fairings covered the front and protruding rear suspension elements. In contrast to the first prototypes, all racing vehicles had a headrest extension running to the rear on the rear-end fairing. Other body components also formed part of the quest for the best possible aerodynamics. Depending on whether races or record runs were on the agenda, there were radiator openings of different sizes and, for the latter case, even a closed, forward-folding cockpit bonnet with a three-part windscreen. For racing with an open cockpit, differently dimensioned racing panes were also available, depending on the driver's physique.
For the races and record runs of the 1934 season, a total of six W 25s were built and equipped with M 25 A or B engines. Before the first races, which included circuit and road races as well as hillclimbs, an extensive test programme was started in January, lasting almost six months and taking place not only on the domestic Nürburgring and AVUS circuits, but also in Italy and France. Right at the first start, the Eifel Race held on 3 June, Manfred von Brauchitsch in his W 25 won against Hans Stuck in the likewise new mid-engined Auto Union and Louis Chiron in an Alfa Romeo P3. Star driver Rudolf Caracciola was not yet ready for action: he was still suffering from the hip injury he had sustained in April 1933 in an Alfa Romeo during practice for the Monaco Grand Prix.
This first race of the new Mercedes-Benz racing car not only marked the beginning of a unique success story, it was also the starting point for one of the most spectacular legends in automotive history. Documented in the memoirs of racing director Alfred Neubauer and through statements by Manfred von Brauchitsch, on the eve of the Eifel Race the new racing car was apparently one kilogram over the upper limit defined by the regulations. Since all possibilities for saving weight had already been exploited during construction of the car, Neubauer and von Brauchitsch apparently had the idea of sanding off overnight the white paint with which the cars were traditionally painted. The rest is history: the now shiny silver cars took to the grid, and Manfred von Brauchitsch's victory marked the start of an unprecedented series of victories for the Mercedes-Benz racing cars that were subsequently dubbed Silver Arrows.
However, the first tussle with the competition at the highest possible level, the Grand Prix de l'A.C.F. in Montlhéry, which took place about four weeks later, ended in disaster: all three W 25 cars that started, with Caracciola, von Brauchitsch and Fagioli at the wheel, had already retired after 15 of the 40 laps to be completed. All podium places went to Alfa Romeo, with Louis Chiron winning ahead of Achille Varzi.
Luigi Fagioli was able to make up for this shortcoming at least in part two weeks later in the German Grand Prix on the Nürburgring. With a second place, he impressively demonstrated the qualities of the new Mercedes-Benz racing car, but still had to narrowly admit defeat to Hans Stuck in the Auto Union Type A.
The teething troubles of the W 25 had not yet been completely overcome by August either, yet the success record was solidifying - not least due to the revised and more powerful M 25 B engine. Rudolf Caracciola won the Klausenpass Hillclimb in the racing car class, Fagioli won the Coppa Acerbo, a high-speed race held on a triangular course near Pescara, and finished sixth in the Swiss Grand Prix on the Berner Bremgarten Circuit 10 days later. His team-mates Caracciola and von Brauchitsch did not finish in the points there. Highlights of the W 25's début season, however, were the Italian and Spanish Grands Prix, held in September in Monza and on the Circuito Lasarte near San Sebastian respectively. While Rudolf Caracciola was only just able to keep the competition from Auto Union at bay in Italy with the only W 25 remaining in the race as the winner, the race in Spain turned into the first major triumph for the Mercedes-Benz racing team with a double victory by Fagioli and Caracciola.
With this outcome for the year, the first racing season of the new 750 kg racing car was quite promising - even though only the successes in individual races were important for 1934, because the AIACR had not announced a higher-level championship, such as a European championship.