Mercedes-Benz Grand-Prix racing car W 125, 1937

Mercedes-Benz Grand-Prix racing car W 125, 1937

The withdrawal of the Mercedes-Benz racing team from the battle for the European Grand Prix Championship at the end of August 1936 after the Swiss Grand Prix reflected on the one hand the inferiority of the 750 kg Formula W 25 racing car in that season, but was the only correct strategic decision on the other. After all, the focus was already on the following season, where the team, once again equipped with powerful material, planned to take on the challenge of the V16 mid-engined cars from Auto Union.

Although time had now been gained, it was not enough for the complete new development and testing of a 750 kg Formula racing car capable of winning. It was clear from the outset that the familiar eight-cylinder inline engine with Roots blower would continue to be used as the power unit, the last evolutionary stage of which, with a maximum output of 494 hp/363 kW at 5800 rpm, had essentially been maxed out and had reached its limit.

Against this backdrop, there was only a limited range of measures available for the further increase in performance, which was urgently needed in view of the competition from Zwickau. This included, first and foremost, a further increase in displacement to 5663 cc, which was achieved by enlarging the bore by 8 mm to 94 mm. In order to create the necessary space, a new engine block had to be cast. The larger displacement was flanked by valves whose disc diameter was now 40 mm instead of the previous 37 mm, by more performance-oriented valve timing and, depending on the engine, increased compression ratios of up to 9.4:1. The new evolutionary stage was given the designation M 125.

The further increased mechanical load on the long inline eight-cylinder led to a reinforcement of the most important moving components inside the engine. More robust connecting rods with a thickness of 21 mm were used and, after some tests with a crankshaft that still only had five bearings, a variant with nine main bearings was also used.

The other focus of the development work was on the intake side of the engine. Since the Roots blowers also quickly reached their load limits at higher speeds, the M 125 was dimensioned with a vane height of 255 mm - a dimension that had already been used in the previous version. However, the vane diameter was significantly enlarged, growing from 106 mm to 124.6 mm and thus providing increased throughput.

In terms of performance, however, the - albeit initially hesitant - move away from traditional pressure carburettors and so-called dry supercharging, in which the mixture was prepared by one or two pressure-tight carburettors between the compressor and the engine intake, had a much more lasting effect. Test stand trials had clearly shown that float carburettors positioned upstream of the Roots blower, which had to draw in their combustion air instead of being supplied with already compressed, correspondingly heated and thus oxygen-depleted air, generally resulted in higher power output. In addition, so-called wet charging - the compression of an already ignitable combustion mixture - also had considerable advantages: its cooling properties had a favourable effect on the efficiency and temperature of the compressors.

As far as the actual performance data of the M 125 engines were concerned, there was a relatively wide spread, according to in-house measurement reports. The engines used under the same conditions in the last two races of the season in 1937, burning the same performance-enhancing fuel, provided between 551 hp/405 kW and 585 hp/430 kW at a nominal speed of 5800 rpm in each case. As far as is documented, engine no. 9, which was fitted with an additional carburettor and for which a measurement report showed the frequently quoted peak output of 646 hp/475 kW, was never used in this form in circuit races.

The heavily revised Mercedes-Benz inline eight-cylinder engines did not need to shy away from comparison with the V16 engines of Auto Union, which for the 1937 season had an enlarged displacement of 6333 cc and an output of 545 hp/401 kW - neither in terms of absolute hp/kW output nor in terms of maximum torque, where the large-volume V16 units had always been clearly superior: in each case, at least 900 Nm were achieved as the best value.

While only limited further development was possible on the engine side, as the familiar basic design had to be used, there was considerably more scope for extensive improvements to the chassis and suspension of the new car for the 1937 season. In addition, the organisational framework had changed decisively. The establishment of an independent racing department under the 30-year-old graduate engineer Rudolf Uhlenhaut, which had been floated out of the testing department, made the processes much more efficient than before. Uhlenhaut proved to be less of a theoretician at the desk or drawing board and more of a high-ranking practical person: his analytical approach was based primarily on his own driving tests with both the racing cars currently in use and those in the testing phase. Uhlenhaut's distinct driving talent, which was in no way inferior to that of the factory racing drivers, allowed him to make concrete technical statements after test drives and to draw the necessary conclusions for further vehicle development.

His findings in the case of the unsatisfactory driving characteristics of the last evolutionary stage of the W 25 focused on three essential points: lack of torsional stiffness of the pressed steel frame, likewise lack of torsional stiffness of the axle tube of the de Dion rear suspension and insufficient spring travel on the front axle.

The first draft for the new W 125, dated early September 1936, already showed that the design deficits of the W 25 pointed out by Uhlenhaut had been eliminated. Instead of the pressed steel frame that had been used for decades, a ladder-type frame made of oval longitudinal tubes with transverse bracing made of round tubes was now planned. The concept of the de Dion rear-axle design had also been comprehensively advanced, and instead of the traditional quarter-elliptic springs, modern torsion bars with hydraulic dual-piston lever-type shock absorbers now controlled the axle and wheel movements. At the front axle, dual trapezoidal control arms of unequal length were used instead of double wishbones. The W 25's ill-fated coil springs, housed in the front axle tube, made way for two directly actuated, vertically positioned coil springs in the W 125, which were now also kept in check by hydraulic dual-piston lever-type shock absorbers instead of antiquated friction shock absorbers.

The brake system of the new 750 kg Formula racing car had also been modified and adapted to the higher output volume. Although the front and rear drum diameter remained at 400 mm, a widening of the radially heavily ribbed brake drum now allowed the use of brake shoes measuring 72 mm.

The shape of the W 125 body bore a strong visual resemblance to that of its predecessor. What was striking, however, were the much more balanced proportions, modified by an extension of the wheelbase by 293 mm to 2798 mm, and the enlarged air intakes at the front of the car. In addition, thanks to aerodynamic fine-tuning, the designers had succeeded in lowering the Cd value of the W 125 by over 5% to 0.589.

The first example of the new racing car was completed at the beginning of 1937. In early February, extensive testing began with it in Monza. Further comprehensive testing took place in several stages up until the beginning of April, and already involved two W 125s. From March onwards, the new M 125 engine was used for the first time. The only major modification made as a result of the tests was the fitting of additional friction shock absorbers to the rear axle to eliminate any remaining slight trample tendency.

A total of eleven W 125s were built in Untertürkheim, nine of which were entered in circuit races during the 1937 season. The automobile umbrella organisation AIACR (Association Internationale des Automobile Clubs Reconnus), also the supreme motorsport authority, had qualified five races, the Grands Prix of Belgium, Germany, Monaco, Switzerland and Italy, as races counting towards the European Grand Prix Championship. In addition to these, the Mercedes-Benz team's race calendar included starts in six other events, the Tripoli Grand Prix, the Eifel Race, the Vanderbilt Cup in the USA, the Coppa Acerbo in Italy, the Masaryk Grand Prix in Brno, Czech Republic, and the Donington Grand Prix on the English circuit of the same name.

The season opener for the Mercedes-Benz racing team was the Tripoli Grand Prix on 9 May 1937 on the Mellaha Circuit in Libya, which was then part of Italian territory. The Untertürkheim team competed with five W 125s: four racing cars and one practice car. The driver team consisted of Rudolf Caracciola, Manfred von Brauchitsch, Hermann Lang and the newly added Briton Richard "Dick" Seaman. It became apparent that the powerful new 750 kg cars were absolutely competitive, but that the only serious opponents, the Auto Union Type Cs, would fight on equal terms for the victories. In the end, Hermann Lang narrowly retained the upper hand and crossed the finish line in first place ahead of a phalanx of four Auto Unions. Caracciola and Seaman finished sixth and seventh respectively.

The successful premiere of the W 125 in North Africa was followed a good three weeks later by the first disillusionment: in the Eifel Race on the Nürburgring, Bernd Rosemeyer in the Auto Union Type C won by around 50 seconds against the Mercedes-Benz drivers Rudolf Caracciola and Manfred von Brauchitsch. Sixth and ninth places went to Hermann Lang and the Swiss junior driver Christian Kautz, newly signed by the Mercedes-Benz team.

Next on the agenda at the beginning of July 1937 was participation in the always highly prized Vanderbilt Cup on the Roosevelt Raceway in New York, where European and US racing culture met. The result showed a clear dominance of the European cars, with the first six places in the final classification going to German and Italian marques. The Mercedes-Benz racing team had embarked on the lavish overseas excursion with only two W 125s, which were intended for Rudolf Caracciola and "Dick" Seaman. However, the victory and the princely prize of 20,000 US dollars were once again won by Rosemeyer in the Auto Union mid-engined car, with Seaman just under a minute behind in second place.

Over the following eight weeks, the five Grand Prix races that counted towards the European Championship were held in quick succession. The first event was the Belgian Grand Prix on the Ardennes circuit in Spa-Francorchamps on 11 July. As Caracciola and Seaman were still on their way back from the USA, the Mercedes-Benz team comprised only three starters: Lang, von Brauchitsch and Kautz. Auto Union fielded four cars and also had to do without its new star Rosemeyer for the aforementioned reasons. In addition to Rudolf Hasse, who had already been under contract since 1936, the Zwickau team had signed another ace from motorbike racing, H. P. Müller. On the extremely fast track, Hasse proved his qualities and, together with his second-placed team-mate Hans Stuck, clinched a one-two for Auto Union ahead of Lang and Kautz in the W 125s.

Two weeks later, in the German Grand Prix on the Nürburgring, the Mercedes-Benz team managed to turn the tide. At the finish, Caracciola was 46 seconds ahead of von Brauchitsch and just over a minute ahead of Rosemeyer. Kautz and Lang finished the race in sixth and seventh place. "Dick" Seaman had collided with Auto Union factory driver Ernst von Delius on lap 6 and had to retire. Von Delius, who had suffered significant but not life-threatening injuries in the accident, died one day later in hospital due to sudden complications.

The next race in the European Championship, the Monaco Grand Prix on 8 August, had already been dominated by Mercedes-Benz 750 kg racing cars in 1935 and 1936. This picture did not change in the 1937 season either. In the order of von Brauchitsch, Caracciola, Kautz, the Untertürkheim racing team achieved its first triple victory with the W 125 in the principality. Italian Goffredo Zehender, who was a substitute driver for Seaman, completed the triumph for Mercedes-Benz with fifth place.

Although the Coppa Acerbo held near Pescara on 11 July 1937, which did not count towards the European Championship, had brought a defeat for the Untertürkheim team in the interim, the next European Championship race, the Swiss Grand Prix, held on 22 August on the Bremgarten Circuit near Bern, saw a repeat of the Monaco debacle for Auto Union. The places on the podium were shared exclusively by Mercedes-Benz drivers: Caracciola first, ahead of Lang and von Brauchitsch. Christian Kautz finished sixth in his home race.

The final race of the European Championship, the Italian Grand Prix in Monza on 12 September, had to decide the title. Rudolf Caracciola and the steadily emerging young star Hermann Lang duelled throughout the race and pulled away from the rest of the field. In the end, the experienced driver from Remagen won by a hair ahead of Lang and secured the European Grand Prix Championship for himself and Mercedes-Benz for the second time after winning the title in 1935. Rosemeyer in the Auto Union Type C finished third, almost two and a half minutes behind.

Even after the successful conclusion of the rounds for the European Grand Prix Championship, the season was by no means over for the Mercedes-Benz racing team. Two events were still on the schedule: the Masaryk Grand Prix and the Donington Grand Prix. The race in Czechoslovakia, which was held at the end of September on a closed public street circuit near Brno, saw the continuation of Mercedes-Benz's winning streak that had begun in August. The new European Champion Rudolf Caracciola and Manfred von Brauchitsch scored a double victory with their W 125s, a good 40 seconds ahead of the Auto Union Type C of Bernd Rosemeyer who, after an accident with Hermann Lang, was driving the car of H. P. Müller. "Dick" Seaman finished fourth in the third W 125.

On the hilly racetrack of Donington Park in the British Midlands, the Untertürkheim works team competed at full team strength. While Caracciola, von Brauchitsch, Lang and local hero "Dick" Seaman rolled up to the start in their W 125s, Christian Kautz, the fifth member of the driving team, acted as a reserve. In the race scheduled over 80 laps on the first weekend of October, the only event in the British Isles contested with Grand Prix cars, Bernd Rosemeyer in the V16 mid-engined Auto Union managed a small revenge for the defeats suffered in recent months. He crossed the finish line as the winner ahead of the two Mercedes-Benz drivers von Brauchitsch and Caracciola. Seaman and Lang had to retire early from the race after an accident and a technical defect respectively.

In contrast to the previous year, Daimler-Benz AG was able to look back on an all-round positive result in the 1937 racing season, once all races had been completed: in the last year of the 750 kg Formula the W 125, developed from a difficult starting position, had demonstrated its superiority against the fiercest competition in impressive style. Now the task was to put a competitive vehicle together for the 1938 season with its fundamentally changed regulations.

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