Lord K's Garage #133: Silver Arrows

Here is a story of one of the most successful 1930s racing car types - the Mercedes-Benz W25.

For Mercedes-Benz the arrival of a new German government in 1933 and a thoroughly revised rule-book for the 1934 season provided the final push to build a Grand Prix car for the first time the Daimler and Benz merger. Appalled by the defeat at Avus in May of 1933 to the French and Italians, the new 'Reichs-Chancellor' made a substantial amount of money available for a German Grand Prix team. Surprisingly, Mercedes-Benz was not the only German manufacturer to jump on the opportunity; the newly formed Auto Union conglomerate also ventured into Grand Prix racing with a Ferdinand Porsche designed machine. Devised to slow the cars down and make racing saver, the new regulations dictated that a Grand Prix car could have a maximum dry-weight of just 750 kg. Work on the new Mercedes-Benz 'W25' Grand Prix racer had already started back in March of 1933, so it seems the government incentive offered after the Avusrennen was just an added bonus.

The new regulations meant that the German engineers had to find a balance between outright performance and weight. Simply put; they had the fit the biggest possible engine in the lightest available chassis. Yes that does not sound very safe at all. Following the successful Alfa Romeo and Bugatti Grand Prix engines, a supercharged, twin-cam, eight-cylinder engine was chosen. Unlike the contemporary designs, the new 'M25' Mercedes-Benz featured a head with four valves per cylinder. Following Daimler-Benz design practices, the engine itself was constructed from two groups of four separate forged-steel cylinders.

The sheet-steel water-jackets and valve-ports were welded to the individual cylinder. Also typical for Daimler-Benz was the unusual carburation; the supercharged air from the single Roots-type blower was fed through the carburetors. Just like the earlier S-Type Mercedes', the new W25's supercharger produced a very recognizable whine. Displacement was just under 3.4 litre, which was enough to produce 302 bhp on petrol. When fed with more exotic, alcohol-based brews, the output rose to 354 bhp.

The M25 engine alone already weighed in at just over 200 kg, so saving weight on the rest of the car was paramount. Using two longitudinal box-section beams, the chassis itself was a rather conventional affair. The two box-section frame was reinforced by several cross-members and a cross-tube at the front. Its lightness was ensured by drilling numerous holes in the frame. The all-round independent suspension did break new ground. At the front very short double wishbones were fitted, while the rear-end featured swing-axles. Springing at the front was by coil springs that were actuated by bell-cranks and fitted horizontally inside the cross-tube. The bottom wishbones were attached to friction dampers. The swing axles were controlled by short transversely mounted quarter-elliptic leaf springs and friction dampers. The four-speed gearbox was combined with the final drive. Braking was provided by hydraulic drum brakes sourced at Lockheed. Like the chassis, all of the small bits were cross-drilled to keep the W25 under the 750 kg maximum. The rolling-chassis was covered in a thin alloy skin. Cowlings over the radiator and suspension gave the car a very aerodynamic appearance.

Like the altogether more adventurous, V16 powered, mid-engined Auto Unions, the new Mercedes-Benz W25 debuted at the 1934 Avusrennen. During practice the cars had major carburetor problems and the W25s were withdrawn from the race, which was duly won by an Alfa Romeo after the very quick Auto Unions had retired. With the engines fully sorted, the cars appeared next at the Eiffelrennen on the Nürburgring.

Popular myth has it that the legendary 'Silver Arrow' was born here. According to the memoirs of both team-leader Alfred Neubauer and lead-driver Manfred Von Brauchitsch the white paint had to be scraped off the W25s overnight to get them underweight for the race. They re-appeared in bare-aluminium on race day and Von Brauchitsch won the race. This story only appeared in the 1950s and a little digging revealed that the name 'Silber Pfeil' had first been used by radio commentators in 1932 for the silver Mercedes-Benz raced by Von Brauchitsch himself, the Auto Unions were already 'painted' silver at Avus and the Eiffelrennen event was held under 'free formula' regulations so there was no need to get the weight down in the first place. Regardless, Von Brauchitsch' victory did herald the dawn of the 'Silver Arrow' era.

Manfred Von Brauchitsch, 1932

Throughout the 1934 season the two German manufacturers proved to have the quickest cars but reliability issues still handed numerous victories to the competition. The W25 nevertheless won the Klaussen pass hillclimb, the Coppa Acerbo and two Italian and Spanish Grands Prix. The original M25A was continuously developed. The first evolution was the M25AB, which featured a slightly larger bore and a boost in power to 348 bhp.

By the end of the year the 4-litre M25B engine was good for 370 bhp but weighed only 4 kg more than the original M25A. Now fully reliable, the W25 dominated the 1935 season, scoring 9 major victories. A painful exception was the loss at the German Grand Prix to Tazio Nuvolari in an Alfa Romeo in front of the Reichs Chancellor. Rudolf Caracciola was well-deservedly crowned European Champion at the end of the season. By that time the M25 had evolved into the C specification, which displaced 4.3 litre and produced just over 400 bhp. The Mercedes-Benz engineers also experimented with different aerodynamic tweaks and for the high-speed Avusrennen a streamlined version with a closed cockpit was tried.

In 1934, Caracciola succeeds in establishing several class records on the Gyon speed track in Hungary and on the Avus in a modified W 25. On 30 October he establishes a world record, covering a measured mile in 30.71 seconds with a standing start. The performance of the record car was increased to 430 hp, its transmission ratios were changed, and its cockpit was equipped with a lining which made Caracciola dub the car "racing limousine", but apart from that, it had the same design features as a standard W 25 racing car. It achieved a speed of 317.5 km/h.

Although still referred to as the W25, the 1936 Mercedes-Benz Grand Prix car incorporated enough changes to warrant a new type name. The rear-end of the chassis was completely altered; a transversely mounted gearbox was used and the swing-axles were replaced by a more sophisticated DeDion-axle. To keep the weight under the maximum 25 cm was cut out of the center of the car. A new V12 engine had been designed but it was overweight. As a stop-gap the M25C (below) was evolved into the 4.7 litre and 453 bhp ME25, which was actually lighter than its immediate predecessor.

The W25K (for Kurz or short) failed to fill the footsteps left by the original W25, particularly due to its poor handling characteristics. Caracciola managed to win only two Grands Prix in a season dominated by Bernd Rosemeyer in his 6-litre Auto Union Type C.

Mercedes-Benz got things right again in 1937 with the W125 that used an all new chassis and an even larger engine. The last hurrah for the W25, ironically, came at the Avusrennen. The 1937 edition was run under free formula regulations and attracted some of the wildest Grand Prix cars ever constructed. Hermann Lang eventually took the victory in a lengthened 1936 W25 fitted with a fully enveloping body and the stillborn V12 engine that produced a staggering 736 bhp.

Sources: Wouter Melissen @ Ultimatecarpage, Daimler Benz AG, Bundesarchiv