Every company has a number it cannot escape. For Mercedes-Benz, that number was written in 1937, and it sat there, untouched, for the better part of fifty years while the rest of the world caught its breath.
By the end of 1936 the racing department had lost its nerve. The W25 had been shortened, lengthened, re-engined and re-bodied into a car nobody fully understood any more, and Auto Union's mid-engined machines were taking it apart weekend after weekend. Alfred Neubauer, the man who invented the modern pit signal because he could not bear not knowing what was happening on the far side of the circuit, was running a team that had simply stopped frightening anybody.
The answer they came up with was unusual, and in hindsight it was the most important decision the company made that decade. They pulled the racing department apart and handed the chassis to a thirty-year-old engineer from the passenger car test division named Rudolf Uhlenhaut. He was young, he was not a racing man, and he did something none of the senior engineers had bothered to do. He put on a helmet and drove the car himself, hard, over and over, until he could describe exactly what was wrong with it.
What he found was not the engine. The W25 was sprung like a bank vault. The thinking of the day held that a racing car should be rigid, held flat and unyielding, and that anything softer was a compromise for road cars and old men. Uhlenhaut concluded the opposite. The car was so stiff that it spent half of every bumpy circuit skating over the surface rather than pressing into it, and a tyre that is not touching the road makes no grip at all. No amount of horsepower repairs that.
So he built the W125 soft. Long travel, properly controlled, a chassis that breathed with the road instead of fighting it. It is now the first principle of every competent suspension on earth, and at the time it was close to heresy. The reputation of the W125 rests almost entirely on its engine, and its real engineering significance rests almost entirely on its springs.
Still, the engine. The formula of the day capped weight rather than displacement, which is the sort of rule that only ever produces one outcome. What went into the W125 was a supercharged straight-eight of nearly six litres, running a fuel blend closer to industrial solvent than petrol, making something in the region of 595 horsepower. Consider what that meant in 1937: drum brakes, tyres narrower than those on a modern hatchback, no seatbelt, no roll structure, and a driver sitting upright in the airstream with his sleeves rolled up. It would be the most powerful Grand Prix car in the world for the next five decades.
Rudolf Caracciola took the European Championship with it. Hermann Lang, Manfred von Brauchitsch and Dick Seaman filled in the rest of the results sheets. Then the formula changed for 1938 and the car was finished, retired after a single season of service, which is a strange fate for the most extraordinary thing anyone had built.
But the story does not end on a circuit. It ends on a public road on the morning of 28 January 1938, on the autobahn between Frankfurt and Darmstadt, in ordinary January weather. Mercedes-Benz and Auto Union had carried their rivalry off the track and onto the highway, and both had built streamlined record cars for the purpose. The Mercedes was a W125 underneath, its body drawn tight and low over the chassis until the thing looked less like a car than a dropped bomb.
Caracciola went out early, while the air was still. He recorded 432.7 km/h over the flying kilometre. Two hundred and sixty-eight miles an hour, in 1938, on a road that ordinary people drove to work on. It would stand as the fastest speed ever set on a public road for some eighty years.
Bernd Rosemeyer went out that afternoon to take it back. The wind had risen across the open ground by then, and Caracciola, who understood better than anyone what he had just done and how narrowly he had got away with it, advised him to wait. Rosemeyer did not wait. The Auto Union left the road at close to its maximum speed and he was killed instantly, twenty-eight years old, the finest natural driver of his generation.
Nobody celebrated the record after that. The number went into the books, Europe found other business by 1939, and by the time anyone went motor racing properly again the men who built these cars were scattered across a ruined continent. Uhlenhaut survived, of course, and went on to give the world the 300 SL and the car that still carries his name, but the particular madness of the late thirties never came back to the factory.
It came back somewhere else. When Aufrecht and Melcher started pulling Mercedes engines apart in a converted mill in Burgstall, when Koenig and Gemballa and Brabus began bolting on blowers and turbochargers that Stuttgart would never have sanctioned, none of that was rebellion against Mercedes-Benz. It was a return to a version of Mercedes-Benz that had quietly stopped existing. The tuners of the eighties were making an argument that the factory had gone conservative, and the W125 is the evidence that they were right all along.
Five hundred and ninety-five horsepower, in 1937. It took the factory the better part of half a century to sell you anything like it, and when it finally did, the badge on the boot lid said AMG.