On 9 May 1937, the 750-kilogram W125 racing car from Mercedes-Benz celebrated its racing debut with a victory at the Tripoli Grand Prix. Following this opening triumph, it went on to dominate the entire season, which would prove to be its one and only season. This was because 1937 was the last season in which the 750kg weight limit was the formula applied for the Grand Prix European Championship.
The W125 drivers from the Stuttgart-based racing team won four of the five Grand Prix races that counted towards the European Championship. This included two 1-2-3 and two 1-2 finishes. At the end of the season, Rudolf Caracciola was crowned European Champion for the second time. The W125 also racked up some great successes in other races and on record-breaking drives.
The goal was an extremely ambitious one: to produce a brand new Mercedes-Benz racing car within the space of just a few months, in time for the 1937 Grand Prix European Championship. This was the task that the young engineer Rudolf Uhlenhaut, who was technical director of the Stuttgart brand’s newly created racing department, gave to his team of design engineers in the summer of 1936.
It was necessary because the first of the legendary Mercedes-Benz Silver Arrows models, the W25, could no longer compete with the Auto Union racing cars, its fiercest rivals.
It was quite clear what the new racer, christened the W125, needed: a more torsion-resistant frame, different suspension tuning, a more powerful engine, plus further modifications besides.
In August 1936, the design engineers got to work on the development of the racing car, which would compete in 1937 as the W125. The established concept of the first Silver Arrow car needed to be completely rethought.
By 9 September 1936, Josef Müller already had an overall design ready that included all the characteristics of the final racing car. The wheelbase was extended from 2505 to 2798 millimetres, and fitting oval tubes as longitudinal members and circular tubes as crossmembers increased the frame’s torsional stiffness by a factor of 2.65 compared with the W25.
The suspension’s setup featuring soft springing and strong damping was nothing less than revolutionary. The developers of the W125 achieved this by using a trapezoidal-link front suspension with coil springs and a de Dion rear suspension with reduction of the roll centre. Front and rear spring travel grew significantly, with an increase of 50 millimetres at the front axle to 140 millimetres. The aerodynamics of the body, with its distinctive three cooling openings in the front end, was also improved, lowering the Cd figure from 0.620 to 0.589.
When considered from a modern standpoint, the development of a brand new engine bears particular testament to the visionary power and boldness of the project. For 1937 would be the last season in which the 750kg weight limit was the formula applied – a new formula based on engine displacement was to be introduced in 1938.
This meant it would no longer be possible to use the new power unit in motor racing’s most elite event. But all those involved, from the company management down, gave their full backing to the plan. With engine designer Georg Scheerer overseeing development, the result was the M125 – an 8-cylinder inline engine with a Roots-type supercharger and a displacement of 5660 cubic centimetres.
In the races held during the 1937 season, the W125 attained speeds in excess of 300km/h. In the process, its engine generated between 556bhp and 585bhp, while on the test stand, the output was even higher at 646bhp. 
Check out the 2017 Mercedes-Benz Formula 1 “Silver Arrow” racing car.
