Mercedes-Benz SSKL: recreating the first Silver Arrow

By Kyle Fortune

| 30 Oct 2023

Classic & Sports Car – Mercedes-Benz SSKL: recreating the first Silver Arrow

Approaching the corner at the end of the main straight on the Mercedes-Benz Untertürkheim test track, near Stuttgart, we briefly witness 130kph.

Lifting off, the supercharger’s banshee howl abates, giving a brief moment of quiet serenity before the quick, dreadful realisation that the rod-actuated drum brakes aren’t as up to the task of shedding speed as convincingly as the supercharged 7-litre straight-six engine is at gaining it.

We’re not on the full, high-speed test track today, the famous banked corner at the top end is closed (though we did manage to get a few shots on there earlier with a member of the Mercedes-Benz team driving), but even this shorter track is quite enough to get an idea of just how heroic – or, more likely, unhinged – racing drivers were in the 1930s.

Coachwork specialist Walter Vetter with the Mercedes-Benz SSKL, before mechanic Willi Zimmer drove the car from the Cannstatt works to the Avusrennen in Berlin

Alongside the straight that we’ve just driven along, behind high fencing to hide Mercedes-Benz’s modern prototypes from view, are the grounds for Stuttgart’s American football team, named The Silver Arrows – which is somewhat fitting, given the car I’m driving today.

It’s a Mercedes-Benz SSKL, albeit one wearing an unpainted, streamlined body.

This machine, built on a genuine 1931 chassis, is an exact recreation of the car with which privateer racer Manfred von Brauchitsch arrived at the 1932 Avusrennen in Berlin.

It took Mercedes-Benz Classic seven months to build this exact recreation of the rebodied SSKL racer

It still looks unusual today, but back in 1932 it must have appeared other-worldly, its unique, hand-shaped bodywork marking a significant departure from the conventional SSKL.

It was Mercedes-Benz Classic project leader Michael Plag who recognised the historical significance of this unique car and, with the original having been lost – an investigation by German journalist Gregor Schulz suggests that the factory body may have been put back on to the streamliner’s chassis – Michael lobbied Benz management for many years to build it.

His meticulous research, using the original technical drawings, period photographs and historic documentation, would eventually lead to the Classic team being given the opportunity to recreate the lost streamliner, and to reinstate its significance not just in Mercedes-Benz motorsport history, but also in that of the development of automotive aerodynamics.

The Mercedes-Benz SSKL replica’s mighty 7.1-litre straight-six

The build was completed over a seven-month period, just in time to be shown at Goodwood Festival of Speed in 2019, not long before the pandemic stopped the world turning.

The standard SSKL was a formidable racing car when it was introduced back in 1928.

In the hands of Mercedes-Benz works drivers it won races such as the Mille Miglia – and, indeed, the Avusrennen in 1931, with factory ace Rudolf Caracciola behind the wheel.

The following year he was back at the Berlin event, this time in a new challenger, the Alfa Romeo Monza, with a point to prove to his former employer.

The 18-fin ‘Elephant kompressor’ squeezes an extra 60bhp from the engine

The Stuttgart team’s works operation had been put on hold, leading Caracciola and his former teammates to seek drives elsewhere, though privateer drivers in Mercedes-Benz vehicles would still get limited factory support, albeit somewhat unofficial in nature.

With the absence of a works Mercedes-Benz team, and rivals such as Alfa Romeo bringing newer cars, the Stuttgart machines weren’t the favourites in 1932, despite their previous success at the event.

Alfa and Caracciola were touted as potential winners and, with Maserati and Bugatti also considered strong contenders, the privateer Mercedes-Benz SSKLs were looking as if they might have had their day.

The Mercedes SSKL’s streamlined body was only interrupted by the detachable Bavarian numberplates

Reinhard Baron von Koenig-Fachsenfeld, a German engineer and motorcycle racer with a number of speed records to his name, would suggest a possible solution.

The automotive aerodynamics pioneer proposed rebodying the powerful SSKL to improve its odds against its newer, lighter, faster and more agile rivals.

Von Koenig-Fachsenfeld had calculated that the benefits of reducing drag over the regular SSKL would be equivalent to an 80bhp increase in engine output, with the potential to increase top speed from 210 to 230kph (131-144mph).

The rounded nose and enclosed cockpit made the streamliner more slippery than the standard Mercedes SSKL

Impressive though the potential gains were, budgets and timings would prove problematic, with racer von Brauchitsch having to borrow money from his mechanic, Willi Zimmer, to proceed, and only doing so after Mercedes-Benz racing manager Alfred Neubauer had given the project an unofficial green light.

Streamlined coachwork specialist Walter Vetter, in Cannstatt, was tasked with building the body, which also contributed to the cost of the conversion, and the extensive work was completed – incredibly – in less than two weeks.

Such was the rush to prepare the car, there wasn’t even time to have it painted before Zimmer drove the road-legal racer to Berlin, arriving with its light, hand-formed aluminium body still finished in raw, unpainted form.

The aeroscreen directs air up and over the cockpit

The new car looked radically different not just from the regular SSKL from which it was derived, but also its contemporaries at Avus.

Aerodynamics in the automotive sphere was still very much in its infancy, with even the expert von Koenig-Fachsenfeld’s redesign accomplished using relatively rudimentary understanding of how to cheat the wind.

On seeing the car for the first time, Neubauer was reportedly rather shocked by its appearance, though that shock would change to awe when he began noting the lap times.

The stopwatches would reveal that the bespoke-bodied SSKL was able to achieve Koenig-Fachsenfeld’s anticipated increased 230kph top speed, a useful 20kph faster than the regular car, allowing the heavy (1445kg) SSKL a chance against lighter, more nimble competition.

The original Mercedes-Benz SSKL was a pioneer in the early days of automotive aerodynamics

That higher maximum would prove to be a huge advantage at Avus, with the race around the high-speed circuit running to nearly 300km, up and back via two 9.5km-long straights, joined by corners at each end.

A test for any car, the sustained high speeds played havoc with reliability and pushed the tyres to their limits.

Continental had brought a new tyre to the 1931 race, which had lasted well, but it was now being taken to breaking point by the greater speeds in testing.

During the pre-race practice session von Brauchitsch demonstrated the effectiveness of the new car’s streamlined coachwork, but, in a bid to ensure longevity over the gruelling race distance, Neubauer suggested a change in the axle ratio.

The Mercedes-Benz SSKL being built at Vetter’s Cannstatt body shop

Allowing the car to run at higher speeds but at lower revs, this would aid cooling and, he hoped, reliability, although the approach would sacrifice acceleration out of the slow corners at either end of the lengthy straights.

That change was implemented prior to the race, which took place in front of more than 200,000 spectators, with countless more throughout Germany and the rest of Europe listening in on the radio.

The curiously bodied new SSKL caused a sensation.

Affectionately dubbed ‘The Gherkin’ by the assembled crowds, it was very much the talk of the event.

Alongside a standard Mercedes-Benz SSKL, the streamliner’s radical body style is clear

Race commentator Paul Laven, presenting for Südwestdeutsche Rundfunkdienst AG (Southwest German Broadcasting Service), eschewed the nickname the masses had given von Brauchitsch’s SSKL, instead describing it as a “Silver Arrow” as it sped down the straights.

The recording of the broadcast is thought to be the first instance of a Mercedes-Benz racing car being referred to as such, with Stuttgart’s race cars usually having raced in the traditional German racing colour of white.

Indeed, the dominant Model S, SS, SSK and SSKL racers had previously been referred to as ‘White Elephants’ – that somewhat unflattering moniker perhaps having helped accelerate the switch to silver following the Avusrennen.

Manfred von Brauchitsch’s Avus win is commemorated on this faithful recreation

In the early stages the race was led by Albert Divo’s Bugatti, but on lap six the Type 54 would break an oil pipe, leaving Caracciola’s Alfa Romeo to take the lead with von Brauchitsch in hot pursuit.

The Gherkin wowed the crowds outside Berlin as its top speed gave it a useful advantage on the straights, but it would prove unwieldy in the bends, where higher entry speeds provided a big challenge for the brakes.

While von Brauchitsch had to adapt his driving style to suit, Caracciola in the Alfa Romeo had more effective brakes, lighter weight and better agility, as well as a sweeter-revving engine and faster initial acceleration out of the corners.

There was no time to paint the original Mercedes-Benz SSKL, such was the rush to complete it

Von Brauchitsch would manage the Mercedes-Benz’s greater maximum speed, while taking advantage of the expert tutelage of Neubauer.

He advised restraint with using the engine on full power for reliability reasons, as well as to keep a little in reserve, the tactician suggesting that he only push really hard late in the race to press home the SSKL’s high-speed dominance.

The event was a real spectacle, the two cars swapping positions several times over the 15 laps, with von Brauchitsch happy to shadow the Alfa for long periods, waiting for the chance to overtake the Italian car.

A supercharged SSK was the basis for the streamliner

In an audacious move during the final lap, von Brauchitsch surprised Caracciola on the back straight, before defending his lead through the last corner and down the long straight to take the win, crossing the line 3.6 secs ahead of his countryman.

The Bugatti Type 51 driven by Hans Stuber came home in third – some three and a half minutes later.

The second SSKL, with a conventional body and driven by Hans Stuck, was fourth, with the final position in the over-2-litre class taken by Ernst Kotte in a Maserati, the other 16 starters having retired or crashed.

Across 294.4km, von Brauchitsch’s SSKL averaged in excess of 194kph (121mph) for a little more than one and a half hours.

Taking the unique streamliner recreation for a lap of the Mercedes-Benz test track at Untertürkheim

The curious SSKL’s aerodynamic bodywork showed its considerable advantage over Stuck’s more conventional car in fourth, which posted an average speed around 10kph lower, and proved instrumental in allowing the older, heavier Mercedes-Benz to beat the much-fancied combination of Caracciola and Alfa Romeo.

The victory caused a sensation, turning von Brauchitsch into a household name in Germany, his ‘Silver Arrow’ having brilliantly – and very publicly – demonstrated the effectiveness of aerodynamics in racing cars.

Von Koenig-Fachsenfeld’s idea to rebody the SSKL gave the ageing Benz a competitive edge against newer, technically superior rivals.

A re-routed exhaust improved the Mercedes’ aerodynamics

While the Avus victory is certainly not the first instance of aerodynamics having been explored with racing cars – Mercedes itself having done so with the likes of the Benz-Tropfenwagen and 1909 Blitzen Benz – this was a graphic display of their emerging importance and application within the automotive sphere.

In addition to von Brauchitsch’s win, the ‘Silver Arrow’ also set a number of speed records, with the international benchmark over 200km broken during the Avus race, thanks to the enclosed SSKL’s 194.5kph average.

That von Brauchitsch wrestled the big Benz around Avus for just over an hour and a half at speeds significantly in excess of what I’m experiencing today is difficult to comprehend.

Those rod-actuated brakes have a physical link to the drums, but even the most prodigious of pushes does little to slow the weighty Mercedes-Benz.

Manfred von Brauchitsch and the rebodied SSKL beat all comers at the Avusrennen in 1932

Pleasingly, the steering rewards with surprisingly deft immediacy, those big, exposed wheels out front, bobbing about on their suspension, generating a quick direction change that feels at odds with the physicality required elsewhere.

The gearshift is a challenge, the non-synchro ’box requiring a swift double-declutch to avoid clumsily crunched cogs.

Mercifully, such is its tractable, low-rev urgency, once you’ve worked your way up through the gears you seldom need to shift down around this relatively small track.

‘Commentator Paul Laven described von Brauchitsch’s SSKL as a “Silver Arrow” as it sped down the straights’

The engine revs at a maximum of around 3600rpm, but it doesn’t need to be worked hard to deliver vivid pace, thanks to the 7.1-litre straight-six producing its peak torque at around 1900rpm, so it shifts its not-inconsiderable bulk with relative ease.

Push through the first portion of the central accelerator pedal’s travel and the Roots-type supercharger spools up, forcing another 60bhp on top of the normally aspirated unit’s 240bhp.

That additional push is accompanied by a wailing scream that’s more frightening – and addictive – than the increased velocity it delivers.

A victorious von Brauchitsch celebrates with mechanic Willi Zimmer

It’s enough to have you forgetting the brakes’ lack of bite, something von Brauchitsch would have had to manage, repeatedly, as he came down from far higher speeds at Avus; his legs must have been like the pistons of the big ‘six’ to race this car at the speeds he did, for the time he did.

The drive for streamlined cars, both on track and road, accelerated markedly in the ensuing years, which makes von Koenig-Fachsenfeld and von Brauchitsch’s missing-link SSKL hugely significant.

Not only did it inform every racing car that followed, but also gave birth to a name that is still used to this day.

Thanks to: Michael Plag, Mercedes-Benz Classic

Images: Mercedes-Benz Classic

Tunnel vision: The Mercedes-Benz SSKL in the wind

The Mercedes-Benz streamliner replica with a W25 (left) and standard SSKL (right)

This SSKL presents a blend of radical thinking mixed with old-fashioned technology, with its race win being proof of concept, but just how effective von Koenig-Fachsenfeld’s body revisions are had never previously been measured – at least outside the spectacle of that 1932 Avusrennen weekend.

Until today, that is, because the SSKL now sits in Mercedes-Benz’s own wind tunnel to be scientifically measured for the first time.

The venue itself is interesting, being the oldest automobile-specific wind tunnel.

Constructed in 1939, the 8.5m-wide tunnel is capable of up to 250kph (156mph) and is still used by Mercedes-Benz today.

Post-war, when it ran at full speed it could draw so much power that it had the potential to – and occasionally did – kill the floodlights in the Stuttgart football stadium.

“The way our club currently plays I wish we could still turn them off,” mutters the technician, wryly.

That’s not going to happen today, though the SSKL will be put ‘in the wind’ for measurement, alongside a regular SSKL and a later 1934 W25 GP car for comparison.

In the wind tunnel, Mercedes’ streamlined SSKL recreation revealed much about early automotive aerodynamics

Aerodynamicist Martin Konermann has run the numbers on each, pointing out that the unique body on the SSKL immediately presents a smaller frontal area in comparison to the standard car, aided by the removal of the headlights, the fully enclosed chassis, rounded nose, enclosed cockpit with aeroscreen, tapered tail, faired-in rear axle and re-routed exhaust.

Its improved slipperiness is obvious when the smoke is turned on, with the trails passing over to reveal fewer swirls and vortices compared to the standard car.

Martin also highlights that engine cooling is significantly enhanced, though less advantageous is the fact that von Brauchitsch would have been busy at speed, thanks to a fair bit of lift over the rear axle, which makes his efforts at Avus even more heroic.

Overall, it presents 1.6m² surface area compared to 1.7m² for the standard SSK, and the coefficient of drag is reduced from 0.914 with the SSKL to 0.616Cd for the streamliner.

In comparison, the later 1934 W25 Grand Prix car offers a 1.2m² area and 0.614Cd.

Those numbers would be poor on a truck today, but in the early 1930s, when automotive aerodynamics was in its infancy, the improvements the SSKL streamliner made were significant, and well proven on the track.

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