Air Cap tweaks new E cabrio

A quirky new aero tweak at the top of the windscreen could be set to turn Mercedes-Benz's upcoming E-Class Cabrio into the first year-round four-seat convertible.

The Air Cap, which threatens to overshadow everything else about the latest member of the new E-Class family, pops out of the top of the windscreen frame at speed and pushes the wind up and over the occupants -- even those seated in the twin rear seats. An idea-in-progress at Benz for more than 20 years, it not only removes almost all buffeting from all four seats at 100km/h, but it drops the car's aerodynamic drag as well, helping to lower its fuel consumption.

Mercedes will debut the technology on the E-Class Cabrio at the Detroit Motor Show in January, but the Carsales Network was given a sneak preview of how it works in Benz's historic Stuttgart wind tunnel last week.

The Mercedes design team has softened the E-Class sedan's confrontingly angular styling in its rework of the Cabrio. The result is a dedicated four seater that, at around 1645kg, weighs in at just 70kg more than the coupe.

Benz has eschewed a folding hardtop for a more traditional cabrio look. The new two-door's seven-layer cloth roof can be raised or lowered at up to 40km/h in 20 seconds.

But the real highlight is the Air Cap. In Benz's wind tunnel, the rear-seat buffeting was troubling at 80km/h, annoying at 100 and unbearable at 140. With the Air Cap in place and an integral windblocker installed between the rear headrests, we can vouch for the fact the wind is barely noticeable at legal speeds. In fact, we could still hold a conversation between the front and rear-seat passengers at 100km/h and the buffeting at 140 was no worse than the non-Air Cap result at 60. At 120km/h, too, you could still feel the warming effects of the car's climate-control system on your face, rather than the constant assault of turbulent wind.

Air Cap helps to reduce the E-Class Cabrio's drag co-efficient to just 0.33Cd. This compares to 0.28Cd when it's running with the roof in place (and 0.25CCd for the very slippery E-Class sedan).

It's this drag number of which Benz's Senior Manager of Aerodynamics, Dr Teddy Woll, is particularly proud.

"From 70km/h, the air resistance to a car becomes greater than the rolling resistance, and therefore the determining factor for fuel consumption," he told the Carsales Network.

"At 100km/h, a 0.03 improvement in the Cd figure lowers the fuel consumption by as much as 0.2 litres/100km, and it's 0.6 litres at 160km/h and a full litre at 200.

"We make the car smoother through the air because resistance that does not need to be overcome does not require any engine output and requires no fuel consumption," he explained.

But more than that, cleaning up the Cabriolet's airflow makes it a more comfortable vehicle to spend time in and encourages people to drop the roof.

"Our research has shown that most four-seat convertibles are really only suitable as two-seaters in day to day use," E-Class Cabriolet Senior Product Manager, Wolfgang Bremm, admitted.

"That's because the traditional wind-blocker is often awkward to install, hard to store and means you can't sit in the back seat -- or even drop your handbag or briefcase there."

The Air Cap, he says, will make wind blockers redundant, because at the push of a button, it juts out 60mm above and in front of the top of the windscreen surround, acting like a 'spoiler' to push the turbulence over the top of the cabin area.

It works at up to 250km/h and needs 211 different parts, including a six-step electric motor and a four-link "suspension" system, which allows it to sit inside the shape of the normal window frame.

Benz took out 20 patents during the system's development, which Woll admits was the aero department's fourth attempt at getting it into production since the early 1990s.

While Mercedes has yet to confirm the E-Class Cabriolet pricing, its German pricing is expected to be around €5000 dearer than the Coupe.

In Europe, the base cabrio will start with the E250's 150kW diesel engine, then there will be the 170kW E350 diesel, the E250 and E350 direct-injection petrol engines and the range-topping E500 with a 285kW V8.

From the back seat -- inside the wind tunnel
There's something extremely bizarre about wind-in-the-hair motoring when you're sitting still, with the engine off, inside a concrete box. It's stranger still when you begin to absorb what's been in here before the E-Class Cabriolet and I. After all, the tunnel was built as a research facility in 1939 (though Benz didn't buy it until relatively recently), and it saw service in countless military applications in its first few years.

Where BMW's sparkling, ultra-clever new wind tunnel in neighbouring Munich is a model of contemporary thinking and engineering, the Big Wind Tunnel (as they call it at Mercedes) is a working museum. Modern wind-tunnel fans mount their electric motors inside the hub, but they couldn't build electric motors small enough back then, so they sat outside the building and turned the blades through a long driveshaft. They made them tough back then, though, and Benz only replaced the last of the original motors three years ago.

However old it is, it still blows a lot of air. Sitting in the back of the Cabrio, with the trick Air Cap tucked up inside the windscreen surround, Dr Woll calls up 80km/h. And it's fast enough, to be honest. You have to lean forward to hear sporadic snippets of front-seat conversations, but it's better than being pummeled by the wind.

At 140, though, it becomes utterly unbearable. You can't hear anything, it's cold and the wind's punches come at you from every direction.

As soon as Woll pops the Air Cap button, though, it all changes. At 120km/h, it's just as relaxing as sitting in the front. There's some minor wisps of air flicking through your hair, but otherwise, it's all calm and, relatively speaking, quiet.

"On one hand, you want to feel the wind in your hair," the good doctor said. "On the other hand, wind turbulence and draughts in the interior can become a real nuisance on longer journeys, especially at high speed."

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