Avoiding the wind - in a convertible!

JIM KENZIE FOR THE TORONTO STAR

The Mercedes-Benz Cabriolet, the first E-Class convertible in years, won't be formally introduced until early 2010.

Jim Kenzie

SPECIAL TO THE STAR

STUTTGART – Can I tell you what a car is like without having actually driven it?

Yes.

Provided that:

a) it is a convertible;

b) one of its main features is a clever bit of engineering designed to stop drafts inside the car when the top is down;

c) I have access to a wind tunnel.

Check, check and check.

The Mercedes-Benz E-Class Cabriolet, which doesn't formally get introduced until early next year, is the first E-Class ragtop for what seems like forever, and is based on the two-door E-Class Coupe.

The clever bit of engineering is called "Air Cap," and stems from a problem the aerodynamicists have known about for some time: wind cascading over the windshield curls back down behind the passenger cabin and enters the car from the rear, causing annoying drafts.

In a sports car or other two-seater, this re-entry point is often far enough back so as to not cause undue hardships. Besides, the wind-in-your-face (or on the back of your neck) is part of the deal when driving a sports car.

In a four-passenger convertible, typically used more for Grand Touring than elemental sporty driving, it isn't as acceptable.

Wind blockers, typically mesh screens that install over the rear seats, reduce this problem at the obvious expense of turning the four-seat car into a two-seater.

The aero boffins have also known for some time that if the windshield could be significantly taller it would push the air far enough up and away from the car that the re-entry point would be far enough back that drafts would not be an issue.

But no convertible stylist worth his or her Faber-Castell mechanical pencil is going to go for a look that shouts "snowplow" on a car that is supposed to ooze glamour.

The Air Cap solution proposed by the E-Class Cabriolet consists of an electric motor and a set of linkages that shove the windshield header cover – the piece of plastic that runs across the top of the windshield and remains in place when the top is folded – about six centimetres forward and up at the touch of a button.

This drives the air stream upward.

The gap between the deployed header and the top of the windshield is filled with a black mesh fabric, which allows just a smidgen of air to leak through.

At the same time, the rear seat headrests glide upward and between them is another chunk of black mesh fabric. This prevents any wayward back drafts from entering the cabin. Together, the air currents create a "roof" made of air.

Not only does this "cap" reduce drafts coming into the car, it helps keep conditioned air – heated, if you're driving in winter; cooled, if you're driving in summer – inside the car where it belongs.

The E-Class Cabriolet can also be fitted with the Air Scarf, a device that pumps warm air up through the seat back and down across your neck and shoulders. My "test" consisted of a run in a wind tunnel. I first sat in the back seat while the techies spooled the tunnel's wind velocity to 80 km/h.

We hit the button to deploy the system. In just a few seconds, the drafts stopped almost completely.

Cranking it up to 120 km/h, the difference seemed even more prominent, to the advantage of the Air Cap.

This trick really works.

We'll have more on the E-Class Cabriolet after we've had a chance to actually drive one, which will probably be next spring.

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