Safe without roof

CONVERTIBLE DEVELOPMENT When its sheet metal roof is removed, a vehicle loses around three-quarters of its stability. Thanks to Technical Calculation, IVM Automotive puts the stiffness back into the convertible and cuts weight and development time at the same time.

FROM COUPÉ TO CONVERTIBLE

We don’t know which team of vehicle developers transformed the Audi TT coupe into a road- ster. What we do know, how- ever, is how much know-how and effort it takes to optimize a topless derivate in terms of static and dynamic stiffness and crash safety. At IVM Auto- motive, this bodywork development process is based on modern simulation techniques. By the way: at least the soft top of the Audi TT roadster is made by Edscha.

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W hen was the last time you got exasperated by an attempt to stow your collected odds and ends in an open

shoe box and carry it around with you? With its lid on it was relatively stiff. But now, without the

lid, it gets distorted and mis- shaped, and it doesn't give your treasures much support.

A similar thing happens when a coupé loses its sheet metal roof. Both the flex- ural strength and the torsion stiffness of the body drop to around a quarter of the values for the coupé – with drastic consequences for driving dynamics, driving comfort and occupant safety.

No convertible owner fancies being trapped in his vehicle just because one wheel is up on the sidewalk, thus distorting the body and jamming the door – whatever the joys of fresh air. And he'd probably be even less happy if the forces of a frontal impact, instead of being transmitted into the vehicle structure via the roof, caused the A pillar to come alarmingly close to his head.

A major challenge for the bodywork experts at IVM Automotive, who are being commissioned more and more often with the body development of vehicle derivates like convertibles: they have to compensate for the lack of support from a roof, yet without substantially increasing the weight of the vehicle or the manufacturing costs. This means that a standard target figure of about 20 percent additional stiffness-enhancing mass relative to the closed vehicle is not exceeded; and also that as many parts as possible from the original series-produced vehicle can be used without

modification. The development time too should of course be kept as short as possible.

This is why the bodywork development process at IVM Automotive is now determined by the Technical Calculation department. It's only under their guidance that the heavy demands placed on static and dynamic stiffness and crash safety can be satisfied with a short development time and with safe designs in which problems don't go undetected until the testing stage. The Technical Calculation people first conduct simple potential studies to pinpoint quickly and easily the areas in the original body that need optimization. Then the design team analyses the package and the feasibility of these approaches. In detailed calculations, the precise measures necessary to achieve the target values are then ascertained. An essential condition for this new method: powerful computer hardware.

The designers too benefit from this ap- proach; whereas in the past their concept proposals could not be evaluated until much later on in the process, nowadays this is often possible after just hours on the basis of the computation results. Corrections and optimizations can be incorporated very quickly into the development work. A gain for everyone, and particularly for the OEM: this method gives him a few more months of time-to-market.

And there's another competitive advantage of convertible design by IVM Automotive: cooperation with the Convertible Roof Systems division. The Edscha Group can optimize both the body and the roof system at the same time and at the same place – and in this way offer convertible derivates as turnkey projects.

Jürgen Gumpinger, Dr. Gunther Wisinger