"Convertibles, cabriolets, drop-tops, open-tops, ragtops, retractables - call them what you want, they all spell one thing: fun," said Paul Wilbur, president and CEO of ASC Inc. He has got a point. But those designing such roof systems regard it as an "engineering challenge", too. Matthew Beecham reports.

Roof systems are fast becoming part of a car's identity. Panoramic sunroofs stretching the length and breadth of a vehicle are becoming increasingly popular on both sides of the Atlantic. In fact, manufacturers report an "unbelievable" amount of interest in large sunroofs. The trend is toward roof systems, mainly of glass, with big openings that can be adjusted even during driving. These designs give the best of both worlds: saving on cost (compared to a convertible) and adding to the versatility of the car.

In fact, an increasing number of automakers are offering models featuring large glass roofs. One reason is that large areas of glass give a sense of openness and freedom. The selection on offer nowadays is varied, ranging from the closed panoramic glass roof (Peugeot 307 SW, 407 SW, and Citroën C4) and so-called twin panels or multi-panels (Renault Espace IV) through to the lamella sunroof (Fiat Stilo). According to Saint-Gobain Sekurit, in the early 1980s, the average glass surface area in a car was about 3-m². Today, this figure is about 4.5-m². The clear view of the sky, the feeling of freedom and the large amount of light are the key selling points to consumers, says the French glassmaker

"The production of larger sunroofs does not mean that the stipulated requirements in respect of safety and comfort are no longer guaranteed," said Bruno Pouillart, operational marketing manager of Saint-Gobain Sekurit. "Any type of glass used in a car - whether it be the windscreen, glass in a side window or glass roof - has to meet certain safety standards which are prescribed by the EU." In order to reduce the risk of glass breakage in sunroofs, the glassmaker recommends glass roofs are made from laminated glass (particularly over large areas) or specially coated glass. The consequence is that the larger the sunroofs, the greater the proportion of laminated glass used. Another aim for developers is therefore to make the panes of glass increasingly thinner and also lighter while maintaining the same safety standard.

The desire of many car designers to include increasingly large amounts of glass in their cars presents glassmakers with new challenges because glass cannot be shaped at will like steel or plastic. Other trends include producing a smooth roof line using several glass panels, e.g. producing windscreens which extend up into the glass roof.

For its part, ArvinMeritor used the 2007 Frankfurt motor show to unveil its new convertible-like, large-opening multi-panel roof system installed in the Porsche Cayenne and Audi Q7. The company claims that its adaptable modular roof system allows automakers more styling options from the same basic body-in-white. The system can accommodate a choice of glass panel configurations from a simple, one-panel layout to a complex system featuring multiple moveable panels. To minimize buffeting and noise in the cabin, a side-guided roller blind or a glass wind deflector can be incorporated. The company points out that, to improve flexibility for the OEM's assembly methods, the large-opening multi-panel roof system can be either top- or bottom-loaded and can be incorporated into ArvinMeritor's long-fibre injection roof module allowing for integration of other components such as roof-rack fixings, antennae, roof beams and interior headlining parts.

Since then, Webasto used the 2008 Geneva motor show to present a "new category" of roof design for convertibles, known as the Webasto LigHT Concept. The roof system is a so-called Hybrid Top, i.e. a combination of hard and soft top which uses new materials. "The description LigHT reveals the essential key to this roof system", said Franz-Josef Kortüm, CEO of Webasto AG. "We are using it to show for the first time what an innovative, simple and light hybrid top could look like when a retractable hard top is combined with a soft top." Whereas metal rods and joints provide stability on conventional roof systems, here Webasto relies on self supporting surface cross members connected by film hinges. Apart from these film hinges the LigHT Concept has one additional rotational point, which is used for moving the roof. The entire substructure is manufactured in one piece. When opened, the roof is positioned on the rear section of the car and hence does not affect the size of the boot. The project was conducted in cooperation with IED Automotive, a spin-off of the Italian design school IED (Istituto Europeo di Design). The concept car was styled by Andrea Militelo, chief designer of IED Automotive. The surface elements of the roof are produced using Compound Spray Moulding, stemming from the further development of Webasto's series-tested polyurethane technology. The company points out that since polyurethane combined with paper honeycomb-like structures weighs far less than conventional solutions, very light and robust roof systems can be realised in one piece. The rear window is a polycarbonate panel produced in an in-mould decoration process. The material is very transparent and wear resistant. Polycarbonate windows absorb 100 percent of UV radiation and do not splinter in the event of an accident. Thanks to the newly developed heating function, they achieve defrosting speeds comparable to a glass window. Webasto is one of the main automotive suppliers using the plastic technologies polycarbonate and polyurethane. The company operates its own synthetics facility in Schierling near Regensburg, Germany. This is where currently biggest polycarbonate automobile roof (1.2-m²) is being produced for the Smart fortwo.

Over the past few years, novel retractable hard-top (RHT) designs have enabled automakers achieve that appearance of a coupé yet with four comfortable seats.   "OEMs are now able to achieve designs which are much closer to traditional coupé shapes as opposed to a two-part retractable hard-top on a four seater car which often resulted in a design with a very flat windshield," said Andreas Weller, marketing director, Webasto Roof Systems Inc.  "Today's retractable hard-tops designs offer more design freedom, enabling OEMs to achieve the traditional coupe look with a retractable hard-top.  That is a very exciting development in the convertible industry."

Webasto worked with VW to develop a unique five-piece steel and glass roof with a sunroof mode sandwiched in between. "The VW Eos is a leap forward in convertible roof design, combining a sunroof, retractable hardtop and coupe all in one," said Brett Healy, vice president Business Development, Webasto Roof Systems Inc. "VW refers to the Eos as a CSC - Coupe, Sunroof, Convertible.  VW / Webasto are the first to offer such functionality in a retractable roof.   So that is a pretty big leap. As far as tomorrow's retractable hardtops are concerned, we have additional innovations in our pipeline." While the Eos bears some relationship to the VW Cabrio and resembles the Golf and Passat in terms of size, VW says it is not a convertible created by removing the roof of a hard-top.  Rather, says VW, it was designed from the base up to be a convertible coupé.

Edscha estimates that currently about half of all convertible drivers opt for a soft-top. The other 50 percent prefer the metal roof - at least in Europe. "Convertible drivers in the US are still fixed on soft-tops," said Walter Pecho, managing director of Edscha Convertible Roof Systems. "In the North American market the French are not represented. But they were the ones that performed pioneering work in the area of retractable hard-tops. With the boom in RHTs the question often arises whether soft-tops are threatened by extinction in the medium term. I don't think there's any need to worry about soft-tops. They will continue to have their justification. Many convertible drivers swear by soft-tops, since they convey a genuine convertible feeling even when they're closed - something they often miss in RHTs. Whereas RHTs frequently are used all year round, soft-tops usually are second cars. On the other hand, soft-top development is so far advanced meanwhile that soft-tops are totally fit for winter use as well. Edscha's product portfolio covers all areas: whether soft-top or RHT, volume-production vehicle or low-volume luxury convertible - we have everything in our line-up and consequently can play the game and score in all areas."

Karmann sees a balance between the popularity of RHT and the soft-top. "The RHT boom is partly rooted in the classical coupe segment and not in the convertible segment," said Karsten Huelsemann, executive director, New Business Development, Marketing and Communications, Karmann. "To many RHT owners, the perception is that their car effectively is a coupe with the possibility to open the top. Besides, RHT drivers are emotionally convinced that an RHT is less prone to attracting vandalism. The soft top has remained -- or been revived -- especially in the premium brands. Drivers of soft top convertibles often like to showcase what they drive as an open-top vehicle, deliberately choosing the roof in a colour different from the vehicle paint."

Pecho believes that the market for retractable hardtops in Europe is growing, and the success of retractable hard-tops is no temporary phenomenon. He said: "According to my estimation the trend towards the metal roof will settle down with a ratio of about two to one in favour of RHT convertibles. Currently about 1 million open vehicles are sold per year - by 2014 the number will be 1.4 million. The popularity of the RHTs is still very different on the regional level. They are well liked in Europe and in Japan, and the trend towards RHTs will soon spill over the Atlantic. If the drivers of American cars acquire a taste for retractable hard-tops, I see even more potential."

It is clear that the expanding convertible segment is crossing all vehicle segments. It is no longer a preserve of the luxury class. Roof makers say that growing consumer acceptance as well as all-weather convertible top concepts such as its retractable hard-tops will contribute to the market's expansion. While electrically operated convertible-roof systems are industry standard for luxury vehicles, roof makers are continuously working toward developing smaller, lighter and cheaper systems.

Matthew 'Beechy' Beecham

See also: Global market review of automotive roof systems - forecasts to 2014 (download)