Convertibles are big business these days, and their size and sophistication are continually increasing. Ian Adcock looks at the ups and downs of an expanding market
That most traditional of British brands, Morgan, is the simplest form of convertible, a conventional fabric hood secured to the windscreen header by two over centre clips. It is a one-handed operation to lower it and enjoy instant open-air motoring, while it takes only a few seconds to raise it against the elements.
A decade ago the Morgan would have been just one of many convertibles that had a simple, manually operated fabric hood. Now its in the minority as OEMs choose more sophisticated roofing systems to combine the virtues of open and closed motoring.
The trend really took off in 1996 when Mercedes-Benz introduced its new SLK. This featured the first modern generation folding hardtop, delivering two cars in one, a convertible and a coupé. The trouble was that with the roof folded away there was little room left in the boot for much beyond a toothbrush.
Four years later Peugeot brought the folding metal roof to the masses with its 206CC and since then there have been a number of models featuring interpretations of the technology.
Many predicted that folding roofs would sound the deathknell of the more conventional fabric roofs but this has been far from the case, although it does appear to have polarised the market to an extent.
Manufacturers of sporting, high performance cars such as Jaguar with its XKR and Porsche with the 911 are not prepared to compromise on the two main drawbacks of solid folding roofs, weight and the packaging space required, while at the top end of the luxury market, fabric hoods are the preferred option for Bentley and Rolls-Royce.
Development of fabric hoods has not remained static. The latest Audi TT roadster has an electrohydraulically powered hood that features a new Z-fold system that stacks the rigid front section on top of the folding cloth portion to a form a completely flat cover. The operation takes just 12 seconds and, like the 911, can take place at speeds up to 30km/h. The system also features a new steel-aluminium framework to save more weight.
At the other end of the price scale the Karmann-developed hood for the Bentley GTC launched last autumn features seven bows in the roof.
We didnt want it to look like a starving cow, says Bentley engineering director Dr Ulrich Eichorn. The outer fabric is 30% heavier for improved NVH performance. Although its operating time is a comparatively leisurely 25 seconds, this was done in the interest of grace, says Eichorn. We could have made it faster, it just wouldnt have been so stylish.
As well as offering a conventional cabrio, Porsche also offers a targa roof system. Although Porsche was one of the early pioneers of removable targa roof panels, its latest iteration actually features a large glass panel that can be motored back to offer the occupants everything from a fully open roof to just a small gap. Developed in conjunction with CTS – now part of the Magna group – it features thinner glass than its predecessor for weight saving.
We looked at polycarbonate, but it isnt stiff enough and becomes brittle below –25ºC, says Porsche engineer Thomas Christiansen.
The shortcomings with this are that it restricts rearward vision – especially for shorter drivers – and, despite Porsches best efforts, theres noticeable wind noise at speed.
Magna Car Top Systems, as the group is now called, has taken the concept of a folding metal roof one step further with its Libertin, developed in co-operation with Italian styling house Bertone.
Based on an Opel Zafira MPV, the roof is divided into three section which stack one on top of the other then slide down the rear of the car, with the door open, to be stored under the rear parcel shelf where the rearmost seats would be. In this configuration seating capacity is reduced from seven to five.
Its second concept, the Suagna, also developed with Bertone, is based on Fiat Punto proportions and features a two-piece folding roof stored in the boot that still allows seating for four adults. Previously, this type of folding roof system had been restricted to small two-seaters.
In the past Finnish specialist Valmet has demonstrated a folding metal roof on an Audi A4, again preserving the full seating layout even if at the expense of some boot capacity.
The complexity in which these roofs fold usually demands a boot that can be opened conventionally and at the rear edge to allow the roof components to be stored.
Heuliez, at the 2006 Geneva Salon, introduced a new interpretation on this theme with its four-door, four seater Macarena concept developed from a Peugeot 407 saloon.
This featured three glass panels, which can also be used as a sliding sunroof by opening one or more. It retracts into the space where the rear parcel shelf would normally be, negating the need for a double-hinged boot lid.
German body specialist Karmann teamed up with ZF to produce the Sport Utility Cabrio employing its ZFolding system, which is stored immediately behind the rear seats. The 4m2 fabric roof is one of the largest demonstrated, but can be stored in just 13 seconds. Although Karmann wont reveal if it has any takers for the concept, the system is unlikely to reach production before 2009.
Large roofs like this concept and the one seen on the Mercedes-Benz Ocean Drive concept that debuted at Detroit earlier this year present their own problems. This soft top posed a special challenge for supplier Edscha, not least due to its dimensions; with a length of 2,182mm, it forms a roof arc of 2,220mm and covers an area of approximately 3.2m2.
The soft top is opened and closed by a sophisticated electric retraction mechanism. First an electric motor opens the catch on the windscreen frame, then the rear bow unlatches. The roof folds up and the boot lid opens backwards, together with the parcel shelf, to accommodate it.
The retraction mechanism displayed on the concept vehicle is new in this form and makes it possible to dispense with the usual transverse join.
The complex sequence of movements is driven by a variable-power hydraulic system. This provides the motive power for the opening and closing operations, while control and co-ordination is for the first time provided by unique proportional valves. Unlike the type of valves previously used, which carried out the various operations consecutively and at uniform speed, the proportional valves use a more intelligent control strategy whereby the different opening and closing phases can take place at different speeds and in parallel. This means the different movements can be more smoothly and mention efficiently choreographed.
The six separately controllable proportional valves used in the Mercedes-Benz luxury convertible allow the roof to be opened in approximately 20 seconds, quick for a soft top of this size.
For the soft-top material, a unique high-quality fabric was chosen. Since the warp and weft threads are in lighter and darker shades, an alternating matt/shiny effect is created which complements the colour-shifting effects of the paintwork.
The soft top has a layer of intermediate padding and a light-coloured interior lining which preserves the light and airy interior ambience with the top up.
Fighting it out with Mercedes-Benz for top honours at Detroit was Rolls-Royce with its Phantom drophead coupé. Its 2.8m long fabric roof was also developed by Edscha and features patents for its acoustic insulation and for the way the rear window is bonded in place.
As with most technologies there will be no clear winner in the battle between convertible roof systems. For folding solid roofs the OEMs will continue to demand lighter, more easily packaged systems that require less storage space and do not compromise the overall design. And there will always be a place for fabric roofs but they will become more complex and, as with BMWs 6 Series, the design will evolve to become more integrated. One thing is certain: it's an expanding market.
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