There’s a story told that may well be simply an automotive journalists’ urban legend but it concerns a gent, now a Very Senior Editor at a Well Known Motoring Magazine who was, some 30 years ago, inadvisedly left alone – with a photographer – in a rather secret section of a Jaguar facility.
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Surrounded by vehicles draped in dust cloths, he and his colleague did the obvious thing. Under one cover they surveyed massive flying buttresses, a tiny rear window, a great long flowing nose and most un-Jaguar-like rectangular headlamps. Glances were exchanged. “Nah, Jag would never build something like that.”
“Yeah, think you’re right.”
The camera remained holstered, the dust cover was stealthily replaced, the PR minder returned and the factory tour continued.
A year or two later, Jaguar, of course, unveiled the XJS coupe with its massive flying buttresses, tiny rear window, great long flowing nose and most un-Jaguar-like rectangular headlamps and a certain journalist and snapper probably still occasionally think wistfully of the Scoop of the Decade that Never Was.
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By GlobalDataThat particular tale returned to mind, writes just-auto.com deputy editor Graeme Roberts, when BMW recently revealed its new 6-series convertible, marking the first use of those now-famous buttresses and a small, almost vertical rear window since Jaguar retired the XJS, replacing it with the XK8 in 1997.
Apart from the instant recognition, why did BMW adopt this styling, and why did the engineers not switch to the folding metal roof you find on other luxury coupes such as Mercedes’ SL, the Lexus SC and Cadillac XLR?
As always, the Germans have an answer or two.
Despite noting the fad for fancy folding metal roofs, they preferred to advance the art of the folding soft-top because such systems afford greater flexibility, more boot space and contribute to lightweight engineering. And they claim that they’ve advanced convertible roof applications with what on the 6-series convertible is called a “fin structure roof with independent vertical rear window”.
The multi-layered folding roof is made of a rubberised cloth inner and outer with a polyurethane material called PUR sandwiched between. This results in virtually the same high standards of acoustics and thermal insulation as a folding hard top or fixed-head coupe, reducing wind noise by up to 3dB(A).
Then, of course, they launched it to the UK motoring press in up-to-23-degree Malaga, southern Spain, after most of the hacks had just been snowed on at home. The one time we could bear to interrupt our unexpected March sunbathing session and put the roof up, about 10 minutes from the end of the final drive session, we noted that it sealed very tightly and felt as solid to a prod as a metal top.
Suppression of outside noise seemed to be almost as good as a tin top but I hold the best test of a soft top is whether or not the cabin feels like a blast freezer after a frosty UK December night and that is impossible to ascertain in Spain this time of year…
The new BMW hood is also distinctive because it looks really good when raised, a position most British convertible hoods spend most of their time in.
BMW reckons the fin structure roof stands out from the crowd because, in profile, the fins offer the same visual effect as wide C-pillars, maintaining a coupé-like silhouette. This design also marks a first – the retention of the ‘Hofmeister kink’ in a BMW soft-top. That means the traditional design signature at the trailing edge of the rear side windows is featured as a subtle kick-back of the soft roof where it meets the body.
With the roof open, the shape of the fins is replicated by the tonneau lid covering the roof compartment when it’s folded away. Made from watertight, dust-proof SMC (sheet moulding compound – a highly durable and scratch resistant plastic) it co-ordinates with the colour of the interior, coming in either grey or black, and its shape and lines complement the rest of the car’s design.
Steel bars in the roof structure help provide the 6 series convertible with excellent strength and rigidity, but a most significant advantage of this rigid steel skeleton is its compact dimensions. Although the roof is necessarily long to cover the generous 2+2 passenger compartment (it really can seat four adults if those in front aren’t too greedy over legroom), it folds tightly into a very compact retractable roof compartment in the boot. Even better, with the roof up, you can swing on a fold-out handle and flatten the empty roof cartridge against the ‘roof’ of the boot, liberating another six inches or so of luggage compartment height.
This flexibility contributes towards the car’s impressive luggage capacity of 300 litres with the roof in the down position, which increases to a class-leading 350 litres when up.
And here’s the reason for going with fabric rather than metal: a folding metal roof would occupy considerably more space when folded and weigh substantially more than the fabric roof.
The fully automatic roof – it takes care of its own catch release and latching at the top of the windscreen – opens by remote control from the key or via a button at the base of the centre console. Six hydraulic cylinders perform the opening and closing movements on both the roof and its compartment lid, and the complete process of opening or closing takes around 20 seconds and, within this time, all windows are lowered or raised as required.
The roof is electrically locked on to the windscreen frame to complete closing, or the compartment lid sealed electrically when the roof is fully open, and the driver notified via a check control message.
Even better, as we found when surrounded by smoky lorries on a dusty, traffic-snarled road, you can close (or open) the roof on the move at up to 20mph while the roof continues its automatic process. Should the driver exceed this speed limit a warning signal reminds the driver that the roof mechanism is still in operation.
Now, about that vertical rear window.
The 6 series convertible is unusual in offering a small, fully independent and retractable vertical pane. It is heated and is claimed to offer better visibility than traditional large, sloping convertible windows though we don’t think there’s much in it, as the flying buttresses act like big C-pillars and block your quarter view in any case.
More to the point, the window plays a key role in maintaining the distinct coupé-like roof profile – a key principle of the new car’s design.
BMW says that, traditionally, a conventional glass rear window would have to be very large, in order to maintain rear visibility, if the car’s design is to adhere to the coupé profile. However, this would occupy an impractical amount of boot space when folded. The answer is usually to offer a smaller window on a steeper angle, which affects the pure, sweeping coupé lines.
At the same time visibility to the rear is often impaired because the necessary tightening bar that extends from one side of a conventional soft-top to the other obstructs a clear line of vision.
So their answer is the fin structure roof. Its key features are the two supporting buttresses in the C-pillar position that double back towards the cabin providing physical support and allowing the design to remain unadulterated. Crucially, the inherent strength provided by the fin structure also renders a tightening bar superfluous and allows the rear window to remain independent of the roof.
Another advantage of the independent rear window is that, at the touch of a button, it may be raised or lowered from its compartment just aft of the rear bulkhead which provides draught-free interior ventilation with the roof up (though we’d stick with the excellent climate control a/c system). BMW claims the effect of a lowered rear window is every bit as efficient as a sunroof on a hard-top and significantly quieter.
They have thought it through, though. The company says that driving with the roof down on particularly hot, sunny days can be uncomfortable, so the ability to keep the roof up, but allow rear ventilation, is a real benefit. By lowering all four side windows and the rear screen, passengers can take in all the fresh air they want but at the same time remain safely in the shade. Raising the rear window with the roof down also serves as an effective wind deflector for rear passengers.
Rather than adding two extra buttons for all-windows-down and the rear window, BMW has dual-functioned the rear window switches. You prod a button and then use them while a green LED is glowing. A timer switches the button back to controlling the side glass after a moment or two.
BMW adds that one further benefit of the vertical window is that in winter the heated elements de-ice the glass quickly allowing ice or snow to slide off its vertical plane more readily.
