by: Gavin Conway A roof over one's head Grand touring car? This, or the SL350?

In its transformation from coupe to convertible, the big 6-Series has lost none of its overt, aggressive machismo. Not a hairdresser's car, then. Whatever you think of the controversial styling, here is a car with massive presence, a big BMW that commands respect on the road. Not content to stir things up with its challenging style, BMW has come up with a quite spectacularly eccentric roof mechanism. Bucking the trend toward steel folding roofs as practised by the latest Mercedes Benz SL convertible, the 645Ci uses a fabric roof. BMW reckons that the weight saving alone justifies such an approach. But the 645Ci's real party trick is the rear bit of the convertible roof - they call it 'fin structure', and it's basically a pair of flying buttresses where a sloping rear window would usually reside.

This arrangement allows for a perfectly vertical glass rear window, which looks much like the old Jaguar XJS. The idea is that in profile, the roof-up appearance of 645Ci mimics that of the coupe. And because the rear window is much smaller than a conventional sloping item, much more room is liberated in the boot when the roof is folded away. And the really neat bit is that the rear screen can be powered down into a slot behind the passenger seats, providing natural ventilation with the roof up, and in the raised position, a windbreak when the roof is down.

Rag-top opens in 20 seconds

With a truly one-touch operation, the roof folds away in about 20 seconds. And in order to save you looking like a total prat after being caught short at a traffic light, the 645Ci can be driven at up to 20mph while the folding operation continues. Once folded, the roof lives under a solid tonneau cover, which means the BMW's lines look smooth and uncluttered.