What's this then?

This is the BMW i8 Roadster.

Hmmm. Looks a bit... different to the coupe.

That's because this is the bare chassis, rather than the complete car. BMW is keeping the styling of the i8 Roadster under wraps for now (even though it's already shown the car in concept form, as well as in plenty of teaser images wrapped in camouflage), so this is its bare-bones carbon fibre and aluminium structure. In the video you can see the frame being put through endurance testing on a rig at BMW's factory.

Is the i8 Roadster the same as the coupe?

Aside from the hole where the roof once was, yes. It has the same 1.5-litre turbo three-cylinder engine and twin-electric motors that makes 362hp, the same four-wheel drive system and the same plug-in hybrid capability. This drivetrain propels the i8 coupe from 0-62mph in 4.4 seconds and on to an electronically limited top speed of 155mph, and the Roadster is likely to have similar figures.

BMW claims that the i8 Roadster can return 141mpg on the standard economy cycle, and it has CO₂ emissions of just 46g/km, too. These figures are better than the current i8 because of developments with the electric drive system, and the coupe will benefit from improved economy figures when the 2018 range launches.

The hole in the roof is covered (or will be) by a simple fabric set-up that pops out and rolls up by hand, much like that of a Lotus Elise, although being a BMW it's likely to offer better weather protection. The folded top stows behind the front seats, as the i8 Roadster lacks the (small) back seats of the coupe.

When can I buy one?

Not quite yet. The i8 Roadster will debut at the Los Angeles Motor Show at the end of November, and is scheduled to go on sale in 2018.