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This open top BMW i8 Spyder concept will be turned into a production variant of the petrol electric hybrid supercar in 2015.

 The i8 Spyder is the third pre production version of the revolutionary supercar. It’s set for its first public airing at the Beiging motor show later this month. This concept is said to give extra clues to the production coupé, which is due to go on sale in late 2014. Of note are the solid doors, which replace the transparent sides of the previous concepts.

There are other subtle differences from the two previous concepts. The kidney grille is blanked off to cut aerodynamic drag & the ‘layered surfacing’ styling treatment has evolved slightly with its open roof design, the i8 Spyder has a new, free standing windscreen & conventional door glazing that integrates with fixed rear three quarter side panels.

To provide roll over protection & boost chassis stiffness, there are prominent safety hoops, faired in by body panels, at the rear of the cabin. Although this concept leaves the design of the roof unclear, BMW insiders suggest that a removable panel, probably fashioned from carbonfibre, is the favoured solution.

The rear end has also been modified, with a flat, transparent engine cover in place of the sloping coupé roofline. It doubles as the luggage cover, over a wide but shallow stowage space.

As with all future BMW ‘i’ sub brand models, the Spyder is based around a lightweight carbonfibre reinforced plastic structure clothed in body panels of the same material. It is 4480mm long, 1922mm wide & 1208mm tall. That makes it 130mm shorter, 142mm wider & 187mm lower than a 335i cabriolet.

The snug 2+2 layout of the i8 coupé has been ditched for a pure two seat cabin, with the rear bulkhead moved forward to provide added space for the engine. With a 2650mm wheelbase, accommodation is described as “generous”.

The i8 Spyder has the same plug in petrol electric hybrid system used by the i8 coupé. The 1.5 litre three cylinder turbo petrol engine is mid/rear mounted & delivers up to 220bhp to the rear wheels. The same engine will be offered in future Minis & a new front wheel drive entry level BMW.

In the i8 Spyder, the 1.5 litre petrol engine is mated to a front mounted 129bhp electric motor driving the front wheels via a fixed ratio gearbox.

The i8 powertrain can operate on petrol or electric power, or a combination of the two & in either front, rear or four wheel drive. A range extender function recharges the battery on the move. Together the petrol & electric motors make 349bhp. Computer simulations suggest the 1630kg i8 Spyder’s 0 100kph will be 5.0sec. Top speed limited at 250kph.

With the lithium ion battery sited within the centre tunnel & the petrol tank behind the rear bulkhead, BMW is claiming 50:50 weight distribution. Technical details of the battery remain secret, although BMW reports a 1hr 45min recharge using a high voltage supply. A recuperation system is also included to collect kinetic energy produced under braking & on a trailing throttle.