Ultra-Lightweight CFRP Proves Problematic For i8 Spyder, BMW Admits

Carbon Fibre Reinforced Plastic is both lighter and stronger than steel, uses less energy to make, and is easier to repair, making it one of the hottest new construction materials in the automotive world today.

Engineering the i8 Spyder's CFRP passenger module is proving harder than BMW thought.

Engineering the i8 Spyder’s CFRP passenger module is proving harder than BMW thought.

It’s no surprise then that performance-oriented BMW — whose i3 electric hatchback and i8 coupe plug-in hybrid sportscar both extensively make use of CFRP in their construction — views CFRP as a way to improve the range and performance of its EVs. As BMW admitted this week to Autocar however, CFRP isn’t always easy to use in every situation.

Talking to the British motoring magazine, BMW engineers admitted that while the lightweight CFRP made an ideal construction material for the rigid ‘Life Module’ passenger cell of both the production i3 and i8, CFRP was giving them a few headaches readying the i8 Spyder first seen at the 2012 Bejing Auto Show as a concept car for production as a 2016 model year.

That’s because CFRP derives a lot of its intrinsic strength from the shapes it is moulded into, not just its mechanical composition.  While the BMW i8’s CFRP passenger cell is incredibly rigid and strong thanks to its fully-enclosed design, removing the roof from the i8 in order to make the i8 Spyder has forced BMW to heavily modify the lower part of the i8’s CFRP passenger cell to get the necessary rigidity needed for crash protection and road holding.

“The last architecture presented some fairly major challenges, to be able to produce a convertible in that format,” a BMW spokesman confirmed.  “The rigidity was hard to find with that particular architecture.”

When it finally launches — probably some time towards the end of 2015 — the BMW i8 Spyder will use the same plug-in hybrid drivetrain found on its roofed sibling. That equates to a 0-62 mph (0-100kph) time of around 4.4 seconds, all-wheel drive capability thanks to twin front-wheel drive electric motors and gasoline-driven rear-wheels, and a combined real-world fuel economy of around 80 mpg.

For now however, if you want a BMW i8, your only option is the $135,925 (£100,000+) coupe, which will arrive at dealers late next year.  And if you can’t wait that long, there’s always the all-electric replica we told you about yesterday…


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