We’ve finally driven the production 2015 BMW i8 plug-in hybrid sports car, and we came away highly impressed. But one thing stuck out: The car’s size. As it turns out, the i8’s footprint is no accident. The BMW i8 is literally engineered around its battery pack, which is about the size and shape of a tipped-over grandfather clock. To achieve the i8’s targeted 22-mile electric-driving range, BMW assembles 96 Samsung-supplied prismatic lithium-ion battery cells into a 57.5 x 14.4 x 13.0-inch die-cast aluminum box.
Even though the battery pack’s total energy capacity is 7.1 kWh, normal usage is limited to 5.2 kWh to ensure a satisfactory service life. To guard the 216-pound pack against impact damage and to make it readily removable for service, it—and its aluminum box—runs lengthwise down the middle of the car. It could be only so wide and so tall to avoid overwhelming the interior, so as a result, it became very long to provide the desired energy capacity and maintain ample space for people.
Tacking a powertrain (one gas, one electric) to each end of the box yields a 110.2-inch wheelbase, only fractions shorter than what’s found in BMW’s 4-series coupe. The span between the i8’s axles make it lanky for a sports car, so adding a pair of rear seats and christening it a “2+2” helps justify its outsized dimensions.
The battery box plays no significant structural role; attaching it rigidly to the surrounding structure would frustrate its removal for service. Thus high, thick sills are necessary to provide the requisite bending and torsional rigidity, as are formed-aluminum structural reinforcements throughout the molded-carbon-fiber central body. Add together the 13.0-inch width of the battery box, adequate space for two front occupants, and the girthy sills, and you get an overall width of 76.5 inches, more than five inches greater than a Porsche 911’s.
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Since the entry aperture’s vertical dimension is constrained by the fashionably low roof (the i8 is in Corvette and 911 territory in terms of overall height) and the high sills, the opening must be long to provide access to the rear seats. But a long aperture demands awkwardly long doors, and the only practical means of opening such a long door—without using extra-wide handicapped parking spaces—is to employ unconventional hinging. Scissors and gullwing options are both awkward. The best compromise is the one BMW selected: hinging the doors off the steeply raked A-pillars.
But when an i8 arrives at a swank watering hole and the drama doors swish open, civilians expect James Bond or Emma Stone to step out. That’s the price when high fashion collides with advanced technology in the sports-car lane.
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