<P> The drag coefficient is maximum 0.35 . </P> <P> BMW unveiled a concept coupé version of the Z4 at the Frankfurt Motor Show in 2005 . The design of the Z4 and Z4 coupé has variously been ascribed to Anders Warming, Chris Bangle, the controversial former BMW Head Designer, and Adrian van Hooydonk, current BMW design chief, and BMW designer Tomasz Sycha . The company announced in 2005 that the two - door fastback coupé would be available for production including the return of the M Coupé . The production cars were introduced at the New York Auto Show in April 2006 as a 2 - door fastback coupé . </P> <P> Thanks to its hatch design, the Z4 Coupé offers 10.1 cu ft (0.29 m) of trunk space, compared with 8.5 cu ft (0.24 m) for the roadster . </P> <P> The Coupe's fixed roof increases torsional rigidity, resulting in a stiffness of 32,000 N m (24,000 lb ft) per degree of body twist on the coupe (compared to 14,500 N m (10,700 lb ft) per degree on the roadster), which improves turn - in and overall handling response . The roof has a' double bubble' contour which serves as an aerodynamic aid and offers more headroom than the roadster with top closed . The Coupé has a sleek fastback rear window that slopes down to an integrated spoiler shaped to deliver downforce to the rear axle at speed . </P>

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