The new BMW Z4 looks stunning as a soft-top convertible, but while its predecessor was also sold as a hardtop, BMW Z4 design leader Calvin Luk has ruled out the possibility of the new roadster getting the hardtop treatment.

"From the beginning we knew that it was going to be a soft-top mostly just because of the performance advantage in terms of the weight, so we didn't need to mock up an alternative to that," Luk told Wards Auto. Then again, it would be difficult for BMW to justify building a Z4 hardtop when it helped Toyota co-develop the new Supra coupe.

The design chief went onto say that the decision to only sell the new Z4 with a soft roof was made by BMW's engineering division and not the designers. "More coming from engineering," he said. "I think mostly engineering, there may have been some market feedback influence on that, but I would expect it to be engineering – performance-related."

According to Luk, the soft-top was introduced to save weight for the new-generation roadster, which also uses metal panels between the layers of fabric to keep the car looking sleek when the roof is up.

The interview also reveals that Luk was given a lot of creative freedom when designing the Z4 since BMW considers it a niche model. The design was inspired by the quirky Z8 roadster produced from 2000 to 2003, which is evident in the headlight design that features stacked semi-hexagonal LED headlights rather than a horizontal design like other BMW models. However, it took 18 months for Luk to persuade BMW executives to use a mesh-style twin-kidney grille instead of the traditional horizontal slats.

In the US, the Z4 is available in two flavors, including a range-topping M40i with 382 horsepower and 369 lb-ft of torque and the Z4 sDrive30i with a 2.0-liter turbocharged four-cylinder that produces 255 hp and 295 lb-ft of torque.