The 1987 BMW Z1 concept car featured what innovative door mechanism?

The 1987 BMW Z1 concept car featured vertical sliding doors.

BMW’s Z1 was developed by BMW Technik and introduced as an innovative two-seat roadster. Its doors did not swing outward, upward or diagonally. Instead, the complete door panels descended vertically into the deep side sills, allowing occupants to step over the sill into the open cabin.

The design depended on unusually high structural sills, which helped provide crash protection even though the doors could be lowered. Electric motors and toothed rubber belts operated the doors and windows, while emergency manual operation was also possible. The Z1’s plastic body panels could be removed, another experimental feature that distinguished it from ordinary BMWs.

The Z1 began production in 1989, so 1987 refers to the concept and development period rather than series manufacture. It shared its 2.5-litre inline-six and five-speed manual transmission with the E30 325i. Butterfly, scissor and gullwing doors all move upward or pivot in different ways; none accurately describes the Z1’s doors, which slide down into the sills.