Most convertibles of the 80s were troubled by poor aerodynamics and chassis rigidity. The Z1 on the other hand had a good drag coefficient at 0.36 with the roof up and 0.43 when the roof is down, thanks to a smooth body and a flat composite undertray covering the bottom of the car. The undertray drew air towards the integral rear diffuser, which pulled the car towards the ground, resulting in stability at high speed without needing spoilers. Diffusers are common today, but in 1988 it was a unique feature on a BMW roadster.