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The Z1 has remarkable aerodynamics for a roadster. The design of the underbody has a major influence on this. The whole of the underneath of the car is covered by a flat composite undertray which is designed to smooth the airflow to the rear wing, which is the silencer and bodywork at the rear of the car.

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Rear downforce is created by a low pressure area generated by the effect of the Aerofoil section of the transverse rear silencer box, and it's nearby body parts which work as Aerofoil slats. Frontal downforce is created by a high pressure area at the front edge of the bonnet (hood). The forward part of this body section is a concave curve, before turning convex as is more normal. A high pressure area is created just above the front wheels.
Air flow pattern around the Z1 rear exhaust box and body panels. The rear silencer is an inverted wing, and the rear fender and undertray work as aerodynamic slots or flaps to increase the negative lift at road speeds.
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.
The Z1's aerodynamics whilst heralded above were also nail bitingly incomprehensible, the wind on the right side of the cabin was like a tornado, whilst the driver's side was a mild storm. The passenger if sat with right hand on sill and held your arm at a certain angle, the buffeting was reduced significantly, lean forward, and it was reduced even more, sit normally and the tornado returned. Whereas the driver has the steering wheel nd driving position with both arms up to mitigate the noise.