It was a quantum leap when BMW unveiled the Z1 to the international motoring press in the Italian town of Punta Ala back in autumn 1988.
The Z1 truly was different from the ground up. A self-supporting monocoque construction made up of individual sheet-steel parts constituted the car's backbone. After it had been welded together, the entire frame was hot-dip galvanised in an immersion bath. Not only did this ensure seamless corrosion protection for the monocoque, it made it substantially more rigid too: the coating of zinc that was applied acted as a connecting and supporting element, especially around panel seams and joint overlaps. The effect was an increase in the monocoque's torsional resistance of around 25 per cent.
The second peculiarity of the Z1 bodyshell was the vehicle floor, which was bonded with the frame and partly bolted to it – and made of plastic. In collaboration with specialists from MBB – the Z1 engineers had developed a material which combined low weight with high load-bearing capacity, was immune to corrosion, safe in a collision, and produced smooth underbody contours. The solution was a combination of fibre-composite materials sandwiched together. The resulting structure of two layers of glass fibre-reinforced epoxy resin with polyurethane foam in between produced a floor assembly with a weight of just 15 kilograms.
Although the monocoque was roadworthy as it was, it was cloaked in plastic: indeed, the Z1 was the first model ever to feature a plastic exterior whose vertical parts were all made of injection-moulded thermoplastics. Resilient and proven to be immune to damage, the panels were bolted into place. In theory, with a complete second set of outer panels it would have been possible to convert a Z1 from red to blue in the space of an hour using nothing more than a screwdriver.
Different types of plastic were used for the panelling parts depending on their task. The front and rear sidewalls, the doors and the side sill covers were made from a high-tech thermoplastic that was renowned for its high impact strength, almost completely eliminating the risk of bumps and dents resulting from minor knocks. The panelling for the front and rear bumpers, on the other hand, possessed very different properties – the highly elastic plastic used here was able to fully regain its former shape following impacts at up to 4 km/h (2.5 mph).
The bonnet and boot lid as well as the soft-top compartment cover were made from special fibre composites. During the production of these plastic components, different glass fabrics (depending on the demands that would be placed on the component), foam core sections as well as the necessary fastenings, such as bolts and braces, were placed into the mould in a dry state. Once the mould had been closed, epoxy resin was injected into it and compressed while being subjected to carefully controlled temperature sequences. Components manufactured in this way combined excellent surface quality and high strength with precisely defined energy absorption in the event of an accident.
The four possible paint finishes for the Z1 were not mere colours bearing the highly descriptive names of nature Green metallic, Dream Black metallic, Fun Yellow and Top Red, their chemical composition was different too. Development work carried out in collaboration with the suppliers had led to the creation of the Varioflex paintwork system, which catered to all the requirements when applying the paint to different plastic materials.
The conventional painting techniques for metal at that time hardly placed any special requirements on the paint in terms of its flexibility, as it was relatively easy to obtain a high surface gloss and keep the colour consistent. Due to the differing requirements depending on the specific component involved, the various plastic materials used on the BMW Z1, on the other hand, called for a paintwork system offering three different levels of flexibility: high flexibility for the bumper and side sill panelling, medium flexibility for the doors and wings, and a hard coating of paint – just like on a metal body – for the bonnet, boot lid and soft-top cover. While all components received the same base coat providing the actual colour of the car, the parts were then painted with different clear coats, depending on the degree of elasticity required.
Viewed from the outside, the Z1 therefore had all the makings of a sophisticated-looking, but ultimately conventional roadster – if it hadn't been for its doors. These retracted electrically into the side sills, allowing both driver and passenger to cruise along with the door open if they wished, a feature which has never been emulated since.
The mechanism for the doors and side windows was driven by two electric motors and a toothed belt. The motors incorporated a freewheel function to allow the doors and windows to be operated manually if necessary. To avoid burdening the actual door with additional weight, all of the electrical and mechanical components were integrated in the car's body. A double mechanical lock acting on the doors in both opened and closed position prevented rattling noise and excessive door movement while driving. The side windows could, of course, be lowered separately from the doors: after the doors had been opened and closed again, the windows automatically returned to their previous position.
The powertrain and chassis of the Z1 were largely inherited from the 3 Series. Hard at work under the bonnet was the classic BMW straight-six engine, which mustered 170 hp from its 2.5-litre displacement and was fitted in a front-mid position. A central aluminium tube ran from the five-speed manual gearbox to the rear differential to produce a torsionally and flexurally rigid link between the two. The single-joint spring-strut front axle taken from the 3 Series handled the task of wheel location at the front, but a brand new construction was employed at the rear: a multi-link axle comprising two transverse control arms and one longitudinal control arm.
After a run of 8,000 BMW Z1 models, production came to an end in June 1991. It had breathed life back into a segment in the BMW portfolio which continues to enjoy immense popularity today but 25 years on, these cars still have a futuristic look about them and are still in active service: at least one Z1 has a certified mileage of over 330,000 kilometres.