Designing an All-Aluminum, Monocoque Body

A sketch of the NSX’s exterior, with its sleek profile based on an F16 fighter jet.

<< 1. Let’s Build a Sportscar!
<< 2. A Sportscar in the Image of Honda
<< 3. Designing an All-Aluminum, Monocoque Body
<< 4. Subjecting to Severe Tests at Nurburgring
<< 5. The Next-Generation Sportscar
<< 6. A Dedicated Plant: The Dream Takes Shape
<< 7. NSX: A Constant Evolution

Prototype I was completed in mid-1986, representing the team’s first construction using the Milky Way diagram. At this stage, they were considering both sheet steel and aluminum as potential body materials. Of the two, steel sheet was less favored, since it would make the target running performance of a midrange, F-1 class car more difficult to achieve. Moreover, to counter the added weight a larger, heavier engine would have to be used, thus pushing the car right out of the midrange category. Sheet steel would also be a liability in terms of the car’s purpose, which was to break from the conventional image of a sportscar as a compromise in comfort and safety for the sake of superior slalom statistics. Of course, the team was planning to furnish the car with cutting-edge accessories and devices such as power windows, full-automatic air conditioning, traction control and antilock brakes systems (ABS). To accomplish this, however, the weight of the vehicle would have to be minimized. Thus, it was decided that the NSX would have the world’s first all-aluminum, monocoque body.

No other atuomaker had yet to build a car primarily of aluminum, however, even though the material was pollution-free and in abundant supply. It is said that among the mineral resources the amount of estimated aluminum reserves is three times that of iron. Moreover, aluminum has one-third the specific gravity of iron, is resistant to corrosion, and is much easier to recycle. Despite such powerful advantages, the material has several drawbacks, particularly a high cost and proportionately higher technical requirements in molding and welding. But the development team’s greatest hurdle was the need to build a dedicated plant simply to produce the car’s aluminum body.

The project required that the development team make frequent trips between the R&D centers at Tochigi and Wako. During one such trip, aboard the Shinkansen, they realized that the bullet train was itself made of aluminum.

Aluminum has proved very adaptable to the Shinkansen, in fact. So, the team members saw no reason why it couldn’t be used for the body of a sportscar. And though there might be problems along the way, they made a promise then that they wouldn’t shy away from the challenge of creating a car for the new era.

Kobe Steel and other material suppliers appeared somewhat perplexed at the development team’s request that they collaborate in the creation of an all-aluminum car body. Because aluminum is prone to buckling during stamping work and is difficult to weld, its use in mass-production cars was limited to a relatively small assortment of parts. The suppliers were confused by the team’s decision to make the entire body out of aluminum, and doubted the seriousness of Honda’s intentions. However, the staff offered their earnest explanation to the skeptics, saying they needed an aluminum body to build their new sportscar. It was the kind of honest enthusiasm that eventually led the suppliers to wonder, why not?”

Various types of aluminum were considered, but the suppliers selected the 5000 and 6000 series. The former was already being used in the automotive industry, while the latter had relatively high strength despite its lower formability. Still, various enhancements would have to be performed. The supply company personnel in charge of development spent many days anguishing over the situation, working feverishly to meet a series of conditions stipulated by Honda in order to ensure productivity in stamping, forming, welding, coating and other processes. In fact, the hours were so long and arduous that toward the end of development they would often spend the night right in their factory.

Problems inevitably arose, and with them came headaches and delays. In particular, processing of the side sill was proving difficult, since aluminum, unlike iron, wasn’t suitable for deeply drawn press work. Therefore, a new forming process was devised whereby the aluminum was heated to 600 degrees, poured into dies, and extruded from the dies while it was being drawn. But this system created a super-strong, highly rigid honeycomb frame, and this technique ultimately became the assurance that their all-aluminum body would satisfy the rigors of high performance on the open road.

A selection of five aluminum alloys was eventually incorporated into the vehicle. This thorough attention to detail—along with numerous other efforts—soon led to a body-weight reduction of 140 kg and nearly 200 kg for the entire car, as compared to a steel-bodied equivalent. It was quite an achievement in the eyes of the material suppliers, too, who were amazed to see just how much of a car could be made with aluminum.

Concurrent with the material development of mid-1986, the development team collaborated with the Suzuka factory in building an aluminum-bodied prototype CR-X. Performance tests were conducted with the prototype, then the results and problems they identified were examined to determine whether it would be practical to build a car with an aluminum body. Next, the team studied the basic frame structure of an aluminum body for a midship sportscar using Prototype I. Several test drives and crash tests were carried out using the prototype, in order to obtain thorough verifications covering everything from rigidity to repair techniques. The results were reflected in Prototype II, which was much closer to a final product. With Prototype II, interior accessories, equipment, passenger comfort and environmental adaptability were considered as part of the car’s overall performance picture.
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