Returning to the World Motorcycle Grand Prix / 1979

Regaining the Summit

Returning to the World Motorcycle Grand Prix / 1979

The Unconventional: Adopting a "Shrimp Shell" Frame

It was not just the engine that was unconventional. To increase its competitiveness, the NR500 also employed a frame technology that was simply unheard of in the conventional realm of engineering.

Tadashi Kamiya, a research engineer in HGA's Third Research Block who had joined the NR program in the summer of 1978 as chief of the test group for completed product, proposed several ideas that he was thinking could be adapted for use in production cars. One of them was an aluminum frame called the "shrimp shell."

The shrimp shell, which integrated a monocoque structure with the cowling to form the frame and body, virtually encased the engine, which was then inserted through the rear like a cassette. The engine was fixed with 18 six-millimeter bolts inserted at both sides. Although the panel was paper-thin at just 1 mm, once the engine was in place the frame was able to ensure the required rigidity. Additionally, the frame weighed just 5 kg, which was about half the weight of an ordinary tubular steel frame.

The idea, not surprisingly, encountered outright confusion among the members of the NR Block. In order to convince his colleagues Kamiya created a prototype model and, the members understood the concept of his shrimp-shell frame.

The second idea involved the wheel. Kamiya thought of adopting a 16-inch wheel instead of the mainstream 18-inch wheel.

"By reducing the tire diameter," Kamiya said, "we could reduce the machine's weight by around 4 kg and vehicle height by around 5 cm. The lower air resistance and smaller disc diameter also contributed to a lower moment of inertia, enabling the machine to accelerate faster. Moreover, the reduced frontal projection had the effect of increasing output by several horsepower.

"When I compared the 18-inch wheel with the 16-inch wheel, I asked myself which of the two would cross the finish line first in actual races, where average speeds often exceeded 200 km per hour, I was convinced that the 16-inch wheel had greater potential, not simply from the standpoint of partial speeds at corners but by putting all the elements in proper perspective."

Kamiya also adopted a vertical standing screen to be attached to the cowling, instead of the regular semispherical type.

"You could call it an invisible cowling," he said. "With this design we can still achieve sufficient aerodynamic effect. Since the wind is directed upward after hitting the screen, the rider is subjected to less wind resistance at high speeds. Also, the area of frontal projection becomes smaller."

Another idea involved the swingarm and drive sprocket, which were positioned along the same axis. Because chain length was no longer affected by the upward or downward movement of the swingarm, there was no need to provide extra play. This meant an advantage in reducing shock due to acceleration or deceleration, thus stabilizing the suspension's performance over an entire course.

Kamiya believed that no 4-stroke engine, regardless of its merits, could win with a frame design based on conventional thinking. Thus, the NR's frame was constructed from the standpoint of minimizing volume and weight. The staff sought to create a 125 cc frame capable of carrying a 500 cc engine. These were just examples of the many unconventional, even outrageous, ideas being implemented in the new machine. However, they were all based on strategies that had been calculated to an absolutely meticulous degree.

The NR500 was put to the first test ride in Yatabe, Ibaragki Prefecture, in May 1979. Although the road test brought up problems that the team had failed to identify in bench testing, these were gradually resolved through refinement of the engine in repeated tests at Suzuka Circuit and on the course at Tochigi. The NR500 became more complete with each passing day, and soon it would be ready for its first World GP event. Due to a significant delay in the NR500's overall development schedule, though, its comeback had to be pushed back to the British Grand Prix, which was to be held at Silverstone on August 12.

The NR500 was at last completed in July 1979. Equipped with a 4-cylinder V-engine and 100-degree cylinder banks, the machine had a maximum output of 100 ps at 16,000 r.p.m.

Photo

The NR500 evolved as a machine designed to claim the summit of motorcycle racing. It incorporated numerous original technologies, including the oval piston engine and 'shrimp shell' frame.

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