5Uncharted Territory: Engineering from the Ground Up
We finally got started on the bike’s structural design, but this was our first-ever attempt at making something from the ground up. So to begin, we gave a great deal of thought toward deciding the order in which what should go where and how. We then decided to create the layout in the following order:
1. Riding Position
Design the triangle formed by the points at which the rider contacts the machine, i.e., the seat, handlebar grips and footpegs, and their respective dimensions to create a sporty, forward-leaning riding position. (We referred to the methods used when setting dimensions for the area around the driver’s seat in a car. Of course, this was the first time this kind of method was used for a motorcycle.)
2. Seat Height
Choose a seat height that improves ride stability and sets the bike’s center of gravity as low as possible. Also, make it low enough to allow riders to comfortably reach the ground with both feet.
3. Front Wheel Positioning
There was much consideration about how to achieve the proper balance for the rake and trail, but we concluded that the final settings chosen should place more emphasis on ride stability than things like the bike’s agility for leaning.
4. Rear Wheel Positioning
From our guiding principle of building a compact machine, the wheelbase should be shortened as much as possible, i.e., to the very limit where the desired front-rear weight distribution is still maintained.
5. Wheels and Tires
Aim to use wide tires and small-diameter wheels, and decide sizes based on our manufacturing capability. (We went with 16-inch tires, making this the first motorcycle in Japan at the time to use small-diameter tires.)
6. Frame Structure
Although we felt the pressed-steel monocoque frame offered the advantages of light weight and straightforward engineering, it also requires a large press to construct. And if we were to use an all-new frame structure like this, we would have to be able to thoroughly strength-test it beforehand, so going this route would present serious difficulties. However, we didn’t want to build a conventional steel pipe cradle frame either (almost all motorcycles at the time used them).
In the end, we took up the challenge of building a completely new kind of frame. It used a single backbone of large-diameter tubing mated to a monocoque rear section integrated with the rear fender. With this design we could perform adequate strength testing and it allowed us to calculate the section modulus of the frame’s various reinforcement components. We also judged that this design would make determining the amount of flex and rigidity needed at different locations clear and simple.
7. Front Fork
We focused on both good handling and stability, so instead of using the leading-link fork that was catching on at the time, we went with a normal telescopic fork. However, we increased travel by 20–30% of the norm for good stability on rough roads.
8. Rear Suspension
Rather than the plunger suspension used on the YA-1 and YC-1, we used a swingarm for the rear suspension for superior handling stability.
With a priority on sportiness, stability and a riding position that wouldn’t change too much at speed or in different road surface conditions, etc., we used a fixed seat—the first of its kind in the Japanese motorcycle industry at the time—instead of the common sprung saddle seat. Added impact absorption to compensate when riding was provided by the aforementioned longer fork travel and the swingarm at the rear.
10. Fuel Tank
After the items above for the layout are finalized, the remaining space the fuel tank needs to fit into naturally becomes clear. In line with our plan to design a compact bike, the tank would be shorter than existing models, but we also wanted it to have a roughly 15-liter capacity, slightly larger than what was on other 250cc motorcycles of the time. To achieve this we would have to make the tank taller, but then the handlebars would come into the equation. This meant we had to make the rear of the tank taller and it ended up looking like it had been put on backwards compared to conventional fuel tanks. The completed YD-1’s tank would later be called Bunbuku Chagama from the Japanese folktale of a raccoon dog that could transform into a teapot. This is a really good example of how a motorcycle takes on its own unique character when form (design) and function are in sync. Because a motorcycle’s fuel tank is a very important design element, the YD-1’s left a powerful visual impression.
We used a 2-stroke twin-cylinder engine, the same format as the Adler MB250 we initially planned to duplicate. This made the YD-1 the first Japanese motorcycle with a twin-cylinder engine.
It was in this order that we began designing the motorcycle, and then started engineering the structure, working out the detailed specifications and finalizing the exterior design, all based on shared principles and ideals. We worked past eleven every night, sometimes even sleeping at the company. It was incredibly hard work, but probably because we were young and the job was interesting, we never found it burdensome. We were a small group of young engineers given a huge responsibility, so we gave it everything we had.