In 1967, when the Japanese manufacturers were still focused on the World GP, Honda unveiled its vertical-twin CB450 model, the first big-displacement production bike with a DOHC cylinder head. After cementing its image as the world’s supersport bike manufacturer with the CB72 (250cc) and CB77 (305cc) models, Honda moved on to its next challenge. Kawasaki also entered the big-bike market with its W1 model—which used the OHV vertical-twin engine the company acquired after absorbing the Meguro Manufacturing Company and boosted its displacement to 650cc—and the Mach III with its 2-stroke 3-cylinder 500cc engine.
In actuality, a number of Japanese manufacturers had already been producing 500cc bikes or bigger back in the nascent period of the motorcycle industry before the 1960s. However, these were no more than copies of British or German bikes and were products targeting Japanese users who could not afford the expensive and few imported models available on the domestic market. In terms of performance as well, they were not of a quality that could be exported.
Needless to say, however, when the industry began developing and producing big bikes again in the latter half of the 1960s, it was most certainly with the aim of targeting overseas markets—with the United States as the prime target. For motorcycle manufacturers at the time, the country was the biggest motorcycle market in the world during the 1960s and 1970s. There, it was popular among the younger generation to ride big, 650cc class British-made bikes like a Triumph or BSA. Hoping to ride this wave as well, BMW launched a 750cc model on the U.S. market with “Americanized” coloring.

Yamaha’s first 4-stroke big bike was the XS-1 that debuted in 1970 mounting a 650cc, SOHC vertical-twin engine. It was a bike with beautiful styling inspired by the British Triumph and BSA models, but it was developing the machine’s handling that caused the engineers the most headaches. “In developing the XS, the first engine we had wasn’t reaching the performance standards that we’d set, so that was the first problem our engineers tackled,” says [the late] Takafumi Fujimori as he looked back on those times and explains the circumstances the team faced.

(Taken from an interview at the time of Chief Engineer Takafumi Fujimori from Yamaha’s Road Testing Unit, 2nd Project Engineering Division)

“At the time, I often went to see what was happening at the engine bench. Unlike the intake and exhaust ports of a 2-stroke, they said 4-strokes were all about the cam profile and they were changing it by welding on additional ridge height and then using a grinder to grind it down again. It was really like we were feeling our way through the dark,” continued Fujimori.
Trying one thing after another, the team gradually approached the target engine performance goals and started test-running the bike. Just around that time, the Yamaha Fukuroi Test Course was completed. Thanks to this course where development teams could test the bike on the high-speed straight and ride through the turns, the reputation of Yamaha’s motorcycles for good handling would be further strengthened over time. Needless to say, however, the XS-1’s stage of development hadn’t reached that level of testing yet. “At first, handling stability was tested in the 140–150 km/h speed range. It was later raised to about 180 km/h, but we started the development in the former range. There were no particular problems at that speed. But, as the prototype’s performance and speed increased, we began to get shaking at the handlebars.

When you have this kind of shaking known as speed wobble, in many cases it increases rapidly in oscillation, and if you react by cutting back on the throttle suddenly, the built up stress on the bike will immediately rebound, causing the handlebars to wobble even more violently to the point where it can cause a fall.
“After testing it, I went back to the engineers and reported to them that it really shakes badly. This was a new experience for them too, so they turned around and asked me, ‘What should we do?’ Since it was shaking, the obvious conclusion was that the chassis wasn’t rigid or strong enough. So, they tried rebuilding everything from the engine mounts to the swingarm pivot. On the frame they welded in a cross-pipe and gusset plates at the pipe joints to increase the rigidity. When they did that and I tested it again, I could feel how it’d been stiffened, and began to get a feeling for what the difference was. This was probably the first time that we treated handling stability as a basic fundamental element of ‘technology’ in developing a production model. It was because we were now working on a big bike that forced us to deal with this problem, and afterwards, I often found myself thinking that it really was fortunate that this problem came out at such an early stage in our big bike development.”
Back then, the general machine design would come first, then the layout and format of the engine would be decided, and after that testing would begin. Since at the time there was no know-how like today that could enable the engineers to first decide on chassis specs that would prevent these kinds of technical problems from occurring later in the development, it was no wonder that things turned out that way. Also, I’m told that the way targets for performance and handling were set was to look at what the competitors had already done with their models and judge what was good enough based on what they had achieved. In that sense, since there was also speed wobble occurring in the Triumph and BSA bikes at higher speeds, they hadn’t taken the problem so seriously in the early stages of the XS-1’s development.
(Continue to Part 2)