CBX design article from Japan: A Challenge To Dream

Honda climbed all the way up to the top with CB750Four. While feeling easy for long time with that SOHC engine, Kawasaki that got into the business later made Z1 (DOHC900cc) and threatened the premier position of Honda. Honda always flattered itself as the top maker for the motor sports, and it was urgently necessary to make the model which excelled Z1.

CBX was announced on the market as the ultimate motorcycle in December 1977. Detail for the birth of CBX is well known. Kawasaki Z1 posed a real threat to Honda which was at the top with CB750Four in mid-70's, and Honda needed the new model which would sweep away the mood.

The LPL(project leader) for CBX was Shuji Tanaka at Honda Technological Laboratory. He said, "Before the project of new models, we have the evaluation meeting for it. The project teams are called 'S.E.D.' S stands for sales and service, E for engineering, and D for development. Each team discuss on the matter.

Will the engine of GL1000 do, or can it have more power? Or what about much stronger engine with in-line 4 cylinders? Further more, what about the third way as Honda's way? We chose the third way to design a model which was far superior to the opponent as to neglect even its existence.

Honda's way means high revs and high power. In mid-60's, especially high pitch exhaust notes from RC with multiple cylinders had fascinated a lot of maniacs. Starting by the participation in Man TT race in 1959, Honda was the first maker in Japan that challenged the world, and won the maker champion in 50/125/250/350/500cc. On that occasion, RC166 (250cc) and RC (350cc) had air-cooled in-line engines with 6 cylinders, and the image of the ultimate motorcycle that they were going to make was overlapped with them.

Takanori Onda, engine designer, said, "The layout of CBX engine was made only by two of us. The evaluation meeting was set on Monday. We were told to finish it by then, and we sat up all night on Saturday and Sunday. There were 3 designs of CBX engine at first. They were 1200cc and 1000cc with 6 cylinders, and 1000cc with 4 cylinders. The one with 4 cylinders was not that modification of conventional engine, and was specially made for CBX. We put the first
experimental engine on the 750cc frame, and found that it has too much horsepower and too rough as if vicious horse. So it was rejected. Four cylinders engine did not have freshness anyway.

Why the CBX engine had 1047cc, a fragmentary number in volume? "I looked back, but I could not find the exact reason", goes Onda, "Maybe because, in order to gain bank angle, we made cylinders sit as close as possible and shortened the length of the crank". The width of CBX engine is 585mm. No additional devices on both sides of crankshaft, and crank web on both sides as one half width as others made that result. The crank is supported by plain metal by 7 spots.

The DOHC in-line 6-cylinder engine which leaned forward at 30 degree had a strong resemblance to RC.

Tanaka said, "We paid close attention to make a body which should bring out the engine potential with the greatest possible care. Because we first determine the tire size, wheel base, and the suspension travel length, the engine has to be fit in the space left. Double cradle frame makes long wheel base, and the good movability is not expected. The Diamond frame was suggested from a bridge
design, and its truss structure makes it sufficiently strong, rigid, and light".

The width of the CBX engine is 50mm wider than that of CB750Four, but the width of steps of CBX is narrower than that of CB750. What made it possible to shorten the crankshaft of 6-cylinder was the idle shaft which was driven with crank at the rate of 1:1. The AC generator of 320W/5000rpm locates on the left end of the shaft, and 3 sets of transistor pulser locates of the right end. The ignition is derived from battery/coil. ACG is heavy and it has large rotating mass. The idle shaft is driven by hyvo-chain of 34mm wide. Onda said, "Because the turning velocity is directly transmitted to the backside, considerable strength is required to reduce a noise of the idle shaft and to prevent a gap on the ignition timing. When the powerful torque is instantly transmitted, a slipping clutch inside
of the ACG releases that torque, and reduces a strong burden on the shaft. If the reduction between the crank and the idle shaft is large, the transmission has to bear huge torque, and accordingly the drive system and all others should be strong and heavy to withstand the torque. That is why the ratio of 1:1 met the ideal". Tanaka states, "The engine with 6 cylinders runs smooth, and the frame
has less stress, so it is profitable to make a light body. Well, some of us claimed that it was too smooth to feel the torque, and that it was not fun, though".

The background on the birth of CBX was explained by Morioka, who now works at American office (HRA).

"We were fighting against heavy odds in Europe at that time, so we made a project team which consists of some members including the president of our lab. We flew to the Europe in a hurry, and began to rearrange the sales promotion for big sports bikes. We named the project 'Operation Normandy '. It was CBX which was the flagship among 14 models. We developed a series of Euro-sport

models such as 900/750 in-line four simultaneously".

About the design for CBX, Morioka said, "We made all possible effort to make the 6-cylinder engine look less wide, shortening the width of the crank case. Besides that, we paid attention to make the engine look more compact. For example, there is a gusset (?) of engine bracket on the cylinder head, we widened its width and the width of gas tank rail of the frame on purpose, and tried to lessen the exposed area of the head cover. The affair still comes to my mind now and then, however, we had no way to hide the size of the engine after all. By the way, it was CBX that for the first time we used duralumin forging for a sport bike. Handle bars, steps, a brake pedal, a shift pedal, and so on".

The flagship, CBX, for which Honda set foot on European soil again, standing on the threshold of a new deployment, was sold in American, European, and Australian markets.