Basic Motorcycle Engine Architecture—Transverse Inline-Four

Decades before Honda launched the CB750, two Italians aimed for the stars.

1969 Honda CB750 KO
Honda, having made the expression “Honda four” a household word through its 1960s Grand Prix roadracing success, followed through in 1969 by being first to bring transverse fours to a wide market. MV Agusta had made a very few premium-priced touring fours, but Honda cracked the mass market.Honda

What is most remarkable in the origin story of the transverse inline-four is that its creators gave this engine most of the features that its best examples have to this day.

When you think “four cylinders,” you think “wide,” but in 1923 two young Italian engineering graduates, Carlo Giannini and Piero Remor, drew their very first such four with nothing on the ends of its crankshaft but its outer main bearings. It was narrow! No ignition on one end and primary drive on the other to make it even wider. The ignition was behind the cylinders, atop the gearbox. The primary drive—by chain in their first design—was at the center, next to the central flywheel. The drive to that engine’s single overhead cam (SOHC) was by a shaft and bevel gears, not located between cylinders, where it would add width, but ahead of them.

I put that word “transverse” in the title so I wouldn’t have to natter on about early fours that were mounted longitudinally, their crankshafts parallel to the direction of motion. That was only practical with a) water-cooling or b) back when air-cooled engines made so little power that their rear cylinders did not immediately suffer heat death in the hot air flowing back from the cylinders ahead of them. The latter made possible the long-wheelbase fours from FN in Belgium, Henderson, and, later, Indian in the US.

Giannini and Remor were seeking specific solutions: 1) to cool all cylinders equally; 2) to avoid the skid-prone and slow-steering long wheelbase of the longitudinal fours; and 3) to achieve the propulsive smoothness that was impossible when one cylinder propels a vehicle by a series of distinct shoves.

Gilera four engine
"The Gilera four engine is a wonderful piece of machinery, which set an example in the multi-cylinder racing technique, later to be copied by several Italian and foreign factories," Carlo Perelli wrote in "Anatomy of the Gilera Four," published in the May 1965, issue of Cycle World.Cycle World archives

Those young engineers weren’t seeking showroom success; 46 years would pass before Honda commercialized the transverse four as its 1969 CB750. They were seeking an ultimate motorcycle. Fortunately, in 1924, they had found a backer, Count Luigi Bonmartini, an enthusiast who provided resources.

In their 1926 design, the single cam’s shaft drive was replaced by a stiffer train of spur gears at crank center, between cylinders two and three. Two carburetors served the four cylinders.

In 1928, they adopted double overhead camshafts (DOHC), one for the exhaust valves, another for the intakes, operating through inverted bucket tappets like those of so many modern four-cylinder bikes. Because their cylinders were vertical, and now the space between the two cams was not easily reached by cooling air, they next elected to cool the cylinder head with water and a small radiator, leaving the four separate cylinders air-cooled as before. With a 90-degree angle between the valves and a normal-for-the-time compression ratio of 6:1, the 51mm x 60mm 500cc engine made just over 30 hp at 6,400 rpm. That’s one horsepower per cubic inch.

In 1928, Giannini and Remor met the talented rider/driver/engineer Piero Taruffi, who had ideas and connections.

In the world of auto racing, supercharging was rapidly pushing power upward. A completely new inline-four was now designed, water-cooled, supercharged, and with its cylinders inclined forward at 45 degrees. To reduce fore-and-aft length, its gearbox shafts were located one above the other—vertically stacked in modern ad-speak—something you can see on any late-model production four.

As so often with small organizations, the three engineers had created a design beyond maintenance and development resources. Entering races, they ran into reliability problems.

Valentino Rossi
As Yamaha and Suzuki MotoGP riders Valentino Rossi (46), Fabio Quartararo, Maverick Viñales, and Álex Rins continue to show, the inline transverse four-cylinder engine remains competitive to this day.Yamaha

Presently, Giuseppe Gilera, a self-made man whose motorcycle manufacturing was now well-established, saw possibilities in the design but was not interested in anything short of success. “Clean up that design and stop the two-bit crankshaft failures. Then I might have a place for your project.”

The crank became successful as a pressed-together assembly of separate elements, allowing use of caged roller bearings that increased crankshaft life to a whole season. What had previously borne the name “Rondine” (swallow) now became the Gilera four. Just as in production fours from the Kawasaki Z1 onward, the primary drive was implemented by cutting gear teeth into one of the inboard crankshaft flywheels. The cam drive remained at crank center. In this form, the Gilera four in the hands of Dorino Serafini was able to defeat BMW’s supercharged twin to take the 1939 500cc European championship.

After World War II, the use of supercharging in racing was banned so Ingengnere Remor designed an entirely air-cooled 500cc four. It took its first world title in 1950 and, after 1951, when Norton’s single in a new-technology chassis carried Geoff Duke to the title, Gilera dominated 500cc roadracing.

A roller crank, pressed together as these were, has only limited torsional resistance to slippage.

When, in the late 1950s, Honda’s Kimio Shinmura designed that company’s first racing four, he drove its DOHC by shaft-and-bevel gears. On the second design in 1960, he not only adopted Gilera’s central spur-gear cam drive but through a jackshaft also drove the clutch from the center of the crankshaft.

Why this seeming complication? A roller crank, pressed together as these were, has only limited torsional resistance to slippage. Therefore, taking the clutch drive from the center minimized the number of press-fits through which power had to be transmitted.

Shinmura also added the innovation of four valves per cylinder. Four smaller valves had proven better able to deliver reliable action at high revs. By contrast, the Italian two-valve racing fours from Gilera and, from 1950 onward, MV Agusta, often had only 300 rpm between peak power and valve float; their riders had to watch the tach, for there were no electronic rev limiters in those days. The Hondas, by large contrast, gave their riders a safety zone of at least 2,000 revs beyond peak.

As on the Gileras, ignition was behind the cylinders. Shinmura’s second design also inclined its cylinders forward as Gilera had done, giving cooling airflow good access to the hot between-the-camshafts region of the head. The designer of Honda’s fabled six-cylinder 250cc and 297cc racers, Shoichiro Irimajiri, would cite such cooling-air access as sufficient reason to splay his engines’ valves at wide included angles.

Honda took a bold step in building the CB750 mass-market four in 1969.

What Honda had that Gilera never did was endless industrial resources. Honda went on to win many championships with twins, fours, a five-cylinder, and sixes, all quite similarly constructed, some of them making peak power at beyond 20,000 rpm.

Honda took a bold step in building the CB750 mass-market four in 1969. Honda knew that with its up-to-date manufacturing methods, such a complex-looking engine could be sold at a price competitive with simpler existing British designs. It was a success, and the market for four-cylinder production motorcycles expanded rapidly.

One basic difference in the CB750 was to completely eliminate the tedious manual assembly of a multi-piece roller crank and put in its place a forged one-piece plain-bearing crankshaft like those found in millions of automobiles. The new CB drove its gearbox and SOHC by chains and had only two valves per cylinder.

Kawasaki, in its 1973 903cc Z1, took a mixed approach: still rolling bearings for a built-up crank but automotive-style plain bearings for two overhead cams, driven by chain from crank center. Unlike Gilera, Kawasaki did put electrical accessories on the ends of the crank, but like Gilera, gear teeth were cut into an inboard crank flywheel—number two rather than Gilera’s choice, the number-three flywheel.

Then Suzuki added four valves per cylinder to its roller-crank GS series later in the 1970s, bringing to the marketplace the high valve train performance Honda had discovered in racing.

The giant air-cooled fours of the 1970s were replaced in the 1980s by more modern liquid-cooled designs, and the complex roller cranks were replaced by much more durable—and machine-manufacturable—forged one-piece plain-bearing pieces.

One out-of-place similarity to the Gilera original now remained: the central cam drive, with a crank bearing on either side of it, at crank center. Now that one-piece cranks were the rule, this was no longer necessary, so one main bearing was deleted—there had been six—and the cam-drive chain was moved to one end of the crank, making engines narrower.

S 1000 RR
When BMW decided to produce its own world-competitive superbike, the S 1000 RR, the engine architecture it chose was the classic inline-four. The bike has just been updated this year.BMW

I went to my shop with a tape measure and found that Honda’s air-cooled 1981 CB900F engine measures 21-1/2 inches wide, cover to cover, while an equally powerful, and faster around any racetrack, liquid-cooled 1990s Kawasaki 600cc four measured only 18 inches. If the electrics mounted on one crank end of that 600 were moved behind the cylinder in original Gilera fashion, it would measure about 15 inches, just an inch wider than the pre-war 500cc Gilera.

There was still more to be learned. During 2003, Yamaha management put noise, vibration, and harshness (NVH) engineer Masao Furusawa in charge of its flagging MotoGP program. People were saying that Yamaha’s transverse, inline-four YZR-M1 could for some reason no longer accelerate with the Vee engines of Honda and Ducati.

Since the beginning, inline-fours had been given so-called “flat” cranks, with crankpins spaced 180 degrees apart, so that when numbers one and four pistons are at bottom dead center (BDC), numbers two and three are at top dead center (TDC). Furusawa suspected that the resulting need to accelerate and stop all four pistons simultaneously twice per revolution might be imposing an inertia torque that was either upsetting rear-tire grip or creating “noise” that prevented riders from receiving clear signals from that tire. When newly hired Valentino Rossi went quickest with the “crossplane” 90-degree crank Furusawa gave him to test, Rossi rode it to the 2004 world title.

During 1960, Triumph engineer Edward Turner toured Japanese motorcycle factories and wrote a report in which he noted that Japan’s adoption of automated production equipment enabled them to produce bikes with advanced features at affordable prices. That’s just how it turned out: In 1960, no one imagined the massive sales that four-cylinder bikes have achieved since then.

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