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 In 1981, the year after Kenny Roberts won his third consecutive world title on the YZR500, the GP became the focus of attention for new reasons. That year, Yamaha introduced a square-four
model YZR500 (OW54), while Suzuki introduced the RGG500 with a square-four with a level difference between the two front and two rear cylinders. Kawasaki rolled out its KR500 and Honda entered with the
four-stroke NR500. The next year, Honda finally came in with the two-stroke, three-cylinder NS500. As it turned out, the winners in '81 and '82 were Suzuki's Marco Lucchinelli and Franco Uncini.
Amid these developments, Yamaha introduced its last parallel four (OW53) and its first square-four (OW54) in 1981. In 1984 came the second-generation Yamaha square-four (OW60) and its first
V4 engine model, the OW61. "Through 1980 we ran with the parallel fours, but in order to match the power performance of the rival square-fours, we began development of a V4 as our next-generation racer.
There was a lot of risk involved in suddenly jumping in to the races with a V4, so we first developed the same kind of square-four as the competition and fed back used the race data gained with it to
the V-4 development project," recalls Mr. Ito. What proved to be the biggest problem with the V4 design, however, was the layout of the intake system.
The square-four model OW54 introduced in
1981 adopted a rotary-disc valve that definitely delivered more power than the conventional piston-valve type parallel four. However, the added weight of the engine affected the handling stability,
which led to a number of problems. Eventually, the V4 engine OW61 model was the product of development efforts aimed primarily at solving these problems by reducing engine weight and reducing its width
to one comparable to a 250cc engine.
Since a two-stroke engine has a separate crankcase for each cylinder, the width of a single-axis (shaft) V4 ends up being about the same with as a parallel
four. However, adopting a two-axis design enabled a design with less right-left width. It also theoretically offered the advantage of easier voluntary adjustment of the ignition angle, but the problem
became where to position the intake system. In other words, the question was where on a V-4 design could you position the intake system for the rotary disc that was normally positioned on the side
(lateral) surface of the cylinder.
The key that would lead to the solution of this problem came from words muttered by one of the engineers in a development staff meeting. "One day, one of our
engineers was mumbling to himself, 'If only the disc could within the "V" bank between the cylinders.' At that minute the whole picture became clear," recalls Mr. Shiohara. This gave birth to a highly
original concept of throwing out the side-mounted rotary disc with its inherently limited forward projection area, positioning the carburettors in the "V" of the block and having one rotary disc service
two cylinders. Now the race was on to test a prototype and get it ready for use in actual competition. This led to the birth of the rear-surface rotary-disc valve intake system.
"In the early
stages of development we had problems like the fact that the drive portion of the disc began to wear very quickly and that we were unable to exceed the performance of a conventional rotary disc. But,
believing in its future potential, we continued to improve on it through trial and error until it gradually matured as a system. Still, I never imagined that the V4 format we were creating would become
the standard engine format for the next 20 years, says Mr. Shiohara.
This OW61, the first YZR500 to mount a V4 engine was unveiled at Saltzburg in May of 1982. Then, two years later the OW76
appeared with a crankcase reed valve. Eddie Lawson rode this machine to win his first world title in 1984, after which the V4 engine continued to dominate the GP scene.
The biggest turning point
for the two-crank-axis V4 probably came with the OW81 in 1985, '86. This was the machine that Lawson won back his title on after losing it to Freddie Spencer on the Honda NSR500 in '85. "On the OW76,
the first YZR500 to adopt a crankcase reed valve, there were things we still had to work out concerning the effect of the engine characteristics on the machine handling. We needed to improve the gyro
moment of the crank shafts, find measures to improve torque reaction and reduce vibration," says Mr. Shiohara. These things caused the development team to turn its attention to the revolving direction
of the crankshafts.
With the existing two-crank V4, both crankshafts revolved in the direction of forward motion, but the question was raised that that might be affecting the handling stability
of the machine. A prototype was created to test this possibility. "In fact, we had created the OW77 prototype and were already running tests on it since the previous spring. It had a two-crank V4 engine
on which both crankshafts could be revolved in either direction and it was set up to enable actual running tests as well. Based on those running tests it was decided to have the two cranks revolve in
opposite directions, and we built that into the layout." Mr. Shiohara adds.
 Eventually, the structure adopted on the OW81 had the two crankshafts revolving in opposite directions (the forward
shaft opposite to the direction of forward motion and the rear in the direction of forward motion) with the drive force picked up from the rear shaft. This layout reduced the gyro moment effect on
handling stability to a minimum to help make the YZR500 a machine with excellent handling characteristics.
This would not be the only engine development effort aimed at improving handling
stability. On the 1992 model OWE0, simultaneous phase combustion was adopted from the latter half of the season. "This 0-degree / 90-degree simultaneous phase combustion improved the rear traction
characteristics and thus boosted acceleration performance coming out of the curves. On the other hand, however, there were some problems that would never be expected with 180-degree simultaneous
combustion. Combustion force caused increased stress to the gearbox assembly and the clutch assembly and also led to fears about the reliability of the gear assembly. Of course we couldn't send a
machine we had fears about into the GP, so we cancelled our summer vacations and ran repeated endurance tests until we had the durability demanded," recalls Mr. Hiroya Atsumi, an engineer staff of the
OW81 at the time.
"We were eventually able to introduce this machine from the second half of the GP series that year, and we stuck to our firm belief that the first step of handling stability
was having a machine that the riders could use with full confidence in its reliability. In another aside, we continued tests with different ignition timings, including a 70-degree / 290-degree interval.
Among our riders, Luca Cadalora and Kevin Magee especially liked this spec. But, in the long run we eventually settled on the 0-degree / 90-degree spec. This was because of the good torque balance it
provided between the combustion torque and the inertial torque produced by the revolution of the crankshafts," adds Mr. Atsumi.
The development of the two-crankshaft V4 model, which began with
the trial-and-error search for the right intake system, would proceed to a history of crankcase reed valve development introducing high-performance resin plastics, as well as a pursuit of handling
stability and the kind of reliability that lets the rider ride with assurance. This is the foundation that the YZR500 was built on and it is the starting point for any motorcycle. It is Yamaha's
starting point.
The history of the YZR500 is also a history of suspension development. After winning the first two rounds of its debut season in 1973, Jarno Saarinen was again in the lead with
the fastest lap time in the third round at Hockenheim in West Germany when his chain broke and he was forced to retire from the race. The winner that day was Phil Read on his MV Agusta. In the fourth
round at Monza, Italy, Saarinen was the victim of a fatal crash caused by another machine's oil leak in the 250cc race. In honour of Saarinen's memory, Yamaha withdrew from GP competition that year. |
