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During this period, development continued. And, the following year, 1974, the YZR500 returned to the GP in its OW20 spec with the new Monocross suspension that significantly improved the
machine's competitiveness. To ride the OW20, Yamaha signed a contract with former MV Agusta star Giacomo Agostini, who teamed with Tevo Lansivuori. That year, Agostini won round three at Salzburg and
round six, the Dutch GP, while Lansivuori won round eight at Anderstorp. This record brought Yamaha its first manufacturers championship title in the 500cc class for 1974.
The secret behind this
new world-beating performance was the Monocross suspension. This suspension was characterised by the fact that its positioning of the shock absorber under the fuel tank enabled a larger wheel stroke
than a conventional suspension. The basic structure of this suspension was patented by a Belgian professor. Yamaha acquired the rights to the structure and proceeded with research and development and
applied it to its motocross factory machines on a trial basis. It was unveiled for the first time on the YZM125/250 machines Yamaha entered in round one of the All Japan Motorcross Championships series
at Yatabe. The following year, 1974, it was also adopted on the OW20 that would win the manufacturers championship title along with Agostini's rider title that year.
Of course the same
motocross-use suspension was not simply transplanted as it was to the road racer. "On the OW20, the accumulator had the same basic ball structure as that on the motocross machines, but we need to make
significant changes in the settings, adjust the overall length by introducing an adaptor and fine-tune the damping force adjustment mechanism for the compression stroke. On the OW20, the monocross
suspension provided increased cushioning performance and a better concentration of mass at the centre of gravity that contributed to improved handing stability," says Mr. Makoto Sugimoto, who was in
charge of performance testing.
Furthermore, the Monocross suspension adopted on the OW23 spec the following year featured a new accumulator design that contributed to more stable hydraulic fluid
pressure, as well as improvements including revised settings, new aluminium and magnesium parts to reduce unit weight and a screw-type initial load adjustment mechanism for the spring. In these ways, the
suspension was improved year by year and became an important asset contributing to Kenny Roberts' three consecutive GP titles. And, the improvements would go on in the years to come as the YZR500
continued to evolve.
The early '80s saw another big development in the suspension. It came with the introduction of the second-generation YZR500 with a square-four engine, the OW60 of 1982. The
OW60, which won its debut race in the opening round of the GP series, was a full six kg lighter than the OW54 of the previous year. At the same time, this spec adopted successful new suspension
technologies that included the use of a "bell crank" that produced progressive effect to move the shock absorber.
"With the introduction of the Monocross suspension Yamaha had gained a
definite advantage over the competition. But the competitors then came back with link-type suspensions that developed the possibilities of a rising-rate effect. In response, Yamaha began testing a
rising-rate approach with the OW53 only to find that adding a link mechanism to the existing shock unit was not enough to render a significant effect. With the OW60, Yamaha stuck to the basic structure
that identified the "Monocross suspension" with its shock positioned under the fuel tank and added a bell crank to move the shock absorber. Also a lighter unit was developed for the shock absorber
itself," comments Mr. Shigeto Kitagawa, one of the engineer staff of this project. In this way the OW60 became the groundbreaker for the next development of the Monocross suspension.
 The
excitement of the one-two finish by Kenny Roberts and Barry Sheene on the bell crank-fitted OW60 at the season opener in Argentina on March 28 had barely time to subside before Yamaha called a press
briefing on the paddock at round two of the series at Saltzburg on May 2 to introduce a new factory machine the OW61. This OW61 was the first YZR500 to mount a V4 engine and it also feature a new
aluminium frame that eliminated the under-loop. This frame structure would be the roots of the coming Deltabox frame, and here again an innovative suspension was introduced. In it, the shock absorber was
positioned at right angles to the direction of forward motion with links at both ends in a layout that would come to be called the "crab claw." This design featured excellent space efficiency while
contributing to a lower centre of gravity.
In its debut race, Roberts rode the OW61 to a third-place finish, and then to victory at the Spanish GP. But the goddess of victory smiled no more on
the lateral suspension OW61. Eventually, this would be the last machine to use this suspension, but it would be remembered as an example of the kind of taboo-breaking, free thinking design that lies at
the heart of Yamaha machine development.
"A lot of research went into the lateral suspension from a number of directions. We gathered data about how much contract the tire had with the road
surface during cornering, acceleration and over bumps. Then we would take that data to the test lab and use it in analysis utilizing a servo pulser. The results we got in the lab would then be taken
back to the GP circuit and new things tried to improve the system. But we also realized that the space interference resulting from the lateral layout of this suspension was preventing us from making
optimum use of the thinner engine made possible by the rear-surface rotary-disc valve system. Also, we found that the reaction force from the road surface produced undesired flexing in the shock
absorber itself in the case of a lateral suspension. We finally reached the conclusion that a vertically positioned shock absorber placed less additional load on the shaft, and after that we decided to
concentrate our development efforts on a vertical shock layout," says Mr. Takaaki Suzuki, who was in charge of tests.
 For the next model, the OW70, the design was returned to the bell-crank type
suspension used on the OW60. "All the new things we tried with the lateral suspension brought with them an element of uncertainty. We could have chosen to keep working on the detail until we got a
mature system, but we knew there would be considerable difficulties involved, so we decided to return to the proven bell-crank format for the rear suspension. At the same time, we were working on the
chassis development with the focus on keeping the basic framework of the frame within the same width as the V4 engine. This effort would lead to the birth of the aluminium Deltabox frame," explains Mr.
Sugimoto.
As it turned out, the OW70 introduced for the '83 GP season mounted a bell crank format suspension, but by mid-season the OW70 would be given a bottom-link type suspension that further
improved performance. This machine, with the new bottom-link suspension would be the one Kenny Roberts rode against Spencer on the three-cylinder Honda in one of the closest title bouts in GP history.
The OW76 spec introduced from the start of the 1984 GP season mounted another bottom-link suspension with the same basic structure that GP machines use to this day, though with each year's GP this
suspension has continued to be improved and matured.
This was a period that saw the appearance of the Monocross suspension and its development into the bottom-link type suspension. This
development coincided with the change from the square-four to the V4 engine, the adoption of the crankcase reed valve and also the appearance of the first Deltabox frame. Never before had the YZR500
gone through so many dramatic changes in such a short period of time.
The aluminium Deltabox frame first introduced in 1983 on the OW70 became a defining feature of the YZR500 in the years that
followed. This OW70 was the machine Kenny Roberts rode in his legendary GP battles with Freddie Spencer on the three-cylinder Honda NS500. Externally the machine was characterised by the three
connecting points of the chassis' triangular framework, two points at the top and bottom of the steering head and one at the pivot. The aim of this design was increased handling stability and
manoeuvrability.
However, the original aim of the Deltabox configuration was not simply increased frame rigidity. It was born of the quest and research for a frame on which the new V4
engine could be mounted most efficiently.
"On the OW61, the first spec to mount the V4 engine, it was necessary to fit the carburettors and the rotary-disc valves neatly inside the "V" bank of
the engine block. This led to the proposal to adopt an aluminium frame on the OW61 that would offer good engine mounting and efficient space allotment for the intake system," says Mr. Sugimoto.
It was the following OW70, however, that was first to use the frame that is now called the Deltabox. As we mentioned with regard to the suspension development, the lateral suspension adopted on the OW61
prevented the adoption of a slimmer frame that would have been possible with the narrow engine format achieved with the rear-surface rotary disc valve V4. "It was on the OW70 of 1983 that we dealt with
this issue by working to develop a frame that was roughly the same width as the engine while also being lightweight and highly rigid. The answer we came up with was the Deltabox frame."
Some
unique ideas were tried in the process. "At first we thought of using the interior of the main frame for the fuel tank in order to achieve a greater concentration of mass at the centre of gravity. We
tried using a special coating used in aircraft on the inside of the frame sections, but eventually there were regulation-related problems that prevented the use of the in-frame tank in actual GP racing.
The Deltabox frame that eventually appeared in GP competition on the OW70 was a product of further refining the combination of pressed parts and machine cutting parts used in the OW61 frame and then
employing computer analysis to achieve a high degree of lightness and slimness." |
