Suzuki GSX-R750 - CLASSICS REMEMBERED

A look back at the classic motorcycles of yesterday

This is one of the shining examples of successful product planning. Change was afoot in the early-to-mid 1980s. Public taste was evolving. And Suzuki, as always, was strongly motivated to innovate because for them, ideas had to take the place of big budgets.

Honda, the industry Juggernaut, had a multiple personality. One part was progressive, creating the water-cooled Interceptor Superbike of 1983. Another part was traditional, continuing to crank out elaborations of their conservative "iron bikes" of the 1970s –bikes whose distinctive styling vaguely evoked Honda's Grand Prix successes of the 1960s, but which were heavy. Built in this way, a sporting 1000-cc four weighed 600 pounds.

While a 600-lb sportbike had no future, the nature of four-stroke motorcycles was changing as smaller, more agile machines – Ninjas and Secas – found favor with a public that wanted something beyond two-wheeled power stations with killer quarter-mile time and top speed. The giant sit-up liter-bikes that had made household words of the names Eddie Lawson, Freddie Spencer, and Wes Cooley had been exciting to watch, but extensive modifications had been necessary to make them survive even short 50-mile events.

Suzuki planners set themselves the task, not of just joining the new trend, but of leading it. Motorcyclists certainly wanted high performance, but now they wanted bikes with more than one dimension. Honda had built the Interceptor 750 as a homologation special, built in limited numbers to make a Superbike AMA-legal. But the public loved its new agility and all-around performance. Yet it too was an “iron bike”; American Honda fabricator Todd Schuster had said at the time,, “There’s only one chassis heavier than this one in the history of racing – the 1953 Hudson Hornet.”

Honda's bold-and-musical CBX six-cylinder and Yamaha's monster XS1100 had both suffered in road manners from too much engine, too little chassis – They were flexy, had dinky fork tubes (35-mm on original CBX, 37 on Interceptor, XS11), and weak swingarms.

Suzuki had reason to know the handling value of stiffer chassis. An example is the fork tube diameter they gave their RG500 square-four two-stroke GP bikes, starting with 35-mm in 1975, rising to 40-mm by 1981 (roughly twice as stiff). This was not fashion. This was necessity. The more power a bike had, and the greater its acceleration, the more chassis stiffness it needed to remain controllable. Suzuki's new bike would have a chassis based on what RG development had taught them.

Other things being equal, acceleration equals thrust divided by weight, making light weight essential to rapid acceleration. Light weight had already been tried using conventional construction; when Kawasaki had tested a lightweight H1-R chassis made of 7/8" steel tubing, its floppy handling was so frightening that they sawed the engine mounts out of it so no one could use it – even by mistake. Something better was needed.

At first look, aluminum looks no better than steel for lightweight construction, for although aluminum is 1/3 the weight of steel, it also has only 1/3 of steel’s stiffness. This changes when we make tubing of the two materials. Aircraft fuselages are large thin-walled tubes. They are not made of steel, even though steel could be rolled to a thickness 1/3 that of existing aluminum fuselage material. But steel that thin would be foil, lacking adequate buckling resistance (it is for this reason that traditional steel motorcycle frame tubes are typically made no thinner than 16-gage, or .0625” wall thickness). This is the basis of the aluminum chassis revolution in motorcycling – that for a given weight of tube per foot, a bigger aluminum tube can be made with adequate buckling resistance than can be made from steel. The larger the tube’s diameter, the greater its stiffness – even though its wall thickness must decrease to keep to the given weight per foot of tube.

Although Antonio Cobas began his work with twin-beam aluminum chassis in 1982, Suzuki planners knew that while design must lead public taste, it must not lead it by too much. Therefore the chassis of the new GSX-R750 was of traditional twin-loop pattern, but made of large rectangular-section aluminum tube. This was an important step away from brute-force chassis design (You want it twice as stiff? Then it’s gotta weigh twice as much!) and toward light weight achieved in the new way - through shape effects.

Engines were at this time still quite heavy because existing casting techniques did not yet emphasize preventing entrainment of aluminum oxide films in the pour. To compensate for their weakening effect, cross-sections still had to be thick, increasing weight.

A modern narrow-valve-angle four-valve combustion chamber was essential for power, but that left the problem of head cracking in the hot region between each pair of exhaust valves. Air-cooled four-valves had been built, but a great thickness of metal was required in the head both to conduct heat to cooling fins and store excess heat temporarily. Harley-Davidson's great racing manager Dick O'Brien had insisted upon providing extra head material directly above the combustion chambers of the XR-750 air-cooled dirt-track engine of 1972.

Cooling of a modern four-stroke is further complicated by narrow valve angle, which moves intake and exhaust camshafts so close to each other that there is no longer room between them for placement of cooling fins directly over the hot combustion chambers.

One Suzuki engineer, Masahisa Kamiya, had studied the cooling problems of large air-cooled aircraft piston engines. He knew that their cooling had by necessity been supplemented by circulating a high volume of oil through their rocker boxes. Why not cool the critical areas of Suzuki’s new engine with oil pumped through a large cooler? This could provide greater cooling intensity than air by itself, yet would not require the added weight of water jacketing. Piston cooling oil jets were included.

With regard to the oil-cooled original GSX-R, here's an interesting tidbit, from "Tanks: 100 Years of Evolution" (p. 292), by Richard Ogorkiewicz:

"(In 1974), when the US Army showed renewed interest in diesels, it funded the development of another unconventional tank engine as part of the competitive Advanced Integrated Propulsion System, or AIPS, programme. What emerged out of it was another departure from standard diesel engine practice, the Cummins XAV-28, a 27.56 litre V-12 with a high temperature lubricant acting also as the coolant. The engine was to produce 1450hp but failed to come up to expectations, and in mid-1990 Cummins terminate its involvement with it."

The new GSX-R750 engine took the form of a transverse in-line four. Its cooling was achieved partly via cooling fins and partly by circulated and cooled oil.

I think of Suzuki’s first GSX-R750 as a “design reset” because, by being 388-lb dry it was massively lighter than contemporary big sportbikes, setting a new standard. Honda’s 549-lb Interceptor 750 had been a great step forward in handling, but the Suzuki’s lighter weight and stiff construction pushed the bar much higher.

The GSX-R's engine had a progressive bore and stroke of 70 x 48.7-mm = 749.7-cc, or a bore/stroke ratio of 1.45. GSX-Rs immediately became the sportbike to have and soon dominated AMA's near-production Supersport class. Suzuki's contingency program – paying Suzuki riders for wins and places in regional races – is said to have earned more in parts sales than it cost. A few talented riders like Doug Polen were able to make their living this way, by riding every race they could get to. It was an exciting time.

Yet against the water-cooled Superbikes of rival companies, the oil-cooled GSX-R750 won only a single AMA Superbike title – in 1989 – and no titles in World Superbike. At the end of the oil-cooled era Yosh’s AMA Superbikes carried a lot of extra oil coolers, strongly suggesting that oil cooling (which is only about 40% as intensive as water cooling) was limiting the maximum compression ratio they could use without detonation. Never mind – the big weight reduction of which the oil-cooled engine had been a part won Suzuki most of the emerging sportbike market.

The GSX-Rs became famous for their high-rpm shriek and for their top-heavy powerbands. Cammed as they were to make impressive top-end power, they could be a bit weak down low. GSX-Rs were the most available and widely sold of sportbikes. The engineer most strongly associated with them is the honored Etsuo Yokouchi.

In 1988 came an upgrade, changing bore and stroke to 73 x 44.7-mm in the interest of higher rev capability and bigger valves, plus increasing the rate of oil circulation. But at this higher bore/stroke ratio of 1.63 combustion became inefficient, so the engine reverted to the original dimensions two years later. Then in 1992 the inevitable occurred; adoption of water cooling.

Such details are unimportant when compared with the high degree of market dominance and regional racing supremacy achieved by these epoch-making motorcycles.

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