Birth of the 'Supersport' 600

The 600cc lightweight class is well-established today but it was Kawasaki who got the ball rolling back in 1985 when they launched their nimble GPz600. Chris Pearson reckons it can still hold its own today.

THERE are lots of occasions throughout history when the wrong guy iscredited with a breakthrough or important event. Certainly while researching Kawasaki in general for this article my recollections of events were being constantly challenged. There are lots of references to the later GPX600, both in books and on the web, but precious little about the bike that, for me at least, started it all, the GPz600. The many history books that cover the Kawasaki brand place great store in the 1984 GPz900 as a turning point in motorcycle design. In fact, the term 'hyper bike' is often used in this context but the real event that changed the way we look at machines today happened the following year with the launch of the nippy little GPz600. As a styling exercise it broke many moulds with its racy looks and petite, yet full fairing, well equipped with loads of track-like trickery.

This is the bike that has given us all those exciting middleweight machines of the last 15 years or so. The GPz rewrote the book on how a little bike should look, handle and perform and in the process Kawasaki invented a whole new capacity class, that of the 600 Supersport. Before then the middleweight was set to be around the 550cc mark with many manufacturers producing aircooled, in-line fours, all of similar specification and hence performance, with little or no innovation due to a complacent approach to styling and cautious product placement. Of course, we couldn't have foreseen the crippling cost of modern-day superbike ownership back then. Before then the middleweight class was looked on, certainly in the UK if not the whole world, as a 'toy' capacity class, used by blokes to get to work on and some of the LC brigade - should they have avoided the car, marriage and two kids trap.

Instant hit

KAWASAKI lit the proverbial blue touch paper and away we went, effectively giving the rider exactly what they wanted. Producing around 73 bhp, in a smooth (unlike the strokers of the period) and contained manner, the genuine 130 mph plus, watercooled GPz instantly became a hit with the middleweight 'boy racer' brigade. The strong engine, which freely revved to 11,000 rpm, was allied with a great handling chassis containing all of the mod cons considered important in those technically heady days of the mid Eighties. Sixteen inch wheels, for reduced unsprung weight and gyroscopic forces, and anti dive forks for when the twin discs were implemented, scored maximum points for the Kawasaki salesmen making the GPz literally fly out of the showrooms. The machine looked awesome with a sleek, full fairing in a racing style, inviting the riders of the day to get their head down and go for it. And the finish was very good with Kawasaki's typically generous application of paint to the substantial plastics of the bodywork. Honda quickly responded with the even more stylish CBR, followed some time later by Suzuki with the GSX and eventually Yamaha with their FZR.

The rest, as we say, is history. The Supersport class has been with us ever since and the machines that the breed has spawned always contain technical innovation and performance advancement. As every new season dawns the emphasis would appear to be on that particular capacity for guidance and inspiration. With power outputs well into treble figures these diminutive machines are now very capable and serious motorcycles. It is only the Italians and Americans who have staunchly stayed clear of such machines - even Triumph have recently, and successfully I might add, taken up the gauntlet with the TT600 Daytona.

Similar to 550 model

THE GPz600 power plant was based largely upon the existing GPz550 model. The castings and architecture of the crank casings are so similar as to cause confusion, the extra horsepower coming from the watercooling and the doubling-up of valves. The 16 tiny inlet and exhaust valves are neatly lined up within the narrow head and cam box area. A single cam lobe, via a doubled-up rocker arm for each cylinder in turn, depresses these. Adjustment is by the vernier method, removing the need for trips to dealers. In 1987 the engine design was given a larger bore, and shorter stroke, to take the capacity out to 748cc and give us the GPX750. Despite being slated for 'nervous' handling on the road, this bike was good enough to run in the top six at WSB level and gave Kawasaki their first of many World Superbike wins when French rider Adrien Morillas took victory in the second race of the inaugural 1988 season.

The GPz600 Ninja lived on until 1990, though by this time it was woefully outdated, as well as overshadowed by its new, more powerful stable mate, the GPX, introduced in 1988. This bike was considerably lighter than the GPz with an all-up weight some 15 kgs less than the 195 kgs the Ninja hit the scales with, featuring thicker forks and new multi-piston brake calipers that gave a larger pad area. Quite surprisingly, it was also two kgs lighter than the benchmark CBR of the time while the two shared the exact same power output of 84 bhp. The GPX was not as sure-footed as the earlier GPz, the shorter wheelbase and steeper head angle giving a skittish feel, particularly under power. This 'improved' version of the GPz featured a sharp, angular styling, while the rest of the 600 Supersport class got more rotund and friendlier, resembling a well sucked sweet. The Achilles heel of the early GPz would have to be the flimsy steel frame with its detachable down tubes facilitating engine removal. This main loop, and the lengthy frame rails to the tail section, are easily twisted in all but the lightest of crashes. The later versions incorporated a much stronger one-piece loop and separate rear sub frame enabling that part to be replaced should any damage occur. Running a GPz now is potentially a compromise due to the lack of suitable rubber, although thankfully the companies that still supply 16 inch tyres see fit to continually upgrade their old designs with modern compounds.

Camshaft problems, due probably to the high 5000 rpm tickover speed from cold combined with the lack of a centre stand, causing starvation of oil to the extreme right, were prevalent. Likewise, wear in the Hyvo chain, situated between the layshaft and crankshaft, is another common and noisy problem. This gets a double attack of abuse due to it being the drive for the starter motor on to the crank as well. These rattle mostly at tickover as indeed does the clutch basket and anything else if the carbs are out of sync. Carb icing was a major problem and the only cure was to route hot fluid directly from the coolant system around the carburettor venturi to keep the bodies warm. This was at the expense of power but carb icing could prove lethal if it decided to cut out the motor at the wrong time.

Final piece of the jigsaw

ON starting the bike I noted that the bottom end sounded clunky and the carbs felt to be slightly out. Roger replied that a new set of Davida vacuum gauges have been acquired for the final piece of the home servicing jigsaw to be carried out. Looking around the machine, the small diameter of the wheels give a modern chunky look to the tyres but the 37mm forks are positively spindly by comparison. They do work well though and provide a positive riding experience even as the pace gets a little hotter. The forks are not adjustable, apart from varying the air pressure a pound or so either side of nine psi, so the designers at Kawasaki must have done a great job to have got it so right from the outset. Roger informs me that the GPz is just as well behaved two up. Fitted with new Metzeler rubber front and rear, the bike still feels reasonable taut and fresh. The engine is keen to respond and the gearbox has a reassuring clunk to it throughout the six ratios. This makes the GPz feel considerably younger than its 15 years.

Looking back, it is hard to imagine the thinking behind 16-inch wheels, certainly for the front. It gives a slow drag into and out of corners but, even so, the little GPz is a sprightly performer. It's perhaps a good job that a few tyre manufacturers still provide grippy, if not sporty, rubber as the GPz was always the raciest of its contemporaries, with the greatest ground clearance, whopping angle of lean and virtually no warning before running off the edge of the tyre. Flat out and the GPz does tend to weave its way around rather than follow a straight path. This is a trait of the small diameter wheels and once down into double figures the nimbleness and accuracy of trajectory returns. With speeds into treble figures the GPz starts to take a fancy to white lines, snatching the odd kiss whenever one passes within an few inches of either tyre.

With the seating position placing you firmly in the bike rather than on it, you can quickly get to grips with this lightweight machine and feel confident in its cornering ability. Being sat quite low within the chassis really brings the best out of that small, and inadequate looking, fairing which transforms into a very efficient aerodynamic device with very little in the way of turbulence felt once on the move. Although torquey from quite low down in the rev range, power kicks in for real around the 7000 mark and continues to pull strongly all the way through to the 11,000 red line. This gives an effortless approach to riding without the constant up-and-down-the-box technique to keep the motor buzzing. The motor is at its best on a long run as was quickly shown during this photo shoot. With the need for constant short, high speed runs, the GPz soon got very hot and needed the assistance of its cooling fan. The compactness of the engine, while giving great ground clearance and centre of gravity, does lead to overheating. This in turn leads to the one weak spot that the engine has, the clutch. After several runs for the camera it was starting to give up the ghost. Neutral was increasingly difficult to find when stood still, while up-shifts had started to develop that soft 'automatic' feel before finally grabbing home.

[End of Online Sample]