The Kawasaki GPz550 Launched the Middleweight Revolution

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Kawasaki’s 1981 GPz550 was an off-the-shelf sportbike that street riders and racers couldn’t get enough of. (Cycle World Archive/)

For all practical purposes, 1980 was still the ’70s in the world of motor­cycles. Handlebars were still gener­ally high, footpegs were out front, and sportbikes were, for the most part, hot-rodded standards. The hottest of these was Kawasaki’s KZ550, a handsome-enough sunset-striped middleweight UJM that combined reasonable weight with decent power to rule the box-stock class at local racetracks. In fact, when Kawasaki noticed that its dealers couldn’t keep the KZ in stock and that racers were buying them used for new-bike prices, it put together a bike that wound up changing the entire motorcycling landscape. The GPz550 is arguably one of the most influential motorcycles in history, and its four-year run absolutely launched the sportbike as we know it.

The magic of the bike was simple: Kawasaki simply took what racers were doing to the KZ and put it into production. The 1981 GPz550 got the KZ’s oversquare two-valve 553cc engine, but with higher compression, hotter cams, electronic ignition, and an oil cooler. The rest of the bike got a second front disc, a disc instead of a drum out back, an air fork, adjustable damping on the shocks, lower handlebars, and rearset pegs. A little cafe fairing was fitted to the headlight, the engine and exhaust were blacked out, and Kawasaki sent it out into the world.

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Where the GPz550 basically exploded. In the best possible way, mind you. Once more, Kawasaki found it couldn’t keep the bikes in stock. For about $3,000, just a few hundred more than the KZ550, you basically got the racebike the box-stock guys were building themselves, but with the reliability, warranty, and in deep red with black stripes, the beauty that only comes from the factory.

Kawasaki’s 1981 GPz1100 shared many of the 550’s styling cues but did not find the same success. (Cycle World Archive/)

If this doesn’t seem like a particularly radical idea now, that’s because this bike helped make it routine. At the time, it was a phenomenon. Sport riders and racers alike couldn’t believe their luck. The street guys got a stock bike they could ride reasonably fast all day, with a decent riding position, gearing tall enough to let them relax on the highway, and a fuel gauge, though that was admittedly only about 25 percent more accu­rate than a best guess. The racers got a bike that was almost perfect and easily modified where it wasn’t; the 22mm flat-slide carbs were easily tuned, competitive rubber went right on. It wasn’t perfect: The bike could be a little hard to start when cold; there was no balance tube joining the fork legs, so air had to be added and pressure set individually, an occasionally finicky process; and the seat was merely OK.

If those sound like quibbles, they are. Riders may just as well have complained that there was no ice cream dispenser. And the good news kept coming; test results showed that the middleweight GPz550 was as quick as Kawasaki’s mythical Mach III two-stroke triples, as quick as the still-revered 900cc Z1. It ran the quarter-mile deep into the 12s, and could do so all day. It was, in short, everything anyone had ever wanted.

1981 Kawasaki GPz 550 road test (Cycle World Archive/)

A little anecdotal history may be useful here, just to put the GPz550′s runaway smash hit status in perspective, because the bike was more than the sum of its parts. Note that Kawasaki also introduced the GPz1100 the same year, featuring fuel injection and monster power. It did…OK. Yamaha also introduced its Seca 550 in 1981. It was widely considered a decent-enough middleweight bike, if you couldn’t find a GPz for sale. A year later Kawasaki tried to follow up the success of its 550 with a GPz750, which met with the same reception as the 1100, and a belt-drive beginner GPz305, which won a Ten Best Bikes award. Even so, none of those bikes had the same sheer purity, balance, or value for money.

Neither, for the next couple of years, did the GPz550 itself. The 1981 model was a one-year wonder. The ’82-83 model made big headlines by offering a single-shock rear suspension, a progressive Uni-Trak unit mounted to the swingarm and adjustable for damping and preload. It worked well, but the bike also had different chassis geometry, with rake kicked out and trail kicked back. The power got down better, but the bike was a little less agile, not quite as directly wired into the synapses as the ’81. The engine’s internals were unchanged, but the carbs switched over to 26mm constant-velocity units, which made the bike somewhat less responsive and less tunable. Still, it was a surpassingly good road bike, and Kawasaki sold every one it could stamp out.

For just a few hundred dollars more than the KZ550, Kawasaki provided racers a nearly complete package in the GPz550—less so with the ’82-83 model pictured. (Cycle World Archive/)

But Kawasaki, whether it knew it or not, had really started something. In 1983, Honda introduced its radical V-4 VF750F Interceptor to a huge reception, following it up the next year with the brilliant VF500F middleweight. The 1984 model would be the farewell tour for the now-outclassed GPz550, updated with a body-mounted fairing, a few more horsepower than before, a somewhat annoying anti-dive system, and not much else of note. Kawa­saki, an engine-centric com­pany, had decided to fight Honda with displacement. Its energy was poured into the Ninja 900, which it carefully put under Tom Cruise in Top Gun not long after it put the GPz550 out to pasture.

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The excellent Ninja 600R would follow in the resulting sportbike explosion, as would five-valve Yamahas, stripped-down Suzukis, and bubble-fairing Hurricanes, with technology larded on thick as the scene played out. But the heart of the sportbike revolution was always the middleweights, and it’s impossible to deny that the grassroots-inspired GPz550 started it all.

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[Ian Frog]

Don`t you just love the Americans view of history lol.

Have they completely forgotten about the Z650 ?

I accept the 550 was a lovely thing but it was hardly earth shatteringly new in any significant respect.

The GPZ600 with the camchain repositioned ala the 900 would be a different kettle of fish perhaps.

Cheers

Ian

[S-Westerly]

If it didn't happen in America it never happened at all.