This series began with the 1979 Z500, a scaled-down version of the legendary Kawasaki Z1R. It used a double-cradle steel (Norton featherbed-style) frame with a transverse-mounted air-cooled 4-cylinder engine--a classic universal Japanese motorycle.
Z500/Z550 (KZ550) - This was the first of the line, with a 500cc engine, later bored out to 553cc's and also available as the 398cc Z400J (identical to the Z550). Early Z550 models (1980-1981) had a double disk brake in the front (The US model KZ550 had a single disk brake) and a drum brake in the rear, with a conventional swingarm using twin shock absorbers. Later models (1982-1983) had improved brakes (twin disks in the front, with a single disk in the rear). The original Z500 (1979-1980) differed from the early Z400/Z550 models in having twin front and single rear disks.
ZX550 (GPz550) - Introduced in 1981, this was a slightly more aggressive version, with improved brakes, larger valves, more aggressive tuning, and a bikini fairing. A monoshock rear swingarm was introduced in 1983, along with updated bodywork.
Living with the Z500/Z550 These make excellent city bikes, very easy to manage at parking speeds and very agile in urban traffic. They have safe, neutral, tossable handling and a flexible engine, making them fun to drive without being too intimidating for first-time riders. These machines are also very easy to work on, with ample engine access, a simple electrical system, and a very reliable drivetrain.
Earlier models suffered from inadequate brakes and bad clutch cable routing; these issues were resolved with the 1981-on models, which also featured a simplified clutch cable and electronic ignition. Despite these constraints the original Z500 model was a road-going race bike with a free revving engine, triple disks and it's own race series. Britain's best known formula one champion Damon Hill began his racing career riding a Z500 in competition.
The Z500/Z400/Z550/GPZ550 series developed into the GT550 shaft drive and eventually the 1991-on Zephyr (ZR550) models. With a twenty two year model run in various incarnations they have gained a reputation for reliability and longevity reinforced by numerous examples of GT550 courier bikes covering in excess of a quarter of a million miles despatching in London.