Kawasaki W series

1960s BSA A10 Super Rocket

The Kawasaki W1 is based heavily on the post-war, pre-unit construction, 500cc vertical-twin BSA A7 design inherited from Meguro, but as time passed, the Kawasaki and BSA designs diverged.[14] The BSA engine has a 70 mm (2.8 in) bore and 84 mm (3.3 in) stroke, whereas the W1 inherited its 72.6 mm (2.86 in) stroke from the K2 engine, adding displacement by increasing the size of its bore to 74 mm (2.9 in). This slightly oversquare (i.e., short-stroke) design favors higher engine speeds, while reducing stresses on the crankshaft. In addition, the new W1 had a multi-piece pressed crankshaft assembly with ball bearings and one-piece connecting rods with needle bearings, significant changes from the earlier BSA (and Meguro) engines that used plain insert type bearings and two-piece connecting rods. The BSA and Kawasaki 650cc engines were thus mechanically different, but they looked very similar.[17]

Likewise, in the design of its twin-loop frame, as well as its overall styling, the W1 motorcycle was clearly influenced by classic British road bikes, including shifting with the right foot and braking with the left. From 1966 to 1968 W1 engines were built with a single 31 mm Mikuni carburetor (this is only feasible in a straight-twin with a 360° crankshaft angle). Starting in 1968, the W1SS with two 28 mm Mikuni carbs took the place of the original W1. Also in 1968, the W2 (aka Commander) emerged. The W2SS was a restyled W1SS with slightly more horsepower, and the W2TT was a high-pipe version with twin mufflers on the left side. Due to flat sales in North America the W2TT was discontinued in 1969, the W2SS ended in 1970, and finally in 1971 Kawasaki axed the W1SS.[6][18]

The 650 remained popular in Japan, and although some were exported to Europe in the 70s, subsequent models were produced primarily for the domestic market. The penultimate model in the W series was the W1SA with stylistic changes, but most importantly with the gearshift lever on the left side and the rear brake pedal on the right side, which is the standard configuration for Japanese motorcycles. The final version was the 1973 W3 model (aka RS650) with upgraded suspension as well as twin disc brakes in front. W series production ceased in 1974.[3][19][20]

As soon as the W1 was released, Kawasaki realized that even an improved version of the BSA A10 (itself already discontinued) was at a disadvantage against the newer and faster unit construction British twins, the BSA Spitfire and the Triumph Bonneville T120. The W1 also had to compete with other Japanese twin-cylinder street bikes, such as the Suzuki T500 and the Honda CB450. If the W1 was seen as being behind the times, then Kawasaki came back with a two-stroke engine that was clearly ahead of its time, in the 1967 Kawasaki A7 Avenger with performance at least equal to the W1. The following year the W series (as well as the British bikes) faced a new competitor in a state-of-the-art twin from Yamaha, the XS650.[13]

In 1968 the domination of the inline-twin engine for high-performance street bikes came to an end when Triumph Engineering developed an inline-triple engine for the BSA Rocket 3/Triumph Trident. The 1969 Kawasaki H1 Mach III with an inline-triple two-stroke, and the Honda CB750's Inline-four engine into the bargain, foreshadowed the ascendancy of multi-cylinder engines. The W series engines were oil-tight and reliable, but by comparison they had low levels of performance with high levels of vibration, and were ultimately unsuccessful on the sales floor. At the same time that production of the W series was ending in Japan, Kawasaki came up with a formula for successful four-stroke street bikes in its Z series.[3][14]