1984 Yamaha FJ1100
(Page 2 of 4)
September/October 2008
Story and photos by Robert Smith
Genesis
The basic FJ concept was familiar enough for the time: an inline 4-cylinder engine with double overhead camshafts operating four valves per cylinder. Bore and stroke were conservatively oversquare at 74mm x 63.8mm, but valve sizes, large-ish with 29mm intakes and 25mm exhausts, speak to an engine intended to rev to produce power. Likewise, the four 36mm Mikuni carbs would need to see some pretty rapid pumping to work most efficiently. So this was an engine that had displacement for low-down torque but was also intended to rev to produce its optimum power.
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Minimizing engine width was a prime objective, so the generator went behind the crankshaft, and drive to the gearbox input shaft was by means of a gear straight-cut into the no. 3 cylinder’s crankshaft web. The result was an overall engine width of just 20.6 inches. Drive to the camshafts was by Hy-Vo chain from the center of the crank, a one-piece item running in five plain bearings. Supporting those bearings was a cast alloy crankcase, the webs of which were drilled to allow free passage of air between each cylinder. Yamaha claimed this breathing improved power to the “tune” of 5hp!
Behind the crankshaft in the same casing was the multiplate, diaphragm-spring, wet-clutch and 5-speed transmission, the output of which passed to the rear wheel by a conventional 530 chain. Yamaha engineers were also able to keep the engine compact enough for a 59-inch wheelbase.
Less conventional was the chassis. Built from rectangular section steel tubes, the frame’s upper members curved around the engine instead of over it, a pattern now universally adopted for sportbikes (but now wrought in aluminum alloy). The top tubes continued forward around the headstock, meeting in front of it and triangulated to it by short welded tubes. The result was an extremely rigid front end. The peripheral frame structure also allowed the upper fairing to be bolted directly to the frame without using extra brackets or stays, making for a very solid assembly.
Two more frame tubes ran below the engine, but with a bolted-in center section to allow the engine to be dropped out for repair. A sturdy rectangular, extruded-alloy swingarm attached to a single spring/damper unit for rear suspension. At the front was a conventional telescopic fork, set at what would now be considered a relaxed rake angle of 27 degrees (“… steeper than any other current big sport bike,” noted Cycle Guide at the time) and fitted with an adjustable anti-dive device.
This is one of two period features that mark the FJ as a machine of the mid-Eighties: the anti-dive units, one attached to the bottom of each fork, were designed to prevent excessive fork dive under braking. They used hydraulic pressure from the front brake line to restrict fork travel. So the fork would compress normally when hitting a bump in the road, but its compression would be limited when the front brake was applied. That meant the front brake could be used during cornering without unduly upsetting the steering geometry. It certainly improved the performance of the front dampers of the day — though newer forks with variable rate damping have made anti-dive units unnecessary.