As I’ve been assembling my 1965 Yamaha TD1-B I’ve been impressed again and again by how heavy its parts are. My conclusion is that this weight was dictated by its engine vibration. Control when cracking appears by varying the wall thickness of frame tubes. Ah, thick is best!
Weight and vibration are connected in other ways too. When Honda built its 1997 1,100cc Super Blackbird, it decided to give it secondary balancers. Why on that model and not, for example, on the CB900F—also a transverse inline-four—of the early 1980s? At least part of the answer has to be that those massive earlier engines were so heavy that the shaking forces of their madly back-and-forthing pistons could hardly move them.
Here I’ll introduce the idea of vibratory excursion: The amounts by which piston shaking force could move back and forth the engine of which they are part, if it were a free body floating in space. Clearly, the heavier the engine in relation to its pistons, the smaller its excursion and the smaller the discomfort they can transmit to the rider. This explains why early British twins, which began as 500s, vibrated fairly tolerably. But as the market demanded more power, bigger, heavier pistons and rising rpm produced larger and more tingly vibratory excursions. At the end. Norton’s 835 parallel twin had to be mounted in rubber “Isolastic” mounts to keep the worst of its shaking from reaching the rider.
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