In this video, Kevin Cameron looks at something normally buried inside the engine of your motorcycle: the gearbox.

In very olden times, before the 1930s, there were slidinggear transmissions, where gear teeth actually were “crashed” in and out of engagement. But gear teeth have to be quite precisely shaped to endure transmitting very large forces of contact from one shaft to another. So the idea of sliding gears into and out of engagement was given up a long time ago.

That leaves us with constantmesh transmissions. In order to change from neutral to another speed, there is a part called the shift drum. By rotating it, the positions of the shift forks, which move the gears, are changed. The gears have to be completely under control at all times because the last thing in the world you want from your gearbox is double engagement.

Every other gear is locked to a shaft. The gears between them are free to spin. The dogs, which are engaged and disengaged by the shift forks on the shift drum, lock the desired pair of gears to their respective shafts—clutch and output—so the drive passes through that pair and the others are freewheeled.

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