How Does A Motorcycle Gearbox Work?

In very olden times, before the 1930s, there were sliding-gear 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 constant-mesh 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.

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. Kevin’s unparalleled experience and knowledge of the sport were—and continue to be—prompted by a lifetime of curiosity. His willingness to share that information with anyone who is willing to listen is likewise unique.

Kevin’s greatest strength lies in his ability to present complex subjects in simple terms with clarity and, often, humor. In this video series, shot in his home shop, Kevin draws upon his vast historical references to address modern-day questions. As Kevin has written, “Emotions bring us to engineering, but engineering then becomes a special way of confronting reality.”