How a motorcycle's transmission or gearbox works

What is the purpose of changing gears?

A vehicle's transmission or gearbox allows us to modify the rotational relationship between the engine's crankshaft and the rear wheel. This relationship is called the "ratio," and to put it simply, it works the same way as the shifting of a bicycle with gears: with a small chainring and a large sprocket, we obtain a lot of force (torque) but little speed, which allows us to climb hills with little effort. Similarly, with a large chainring and a small sprocket, we obtain more speed with each turn of the pedals, but we must exert more effort to accelerate.

Pandora's Box

The gearbox allows us, therefore, to adapt the engine's power (torque) and speed (RPM) to different driving conditions by modifying the gear ratio, thus conserving power.

On large-displacement motorcycles, the most commonly used gearbox is the sequential mesh gearbox. It's activated manually, usually by foot, and is sequential because shifting up or down requires shifting through the intermediate gears (1-2-3-4-5-6).

Although it is one of the most mechanically complex devices on a motorcycle, we will try to explain how it works in a very simple and simplified way.

It all comes down to two axes

The gearbox consists of two shafts: the primary shaft, which receives the movement of the engine crankshaft through the clutch, and the secondary shaft, which receives the movement of the primary shaft and transmits it to the rear wheel, through the secondary belt or chain.

Both axles have gears (speeds) that go from larger to smaller (like bicycle sprockets) and that are facing each other and constantly meshing with each other (hence its name constant mesh).

The trick is that only some of these gears rotate in conjunction with each shaft; the others are "loose," meaning they don't rotate, even though the shaft does. These gears have the unique feature of being able to slide laterally on the shaft, and when they do, some protrusions engage with the free gears, forcing them to rotate with them (and the shaft).

Shifting gears

When the gear is in neutral, the input shaft rotates, but since there are no gears connected to the output shaft, it doesn't rotate. When first gear is engaged, gear "C" moves toward gear "B" and inserts into it. Since gear "B" is constantly driven by gear "A," this movement is transmitted to the input shaft, moving the chain and wheel.

In another similar example, if we engage second gear, gear “C” returns to its initial position and at the same time gear “Z” moves and engages with gear “Y”. As this gear is constantly driven by gear “X”, the engine motion passes from the primary shaft to gear “X”, then to “Y” connected to “Z”, which transmits it to the secondary shaft and to the wheel.

It is important to remember that all the blue gears rotate freely, without contact with the shaft, and the red gears move laterally, but are constantly connected to the shaft.

The drum marks the rhythm

The gear shifting movement is done through the shift drum, which has channels through which the shift fork pins move, which in turn move the red gears.

The shift drum rotates via a ratchet mechanism actuated by the shift lever, which is moved by your foot. The drum features a star-shaped mechanism to hold the gears in place, assisted by a spring. This star has a specific groove for neutral.

Frank Burguera

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