How the transmission or gearbox of a motorcycle works

What is the purpose of changing gears?

The transmission or gearbox of a vehicle allows us to modify the spin ratio between the crankshaft of the motor and the rear wheel. This relationship has the name "ratio" and, to understand it in a simple way, it works the same way as the change of a bike with marches: with the small plate and the big pinion we get a lot of force (torque) but little speed, which allows us to climb slopes with little effort and, likewise, with a large plate and a small pinion we get more speed for every turn of the pedals, but we have to exert more effort to accelerate.

The Pandora's Box

The gearbox allows us, therefore, to adapt the force of the motor (torque) and its speed (RPM) to the different driving conditions, modifying the relation of change, thus retaining the power.

In large-cylinder motorcycles, the most frequently used change is the mechanical change of constant constant mesh (gearbox). Its activation is manual, usually with the foot and it is sequential because to go up or down the track you must always go through the intermediate (1-N-2-3-4-5-6).

Although it is one of the most complex mechanical devices of the bike, we will try to explain its operation in a very simple and simplified way.

It all comes down to two axes

The gearbox consists of two axes: the primary axis, which receives the movement of the crankshaft from the motor through the clutch, and the secondary, which receives the movement of the primary axis and transmits it to the rear wheel, through the secondary strap or chain.

Both axes have gears (gears) that are larger to smaller size (such as the pinions of the bike) and that they are constantly faced and engaged with each other (hence their constant name or constant mesh.

The trick lies in that only some of these gears rotate in solidarity with each axis, the others go "loose" i.e. they do not spin, even if the axis does. These gears have the particularity that they can be slid laterally on the shaft and when they do so they fit with the free gears, forcing them to rotate with them (and with the shaft).

Changing gears

When the change is in neutral, the primary axis rotates, but as there are no gears connected on the secondary axis, it does not rotate. When the first speed is introduced, what happens is that the "C" gear moves into the "B" gear, inserting into it. As the "B" gear is constantly moved by the "A" gear, this motion is transmitted to the secondary axis, moving the chain and wheel.

In another similar example, if we introduce the second speed, the gear "C" returns to its initial position and at the same time the gear "Z" moves and fits with the gear "Y". As this gear is constantly moved by the "X" gear, the motor movement passes from the primary axis to the "X" gear, then to the "Y" connected to the "Z", which transmits it to the secondary axis and the wheel.

It is important that you remember that all blue gears rotate free, without contact with the shaft, and the red gears move laterally, but are constantly joined with the shaft.

The drum sets the pace

The movement of movement of the gears is made through the change drum, which has channels through which the teats of the exchange forks are displaced, which in turn move the gears of red color.

The exchange drum rotates through a "carry" mechanism actuated by the shift lever, moved by our foot. The drum has a star-shaped machining for the marches to remain in place helped by a spring. In this star there is a specific slit for the deadlock.

Frank Burguera

#mecanicaharley