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Your motorcycle’s gears affect its acceleration, powerband, and speed.
How your motorcycle’s transmission gears are assembled and configured will differ depending on the type of engine you have.
Horizontally split crankcases are most common, and will allow you to access the transmission gears without disassembling the top end of the engine.
Motorcycle gearboxes are constant mesh, which means the gears are always in contact and spinning. Which gear is producing the engine’s power is controlled by the clutch, shift forks, and shift drum.
Most motorcycle gearboxes consist of two shafts – a main shaft and a countershaft. Certain gears are attached to the transmission shafts, while other gears slide on and are engaged with dogs.
As you move the shift lever, the shift drum rotates and moves the shift forks. The shift forks move gears on the transmission shafts to engage or disengage.
Motorcycle Transmission Gears and Shafts
Before and as you disassemble your motorcycle’s transmission gears, take notes and pictures of how things go together. Your transmission will consist of different gears, circlips, shims, and washers that should go back together in the same way.
If you are unsure, you can look for wear marks depending on which way the gear was oriented.
Make sure to take note of how the shift forks are oriented as well. Shift forks that are installed the wrong way will not be able to shift gears.
If your motorcycle is jumping out of gear or not engaging in certain gears, the dogs on certain gears may be worn and slipping out of place.
If the gear dogs are worn and slipping out of engagement, you can either replace the gear entirely or find a machine shop to undercut the gears for better engagement.
Assemble everything with oil and check that your can shift through all the gears before sealing the crankcase back up. You may need to rotate the shafts by hand as you shift the gears. If something is not shifting as it should, do not use force, go back and check your work. Refer the your motorcycle shop manual for assembly instructions and reference pictures.
Gear Shift Mechanism, Shift Forks, and Drums
To shift most motorcycles, you use your left foot on a lever on the side of the crankcase in a one down, four (+) up pattern, with neutral between 1st and 2nd gear.
When you press the shift lever it moves a shift mechanism that hooks onto pegs connected to a shift drum, and in turn, rotates a shift drum. The shift drum has grooves cut into in where the shift fork pegs sit. As the drum rotates, the forks move left or right, moving the gears around on the transmission shaft.
The shift mechanism can look complicated at first glance, so be sure to take notes and pictures before you disassemble it. If it is installed wrong, it won’t move the shift drum properly.