Transmissions are complicated and consequently can be intimidating for many consumers, not to mention costly. In this article, we will discuss the major components of a motorcycle transmission, how they work, and some common problems we see here at the shop. We hope to offer you a better understanding, so in the unlikely event you have transmission trouble, it won’t sound like the mechanic is speaking a foreign language.
Obviously, not every motorcycle transmission is the same, and we won’t be able to discuss every variation. However, most function in a similar way, with similar parts. First, we’ll break down a list of some of the common parts in a motorcycle transmission, as well as their functions.
BEARINGS
The bearings hold the drum and shafts in place while allowing them to rotate. It’s uncommon for us to encounter problems with the bearings.
SHAFTS
Typically, there are two shafts: a main shaft and an output shaft. The main shaft is driven by the clutch from the motor. The output shaft is driven by the main shaft and usually holds and rotates the front pulley or sprocket. It is extremely uncommon for anything to go wrong with the shafts.
SHIFT FORKS
A shift fork has one end reaching two fingers around a gear and the other end in a channel on the shift drum. The shift fork’s job is to physically push the gears back and forth along the man or output shafts, which in turn changes the gear. In most transmissions, we see three shift forks. The most common reason we replace a shift fork is due to it being bent. This is often the result of the fingers or pockets of the gears having excessive wear, which allows the transmission to jump out of gear and bend them.
SHIFT DRUM
On one side of the shift drum, there is a star-shaped piece. With channels all along the surface, a shift drum resembles a maze. Each channel has one end of the shift fork sticking into it. As the drum rotates, the shift forks follow the channels in the drum, moving them back and forth. The other end of the shift fork pushes or pulls the gear, resulting in a change of gears. When inspecting the drum, we always look for excess channel wear, which hinders the drum from holding the shift fork in the correct location, a phenomenon known as jumping out of gear.
SHIFTER DETENT
The shifter detent consists of an arm with a small metal wheel on one end and a heavy spring pushing said arm on the other end. The metal wheel follows the peaks and valleys of the above-mentioned star of the shift drum as it rotates. The task of the detent is to hold the drum in a precise location (gear or neutral) until it’s time to shift again. We commonly come across springs that have failed.
GEARS
While most people can picture a gear, a transmission gear is slightly different than what might immediately come to mind. It not only has the common teeth around the outer edge but also several fingers or pockets protruding in (pocket) or out (finger) on the sides of the gear. These are what drive the gear and where we most commonly see failure.
It’s worth noting that transmission problems are actually not all that common. The most common issue we repair in a transmission is jumping out of second gear. This makes sense, as second gear is often the gear that takes the most abuse. Typically, this involves replacing two gears, the second and whichever gear it mates with, as well as the appropriate shift fork. The reason for this is that the pockets and mating fingers become worn and rounded where they should be square. The rounded edge is what allows the gear to jump out. Bent shift forks are the result of either jumping out of gear or shifting while the motor is not running.
Spring failure is another issue we see. Although not technically inside the transmission, the shifter detent spring as well as the shifter return spring are both subject to failure. We’re mentioning these springs even though they are not inside the tranny because, most of the time, these problems are described by the customer as a transmission problem.
Six-speed conversions are another transmission job we occasionally perform, which is exactly as it sounds: convert a five-speed tranny into a six-speed. This job is only done on Harleys, and we use a kit made by either Baker Drivetrain out of Haslett, MI or JIMS USA out of Camarillo, CA. This is sometimes done to lower the motorcycle’s RPM at highway speeds.
We hope you now have a better understanding of some of the common parts inside your transmission as well as how they work. Knowing some of the terminology and technical functions of your transmission will better empower you when faced with needing transmission repairs.
If you have any questions or concerns about your transmission, reach out and our team will be happy to help!
Have a project you’re interested in having us work on? Call (740) 747-2299 during normal business hours!
