Whether you’re new to riding a motorcycle, have ridden for years, or you’re simply curious about how a motorcycle transmission works, you’ve come to the right place. It’s always good practice to know how the mechanics work on a machine you’re riding.
So, how does a motorcycle transmission work? A motorcycle transmission works through the engine’s output being tied directly to the clutch through a covered chain/belt. The clutch is tied to the input shaft of the transmission which has an output shaft that is tied to the rear wheel via sprockets and a chain/belt. This is how the engine power is transferred to the wheel.
A transmission is a component that transfers the torque from the engine to the wheels. This is a device that allows the driver to control the speed of the vehicle. Essentially every motorized vehicle uses a transmission and there are different types of transmission among these vehicles.
When it comes to motorcycles and other light vehicles, such as quads, scooters, etc., the transmission is designed more specifically for the application. Lighter vehicles are a bit easier to roll and so the weight and gears are different than what a car or truck would need.
How Does A Motorcycle Transmission Work?
To dig in a bit deeper, one key component to understand is the clutch. Most motorcycle transmissions are manual transmissions, meaning the gear is selected manually versus automatically. A clutch is what ties the engine and transmission together but also is what separates them. A manual clutch is constructed of several plates that alternate being geared to the engine or transmission. The springs of the clutch pack press the plates together engaging them. When the clutch handle is pulled in, the spring pressure is released allowing the engine to spin independently of the transmission.
A motorcycle transmission has sequential gearing, meaning you can only go through the gears in order. Unlike a typical car transmission with an H-pattern gear selection, it is not possible to bypass gears on a sequential transmission. This type of transmission benefits the rider by allowing quick gear selection going up or coming down. Neutral is usually half a click between 1st gear and 2nd gear. Starting in neutral, the shift pattern can be remembered as “one down, four (or five) up,” but some transmissions are inverted to “one up, four (or five) down.”
Inside the transmission, there is an input shaft and an output shaft. The input shaft is tied to the engine and the output side is tied to the wheel. On each shaft are several gears which are in a constant mesh state. Some of the gears, called dog gears, are fixed to the shaft by a spline which allows them to slide and lock into other gears. The sliding of these gears is controlled by shift forks attached to the shift drum, which is a shaft with grooves. The shift drum is controlled by the foot pedal shifter. On the drum is a ratcheting mechanism with a groove notched out for the neutral position. The shafts are suspended in the case by bearings and everything gets lubricated with gear oil to prevent friction and damage.
Here is a visual of what happens inside a motorcycle transmission:
Signs Of A Failing Transmission
The transmission is subject to wear and tear and over time this can lead to other issues. It is important to be aware of small problems before they turn into bigger, more expensive issues. Below are some common failures in a motorcycle transmission:
Clutch Failure
The clutch sees a lot of action whether you’re a beginner or not. Failing clutch symptoms include delayed acceleration, difficulty shifting, or a clunking sound when shifting. Clutches have been designed to be adjusted and/or replaced as needed. An easy first step is to adjust the cables of the clutch handle. If the problem persists, you will have to dig in further. Refer to your owner’s manual for proper procedures and specifications.
Shift Lever Not Working
The foot-shift lever turns the shift drum to select the gear. There is a spring designed to hold the selection which is subject to wear causing the lever not to return after shifting. A simple fix could be lubricating the shift lever to free it up. Otherwise, replacing the shift lever is required to solve this issue.
Bearings Are Worn Out
The shafts of the transmission are suspended by bearings. These bearings can gradually wear out and a worn bearing is most noticeable by the growling sound it will produce. There is a quick way to narrow down where the worn bearing is. If you hear the sound with the clutch engaged and the transmission in neutral, it is likely one of the main shaft bearings. If the noise occurs with the clutch disengaged, it will likely be a countershaft bearing. If the noise happens regardless, it is likely the clutch hub bearing. Replacing bearings is a very involved process and may require a trained technician to accomplish.
Shift Forks Bent
The shift forks are the components of the transmission that move the dog gears into position. They are guided by the grooves in the shift drum, so they see side-to-side movement. Improper shifting can cause these forks to bend. A typical symptom of a bent fork is that the transmission will pop out of gear. If left unfixed, the teeth of the dog gears can round off from being inadequately engaged. Correcting this issue will also require disassembling the transmission.
Gear Clash
Gear clash is a condition in which the tolerance, or space between, the gear teeth has widened from wear. When the clutch is engaged and puts the initial torque on the engine, the gears jump together causing a clunk or tapping sound. To prevent this condition, keep the transmission properly maintained. Older bikes are more prone to have this condition. Unfortunately, there is no quick fix for this and will often require rebuilding or replacing the transmission.
Transmission Servicing
Keeping up on the maintenance of your motorcycle’s transmission is the best way to prevent issues and keep the motorcycle running strong. Transmission servicing mostly involves changing the oil. Some bikes have separate chambers for engine oil and transmission oil while some bikes use engine oil for both sides. Refer to your manual for what your bike needs. If you don’t have your original manual, you can find your specific one here on emanualonline.com.
The process of changing fluids is pretty straightforward. Start by getting the correct oil refill, a drain pan, and tools to open the drain plugs. Before draining fluids, it’s helpful to warm the motorcycle up as it helps the oil drain. Park the bike on a level surface and place the pan under the plugs. Open up the plugs and let as much of the oil drain out as possible. Inspect the oil for any metallic shavings or chunks as this indicates an internal problem.
Another component to inspect is the drain plug. Often the plugs will have a magnet on the tip to collect metal shavings. Some plugs also will have an o-ring seal that will need to be replaced during service. After the oil is drained, reinstall the plug and tighten it snugly.
If your bike uses engine oil for transmission oil, now is the time to replace the oil filter. It is often located on the side of the engine and may require some tools to remove it. Once removed, inspect for metal shavings. There may be a seal that will need to be replaced with the filter. Install a new filter and tighten any hardware.
Once the oil is drained and the plugs reinstalled, it is time to refill the fluid. Make sure to use the manufacturer’s recommended oil type. There are several brands to select from and which brand you use is up to you. Also, make sure to fill the right amount of oil, as over-filling and under-filling both can damage the transmission. It may be useful to have a longer funnel to access the fill ports.
Once filled, replace the fill cap and inspect for any leaks. Start the bike and double-check for leaks again. Take the bike for a quick ride and check again for leaks as well as double-check the fluid level. If the level is full and there aren’t any leaks, your bike is all ready to go.
Types Of Transmissions
As previously mentioned, most motorcycles have manual transmissions. However, there are a few other types found in some motorcycles, namely semi-automatic and automatic transmissions.
The semi-automatic, or auto-clutch transmission, is similar to a manual, but the clutch is automatically actuated. The driver still inputs the gear selection. Some examples of this are the Hondamatic, Yamaha YCCS, bikes with trigger shifters, and several minibike makes/models.
An automatic transmission uses pressurized transmission fluid, valves, and solenoids to change gears. Instead of a clutch, it utilizes a torque converter, which is a series of fans that propel one another. A lot of scooters use automatic transmissions as well as some Honda Goldwings, some adventure-type bikes, and electric motorcycles.