Motorcycle Transmission 101: The Key To Produce BIG Power

I find this topic interesting.

There’s an entire archive of articles online about motorcycle transmission types that all fail to deep dive into “how a motorcycle transmission system actually works”.

This brings the question.

motorcycle transmission system disassembled on table

Is a deep dive even necessary? Or are people simply looking for an overview of how each system works?

I guess you’ll have to be the judge. Because, today, a deep dive is exactly what we’re going to do.

We will discuss how motorcycle transmission systems work, the components they comprise, the varying types of systems used in motorcycles, common system failures, and ways to prolong the life of a transmission with proper motorcycle maintenance.

Are you ready? Great. Rev up your engine, and ride in!

What Is A Motorcycle Transmission System? 

A motorcycle transmission is a series of components that are specifically designed for the purpose of transmitting torque from the engine to the rear wheel. The word ‘system’ is added to describe the function of how multiple interconnected mechanical components work together in a methodical manner to form a process (comprising power input and power output).

In short, the motorcycle transmission system acts as a link between the engine and the rear wheel.

A gearbox, however, simply makes up a portion of this system, as opposed to how it is often referred to as being the entire thing. More on this later…

How Does a Motorcycle Transmission Work? – A Brief Summary

A motorcycle transmission system works by converting large and small inputs of power (from the engine) into higher and lower outputs of power (to the rear wheel).

When a gear is selected, two gears are meshed together (in the gearbox) to provide gear ratios.

Gear ratios allow the transmission to support different directions of power (i.e., engine to the rear wheel and vice versa), provide varying outputs of power, and deliver different ranges of torque.

Larger gears with more teeth provide higher torque and lower top speeds, while smaller gears with fewer teeth provide higher top speeds and lower torque.

A little confusing, but certainly a concept that eventually becomes logical.

How Do Gear Ratios Work?

Ratios can be easily worked out with simple maths.

Example

An engine turning at 6000RPM with a 3:1 gear ratio would equate to 6000 / 3.

Why? Because the ratio is ‘3 to 1’ which also translates to “one-third” of 3. Meaning for every 3 rotations of A there is 1 rotation of B, and vice versa. Meaning, 1 rotation of B would equal 3 rotations of A.

Following this formula leads us to a torque output of 2000 RPM. Because 2000 is “one third” of 6000. Make sense?

Conversely, an engine turning at (the same speed of) 6000RPM with a gear ratio of 1:2 would equate to a torque output of 12,000RPM. Only, this time we are simply trying to find the answer to 6000RPM x 2. Which provides us with the answer.

Check out this quick quiz below to get a better understanding of how this works.

Quick QuizFormula For Gear ratios

image of formula for gear ratios

If an engine is designed with a 3:1 gear ratio and is turning at 12,000 RPM, then what is the output speed?

Take a guess…

If you answered ‘4000RPM’… you were right!

And if this still doesn’t make sense, then do not worry, as I will further explain all of the above (in detail along with visual aids) in the following sections.

What Are The Different Types Of Transmission Systems Used In Motorcycles?

The most common types of transmission systems used in motorcycles are:

Continue reading to learn all about the operations, benefits and disadvantages that each system delivers in an overall riding experience of a motorcycle.

Automatic Transmission Systems

Automatic Motorcycle transmission systems consist of:

CVT transmission diagram

Continuously variable transmission (CVT) – What is it?

To the untrained eye, this is just another twist-and-go automatic scooter. But to a seasoned technician, the Continuously Variable Transmission (CVT) system is an automatic transmission with no predefined or preset gears.

Instead, it features two pulleys, each consisting of two conical plates. One pulley (i.e., the “driver pulley”) is positioned to the front of the transmission, while the other pulley (i.e., the “driven pulley”) is positioned to the rear of the transmission.

Both pulleys are then joined together with a V-belt.

High engine RPM conditions is achieved by increasing the diameter of the front (driver) pulley and reducing the diameter of the rear (driven) pulley.

Conversely, high torque conditions is achieved by decreasing the diameter of the front (driver) pulley, and increasing the diameter of the rear (driven) pulley.

Example – Centrifugal Force

If you’re in any way familiar with a diablo (i.e., the Chinese) yoyo, then understanding the workings of pulleys and a V-belt should be as easy as pie.

diablo yoyo on string doing tricks

Let me explain.

Without getting into complicated physics, this entire system is based on centrifugal force.

Centrifugal force is simply a measure of the rotational force that comes from inertia.

All of this can be understood by simply spinning a coin on a table.

Depending on the mass (i.e., weight), shape, size and let’s not forget the velocity (i.e, speed) of the coin will determine how long it spins for, and what speed it spins at.

Using the formula below would lead you to the answer.

Wollah! Centrifugal force.

And if you are genuinely a nerd, check out the formula below.

F=m\omega^2r
Formula
F=centrifugal force
m=mass
\omega=angular velocity
r=distance from the origin

Interestingly, the CVT system can be also found in other automatic transmissions such as snowmobiles, industrial equipment and most recently, Honda and Toyota motorcars.

The following sections will dive into:

Riding with a CVT

However you look at it, be it, a moped, or a scooter. If it has 2-wheels with twist-and-go automatic transmission, then it’s likely a CVT. Here in the UK, you can legally ride a 50cc from age 16, and there’s a reason for that.

“CVTs make the perfect practice machines for beginners”.

Most 50cc street-legal bikes are mopeds. And most 50cc street-legal bikes feature a CVT. And while engine power is the main factor in new licence limitations, there’s a reason why CVTs have become so heavily appealing to new riders.

For a start, twist and go CVT motorcycles have a short delay in throttle response. Which is simply the result of the rear wheel being designed to use ‘centrifugal force’.

However, for centrifugal force to be created, engine power must first make its way through several transmission components to rotate the rear wheel assembly. As the components of the rear wheel begin to turn faster, centrifugal force is created, but only after a short delay.

The result is that when you rev the engine, there is a short delay before the rear wheel actually begins to rotate. Making these bikes much easier to manoeuvre at slow speeds, less scary or jerky during acceleration, and far more predictable with a consistent power delivery at higher top speeds.

Which is all somewhat unachievable with other transmission types!

Conversely, slowing down or coming to a stop is as simple as rolling off the throttle and applying the brakes.

Not too shabby if you ask me!

And if you live in the city, or in a high-traffic environment, then owning a bike like this could seem like a gift down from heaven.

How does A Motorcycle CVT System Work?

The operation of a CVT system consists of 3 elements. A primary (driver) pulley, a secondary (driven) pulley, and a V-belt (that is used for joining the two pulleys together).

High RPM conditions are obtained by increasing the diameter of the driver pulley and reducing the diameter of the driven pulley, while higher torque conditions are achieved through doing the opposite.


But as a solid metal pulley cannot simply change its diameter. The transmission system must use a variator drive system to achieve the same mechanical characteristics.

Each pulley (in the variator assembly) comprises two conical plates. One plate is in a fixed position while the other is able to slide (side-by-side) on a shaft. Doing so enables the plates to either come closer together or spread further apart, under centrifugal force.

The plates are designed with sloped internal surfaces to allow the V-belt to drive up the pulley, when the plates come closer together, or down the pulley when the plates spread further apart.

The examples below will give you an idea of how all of this works.

pulley stage 1
1. Low Gear (plates together)
pulley stage 2
2. Varying Gears
pulley stage 3
3. High Gear (plates spread apart)

Here’s a side profile image of the same process.

powerful start
High torque, Low RPM
balance of engine RPM and torque
front pulley with large diameter and rear with small
High RPM – Low torque

Note: The contraction and expansion of the conical plates (i.e, the faces on the pulleys) is controlled either electronically or with centrifugal force.

This system enables the ability to achieve an infinite number of drive ratios between the minimum and maximum limits (i.e., size of the conical plates and maximum engine speed or RPM).

The driving pulley – Explained

The sliding conical plate on the driver pulley contains rollers that are designed to remain centred in low engine RPM conditions and roll out in high engine RPM conditions when centrifugal force is increased.

In short, when the engine’s RPMs rise, the rollers roll out.

The walls of the enclosed area that the rollers roll into is made up of (a wall created by) a single fixed ramp plate, and (a wall created by) the rear side of the sliding conical plate.

Check out how this looks below.

rollers and conical plates
Centred rollers on the rear side of the conical plate
rollers moved out under centrifugal force
Rollers move outwards
CVT rear wheel conical plate assembly with rollers
Example of the rear wheel assembly

When the rollers, roll out, the conical plate is forced (to slide) along its shaft, which in turn, causes the V-belt to drive up the pulley and increase in diameter.

The result is higher gear ratios.

The driven pulley – Explained

The rear wheel transmission system consists of a pulley and a clutch assembly.

Inside the clutch assembly are weighted arms (mounted with friction plates) that are held in place by extension springs.

In low engine RPM conditions, the entire clutch system’s internal components remain contracted.

While in high engine RPM conditions, the weighted arms (in the clutch assembly) begin to expand, which in turn, forces the friction plates up against the inside of the clutch housing.

This eventually causes the friction plates to grab and rotate the entire clutch assembly (along with all of its associated components) in unity.

See how this looks in image no3.

CVT clutch assembly GIF
1. Weighted arms expand and friction plates expand
output gear not moving while friction plates are turning
2. Sufficient speed has not yet been reached
output gear turning with clutch assembly CVT
3. Sufficient speed achieved – Output gear turning with clutch

As the clutch assembly is fixed to the output gear (where the arrow is pointing in the image above), power is then transferred to the rear wheel via the gear train.

The Benefits
  • No lag acceleration – (as variable gear ratios return constant power)
  • Easy to maintain, repair and replace
  • Reliable and durable assembly (with the option to upgrade internals)
  • Delivers the best possible fuel efficiency for the speed at which you are travelling
  • Lightweight and relatively compact in size
  • Enables new riders to ride with twist-and-go operation
The Drawbacks
  • More suitable for smaller less powerful engines
  • Requires regular adjustment to maintain the tension of V-belt
  • The V-belt requires regular interval replacements
Motorcycles & Scooters that use a Continuously variable transmission
  • Near enough all scooters (too many to list)
  • Gilera DNA
  • Honda DN-01
  • Aprilia Mana 850 GT
  • CF Moto V5
image of internals on the 2018 Honda Gold Wing Tour dual clutch transmission
Mounted on the 2018 Gold Wing Tour

Dual-clutch transmission (DCT) – What is it?

The entire world has gone digital. Motorcycle transmission systems too! And if you look at the latest production cars, it’s clear to see that a straight manual gearbox is becoming a thing of the past.

A dual-clutch transmission delivers a modern manual gearbox with a touch of automation. The dual within the words ‘dual clutch’ refers to the system’s two-clutch design.

For example, clutch A controls all even-numbered gears, while clutch B controls all odd-numbered gears, allowing the system to make faster and smoother gear shifts.

The semi-automatic gearbox still retains the use of gears and a clutch, just like in a manual transmission, but also delivers the addition of several riding modes.

For example, automatic mode enables twist-and-go operation, while manual mode requires the rider to physically shift a toggle/trigger shifter (usually mounted to the left handlebar) in order to make gear changes.

Note: The first production motorcycle to feature a DCT was the Honda VFR1200F sports tourer in 2010

That being said, the concept of creating an automatic motorcycle transmission is truly nothing new. As manufacturers like Honda have been toying with this idea since the CM400A Hondamatic in 1979.

The difference is that a dual-clutch transmission delivers a better riding experience, improved fuel efficiency, and a lower sacrifice in performance. All in a single system.

Welcome to the DCT!


Digging into this technology, here’s what we will cover:

Riding a motorcycle With A DCT

Dual clutch transmission systems are much closer to a standard manual gearbox than any other type of automatic transmission. The difference is that manual systems rely on human operation, while the DCT features an onboard computer that automates gear changes to match road conditions.

The DCT boasts a simple design and easy operation.

Just fire it up, select your preferred mode, and ride off into the distance. Simple enough. Right?

And while manual mode offers a nifty trigger or paddle shifter to make gear changes, automatic mode enables the rider to ride with no gear shifts at all. In fact, even when coming to a complete stop, gear changes are still smooth, fast, and have very little feedback.

Allowing you to focus on other tasks such as looking at the road ahead.

Some models even feature a 6-axis IMU (inertial sensor) which is used to detect rev range in the powerband, angular velocity, and acceleration for pitch, roll and yaw. Doing so reduces the chances of random gear shifts occurring mid-corner and in uphill.

image showing how the DCT system works
How does a motorcycle DCT System work?

A dual-clutch transmission system features a two-clutch design that alternates each clutch in and out of engagement to produce seamless gear shifts with consistent power delivery.

More specifically, the DCT system can operate two clutches and engage two gears at the same time. Each clutch is assembled side by side, enabling them to conveniently work on the same shaft.

The result is that the transmission can keep one gear engaged (and supplying power), with another preselected gear (rotating at the correct RPM for the next gear change) in neutral, all at the same time.

This seamless operation is accomplished through the use of a linear solenoid that applies hydraulic pressure to the clutch actuator, which in turn, actuates the clutch.

The entire system is run by the motorcycle ECU (i.e., computer brain), which relies on a range of sensors, that can be programmed in a multitude of ways. Even to determine the best time to make gear shifts.

For example, sport mode can be programmed with aggressive gear changes, while cruise mode can be programmed with infrequent gear shifts.  

Truthfully, there isn’t much the system cannot do. And with an ability to consistently push out power, the DCT system is a perfect example of optimising for efficiency in a modern transmission. 

The benefits
  • Perfect for riders that are unable to operate a manual transmission (no left handlebar or left foot controls needed)
  • Frees up mental bandwidth while riding
  • Delivers faster and smoother unnoticeable gear changes
  • Reduces riding fatigue
  • Better fuel efficiency than other transmission types
  • Automatic and manual riding modes are available
  • Easy to learn with for beginners
  • Has optional upgrades of a foot-shift lever
  • Impossible to stall – as downshifts are made by the motorcycle in all modes
  • Easy to tune – Can be set up to work in a variety of ways
The Drawbacks
  • Expensive to repair and usually requires an entire complete replacement of parts
  • Less reliable than regular manual gearboxes
  • More complex setup and uses more sensors that open room for potential failure
  • Can possibly downshift mid-corner
  • Difficult to manoeuvre at a slow speed
Motorcycles that Feature a Dual-clutch transmission
  • Honda Goldwing (Tourer)
  • Honda Goldwing Tour
  • Honda Africa Twin (adventure)
  • Honda NC750X
  • Honda NC750S
  • Honda VFR1200X (the first DCT motorcycle in 2009)
  • Honda Rebel CMX 1100 (cruiser)
  • Honda NT1100
  • Hond NM4 Vultus
  • Honda CRF1100L Africa Twin
  • Honda CRF1100L Africa Twin Adventure Sports
  • Honda X-ADV
  • Honda FORZA 750
image 4
1978 Honda C90

Semi-automatic – What is it?

The topic of semi-automatic transmissions gives me nostalgia.

One of my first and favourite bikes was a Yamaha PW80 that featured a 2-stroke 79cc single-cylinder engine with a 3-speed semi-automatic gearbox. It pushed about 4HP and peaked out at around 50mph, but at just 13 years old, it felt like I was flying a jet.

A semi-automatic transmission, in short, is a manual gearbox with gears and a clutch, but no clutch lever. It’s essentially part automated, allowing you to change gears without having to physically “pull in a clutch”.

In fact, these gearboxes were a big thing from the 1970s to the 1990s. As they featured a centrifugal clutch, which was great for beginners to learn on, as they enabled you to remain stationary, with any gear engaged, and wouldn’t physically move until you rolled back on the throttle.

And shifting gears was even easier!

As you could easily make gear shifts using the enormous heel-and-toe foot-shift lever (#HondaSuperCub) that looked like an old telephone handle. Realistically, minimising the chances of new riders making a mistake.

Fun fact

To this day, the Honda C90 engine is the largest produced production engine in the entire world, with The Honda Super Cub recently passing 60 million units sold.

Here’s a quick list of the topics in this section:

image of old telephone
Old Telephone
The Honda Super Cub with semi-auto shift lever
The Honda Super Cub (semi-auto)
Yamaha PW-80
The PW-80 (semi-auto)

Most semi-auto transmissions feature a 3 to 4-speed gearbox and are commonly used in smaller motorcycles with smaller engines (i.e., under 200cc). They’re also easy to source parts for, cheap to repair and require very little maintenance.

In fact, in my own personal experience, I can’t say that I ever recall experiencing a major issue with a semi-automatic I’ve owned in the past.

This is why I can clearly see how this easy-to-operate, low-cost, low-maintenance transmission has become Asia’s most widely used gearbox.

Truthfully, these bikes are great for tourists, as they’re easy to ride, and great for locals, as they provide a safe, simple method to transport light goods.

Riding a bike with a semi-auto transmission is simple. Fire it up, shift into 1st (via the shift lever) and pull back on the throttle. And when it’s time to stop, simply roll back off the throttle and apply the brake of choice (i.e., front brake lever or rear foot brake).

What more could you ask for?

clutch pack assembly - exploded diagram

How Does a Semi-automatic Transmission Work?

The most important part of understanding the operation of a semi-automatic gearbox is understanding the operation of its clutch. As truthfully, it’s the centrifugal clutch where this system really begins to set itself apart from a standard manual transmission.

So, the real question to ask is,

how does the centrifugal clutch in a semi-automatic transmission work?

Much like the centrifugal clutch in a CVT system, the clutch in a semi-automatic transmission must disengage at idle (or at low engine RPMs), and then re-engage as the engine RPMs increase.

The difference is that a semi-auto centrifugal clutch can additionally slip during downshifts, and act as an oil filtration system too.

All in a single assembly.

centrifugal clutch assembly labelled
centrifugal clutch assembly labelled
Components in A centrifugal Clutch

The components in this system comprise a clutch pack consisting of a clutch basket, an inner hub assembly, a set of friction discs, and a set of steel plates, held into place with a retaining ring. Similar to the setup of a regular manual clutch.

Additional components include a set of lobed (i.e., bob) weights and a bell housing machined with recesses (in the drive plate and centre assembly) that are specifically designed to work with other components in a semi-automatic transmission system.

drive gear and clutch basket assembly
Clutch basket & Centre (containing drive gear) assembly
Clutch Assembly With no rings
Clutch Assembly With no rings
completed clutch assembly
Completed clutch assembly

While being the most complex type of clutch covered in this post, the centrifugal clutch is certainly the most interesting to learn about. So I’ll do my best to simplify it.

Operation of A semi-auto centrifugal clutch

When the piston moves up and down, the crankshaft rotates. But, because the crankshaft and clutch are mechanically connected, when the crankshaft rotates, the clutch also rotates.

The clutch then rotates the drive gear, which then transmits power (i.e., in the form of torque) to the rest of the transmission (ie.., via the input shaft, output shaft, dog gears and sprocket) before finally entering the rear wheel.

Here are a few images to give you an idea of the process.

CT90 engine with labels
CT90 engine with labels (side profile)

Here’s an image of the same engine from a different angle. Only this time presenting the flow of power when a gear is engaged.

example of semi auto CT90 transmission

The centrifugal feature of the centrifugal clutch boils down to the engagement of the lobed weights, and the A plate (i.e., the first plate) in the clutch pack.

In low engine RPM conditions, the lobed weights remain dropped, and out of contact, which is a state that translates to ‘clutch disengagement’.

However, in high engine RPM conditions, centrifugal force causes the lobed weights to swing up, and into contact with clutch plate A. Which is a state that can translate to ‘clutch engagement’.

Lobe weights in centrifugal clutch
Lobe weights engaging & disengaging the clutch
lobes moving in and out
lobes moving in and out

Once the lobed weights and the clutch pack have made contact, engine power can then begin to transmit to the rear wheel.

Conversely, if the lobed weights do not make contact with the clutch pack, then the engine’s power will not be able to transmit to the rear wheel.

Each set of lobed weights is designed with a specific weight that is heavy enough to remain dropped (and out of contact) in low engine RPM conditions. But light enough to easily swing out and make contact in high engine RPM conditions.

Sidenote

Calculating the amount of centrifugal force needed to move the mass (i.e., weight) of the lobed weights against gravity would require complicated physics, and I’m guessing that’s probably not what you came here for, right?

But if you did… then this should help

Multiply mass times acceleration.The force (F) required to move an object of mass (m) with an acceleration (a) is given by the formula F = m x a.

So, force = mass multiplied by acceleration.

From Wiki How

High engine RPMs equal high centrifugal forces.

This means, high engine RPM conditions equal lobed weights coming into contact, which in turn, equals clutch engagement. While low engine RPM conditions equal low centrifugal forces and clutch disengagement.

In short, rolling off the throttle will disengage the clutch while rolling back on the throttle will reengage the clutch.

Enabling the rider to make gear changes without having to physically pull in a clutch.

Congratulations, you now know how an automatic clutch works!

The Automatic Slipper Feature In A Centrifugal Clutch

The automatic slip feature in a semi-automatic transmission centrifugal clutch is a surprisingly simple, but interesting piece of kit.

It features no electronics and instead uses a set of machined grooves (that create channels) in the outer face of the drive gear assembly to automatically rotate the clutch (if met with specific conditions) in or out of engagement.

The specific conditions where clutch disengagement is necessary to consist of (during or immediately after):

  1. Deceleration
  2. Aggressive downshifting
losing traction on motorcycle - GIF

Fun Fact – What is the purpose of a slipper clutch?

The purpose of a slipper clutch is to prevent chatter, shudder and locking up of the rear wheel during downshifts. Which can result in you the motorcycle losing traction.

But, why is this even important?

In short, when you aggressively downshift and try to take a corner, the rear wheel begins turning faster than the engine, and in turn (no pun intended), begins to lose traction.

Commonly referred to as shuddering, skidding, slipping, chattering or wheel locking.

The reason why this happens is that, in normal conditions (i.e., acceleration), the engine must rotate clockwise in order to transmit power from the main shaft (i.e., of the engine) to the rear wheel.

But in deceleration, the entire system begins to rotate anti-clockwise. Which is essentially when the inertia of the rear wheel begins to drive the engine. Often referred to as engine braking, or back torque when making a downshift.

The problem with this is that when the force of the rear wheel meets the force of the engine, there is a breach. As both forces essentially begin to fight. Resulting in sky-high engine RPMs and a loss of traction in the rear wheel.

Understanding this condition has allowed engineers to design the centrifugal clutch in a way that, when power begins to travel anti-clockwise, the clutch can effectively slip, and in turn, retain extremely high levels of traction. Even when travelling at extremely high speed.

This is likely the reason why slipper clutches have become so popular in professional motorcycle racing.

But how does this all work? … Haha, well that’s the genius part!

Similar to a bottle cap that screws onto a plastic bottle, the drive gear assembly features a set of machined grooves (i.e., threads) that house the clutch pack.

In clockwise rotation, the clutch pack is fixed at a distance where the lobed weights (when under centrifugal force) are able to make engagement, which in turn, allows the engine (in high engine RPM conditions) to transmit power to the rear wheel.

Happy days.

However, in anti-clockwise rotation, when the rear wheel begins to drive the engine, the clutch pack is rotated slightly away (like when opening a bottle cap) to a distance where the lobed weights cannot make full engagement.

This results in less friction in the clutch pack, which in turn, provides sufficient space for the clutch discs to slip past one another. Which also prevents the opposing forces of the rear wheel and engine from fighting one another.

Sidenote

Even the slightest space of a few millimetres between clutch plates is enough for them to essentially slip past each other and prevent rear wheel locking.

I told you it was genius.

But let me take this a step further to help you visualise how this process takes place in the transmission.

Example Time…

threads and grooves on a bottle cap
Bottle Cap being opened (anti-clockwise)

Similar to the design of a bottle and bottle cap. Rotating the clutch pack in and out of a distance (to where it can or cannot make engagement), works with exactly the same principles.

If you take a plastic bottle and turn it on its side (i.e., horizontally), then open and close the lid, this concept will quickly begin to make sense.

clutch pack being rotated out of engagement
Rotating the clutch pack anti-clockwise

The drive gear centre assembly (of the centrifugal clutch) contains diagonally machined grooves that act like the threads of a bottle cap. Or even similar to the threads of a nut and bolt.

The channels of the grooves are machined in a way that the clutch pack remains at a distance close enough for engagement, in a clockwise rotation. Or at a distance far enough to prevent full engagement, in an anti-clockwise rotation.

grooves of the drive gear coupling
Disassembled drive gear
Drive gear assembled
Assembled drive gear
Semi-complete clutch assembly
Semi-complete clutch assembly

The image labelled ‘Disassembled drive gear’ above provides a clear example of the machined grooved on the drive gear assembly.

The clutch plates then mount to the assembly to form the clutch pack.

Enabling the clutch pack to now rotate on the machined grooves too.

GENIUS!

The centrifugal clutch Oil filtration system Explained

The pressure plate of the centrifugal clutch bell housing mounts to a gasket that provides two vital functions.

The first function is that the gasket provides an oil-tight seal, and the second is that the gasket is used to filter out unwanted particles from the transmission fluid.

But how?

centrifuge clutch system
centrifuge oil filtration system

The centrifugal clutch system relies on oil being pumped through transmission via several ports at the end of each oil channel in the engine.

Once the oil travels down the recesses of the transmission case, it then makes its way through a hole (i.e., an oil entry point) located in the centre of the clutch to then lubricate the entire clutch assembly (i.e., clutch plates, drive gear, etc).

As the rotational speed of the clutch increases, centrifugal force is created, which then scatters all the heavy components in the oil to the furthest points possible inside the clutch housing, eventually ending up in the gasket.

As the gasket features a material that collects unwanted particles, it collects the unwanted residue and then releases the good oil as centrifugal forces reduce.

The Benefits
  • A very beginner-friendly introduction to riding motorcycles with multi-gear transmission systems
  • Great for riding in traffic as you can remain stationary while a gear is engaged
  • Easy to source parts for and cheap to replace
  • Extremely reliable transmission systems
  • Good for riding in high-traffic and high-congestion areas
The Drawbacks
  • Clunky gear changes and slow gear shifts
  • Lobed weights can get flat spots and cause uneven acceleration
  • Clutch wears out much faster than other clutch types due to the nature of its design
Motorcycles that use a Semi-automatic transmission
  • Yamaha FJR1300AE (confirm this)
  • Boss Hoss Cycles
  • The Honda NC700DCT (confirm this)
  • The Honda Super Cub
  • Pit Bikes 110cc – 125cc
  • Yamaha PW80
MV Agusta Smart clutch system
MV Agusta Smart clutch system

The smart clutch system (by MV Agusta)

Smart clutches are new, modern and among the most interesting types of transmission systems available today. The system still features all the components found in a manual transmission, but additionally provides the ability to ride semi-automatic (at slow speeds), and fully automatic on demand.

The components in this system consist of a rekluse clutch, clutch lever, gearbox and gear shifter (designed for quick shifting). And, they all have the ability to operate in analogue or electronic mode depending on the preference of the rider.

Riding with a smart clutch system is smooth, easy and fun. Especially when using the featured quick-shifter and auto-blipper that comes as standard in most machines.

And hey, who doesn’t enjoy fast gear changes and consistent power delivery?

Check out the list below to jump to your topic of choice:

How does the Smart Clutch System work?

The smart clutch system features a rekluse clutch which is used to enable the rider to start the bike, shift into gear, ride, and come to a complete stop, with or without using the clutch.

The system is also anti-stall, meaning it is completely impossible to stall the engine, even with bad clutch control. Engine braking is still retained (in toggle mode), while the entire setup is assembled with billet machined aluminium components.

All of the above increases the lifespan of parts and improves oil flow, which in turn, reduces operating temperatures, and delivers consistent performance in all riding conditions.

Sidenotes

(1): More oil in the system equals more efficiency, cooler running and longer-lasting components.

(2) Rekluse offers an upgradeable TorqDrive technology that essentially features thinner clutch friction discs and a stronger clutch basket.

The technology works by increasing the level of friction in the clutch system, that in turn, improves slow speed control, torque delivery and overall health of all the associated components.

Similar to the clutch in a semi-automatic transmission, rekluse clutches use centrifugal force to automatically expand and contract a specially designed EXP clutch disk in high and low engine RPM conditions.

How does a Rekluse Clutch work?

The uniquely designed weighted wedges in the EXP disc are positioned circumferentially, enabling them to gradually push out in high engine RPM conditions, and expand the walls of the EXP clutch disk.

However, in low engine RPM conditions (where there is no centrifugal force), the EXP clutch disk remains contracted with a gap of 0.030mm from the pressure plate. Enabling the clutch to effectively slip during downshifts, then re-engage as engine RPMs rebuild.

The images below will provide a clear example of how centrifugal force (created in high engine RPM conditions) can increase the amount of friction in the clutch pack.

wedges slide out in EXP disk
Wedges sliding out under the force
clutch pack assembly overview
Clutch pack assembly overview
ramps in EXP disk
Ramps in EXP disk – wedges push through the ramps of each sidewall to expand the disk

Here’s a GIF that shows how the EXP disc expands using centrifugal force. I.e., the thumb pushing represents the centrifugal force pushing the wedge.

EXP disc expansion
EXP disc expansion

Optional upgrades are also available for the EXP clutch disk. Such as lighter wedges to reduce the aggressiveness of clutch bite, and firmer springs to adjust the timing of initial and final clutch engagement.

However, these same results are also achievable with the smart clutch system by selecting a new riding mode.

Particularly, the MV Agusta features a set of sensors and actuators that assist the ECU to gauge the absolute throttle position and speed of throttle operation.

This information is then used to determine the best time to bite, and the amount of friction to apply in the clutch pack while accelerating.

The benefits
  • Made with high-quality materials (i.e., billet machined aluminium components, and kevlar friction plates)
  • Has a removable gearbox
  • Sill has a completely functional clutch lever at all times
  • Multi-mode riding options to suit a variety of riding styles
  • Beginner friendly (for riders learning to ride with a clutch)
  • Allows the motorcycle to remain stationary while a gear is engaged
  • Features a quick shifter and auto-blipper for faster and smoother gear changes
  • Allows riders to make gear changes without rolling off the throttle
  • Highly tuneable system
  • Retains engine braking – making the system still good uphill
  • Focused power delivery to provide high levels of traction
  • Automatic modulation of the clutch allows riders to ride in taller gears through corners
The drawbacks
  • Very expensive to repair
  • A complex system with a lot of room for failure
  • Requires frequent servicing every 7,500km
  • Allows riders to start the bike in gear – Which can be dangerous
motorcycles that use A Rekluse Clutch
  • Mv Agusta Turismo Veloce Lusso (sport touring)
  • Mv Agusta Dragster RR

Manual Transmission Systems

Manual transmission systems have been around for as long as motorcycles have lived. They are the foundation that all motorcycle transmission systems are built on and are the most common transmission type today.

A manual motorcycle transmission system features a sequential gearbox, meaning that each new gear can only be selected in sequence. I.e., Going up the gearbox 1,2,3,4,5,6, or down the gearbox 6,5,4,3,2,1.

To operate a manual motorcycle, the rider must roll off the throttle, pull in the clutch, shift into the next gear, and then slowly release the clutch lever while rolling back on the throttle.

Easier said than done, and requires a little practice to master.

Unfortunately, with the introduction of new, modern technologies like the smart clutch system and dual-clutch transmission, I cannot personally visualise a future with this particular transmission type. But who knows? I guess we’ll have to see.

>> Learn how a manual transmission works.

The benefits

  • Lower repair costs than other transmission types
  • Lacks complex technology keeping potential faults to a minimum
  • Genuinely fun to operate and ride with
  • Can be upgraded to work with a quick-shifter
  • Durable and reliable system
  • Has engine braking – which is great to retain control at high speeds

The Drawbacks

  • Causes hand and foot fatigue on long-distance rides
  • Requires the use of all hands and feet (making it difficult or impossible for some people to ride)
  • The most difficult transmission types to learn with
  • Does not allow the motorcycle to remain stationary with a gear engaged (unless the clutch is pulled in)
  • Slowest up and downshifts of all transmission types – resulting in inconsistent power delivery

motorcycles that use A Manual Transmission System

Most motorcycles of today feature a manual transmission.

Here are a few of the most popular:

  • Ducati V4S
  • Suzuki Raider R150 Fi
  • Honda TMX125 Alpha
  • Kawasaki Ninja H2
  • Suzuki GSX-R750
  • Harley-Davidson Sportster
  • Triumph Bonneville T120
  • BMW S1000RR
  • Honda Fireblade SP
  • Yamaha YZF-R1M

Electronic Transmission Systems (new)  

The EV industry is booming! Well, it is at least for cars right now.

And while I’m still unsure as to where it’s heading for motorcycles, there is one thing that’s clear. Manufacturers and governments are heavily encouraging this conversion!

EV motorcycles don’t have the typical transmission system but instead consist of a motor (powered by a rechargeable battery) and two sprockets that are linked together with a chain or belt to connect the motor (i.e., engine) and rear wheel.

Most EV motorcycles feature a single-speed transmission. Although, I have also heard that Kawasaki has already begun toying with multi-speed transmissions, in an attempt to increase the maximum speed of EV motorcycles.

In short, power in an EV motorcycle is supplied by the battery, transmitted to the motor, and then transferred to the rear wheel (via the belt or chain).

A simple setup consisting of several complex components.

The benefits Of EV Motorcycles

  • Interchangeable modes for different riding styles
  • Twist-and-go riding style which is great for beginners to learn on
  • Instant and consistent power delivery – as most have no gears
  • Plenty of government schemes to reduce the cost of purchase
  • Tax-exempt
  • Great slow-speed handling – as these bikes deliver instant power
  • Great for city riding and high-traffic environments
  • Features an appealing whining noise on more powerful motors – not the same as combustion though
  • It simplifies the skills required to ride
  • More environmentally friendly

The Drawbacks Of EV Motorcycles

  • Expensive – (i.e., to buy, run, and maintain)
  • Has a low top-end speed – but can be tuned for higher top-end speeds
  • Long charging times
  • Most bikes have a maximum range of 100 miles on a full charge – much less than a petrol bike
  • Not yet profitable in the re-sale market (not many are being resold. Yet!)
  • Generally do not look as good (appearance-wise) as a petrol bike
  • Not designed for cruising long distance
  • Overly smooth power delivery – which is appealing to some, but generally unappealing to riders who love the aggressiveness of riding a motorcycle

A list of Motorcycles that use electric transmissions

  • Harley-Davidson LiveWire
  • Zero Motorcycles (DSR/X | SR/F | SR/S) – see these models compared
  • Energica Eva Ribelle
  • Cake Kalk INK
  • BMW CE 04
  • Onyx RCR
  • Vespa Elettrica
  • Lightning LS-218
  • Arc Vector
  • Pursang E-Tracker
  • Cake Osa Flex
  • Damon Hypersport Premier
  • Evoke 6061
  • Super Soco TC
  • Sondors Metacycle
  • Sur-Ron LBX
  • Energica (Experia | Eva Ribell | Ego)

What Parts Are Contained In The Motorcycle transmission system?  

A motorcycles transmission system begins at the crankshaft and ends in the final drive (i.e., the rear wheel) with every component in-between being a part of the system.

As of writing this, standard manual motorcycle transmission systems are still the most current of all transmission types, so that’s exactly what we’ll talk about.

image of crankshaft, input shaft and output shaft

Shafts (Input & Output)

Definition – A long rod (or pole) shaped component that uses rational force to transmit power from one component to another component

Parts that make up this component:

  • Crankshaft – A cast or billet shaft that supports the engine’s pistons and is used to convert reciprocating motion into rotational motion. This component transmits power to the clutch, which then transmits power to the primary drive (i.e., the input shaft and its associated components)
  • Main/Input Shaft – Transmits torque to the output shaft (via gears in constant mesh)
  • Output/Counter Shaft – Transmits torque to the final drive sprocket
  • Primary drive – A gear (mounted to crankshaft) that transmits power from the engine to the gearbox
image of motorcycle clutch pack

Clutch Assembly

Definition – An assembly made up of several mechanical components that are designed to engage (and transmit engine torque to the gearbox), or disengage (to dissipate engine torque and remove load from the gearbox).

Types Of Clutches

  • Multi-plate – The most common of all clutch types used in motorcycle transmission systems, to date. Consisting of several friction and steel plates, that are pressed together or pushed apart to either engage or disengage the clutch
  • Centrifugal – A clutch that uses centrifugal force to push small weighted lobes or rollers into confined spaces, which in turn, expands components to provide clutch engagement
  • A slipper clutch (Slip-assist) – Sometimes referred to as a ‘back-torque limiting clutch’, the slipper clutch works by reducing the amount of friction in the clutch assembly to allow clutch discs to slip past one another (during aggressive downshifting) partially

Clutch pack

  • Pressure plate – A heavy metal plate that is controlled with springs and a lever, and used to apply pressure on the clutch pack to increase friction in the system
  • Friction plate – A disc that has friction material (made of phenolic resins, metallic powders, compounded rubber, kevlar, or ceramic) on each side and is pressed together with several other discs (i.e., steel & friction plates) in the clutch system to create clutch engagement
  • Steel plates – A steel plate that is pressed together with friction plates and other steel plates to provide ‘clutch engagement’

Other Components

  • Clutch Basket – A bowl shaped basket that houses entire clutch assembly. This component free wheels with the speed of the engine and will only transmit torque to the input shaft when the clutch pack is engaged (i.e., held under force)
  • Inner Hub – A small hub that is mounted to the input shaft of the gearbox and used to transmit torque from the engine into the gearbox by joining with the clutch basket when the clutch pack is under force (i.e., engaged)
motorcycle gearbox

Gearbox

Definition – A mechanical component consisting of dog gears that work in pairs to create gear ratios that increase torque and reduce speed, and vice versa

Types Of Gearboxes

  • Sequential – A non-synchronous gearbox which is designed in a way to provide faster gear changes by only allowing shifts to be made in a sequential order
  • Sliding Mesh – Also referred to as a ‘crash box’, the sliding mesh gearbox involves sliding a gear on the output shaft into engagement with a gear on the input shaft to provide a gear ratios

Dog Gears

  • Fixed – Gears which are fixed to the shafts (usually input or output shaft)
  • Freewheeling – Gears which are on a bearing of the shaft instead of the shaft itself. Enabling the gear to independently turn on the shaft without physically turning the shaft in the process
  • Slider – Gears which are fixed to (i.e., turn with) the shaft and able to slide along the axis of the shaft

Other Components

  • Shift lever – The lever on the external left hand side of motorcycle that is used to shift gears by placing foot over or underneath
  • Ratchet Mechanism – A small mechanical device that is controlled by the shift lever to turn the shift drum, and in turn, move the shift fork into a new position.
  • Shifter shaft – A long pole shaped shaft that the gear lever mounts to one end and the ratchet mechanism mounts to on the other side
  • Shift fork – An aluminium prong-shaped mechanism that is used to slide gears via the coupling sleeve when a gear shift is made
  • Shift Drum – A barrel-shaped component with channels cut into it to enable the shift fork to change position (usually side to side) when a gear change is made
  • Shift Linkage – Also called a shift rod which is a short metal rod that joins the shift lever to the transmission
  • Bearings – A machine element (consisting of a circle or cylinder-shaped metal and several ball bearings) that enables mechanical components to move independently that may not usually be able to do so
  • Sprocket & Chain – A profiled wheel-shaped component with machined teeth around its entire circumferential edge to mesh with a chain
motorcycle clutch failing

Symptoms Of A Bad Or Failing Motorcycle Transmission System

Some makes and models are more prone to particular types of problems, but here are a few common signs of a failing motorcycle transmission.

Clutch Failure

Let’s begin with clutch system failures.

Clutch is “slipping”

A slipping clutch is exactly what it sounds like, it essentially refers to a clutch that lacks friction in the system, which in turn, results in the discs slipping past one another instead of biting and transmitting torque.

Clutch slipping can result from worn clutch discs, a broken or damaged pressure plate, or even weak clutch springs.

Clunking or rough gear change

A healthy transmission system should deliver a smooth, relatively fast (depending on make and model) gear change.

Janky, rough and clunky gear changes are a clear sign of a failing transmission, with the clutch pack being the first place that should be inspected.

Side Note

A healthy clutch should effectively engage and transmit torque from the engine to the gearbox. Anything less is a potential sign of failure.

Snapped or damaged clutch Cable

A cheap and easy repair but certainly one of the most common failures that can occur in a badly maintained motorcycle. A snapped or damaged clutch cable will prevent clutch disengagement via the clutch lever.

warped or bent pressure plate

A broken or damaged pressure plate will reduce friction in the clutch pack, and in turn, result in sluggish acceleration and inconsistent power delivery.

This failure can cause overheating, excessive wearing and a reduced level of lubrication in the clutch assembly.

Bad Or Difficult Gear Changes

Here is a list of reasons behind bad gear changes.

bent shift forks

A bent shift fork can occur from too much pressure being applied to the shift lever. However, it’s likely that the shift lever itself would bend or break long before you damage the shift fork. But it’s certainly possible.

Shift Lever Not Working

The shift lever, shifter shaft and ratchet mechanism must all be intact for a successful gear change. A bad shift lever could need a simple adjustment or may require completely replacing.

The good news is that while a failure of the shift lever is common, it comes at a very low cost to repair. And hey! A broken shift lever could be the perfect time to upgrade for a stronger, better and higher-performing replacement.

worn bearings

The most common causes of a bad bearing are from excessive wear and a lack of system lubrication. Both can develop from exposure to excessively high temperatures or contamination of fluids (i.e., water getting into transmission oil, etc).

This is usually a simple fix (depending on where it is), and always requires a complete replacement of the part.

Worn gear dogs

Worn gear dogs is the most common of all failures in a failing transmission system. Symptoms include clunky gear changes, gears slipping out of engagement and back into neutral, or even worst, no gear engagement at all.

Meaning, when you attempt to shift into gear, the gearbox simply doesn’t allow you to.

Bad transmission fluid and bad riding technique, such as improper clutch control, and a lack of maintenance, are all common causes of excessively worn gear dogs.

In most cases a complete gearbox replacement is needed to repair this problem.

A “Deep Dive” Of How A Motorcycles Transmission System Transmits Power From The Engine To The Rear Wheel

A motorcycle gearbox, or better said, the motorcycle transmission system has one specific job.

Transmit the engine’s torque to the rear wheel.

However, when using a combination of gears in pairs, the transmission system is able to step up torque and increase or decrease maximum speeds by using gear ratios.

This process begins in the engine’s cylinders.

As the engine combusts the mixture of fuel and air, the pistons are forced down.

When one piston moves down in a set cylinder, another piston moves up in another cylinder. This is achieved with a specifically designed crankshaft, and defined as engine ‘cycles’.

Take a look.

4 stroke engine cycle GIF
The power from the engine is converted to rotary motion by the crankshaft

Power from the crankshaft transmits to the primary drive gear, which then leads to the clutch assembly.

Check it out in the image below.

transmission system digram example simple

The clutch basket (housing that is labelled clutch in the example above), turns with respect to the engine’s crankshaft (i.e., the shaft that the pistons are sat on).

The inner hub of the clutch assembly turns with respect to the input shaft.

So how does is the power pass from one shaft to the other?

The answer contains two words. “Clutch discs”.

Clutch discs act as a bridge between the inner hub and clutch basket assembly to provide friction in the system and join the two components together.

Once the clutch basket and inner hub are joined (i.e., by the clutch discs and pressure plate), engine torque is then transmitted to the gearbox, as all components begin to work in unity.

diagram of motorcycle shafts in gearbox

The clutch pack is made up of several components consisting of:

  1. Friction plates
  2. Steel plates
  3. A pressure plate.

The video below will provide you with a full understanding of how power is transferred through the gearbox.


The default state of the clutch pack is always engaged, which essentially means that engine power, by default, can travel through the clutch and into the rear wheel.

Meaning, when the clutch lever is released, and a gear is engaged, by default, engine power is able to make its way to the rear wheel. However, to enable the motorcycle to remain stationary with the engine running, a neutral gear is used.

This means, when you pull in the clutch lever, you are in fact, disengaging the clutch. Not the other way around.

And when you release the clutch lever, you are then ‘re-engaging’ the clutch. Enabling the transmission of torque from the engine to the rear wheel.

How Power Transmits From The Input Shaft To The Rear Wheel

I’ve said it once, and I’ll say it again.

Gears work in pairs to provide gear ratios!

The input shaft and output shaft are located inside the gearbox. And both move with respect to the rear wheel unless a neutral gear is selected.

While most gears mesh with other components, neutral gears lead nowhere. They simply spin on the shaft and join to nothing.

So, when a neutral gear is selected, torque is simply dissipated.

That being said, when a gear, is selected, the input shaft transmits power to the output shaft, which then leads onto the rear wheel via sprockets and a chain.

Torque can then be varied by manipulating throttle input, gear ratios, air flow and the rate at which the system burns fuel.

The often referred to ‘final drive’ is essentially just two words use to summarise the final part of the transmission process before torque is converted to acceleration.

Usually consisting of the final drive sprocket (i.e., front sprocket on chain mounted to gearbox), rear sprocket, chain and other rear wheel associated components.

Once power has reached the rear wheel, it is then put into the ground via the tire.

This is how a motorcycle transmission system works.

Frequently Asked Questions 

Automatic motorcycle gearboxes are the most efficient because they are designed to deliver the best possible performance with the lowest sacrifice in consumption.

Changing gears without using the clutch is a legitimate method for riding a motorcycle when using the correct technique. And when done correctly should result in faster and smoother gear shifts.

Motorcycles that feature a dual-clutch transmission are able to deliver the smoothest and fastest gear changes. Simply because they use two clutches, to engage two gears at the same time. So while one gear is engaged, the next gear is preselected and rotated at the correct speed for the following gear change, all at the same time.

The difference between a gearbox and a transmission system is that a gearbox refers to a contained area made up of two shafts, gears and a shift fork. While the transmission system consists of the gearbox and the addition of all the associated components between the engine and rear wheel.

Lower gears are designed to use higher engine RPMs to provide high acceleration, resulting in larger amounts of fuel being combusted. However, driving slowly in higher gears can result in the same outcome.

The first internal combustion manual motorcycle transmission was the Hildebrand & Wolfmüller produced by Gottlieb Daimler and Wilhelm Maybach in 1885. However, the first automatic motorcycle to hit the streets was the CM400A Hondamatic produced by Honda Motor Company in 1979.

The final drive of a motorcycle’s transmission refers to the last set of gears in the drive train. Usually consisting of the drive sprockets, rear wheel assembly, and chain to join the two together.

Synchromesh is used to describe the processes of how two gears mesh (using a synchronizing unit) with one another, while sequential refers to the order in which gear shifts can be made, in the gearbox.

The perfect time to change gears on most motorcycles sits between 4000 and 7000 RPMs. That being said, this range may be different for particular makes and models.

Dog boxes are designed to use straight-cut spur gears, for strength, with no synchronizer for performance. The result is faster, but more harsh gear changes.

The difference between a DCT and a semi-automatic is the fact that a DCT uses electronics to automate gear changes, while a semi-automatic completely relies on centrifugal force and human operations.

To ride a motorcycle with a dual-clutch transmission the rider must have obtained a full manual motorcycle licence.


Conclusion 

Writing this post completely took it out of me. I do not lie when I say “It has taken me weeks to create this content”. I did the maths, I spoke to the experts, heck, I even rewrote the entire post a few times.

All to ensure it was the best I could create. The images are unique and customised to act as visual aids. I did this with my best intentions of explaining how a motorcycle transmission system works, the varying types of systems that are featured in motorcycles and a little history behind each system too.

If you enjoy reading, this post, then please share it with others! It will help us more than you can possibly imagine.

Leon Angus

Leon Angus

I love bikes. All types, but mainly motorbikes (or motorcycles for those in the U.S.). I also love cars, planes and boats too! I'm a qualified Motorsport Engineer that currently lives in the UK and drives trains for a living (weird combo, don't ask), I love to cook, into fitness and love to smell petrol in my spare time! A true petrol head at heart. This website is my path back into the motor industry and the place where I can shell fuel for fanatics along the way. Learn More about the mission here

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Leon Angus

Leon Angus

I love bikes. All types, but mainly motorbikes (or motorcycles for those in the U.S.). I also love cars, planes and boats too! I'm a qualified Motorsport Engineer that currently lives in the UK and drives trains for a living (weird combo, don't ask), I love to cook, into fitness and love to smell petrol in my spare time! A true petrol head at heart. This website is my path back into the motor industry and the place where I can shell fuel for fanatics along the way. Learn More about the mission here

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