Honda C90 - automatic clutch

15th May 2022
Tags: C90 Engine

The history of the automatic clutch used in Honda's Super Cub motorcycles can be traced back to the 1940s.  Soichiro Honda's early success with auxiliary engines had shown him there was a market in Japan for inexpensive motorised two wheeled  transport.    

By the mid 1950s he had begun to think of an entirely new type of motorcycle,  "a motorcycle anybody would find easy to ride"  and within a couple of years this idea would be realised in the form of the first Super Cub, the C100.

Honda C100 Super Cub launched in 1958

A couple of years before the C100 was launched Soichiro Honda and Takao Fujisawa had travelled to Europe to see how the market was developing there.  At this time mopeds were very popular and they arranged for a number of the most successful models - including bikes made by Zündapp, NSU and Puch - to be delivered to their factory in Japan.

Honda was impressed by the convenient step-through layout but thought he could improve on the designs, firstly by using a 4 stroke engine which would be quieter and cleaner than the 2-strokes used in the mopeds of the time, and secondly by eliminating the need for a hand operated clutch which he believed would help attract customers who had never ridden a motorcycle before.  He told his team[1]:

I want to make it so the noodle shop delivery boy can balance his tray on one hand and operate the bike with the other

This goal would lead to the development of the automatic clutch that was used in  super cubs from 1958 until fairly recently[2]

Honda don't claim to be the original inventor of the automatic clutch though and, as we shall see, the design was actually patented by the Czechoslovakian firm, Jawa.  

Jawa is a trade name of "Zavody 9 Kvetina NP" (this translates as May 9th Racing, State Enterprise). May 9th is the day after Jawa became an independent national company in 1953

Conventional motorcycle clutches

The purpose of the clutch is to provide a means of connecting and disconnecting the engine and transmission while the engine is running.  When the clutch is engaged it permits the transfer of motion between the crankshaft and the input shaft of the transmision and through to the rear wheel.

A standard motorcycle clutch has a series of a alternating steel clutch plates and friction plates. Tangs on the inner edge of the steel clutch plates engage with the clutch centre which is fixed to the end of the main shaft in the gearbox. The friction plates have lugs along their outer edge that slot into the clutch outer.   The driven gear is fixed to the rear of the clutch outer and engages with the drive gear on the crankshaft.   This arrangement means that when the engine is turning crankshaft, and therefore rotating the driven gear & clutch outer, it can turn independently of the clutch centre & main-shaft.  You can see the parts below.

The clutch springs press the clutch plates and the friction plates together, coupling the outer clutch basket to the clutch centre and thereby transferring the crankshaft’s rotation to the main shaft via the driven gear on the base of the clutch outer.  Pulling the clutch lever pushes the clutch rod against the inside of the pressure plate, decompressing the springs and removing the pressure on the clutch plates, thereby disconnecting the engine and the transmission.

Inventing the automatic clutch

A number of problems need to be solved in order to create a fully automatic clutch:

  • it must disengage when the bike is stationary
  • it must progressively engage as you set off
  • it must engage and disengage in synchronisation with gear changes
  • it must engage when the kick starter is depressed (or when the starter motor turns) and then automatically disengage once the engine is turning under its own power

If you solve all these problems then you will have a motorcycle that can be controlled with just a throttle, gear lever and brakes.  The ingenious device that made this possible is described below.

Early experiments  

Jawa and Honda were experimenting with ways to simplify the riding experience in the 1940s.  

Jawa used a semi-automatic clutch that automatically engaged and disengaged in synchronisation with gear changes so that, although you had to use the clutch lever when setting off, once underway you did not need to use the hand lever at all.

Honda's first proper motorcycle, the Dream D-Type released in 1949, did away with the hand operated clutch altogether.  Pressing down on the gear change pedal with the front of the foot would put it in first gear and letting go would return it to neutral. Pressing down and back on the pedal with the heel would put it in second gear.  

Honda Dream D-type

Although this  met Honda's goal of removing the need for a clutch lever, the design was problematic.  In the words of one of the engineers, Kiyoshi Kawashima[3]:

It was extremely popular at first as a very easy-to-ride motorcycle. But then, after a while, people started to complain. The D-Type had two gears, low and high. The way it was made, though, in order to keep going in first gear, you had to keep your foot pressing down on the change pedal. If you were going up a long uphill road, for example, your toes would get tired from keeping the pedal pressed down. It was good not to have a clutch, they said, but this was no good. That was why the sales suddenly dropped. We just got too far ahead of ourselves with the idea of what would be good for the customer, so I guess this was another failure. Coming on top of the economic slowdown, this was real trouble. It was the Honda company’s first business crisis.

...a better solution was needed.

The Jawa automatic clutch

Jawa applied for their  automatic clutch patent in July 1958 and sought protection in many of the countries they exported to.   The GB patent (GB2224559A) was applied for in 1959.

A lot of clutch automation technology already existed.  For instance, centrifugal clutches - which automatically engage when a selected engine speed is exceeded - had been first used in steam locomotives at the end of the 19th century.

Centrifugal clutches use bob weights or rollers that are pushed out by centrifugal force as the engine spins thereby applying pressure to the clutch plates and allowing power to be transferred progressively to the transmission as engine speed increases.   There were also devices like Jawa's and Honda's semi automatic clutches that were actuated as the gear pedal was depressed.  

Indeed, some vehicles were equipped with both types of clutches: one activated by the centrifugal mechanism while the other one is controlled by a hand or foot lever.  This arrangement did remove the need for a separate clutch control, however, because it required two clutches to be installed, it was expensive and bulky and therefore problematic for firms like Jawa and Honda who were trying to create small, inexpensive motorcycles.

Both Jawa and Honda wanted to find a way to combine the two mechanisms into a single part.  

Simple centrifugal clutches are fitted to the crankshaft so the shaft and the clutch outer are splined together.  When torque is transmitted from the crankshaft and the speed of the clutch outer (which holds the moveable weights or rollers) increases, centrifugal forces causes the weights or rollers to push the clutch plates and friction plates together coupling the clutch centre and the clutch outer.  The clutch centre is attached to a drive wheel that then transfers the torque to the transmission.

However, when torque is transmitted in the other direction, from the transmission to the drive wheel on the clutch - as happens when cranking the engine to get it to start - the clutch centre will spin freely and the clutch won't engage.

Thus the inventors were faced with an apparently contradictory requirement where the clutch needs to disengage when the bike is stationary but also engage when the bike is stationary and the engine is being cranked.  It is this requirement that is the source of for the invention described in Jawa's patent.

The invention introduced a screw mechanism, made of two parts with corresponding helical ramps, that was placed between the driver gear and the clutch plates. One part of the mechanism is fixed to the first pressure plate in the clutch so that when torque is transmitted to the drive gear it causes the other part in the screw-mechanism to ride up the ramp.  The resulting axial force presses on the clutch plates and friction plates together causing the clutch to engage.

Jawa 250/350 Panelka (1962)

Although Jawa patented the invention in 1958 their first bike to use the fully automatic clutch was the model 559 (also known as the Panelka) introduced in 1962 and available with 250cc and 350cc engines.

Like the super cub all you need to control the motorcycle is the throttle, gear pedal and brakes.  The video from the Jawa factory includes a demonstration of how the clutch rollers and ratchet work:

Jawa Automatic (1965)

Honda's automatic clutch

Honda released their version of the automatic clutch with the C100 in 1958, the same year as Jawa applied for their patent.  

Honda  used a screw-mechanism in the same way as Jawa had described in their patent. According to this account of Jawa's history, Báječná léta s Jawou (Jawa's Wonderful Years) the firm discovered this shortly after the release of the C100, having seen an article about the bike in the Italian magazine Motociclismo. On reading that it was equipped with an "automatic transmission" they were intrigued and arranged for one to be brought to their workshop.  On discovering the bike was using what appeared to be a copy of their clutch mechanism they insisted that Honda pay a licence fee in all the countries that Czechoslovak invention was protected by a patent, which Honda agreed to.

Did Honda pinch the idea?  Jawa could be forgiven for suspecting so, after all Honda were certainly aware of the Czech makers and in the early years were unembarrassed about copying ideas from other manufacturers.  For example, see the striking visual similarity between the pre-war CZ sport shown below and Honda's 1948 D-type shown above.

CZ Sport ~1937

However, Akira Akima, the Japanese engineer responsible for the clutch, says he came up with the idea of the screw mechanism mechanism on his own:

One day as I was leaving I said, ‘If we used a screw, we could convert the rotation of the kick action to the axial direction, but I’m afraid we might run into trouble owing to co-rotation.’ After saying this, I gave it a bit more thought, ‘Since the clutch has a certain amount of drag, it might just cancel out the co-rotation,’ Just then the Old Man (Soichiro Honda) came running back in saying, ‘Since the clutch has resistance, I’m sure we can make it work! I told him that was just what I’d been thinking, and he retorted, ‘Sometimes we even think alike!’ and we shared a big laugh. Eventually, we applied this method to solving the problem, and I was finally freed from the pressure of his constant visits to our room every morning.”

Mr Kimura, a fellow engineer at Honda added

“Since the automatic centrifugal clutch was a totally new mechanism, it couldn’t be developed overnight, so it goes without saying that its development proved to be difficult and required a lot of time. Mr. Akima never gave up, and single-mindedly came up with eight separate designs. His tremendous efforts were certainly worthwhile. That clutch turned out to be an especially important feature of the Super Cub, and made the bike so convenient and easy-to-handle that women wanted to ride it as well.
Three experimental bikes were made to test out these eight different clutch prototypes, leading to the finalization of the automatic centrifugal clutch that became one of the hallmarks of the Super Cub, and one of its biggest features. ”

Since there is no reason to doubt either Jawa's or Honda's account of the history, the most likely explanation for the situation is that the two companies worked out the technical solution independently, but Jawa got there first with their patent.

How the Honda clutch works

The clutch assembly can be broken down into three sections:

  • Clutch centre & drive gear
  • Centrifugally operated parts & clutch free spring
  • Clutch outer and drive plate
C90 (6v) clutch components - note the later 12v C90s clutches do not share parts with the earlier 6v bike. Note also that the earliest clutches used rollers rather than bob weights. 

Clutch center & drive gear

As the kickstarter is depressed the force is transmitted through the transmission and the drive gear is rotated in the direction shown in the diagram below (left). The screw mechanism causes the clutch centre to be pulled in the direction of arrow A (see the cutaway diagram on the right) and this presses the clutch plates together, engaging the clutch.

This mechanism also maintains engine braking when downshifting (during which the engine may slow sufficiently to cause the centrifugal clutch to disengage).  This works because under these conditions the rear wheel is effectively "pushing" against the engine compression, and the resulting transfer of torque via the transmission to the engine causes the screw mechanism to engage the clutch.

Centrifugally operated parts & clutch free spring

As the engine rpm is increased the clutch weights move in the direction of arrow B (see cutaway diagram above) and pivot on the clutch weights center ring. The centrifugal force exerted on the clutch plates engages the clutch.

The tension of the clutch free springs is sufficient to resist the force of the weights at low engine speed so that the clutch remains disengaged when the bike is not moving.

At ~2500 rpm the force exerted by the weights exceeds that of the clutch free springs and the clutch begins to engage. The pressure of the clutch weights against clutch plates increases progressively with the engine speed, as can be seen in the chart below:

this chart shows the torque and operation of the centrifugal clutches used on the 6v C90s made between the mid 1960s and early 1980s.

At 4200 rpm the travel of the clutch weights is limited by the clutch weight stopper ring.  As engine speed is increased above 4200pm the pressure exerted by the weights remains constant.  The clutch spring tension equals the force of the weights at just when the engine is turning at 4200 rpm and this prevents excessive stress being loaded on the clutch plates.  

Clutch outer and drive plate

The drive plate is splined to the crankshaft and the clutch outer is connected to the drive plate which retain the clutch springs and is held in place by  four screws

Two cam plates are fitted  between the clutch outer and the engine case.  Between the two camplates are three caged balls held in a retainer.  The upper most camplate is prevented from rotating by a pin that fits into the crankcase so, when the gear shift pedal moves it causes the lower camplate to rotate in relation to the upper camplate.  When this happens  the ball bearings will ride up the ramps on the camplates pushing the clutch outer as they do so.

This movement causes the clutch plates to be moved out of the reach of the bob weights so that the clutch disengages. The clutch can be disengaged by either upward or downward movement of the clutch pedal so that the clutch is automatically disengaged when the gears are shifted both up and down.

You can see this mechanism working in the video below:

The clutch damper springs located in the face of the clutch outer are there to reduce any vibration and noise resulting from the clutch outer turning when the bike is idling.


The point at which the clutch starts to disengage when using the gear pedal can be adjusted. Here are Honda's instructions:

You can see how the adjustment works in another one of Michael Mineart's excellent videos:

The purpose of the adjustment it ensure there is a  small amount of play before the gear pedal causes the clutch outer to move and begin disengaging the clutch.   If there is too much play then the clutch might not properly disengage when changing gear and if there is too little play the clutch might remain partly disengaged, causing the clutch to slip when in gear or while trying to kick start the engine.

The adjuster screw is inserted through an unthreaded hole in the case and into the threaded uppermost camplate so that when you turn the screw clockwise it moves the cam plate away from the clutch.  Having screwed in the adjuster and removed the pressure on the clutch outer you then turn the screw anticlockwise, pushing the camplate back towards the clutch until it contacts the ball bearings.  At this stage you back it off slightly so that there will be a small amount of play before the pedal operates the clutch.

Centrifugal Oil filter

Honda included one more clever feature when they designed the clutch by arranging for it to also act as a centrifugal oil filter.  As can be seen in the diagram below, In the 6v C90 oil is pumped through three routes: to the main-shaft and countershaft; to the cylinder head and to the clutch.  

Oil enters the clutch from a passageway in the right crankcase via a spring loaded oil transfer guide located in the centre of the clutch cover bearing.  Because the clutch is spinning at thousands of rpm it acts as a centrifuge and contaminants in the oil are flung outwards where they accumulate in the recess behind the clutch cover.    Centrifugal clutches are very effective[4] and do not use any consumable parts so finding a way to incorporate one into an already feature-packed clutch was an ingenious idea.

Future developments

What happened to this excellent mechanism? Honda still make a lot of  popular small semi-automatic motorcycles but the modern bikes - including the latest 125cc cubs - all use dual clutches rather than the 60 year old automatic clutch.

A dual clutch setup uses a primary centrifugal clutch on the crankshaft and a secondary clutch on the mainshaft in the gearbox.  This second clutch is operated by the gear pedal and used to disengage the primary when changing gears.  Because the transmission is turning more slowly than the engine this tend allow to make for smoother gear changes than when the equivalent action done via the crankshaft mounted auto-clutch which needs to accelerate relatively quickly to match the engine speed in comparison.   I suppose the fact that modern semi-auto bikes are generally quite a lot bigger than the original cubs means there is also more room to accommodate the extra clutch too.

The Jawa/Honda clutch lives on in some chinese copies of the engine,  but the heyday of this engineering masterpiece appears to have passed.


1⏎ see this article by Honda.
2⏎ actually, it was used in nearly all the cubs in this period: in the early 80s Honda ran a shortlived series of fully automatic cubs, the C50 LAE, which had no clutch or gear pedal at all
3⏎ Honda History: d-type
4⏎ A centrifuge running at 6,000 rpm’s generates a force which is 2,000 times greater than gravity and these enormous forces are capable of separating out tiny particles (for instance clinical centrifuges spinning at less than this rate are capable of separating red blood cells from the plasma in blood).