Questions? Looking for parts? Parts for sale? or just for a chat,

The WD Motorcycle forum

WD Motorcycle forum
Start a New Topic 
Author
Comment
View Entire Thread
Re: M20 steel clutch plates etc...

Ian, not pushing you, but have to sort out slipping clutches on both M20s over or after Christmas, so can you give me an Idea as to when these will be available, and a price of course Andrew .h.

email (option): warbikes@gmail.com

Re: M20 steel clutch plates etc...

Hi Andrew...They should be available by the end of next week...The clutch fibre plates (suitable for wet or dry useage) will be £40 for a set of 8 and are the same thickness as the originals (3/32" .0937")....The fork side damper friction discs will be £6.50 a pair...
Both prices are plus shipping at cost...Ian

Re: M20 steel clutch plates etc...

Ian put me down for two sets,of the fibre plates,mail me cost inc pp, to above and how and where to pay you,any body want a couple of clutch covers!!!

email (option): warbikes@gmail.com

Re: M20 steel clutch plates etc...

Ian Wright

The reduction in number of the clutch plates is achieved by removing one friction plate and one plate with external tangs...Despite removing one of these 'external' plates the overall working surface at the face of the tangs when compared to the original set up is still increased by approx. 23% due to the increased thickness of the remaining plates...



Ian, I think we are not on the same page. I am not talking about the effectiveness of the tangs for transmitting power through engaging the driving discs on the inside or the driven discs on the outside circumference. The engagement of tangs to their driven or driving drums is a purely mechanical coupling and you are right that for that calculation thickness makes a difference. I am talking about the friction engagement between the flat side of a friction plate and the adjoining steel plate. The friction disc between each driving disc and driven disc acts as a "temporary glue" through friction, forcing the driven disc to be "driven" when the clutch is engaged and freeing it when the clutch is disengaged (I know I'm telling you something you already know). If you remove one of the eight friction discs you have decreased the total "gluing surface" of the clutch as a whole by one-eighth. Therefore, the remaining friction dics need to have a coeficient of friction 12.5% higher than the original in order to get the same aggregate "gluing" effect. Thickness is entirely irrelevant for this calculation. It's only the total area of contact between friction discs and driven/driving discs that counts. I assume that these clutches were originally engineered with eight friction discs because that's what the engineers' calculations told them they needed in order to transmit power for this bike. Earlier BSA clutches had 7 or fewer friction plates, which leads to the conclusion that it was not by accident but by necessity that the WDM20 clutches were changed to 8 friction discs. This was not a clutch which was designed for a high-power machine (e.g., 15 horsepower; ha, ha) and then used off-the-shelf for the M20, but apparently was specifically designed for this model.

Having said all that, I further assume that today's friction materials are probably better than what they used 70 years ago, so it may be that the 12.5% increase is already built into the material. On the other hand, it is also possible that the better material of 70 years ago is now considered environmentally unsafe or something, and that today's friction materials are not as good as those of yore. I guess it's up to you to figure out which is which.

As for the presence of oil, the manuals of the time for WDM20 seem to say that a small amount of oil on the friction plates does not significantly reduce the effectiveness of the clutch. I have no experience of this on an M20 but I know that my Velocette Venom is VERY sensitive to oil in the clutch. I've often had to do a clutch repair on the side of the road because it worked well when I left home but as soon as a drop or two of oil got into the clutch, a few miles from home, it was completely useless. It seems to work for a couple thousand miles but then go from totally fine to totally useless within a mile or less.

Finally, keep in mind that oil decreases the friction of most materials but it increases the friction of cork. Some models have alternating inserts of cork and fabric, and their oil level must be precicely maintained, so that there is sufficient oil for the cork inserts but not enough to adversely effect the fabric inserts. (Not talking about WDM20s, of course.)

I would like 3 sets of your new discs but, in view of the above, I'll wait until the road reports come in. Sorry.

email (option): jonny.rudge@verizon.net

Re: M20 steel clutch plates etc...

Hi John..Yes, the main thrust of the post was to discuss mods to improve the lifespan of the clutch plate tangs on the external plates...
The clutch plate material was in fact a secondary subject...

These new plates can be used wet or dry and are the same thickness (for the record) as the originals...They can therefore be used either as a direct substitute for the originals, in the same operating conditions and in the same number as in the original set up....Or, they can be used in a reduced number with modified steel plates to achieve the improvement in plate tang life.

As mentioned in my last post it is a common modification to run the M20 clutch with only 7 plates and it operates quite happily under these conditions...So with original plates at least it would appear the clutch was 'over built' to a degree...

The friction material I am proposing to use is employed routinely by the manufacturer in the production of clutch plates, both wet and dry types, for classic motorcycles and is already 'out there' in large quantities...Ian

Re: M20 steel clutch plates etc...

Interesting reading. The M20 puts out about 13 bhp and runs on 8 friction plates. My M33 had the single spring clutch which I replaced with the BSA 6 spring clutch. The B31 putting out a massive 17 bhp had to make do with 4 friction plates. The A10 35 bhp managed on 6 plates, so why does the M20 need so many plates?

Re: M20 steel clutch plates etc...

Hi Ian..I believe the single spring clutch was originally intended for the Empire Star...a bike with considerably more 'go' than the dear old M20...
The M20 also shared the same big end assembly and other parts with more powerful stablemates..

I think a degree of standardisation was an objective due to the obvious production advantages and the M20 benefitted from that thinking..
BSA also had an inclination, at that time, to build the 'working bikes' in their range very substantially, so that they gave long and reliable service and they expected many, if not most, M Series sidevalves to be burdened with the additional load of a sidecar.....

We are talking of a reduction of one friction plate and one steel plate to achieve this modification and as noted, there are many M20s running about already with those two plates removed...my own for example....

Next week I'm going to the laser cutters to get some more steel clutch plates made and will include some of the thicker plates as detailed above, for those who feel they may benefit from the modification...I will certainly fit some during the current rebuild of my own bike.....Ian

Re: M20 steel clutch plates etc...

And also the diameter of the clutchplates from the single spring clutch are considerably smaller than the ones from the 6 spring clutch which I feel must make a huge difference in transmitting the torque. So they had to add a few more plates...which in turn makes separation more critical.

I think the reason for this rather small diameter frictionplates is that there was quite some space needed to create a good seal for the clutchcover/dome.

And when I think of this: should a wet clutch plates need more separation than a dry ones, because of the oilfilm / oildrag ?, just a thought... the hart of a good working single spring clutch comes to a standstill with a running engine which makes selecting first gear easy and without noise...


Regards, Michiel / NL

email (option): m.wijbenga [ at ] hotmail.com

Re: M20 steel clutch plates etc...

I know that the single spring clutch was good as the AA specified them to the end of use of the M21s. When I bought my 1949 M33 it was fitted with the single spring clutch which worked fine but I found it to heavy. I had a spare six spring clutch which I fitted and this gave me a lighter clutch which suited me better.

Re: M20 steel clutch plates etc...

I think a comparison of the surface area of the plates, rather than the diameter, would be a more accurate determination of any clutches ability to transmit power...
The average Japanese sports bike for example doesn't have a large diameter clutch but it transmits a lot more power...(it does however have a lot of plates)

The single spring clutch has full circle fibre plates and more of them...Other clutches only have friction inserts so the single spring clutch plates may well have more surface area per plate, despite being a smaller diameter......

However, I think this discussion could lead on to other things such as the pressure per square inch exerted on the plates etc. etc....

The Triumph clutch...which is a wet clutch.. operates well with the degree of lift provided by the M20s clutch operating arm, so in that case the amount of separation is adequate..even with oil present.

The single spring clutch has more,thinner, plates.. but it also has a deeper clutch basket..
An examination of both clutch types taking into account the available lift and those other factors would be required to determine which, if any, had the greater available space for plate separation at maximum lift....Ian

Re: M20 steel clutch plates etc...

Right. The power treansmission capacity is a function of the total surface area of the friction plates and the total spring pressure of all the springs. Competition cars and sports cars of hundreds of horsepower, for example, mostly have a single friction surface with friction plates on each side (i.e., two friction plates but one touching surface) but they are 14 inches or more across and have six, eight or twelve springs, each of which would make the BSA single spring feel like a weak string, by comparison. And most have a hydraulically-activated clutch, because they need the leverage to disengage it, due to the force of the springs.

email (option): jonny.rudge@verizon.net

Re: M20 steel clutch plates etc...

I think the words 'can' and 'worms' come to mind... ...
It's an interesting discussion though with some thought provoking input...

I can't wait to get a set of these plates fitted and operational to see how their performance compares to the standard set up and the various ideas and questions outlined here....Ian

Re: M20 steel clutch plates etc...

Yes, I should have been more accurate, but my head won't always allow that...

What I was trying to point out is that the (outer)diameter of this clutch is considerably smaller when you compare it to most other heavyweight clutches from that period.

And this small size might even be the reason that it had to be a dry clutch after all to cope with the power of a heavyweight machine.

A look on the partslist tell that the very first 8-plate/single spring design from 1936, used on the Empire Stars didn't have the clutchcover, so was in fact a wet clutch. The cover was added in 1937 when the Val Page-line came (together with the B/M type oil-bath chaincase)

Michiel / NL

email (option): m.wijbenga [ at ] hotmail.com

Re: M20 steel clutch plates etc...

[ 'A look on the partslist tell that the very first 8-plate/single spring design from 1936, used on the Empire Stars didn't have the clutchcover, so was in fact a wet clutch. The cover was added in 1937 when the Val Page-line came (together with the B/M type oil-bath chaincase)']...

Thanks for that bit of information Michiel, I didn't know that...

I wonder, did they add the clutch cover because of a change to the friction material..or because of problems with oil contamination of the same friction material in the '36 clutch?...

Also, if the clutch didn't have the cover in 36 but the plates were the same size, it would seem it was designed from the outset with the small plates and they were not a 'side effect' of designing a clutch with a cover...Ian

Re: M20 steel clutch plates etc.../ some more clutch thoughts

Ian Wright
[
Also, if the clutch didn't have the cover in 36 but the plates were the same size, it would seem it was designed from the outset with the small plates and they were not a 'side effect' of designing a clutch with a cover...Ian

Yes, Think your right about that, at second thought I wouldn't say it was the cover which held the plates small, more likely it was because this clutch was in turn based on the 'medium weight' used in the earlier 30's. The size of the plates seems to be about the same.

Here a drawing of that mediumweight clutch:

 photo SAM_7316_zps279f2991.jpg


Think around the mid 30's BSA had to design a new small diameter heavyweight clutch as the big heavyweight clutches already used on the slopers etc. would not fit the laydown footchange gearbox because the gearbox mainshaft became much lower to the ground as the box was turned. It would only fit if the footchange gearbox was placed higher in the frame otherwise the clutch would stick out below the frame.

It looks like the Empire clutch became the answer but in turn had to become a dry one when it was fitted inside the later oilbath chaincase to cope with given power without slipping.

Again lots of guessing without any proof, hope you don't mind.

Regards, Michiel / NL

email (option): m.wijbenga [ at ] hotmail.com

Re: M20 steel clutch plates etc.../ some more clutch thoughts

Hi Michiel...All interesting information...

I think we often have to work without all the facts and no solid 'proof'... and in that case a 'best guess' is usually the only answer...

Working out what those engineers were thinking 70-80 years ago and why they did what they did is fascinating...and 'messing about' with their ideas seeking improvements can be a risky strategy! .....Ian

email (option): ian@wright52.plus.com

Re: M20 steel clutch plates etc.../ some more clutch thoughts

hi Ian
I have been told that you are making some BSA M20 wet or dry clutch plates for the single spring clutch.
I have been running a triumph 4 spring in my m21 combo but I have picked up a single spring clutch which is going to be a swap for the 4 spring when I get it apart . so I am reconditioning this single spring clutch. I have heard that sometimes they can still get oil in them dispite the dome. so with the intrest of my clutch keeping working on the combo when im abroad your wet or dry plates sounds like a good idea.
have you had the chance to try them out yet ?
an how much for a set + p&p

keep’a’troshin lightningrob of the BSAOC Norfolk branch

email (option): b31rob@aol.com

Re: M20 steel clutch plates etc.../ some more clutch thoughts

Hi Rob...Mail sent...Ian

Re: M20 steel clutch plates etc.../ some more clutch thoughts

Ian, going back to your original query, I don't know BSA clutches but I do know Commando clutches (a little too well) - big heavy things with a large diameter drum.

The Commando drum doesn't have conventional slots but has much finer teeth, pretty much the same size as those on the centre. Friction plates are sintered bronze on steel of the same thickness as the plain plates.

It's a characteristic of these clutches that nearly all the wear arises at the centre. Could it be that the plates which wear most are simply those with the least retaining tabs / teeth ?

It's also accepted wisdom with the Commando clutch that if the friction material is machined from the inside, leaving the outside diameter intact, they will be less likely to slip (they do suffer from oil contamination). Mean diameter of the friction surface is increased and so is pressure.

Re: M20 steel clutch plates etc.../ some more clutch thoughts

Hi Rik...I'm not familiar with the Commando clutch...

However, the M20 clutch has 4 steel plates with 'external' tangs and 3 steel plates with 'internal' tangs. I have found it is the former which always suffer the most rapid wear....

The 8 fibre plates are exactly that...full circle fibre rings with no steel backing plate and no tangs...Ian

Re: M20 steel clutch plates etc.../ some more clutch thoughts

How many tangs do the outers have on the outside...and the inners on the inside ?

Norton single clutches have 8 large tangs on the outside and 16 smaller on the inside plates and with those it's always the larger outer tangs which wear. It seems logical that they're taking twice the force of the 16 inner tangs.

I'd have thought it logical to have the same number of drive tangs on each type of plate.

Norton clutches also sometimes have friction material that's not attached to a steel plate...but it's not intentional !

Re: M20 steel clutch plates etc.../ some more clutch thoughts

Rik, The outers have 8 tangs and the inners 6.

I had a closer look at the construction, the whole package is pressed together on the heart of the clutch, so with the clutch fully released, the inner tangs don't have to transmit the power as the friction between the plates will do that.

The outer tangs however have to transmit all the power on there own all the time.

And the outer tangs will be hammering on the basket by intermittent accelerating and decelerating. And the hammering effect will increase as the play between the tangs and the basket increases. I can imagine this causes most of the wear and burrs on the outertangs.

The earlier heavyweight clutch had a simple solution to create more surface on the outer tangs:

 photo SAM_7498_zps4eefff96.jpg

Regards, Michiel / NL

Re: M20 steel clutch plates etc.../ some more clutch thoughts

to continue: in the earlier/mid 30's middle weight clutch, pictured above a few posts earlier, the chainwheel is part of the clutch'package' and in this design there are only 4 tangs on the outerplates, these outertangs don't have to transmit all the power as the friction between the plates helps here when the clutch a fully released/on.

In the WD clutchdesign the chainwheel is separated from the clutchpackage and the outertangs do have to transmit all the power and the amount of outer tangs is doubled to 8:

 photo SAM_6419_zpsc1857d6d.jpg

The play on the outertangs between the basket determine the play on the whole clutch assembly even when the clutch is released/on, while in the earlier design the clutch can be accounted as more or less one solid piece when the clutch is released.

Regards, Michiel / NL



Re: M20 steel clutch plates etc...

The torque (not power) that a clutch can transmit is determed by:

Torque transmitted = coefficent of friction x number of working surfaces x mean radius x spring pressure.

The M20 clutch attemps to compensate for the low (relative) spring pressure and small mean radius with a lot of surfaces. However the coefficent of 70 year old friction material is some what doubtful and add to that the uneven pressure and small lift of the plates ( the more there are the less they can seperate) result in the clutch we know and endure

Nieuwe pagina 1