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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
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
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
email (option): jonny.rudge@verizon.net
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
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?
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
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
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.
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
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
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
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
[ '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
email (option): m.wijbenga [ at ] hotmail.com
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
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
Hi Rob...Mail sent...Ian
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.
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
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 !
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:
Regards, Michiel / NL
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:
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
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