Hi All
Just Started to restore a early KM20 im looking for images of the upper brake Rod and its connection to the brake cable, i have the Rod and lower linkage, I only have a small nut on the threaded upper but no other connections or fitments.
The spring box and adjusted is present on the forks, but i have none of the bits between where the cable fits and the top of the brake rod.
any help with images and posible source or parts would be appreciated.
mike

email (option): mike.savill@yahoo.co.uk

Hi Mike,

Roy is making all the parts you need for the front brake. You can contact him via mail: roy206@hotmail.com

I have a completely restored KM20 myself. Just send me an email if you have any questions.

Regards,
Bastiaan

email (option): wdmotorcycles@gmail.com

Aren't there two types of front fork, a long and a short one ? With about 1 inch difference ?

Yes, you are right. For a KM20 you need the shorter one, it also has a different partnumber then the longer one.

email (option): wdmotorcycles@gmail.com

The difference in forklength is not that much, it is around 10 mm.

The short fork has a center to center distance of about 42.5 cm between wheelspindle and lower fork spindle.

By the standard forks this is around 43.5 cm.

BR. Michiel

email (option): m.wijbenga@hotmail.com

As far as I am aware, the length difference is 1/2" and the hex spacer for the mudguard is 1/2". I think Roy's repro brake rods are long enough for both types, and you can shorten it for the short forks.

We worked out that the longer forks were supplied from late 1940 although the RAF outfits still used the short forks. Ron

email (option): ronpier@talk21.com

I still haven't come up with a sensible reason for why they did it...I think it's safe to rule out 'improved' ground clearance, half an inch extra would be a laughable improvement...Ian

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

Ian, I have no particular view on the M20 aspect but the Ministry design specs were pretty rigid. There was a minimum clearance under the sump specified. The civilian 16H as originally tested did not pass, hence the 1' higher WD frame - If that 1/2" was enough to meet the standard then they might have done it...When did the change come from 3.50" to 3.25" tyres ? I thought 1943 ? That would have dropped them 1/4".

As far as I now know the longer forks were the standard fitment for the prewar M-line from the M20 to the M23. So the longer/standard forks already existed but returned on the WM20 during the war.

The short forks were used on the prewar M24. These forks are much lighter in weight and being shorter also lowers the centre of gravity, which makes sense on a racing bike.

For some reason the KM and early WM20's were equipped with these lightweight short forks. I guess it was to turn the weight down, probably in order to meet one of those rigid specification from the Ministry ??

Remember these early bikes also had the lightweight M24 frame: thin walled high tensile tubing (reynolds 501??), no sidecarlugs. no pillion footrestlugs.

And guess that when the war intensified and orders increased the weight specification became less rigid and BSA returned to the standard length and tubing for forks and frame.

email (option): m.wijbenga@hotmail.com

Any views on this theory ??

Cheers, Michiel

email (option): m.wijbenga@hotmail.com

My theory which I have mentioned before is a little different.

Pre war the M20 was a sidecar puller, when the Army had used the M20 for a while as a solo they wanted some changes made which included a softer front spring.

The softer front spring made the front end lower so longer forks were made to compensate, but they couldn't make them very much longer without ruining the steering angle and geometry.

The wartime RAF combination spares list returns the spec to sidecar use, so helps to prove the theory.

The only problem is that as yet we haven't found the difference in the two spring specifications?

Rob

email (option): robmiller11(a)yahoo.co.uk

'The only problem is that as yet we haven't found the difference in the two spring specifications?...'

More to the point do the parts lists for the variously equipped models show a different number for the two (possible)springs?....If not then the theory must be questioned I would have thought...

I don't really go with ground clearance improvements or potential weight differences..In both cases the gain would be minute and in the case of weight a pocketful of loose change (or a good breakfast) would probably cancel it out!....

However, shortening the fork slightly to alter the weight bias towards the front wheel for sports use sounds sensible...Though that leaves the question of why the shorter version was also employed for distinctly non sporting machines...Ian

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

Did the M20 ever have the lightweight M24 frame?....The high cost would have made that pretty unlikely I would have thought...

The total M24 production for 1938 and 1939 was only 564 machines so there weren't that many frames....I'd have imagined the M20 frame was just a standard frame with the footrest lugs omitted during production thus making it look like an M24 frame.....

Is there any evidence to clarify this point?...I'd be interested to know....Ian

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

Hi ian

With Michiel & Henk some time back I weighed and measured an early 1940 frame and a late 41 frame I have here

There is a difference in weight of almost 0.75kg between both

Tubing on the early type is definitely thinner wall & the re-enforcement area on the frame just under the headstock is different too

Interestingly, the 41 bike had short forks & a brake rod along with a 1940 frame sequence & a Bakelite damper knob in the side

Job

email (option): jonnyob1@googlemail.com

Yes the part number for the springs used with the longer and shorter forks do differ.

Also in the case of WD Nortons, the Big4 combination had a heavier spring than the solo 16H.

Rob

email (option): robmiller11(a)yahoo.co.uk

You are right Rob. I calculate 66-5051 for the short forks and 66-5053 for the long forks, but I don't think there is any difference in the physical dimensions (maybe just different tensions?) Who knows? As we all fit the same spring if one is needed. I've had several batches made over the years.....copied from an original.....but I've know idea which one and I've never noticed a part number on a spring?

Likewise, a G14 spring has another part number but the spring is exactly the same to look at as an M20.

The Norton 16H and Big 4 springs however are a totally different wire gauge. Ron

email (option): ronpier@talk21.com

Its not really a surprise that a combination needs stronger suspension than a solo, in the case of a rigid outfit of the 1940s, the only springing is at the front and quite a bit of its laden weight and passengers will be levering down on the front corner.

And the post 1940 solo Military M20 which has had all its sidecar fitting lugs removed was expected to perform reasonably well off road, which has got to be easier with some descent springing at the front.

Rob

email (option): robmiller11(a)yahoo.co.uk

Don't think the answer is in the spring. According to the partslist all the prewar forksprings for the whole M-range, including the M24 with different forks, are the same. so doesn't look like the spring has a relation with the length of the fork....

Still think it is a weight issue, will try again: :slightly_smiling_face:

We know that the M24 had a lightweight frame with Reynolds 531 tubing, no sidecar lugs, short headstock lug and no pillionlugs. BSA did a lot to reduce its weight. And according to Georges Wander, M24 specialist, it also had the fork in reynolds 531. This must be the shorter fork which also brings it closer to the ground.

Both my KM20 and early WM20 with original frame and short forks are much lighter than the later WM frame and forks. . These frame and forks, have thinner walled tubing so probably also high tensile steel to compensate. John also has an early lightweight frame and made very clear pictures of the difference in wall thickness.

They also have the exact specification as the M24, no sidecarlugs, no pillion footrestlugs, and a shorter headstock lug. I am convinced these early frame and forks ARE M24's and the only reason I can think off why they used them on the WD-M20 is that they are lighter.

And with Rik's comment that the Ministry had very rigid specification it seems even more logic that it was this reduction in weight which made the bikes fit within these specifications.

email (option): m.wijbenga@hotmail.com

I think it must be weight distribution (more weight on the front end) rather than a saving in all up weight...There would be practically no weight difference between the long and the short forks..

What we are talking about is 1" of tube of one diameter and the same at another diameter (front and rear legs of the forks, I/2" each side)...

I could think of many easier ways to remove that much, or more, weight from an M20 or any other model without rejigging the forks and I'm sure BSA could as well...

It is also worth considering that the M24 alloy top end would have altered the weight distribution compared to the iron M23 etc. and a little more weight at the front might have improved stability at higher speeds....

Regarding fork springs, according to Rob/Ron there are different part numbers and that would normally indicate a different part...However, I can't see how a difference in spring performance could be achieved without altering the number of coils and/or the wire gauge of the fork spring....

Differences such as this and the necessary associated changes to the fork spring scrolls (for a heavier wire gauge) are not mentioned anywhere to my knowledge...There is nothing in the 'standards book' for example...Ian

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

Added to this and something I think we have spoken about before. The G14 spring has a totally different part number 33-5082, but the spring from one of Leon's G14's which he gave me to get copied, looked identical to an M20 to me, which was confirmed by the spring manufacturer, as I'd given him an M20 spring as well. At first I thought the difference in part number was the fact that the G14 spring was chromed, but that theory was proved wrong.

Leon was adamant that his spring was original G14. I assume that a G14 is a heavier bike than an M20 and also used with sidecars.

Is it not possible to have variations in spring tension from the same wire gauge? I'm sure they do it with valve springs?

Ron

email (option): ronpier@talk21.com

"I think it must be weight distribution (more weight on the front end) rather than a saving in all up weight...There would be practically no weight difference between the long and the short forks..

What we are talking about is 1" of tube of one diameter and the same at another diameter (front and rear legs of the forks, I/2" each side)..."


I try again:smiley:

BSA went far to reduce weight on the M24, they redesigned lots of frame parts to make them lighter: delete the sidecarlugs, shortend the headstocklug. They even went so far to make the allready light gearboxcover in electron....

The short forks ARE considerable lighter than the long forks, it is not that short difference in length what accounts for it but mainly the thinner walled tubing.....(same goes for the frame)

I think the reason for shortening them for the M24 was indeed that it improved handling and that justified rejigging.

Being shorter can't be the reason why they were used on the WDM20, it even reduces ground clearance. Being lighter does...

John noted that the difference in his frontframes was 0.75 kg. If I remember well he compared his lightweight frontframe to a later WM frontframe with thicker walled tubes but no sidecarlugs. So the difference with the standard prewar sidecar frontframe would be even greater. Together with a lighter fork AND lighter rearframe the total difference in weight would be considerable.

These lightweight forks and frame were available at that time, a very easy way to gain that weightreduction which brings in the order from the Ministry.....???

BR. Michiel

email (option): m.wijbenga@hotmail.com

Hi Michiel

You could be right its another interesting theory, which I'm not sure even contradicts mine.

I have never weighed any M20 frame or fork parts to actually add any data, I have a set of short forks but they are already fitted to my GPO outfit and I'm not prepared to remove them again at this point.

In BSA and other companies I believe that some parts changed specification for material and manufacturing reasons but retained the same part number, its probably less likely that a specification was ordered without a change of number.

There is also a possibility that there are more than two different tube specs used on BSAs during the period 1939 to 1945. yes some pre war KM24 parts did use thin walled Renolds tubing, but that doesn't mean that the same good quality seamed tubing was used on everything else full stop or that the last of the WM24s delivered in 1940 were the same spec as the KM24s.

You have made a start but more work needs to be done.

Rob

email (option): robmiller11(a)yahoo.co.uk

Also its true that at various times lightness was a definite gaol, according to various sources a lightweight WM20 was manufactured and trialled and the early WB30s did experiment with electron as a replacement to aluminium on some parts, but at other periods of the war mass production was much more important and then again the saving of certain elements for more important roles ruled.

This KM20 from contract A9764 has no rear footrests mounts but still retains the rear sidecar fitting lug undrilled, which could be spun to tell any story we want it to.

And I'm intrigued by the tubing question, it asks all sorts of other question such as inner tube diameters fitting over different outer diameter forgings?

Rob

email (option): robmiller11(a)yahoo.co.uk

I also thought of different quality of tubing but the tubing of the lightweight KM20 of WM20 frame's are so thin...... It is hard to measure exactly because the vernier will not go that deep into the frame but has to be between 1 and 1.5 mm for sure........It has to be higher tensile to compensate



I also had to straighten these lightweight forks and frames a bit and that was very, very though, I had to apply quite some heat to bend it while I usually bend the standard forks and frame's cold and with much more ease. So high-tensile for these parts for sure.

Shame I didn't capture the the weights on paper but difference was easily felt when in hands. I remember being very suprised when lifting those parts.

Cheers, Michiel

email (option): m.wijbenga@hotmail.com

So, possibly, the forks were shortened for the M24 to shift the centre of gravity to improve handling and, perhaps, they could also have been shortened as a contributory factor in reducing over all weight of the machine......

Of course, it may be only one or both of those reasons....

As far as the M20 was concerned weight reduction is the most likely reason they were fitted and IMO the gain would primarily have come from the thinner tube, not the half inch length difference (the same for the M24)...Ian.

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

Sorry I am late to this conversation!

M23,22 and 24 girders are all the same length. They did not shorten them specially for the 24. The only difference is the reynolds tubing. I will run out today and get a scale so I can measure the difference in the blades. I am mocking up a KM24 frame now. When I tear it down I will get a weight on it as I am curious to know. I can tell you The 24 frame feels like picking up a bicycle frame compared to a JM frame!

Is it possible BSA had a stock of reynolds tubing already purchased for the next year and decided to use it up for the war effort??

Adam

If I had to postulate a theory it would be that BSA had a batch of race girders or tubes precut for race girders sitting in stock because the war was going to be over very shortly and civilian production was going to resume.
When it became apparent that this was not going to happen then BSA would have needed to use up these parts.
So a "special order" for the RAF using the lightweight frames would be in order as the RAF bikes were not likely to be running across battlefields at full speed fully laden while being shot at.
Then it would have been a case of we can supply XYZ bikes in 2 weeks if we can use these frames or in 10 weeks if built to the standard spec. And with the "spare" time we can pump out an extra 500 rifles or at least get the current order out before time.

There was a war on

email (option): bsansw1@tpg.com.au

The first KM20 frame book carries a sticker to the effect that "M20 frame was used for a few War M/Cs M23 and M24 Models"



Certainly, in the KM24 engine book, a few engines seem to have been fitted early in September 1939 into frames with numbers far too high to have been proper Gold Star frames. They all seem to have been supplied to the War Office.

If the early WD M20 frame had been identical to the M24 then surely this comment would not have been necessary...unless they had produced a limited number of evaluation machines with the high-tensile frames and had already gone back to standard grades by September 1939...That would seem to rule out any WM20 production though and if there were no proper Gold Star frames for the M24 then presumably they were used up some time before ?

Don't forget in 1938 and '39 competition M24s (in military finish) were supplied to the army for ISDT teams...Ian

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