I was now on the hunt for G15 Matchless parts and I was starting from scratch. We all probably know that building an old bike from bits is more problematic and usually more expensive, but my reasoning to take this route was that I had the intension of modifying many of the engine and transmission components. Collecting the parts took a fair while, but his gave me time to undertake more research on Irving’s theory of using a phased crank in a parallel twin engine.

I will try to explain a little about the theory, but will also try not to get you bogged down with too much physics. I am no expert, and if you want to know more, Youtube will provide you with lots of explanations.

Irving actually worked out that a 72 degree crank stagger would be beneficial. As a snapshot, when one piston is at TDC the downward motion piston is travelling at maximum speed, e.g. “yanking” the TDC piston away from the cylinder head. (See right hand column below)

I don’t know if Irving had the chance to prove his theory in metal at the time, but later on, Ron Valentine (of Wesslake racing engine fame), with the assistance of mathematician Tom Oliver certainly did. He also had a hunch that a 90 degree stagger would be even more beneficial. (See left hand column above)

With a 360 degree crank in a 2 cylinder parallel twin engine, at TDC both pistons are actually stationary (“for a split second”) and begin to move downwards at the same time (The same scenario in a single cylinder engine). At that time the flywheel inertia of the rotating crank assembly transfers kinetic energy to the pistons to get the pistons moving and that transfer direction happens until the crank is at about 90 degrees from TDC. After 90 degrees from TDC, the pistons begin slowing down and their kinetic energy in slowing down is given back to the crankshaft. Remember energy cannot be created or destroyed, just transferred. The same transfers of kinetic energy happen again from the next piston stop at BDC. In theory, if you had a very, very long con rod and no flywheel, which would be difficult, the pistons would stop at TDC and not move again.

With a 270 degree parallel twin, beginning with one piston at TDC and the other at 90 degrees of crank rotation ahead, the second piston is about halfway through its stroke and is in motion. The first piston will begin to move downward while the second is being slowed as it approaches BDC. The first piston is absorbing kinetic energy from the second piston that is giving up kinetic energy. This exchange of kinetic energy between the two pistons is constant so you get a “smooth” transfer, one to the other, through the entire revolution. Less flywheel effect is needed and that permits a lighter crankshaft which enhances acceleration.

Reciprocating engines are liable to produce primary and secondary vibration. This can be quite complicated to understand, so in layman’s terms; primary vibration is the sort of “thumping” vibration and secondary is the tingly stuff. A typical 360 crank engine will produce both. However, there is no secondary vibration with a 270 degree crank so you don’t get the high frequency “tingles”.

The parts started to pile up and I had acquired a 750 Commando crankshaft. I also discovered that Norton had experimented with an electric start much earlier than many people realised. Remember the Ducati salesman couldn’t turn my Commando engine over with the kickstart when I part exchanged the Norton and the factory knew there was a problem.

As I sifted through many crankcases, I found that some Commando timing side cases not only had the usual boss to mount the timing chain tensioner, but also had a similar boss cast-in that would provide a tensioner for an electric start chain. The electric starter would be fitted in the space vacated by the magneto that was now redundant with coil ignition fitted. The story goes that they tried to make it all work without any sort of “backfire proof” sprag clutch and the first kick back destroyed the whole lot. However, that boss got me thinking about fitting a balancer shaft in the future to reduce the primary vibration remembering that, in theory, the secondary vibes would be missing.

The Commando was eventually fitted with an electric start on the MK3 that operated via the primary cases with a sprag clutch. Remember when the British police motorcyclists were supplied with Norton Commandos? I spotted one of the first MK3’s and stopped on my Matchless to ask if it worked. The policeman laughed and then demonstrated the starting technique. This involved him kicking over the engine whilst pushing on the starter button at the same time! It was marginal to say the least.

Current MK3 Commando owners can now thankfully upgrade to a successful solution.

They say a picture is worth a thousand words, so hopefully these photographs of the new 90 degree offset crank will be enlightening. You will observe that the original crank had a huge flywheel that acted as a “spacer” for the two crank “cheeks”. My new spacer is simply that, a spacer. This new spacer-flywheel is actually 1,900 grams lighter than the original, that’s nearly two large bags of sugar. Remember that on my new crank, one piston will “yank” the other from TDC, and so on, so a much lighter flywheel is sufficient.

                                    Original Crank                                                          Heavy old flywheel                         

The flywheel needed eccentric scallops on the periphery to clear the piston skirts at BDC on this long stroke engine and as usual the whole job wasn’t as easy as it might appear.  The two halves of the assembly were balanced individually on knife edges.  (Pictures below.)

ou have probably heard about “balance factors”; quite a science in its own right, so I won’t dwell on it. However, the consensus was that a 50% balance factor would be good. I seem to remember that inn Irvingg’s ‘Tuning for Speed’ book he recollects that when the Velocette factory mounted a nice smooth single into a different type of frame the vibration was terrible. They had to rebalance the flywheels to suit the new frame that had a different “resonant frequency”. It’s a bit like black magic really!!

So now I had my new crank. Other modifications included changing the oiling to pressure side filtration. Using a MK3 timing chest cover not only facilitated an easy change to electronic ignition, but also provided a better anti sumpingystem. A belt primary drive was fitted together with a ‘dry’ Commando diaphragm clutch. I opted for a single system. A belt primary drive was fitted together with a ‘dry’ Commando diaphragm clutch. I opted for a single carburettor for simplicity and purchased a nice new 34mm Mikuni; more about this mod later.

The new offset crank would require two more very necessary modifications. The camshaft would need “retiming” by moving one pair of cam lobes, to be twisted at 45 degrees (not 90 degrees) to the other pair. Take note that a four stroke camshaft spins at half crank speed. All I did was saw the camshaft in half! It was more sophisticated than that. A register was machined on the centre of the shaft between the pairs of lobes. A sleeve was then machined to fit this diameter “post hacksaw”. A jig was made to hold the severed halves at 45 degrees to one another as the whole lot was carefully welded together. The sleeve was also pinned for extra security. It worked!

A Boyer electronic ignition rotor, again modified to 45 degrees, was utilised that gave “wasted sparks” at certain crankshaft positions, but this also worked well.

I made modifications to the frame and ancillary parts that I won’t go into here and eventually I had my “wobbly cranked” Matchless.

So how did it turn out? As I’ve said before, it feels like a “V-twin single”.  It has the thump of a single, but no tingles of a vertical twin. It sounds like a Ducati 90 degree V twin, which I like. The new Mikuni gave a regular tick over with the engine having a nice lilt, but as mentioned earlier this single Mikuni led to a mystery.  John Grew ..... To be continued

What is that mystery, how was it diagnosed and what was the cure?  Find out next time in Chat 21.  Eddy

For you technical types, John has included below a bit of an explanation of causes of vibration in various engine configurations. Eddy

Explanation of gif files.

Here are 4 different twin engine animated pictures. Primary vibration is shown as a BLUE arrow and secondary vibration as a RED arrow. The relative size of the arrow indicates the magnitude of the vibration and how it ebbs and flows as the crank rotates.

The parallel twin belowshowssa large BLUE primary and a smaller RED secondary. Note how the smaller RED arrow is flashing twice as fast as the BLUE but with less intensity, a large BLUE primary and a smaller RED secondary. Note how the smaller a large BLUE primary and a smaller RED secondary. Note how the smaller RED arrow is flashing twice as fast as the BLUE but with less intensity.

 Thee 270 crankk has no RED arrows so higher frequency vibes are missing..

As a comparison the Ducati 90 V  indicates the BLUE primary as constant, meaning perfect primary vibe reduction in theory in the form of a circle. There is a smaller RED secondary at twice crank speed.

Now look at the BMW flat twin. Here we see no arrows. With an opposed piston boxer engine one piston mirrors the other to cancel out BLUE and RED vibes. BMW boxer engines are smooth, but not perfectly smooth because the crank journals need to be staggered, which is why BMW and Moto Guzzi cylinders are “out of line” looking from above. This creates a small ‘rocking couple – a bit like a ‘seesaw’ vibe. (not shown here) The only way to eliminate this is by having ‘knife and fork’ con rods like in a Harley Davidson; difficult with white metal big ends.

Pat Robotham's Motorcycling Memories Part Two.

Eventually we bought an 18th century Malvern stone cottage in West Malvern, the first house we had ever owned, quite cheap and no more rent to pay and I had a workshop in the back scullery, the only downside being wheeling the bike through the main kitchen to get there. The house was down a seriously unmade pebble strewn cul-de-sac with a one in five slope on it. Which made setting off for work and returning quite interesting and probably encouraged my later interest in trials riding.

We both went out for rides together through the Malverns, me on the trusty 6T and Shelley on the newly rebuilt Royal Enfield model G. With no more rebuilding to do I wanted another project and spotted a 1929 model F Ariel for sale in exchange and Mart for £100 just down the road in Ledbury.  I bought it, it was nearly complete missing a front mudguard and cylinder head and exhausts, so it went into the workshop while I decided how to tackle a really old bike. That was my introduction to vintage bikes, I still have it and it is my go to bike for reliable two up vintage riding.  It does not have a speedo but considering I have owned it for nearly fifty years and have used it for commuting and motorway travel I reckon I must have done seventy thousand miles on it, in both solo and sidecar trim. I have personally rebuilt it three times, but once was after a bad accident.  I joined the VMCC and the AOMCC soon after buying it and the rest as they say is history.

Royal Enfield model G

Some weeks after buying the Ariel Shell and I walked into our local pub, the Brewers Arms for a late night drink.  As we had entered I noticed a rather battered BSA M20 leaning against the pub wall in full military trim and loaded to the gunnels.  We got our drinks and went to sit down near to a man in a dark corner wearing a large riding coat and with a pudding basin helmet on the table.  I assumed he owned the M20, and asked him about it.  “I am coming back from work, I live down the Purlieu.  Just popped in for a rum and orange."  I thought it was a bit late for leaving work, “where do you work, are you on the late shift?”  No I work in Tripoli for Save the Children and have just ridden home.” So started my thirty year friendship with one of the most extraordinary people I have ever met, Peter Ward.  One of the very early members of the VMCC, who a year previously had ridden non stop from Lands End to John o' Groats on a 1911 Edwell single speed motor cycle, using A roads and the odd motorway section on Acetylene lights. He completed the trip in about 19 hours.

Shelley didn’t like the Enfield as it was too heavy, so it got sold to a dealer in Worcester.  I decided I wanted a change and sold the 6T, and with the money from both I went for something completely different, an S8 Sunbeam.  Not a good choice, ponderous in performance and handling, and complex to keep running correctly.  I kept it only for about four months.  By this time it was 1978 and I had got the Ariel on the road, my first event in the VMCC was the International Assembly held at Cheltenham race course, Shelley and I went with the bike, I rode it over and she went in Peters van.  About four miles out from home the bike tightened up and came to a grinding halt.  Peter pulled up and walked up to the bike, I described what happened and he felt the engine and gearbox with his hand.  “What is in the gearbox?”  “Grease” as it says on the top.”  “Never put only grease in one of these grease gearboxes, always add about 25% oil, it does not matter if it comes out.”

1929 model F Ariel

Lesson learnt it has never seized again although it does have lubricant on the outside after a decent run.

The VMCC Internationals were held until about 20 years ago either at Harrogate, Cheltenham and later at Beverley.  They were time trials of about 100 miles and very popular.  Always having a good overseas contingent who all took it very seriously.  Over the years the number of people wanting such events declined as did people prepared to organise them so they stopped.  A shame in my view but a reflection of the way the club changed.  The only similar event left is the West Kent International, which I have done a couple of times, but it always seems that the roads around there are too crowded for my liking.

Following a disappointing few months with the S8, I noticed a very odd machine in the window of the dealers in Worcester to whom I had sold the RE.  This was a motorcycle made in Denmark, by the Fisk Nielsen vacuum cleaner company called a Nimbus, an ohc inline four in a riveted spring steel frame with shaft drive.  Made in about 1939.  What was not to like?  So I did a deal with the S8 and brought it home.  Trouble was it had been bought into the country by a Danish VMCC member at the previous International and swapped for a British bike, so it had no papers or reg number.  It also had no proof of import so it cost me an extra 10% import tax just to get it registered.  However it was a really nice machine, smooth good handling and performance but a bit under geared as it had a sidecar ratio final drive. Still I rode it a fair bit, and still had the Ariel for safekeeping.

Nimbus

We also helped Peter ward out on his autojumble stall, he had been doing these for as long as anyone could remember and had a reputation for finding unusual items to sell.  If you mix with dealers you end up dealing so over a few months a number of bikes passed through my hands.  If I liked them I kept them and rode them if not I passed them on.  Ones I remember were a very original D1Bantam which I manged to break my big toe on by hitting the fixed footrest when kickstarting, lesson, don’t use slippers to kiskstart a motorcycle even a 125 two stroke. An Ariel colt 200 cc from about 1954, pretty gutless and unmemorable. I also bought a kit of parts for a 1939 square four, on the misguided basis that if you are into Ariels you had to have a Square Four. How wrong you can be.

Ariel Square 4

By the end  of the year, 1979 I had had to get another job because the contract money had finished, so we moved up to Derby, and I commuted on the Nimbus until I ran the big ends. I stripped the bike in the shed of my digs in Etwall and a real old fashioned motor engineering firm called Cotterals in Derby  remetaled the con rods and caps, and ground the crank.  Making a superb job even though the guy who did it all had to be brought out of retirement because no one else had done anything like this for such a long time.

We bought a house in Mickleover with a big garage that leaked badly and our first son appeared so all things changed again. I also took up trials riding. Pat

To be continued:-

Geek Alert! - Computer Tip:-

Finding something specific in web page can be frustrating, especially if it is a long page. There is an easy way to perform an in-page search. It should work on almost any keyboard.

1. To search quickly within a web page (“Find In Page”), first open the page you’d like to search in your favorite web browser.
2. Press Ctrll+F F (on Windows PC, Chromebook, or Linux system), or Commandd+F F (on a Mac) on the keyboard. The “F” stands for “Find,” and it should work in every browser.

If you’re using Mozilla Firefox, a search bar will appear in the lower-left corner of the window.
If you’re using Google Chrome, a search bubble will appear in the upper-right corner of the window.
If you’re using Microsoft Edge, a search bar will appear in thepear in the upper-left corner of the window.
If you’re using Apple Safari on Mac, a search bar will appear in the upper-right corner of the window.

Below is what it looks like in Firefox.  I'm looking for Eddy, give it a try on this page.

It should also work in many applications too, e.g. Word Notepad, etc. Eddy

There's been quite a bit of technical stuff in this issue so here is something a bit lighter to finish.  It has done "the rounds" many times but it always makes mee laugh when I read it.  (Probably because I recognise it from personal experience. ����)  Eddy