2. An interview with Hooper & John Favill / 1962
Be it in a Cotton, DOT, Greeves or James, something new and highly exciting has emerged from the Villiers stable; and at the London Show at Earls Court the crowds pressed forward to study the Wolverhampton bombshell. A single with twin carbs!
Villiers 247cc Starmaker / 1962 / The Motorcycle 1983
Yes, and there were many other novelties in the design. Full disc flywheels; a nigh-indestructible, all metal clutch; needle-roller bearings here, there and everywhere; a massive light alloy cylinder barrel and head. And to cap it all, a reputed output, of 25bhp at 6,500rpm. The man, principally responsible for the Starmaker was Bernard Hooper, assisted on the transmission side by John Favill.
And at the Villiers factory Bernard and John gave their reasons for the adoption of this or that feature. But how did the project begin?
Bernard Hooper explained, “About three years ago we began to feel that our scrambles engine, the 34A, was reaching the end of its development. We could take it up to 22½ bhp but we needed more”.
“Obviously, a redesign was necessary – it would mean, a virtually new engine, even if we could have incorporated some 34A parts. First, we wanted the smallest possible crankcase volume, which implied completely circular flywheels and close clearances. But the real starting point was the connecting rod. Once the strength and dimensions of that were calculated, then we could work back to the clearances needed for the crank webs”.
Norton Villiers factory Wolverhampton
“The ideal rod would be oval in section, for improved gas flow, but that isn’t the best section strength. The Starmaker rod, a steel component, has an H-section, but with the leading and trailing edges feathered for better streamlining. It is only ¼ in thick at its maximum, so the flywheels can be very close together”.
“In the Starmaker, the crankshaft webs and shaft extensions are one-piece forgings in nickel-chrome-molybdenum steel, while the shouldered crankpin is pressed into place then made to grip the webs more tightly by means of expander plugs forced into the crankpin bore.”
Starmaker / Cotton Conquest / 1965 / Dropbears
“Integral shaft ends and webs seem to me an expensive way of doing things, though, I can see that the principle does result in a really rigid shaft assembly. But on the subject of rigidity, do you feel that a pressed in crankpin, as in the 34A, is sufficient?” I asked.
“Expensive forgings?” echoed Bernard, “Not necessarily, once the tools are made and you can produce in quantity, this is a production engine, after all. Every one will be exactly alike. Scrambling certainly does throw a strain on the crankshaft and we wanted as strong an assembly as we could devise”.
Starmaker / Cotton Conquest / 1965 / Dropbears
“The Starmaker crankpin has a thinner wall than that of the 34A, and we can use larger-diameter expander plugs with a good core strength. This shaft won’t twist, believe me!”.
John Favill chipped in, to point out the main-bearing arrangements, “The shaft,” he explained, “is carried on two lipped roller bearings, placed as near to the middle of the shaft as we could get them”.
“The bearings lie directly under the crankcase walls and – an important point, we felt – there are radial external ribs on the crankcase for added support. On the drive side, as close to the sprocket as possible, there is an additional needle-roller bearing”.
“You also have a needle-roller bearing in the big-end eye. That’s unusual. But what are the advantages over – say – a crowded roller bearing?” I wanted to know.
Again, John gave the answer, “It’s a caged –needle roller bearing, and that’s a much better proposition than ordinary rollers”.
“Where there is a cage, you know where the rollers are going, loose, crowded rollers have to make their own way, as it were, and you get skidding and scuffing”.
Starmaker / Cotton Conquest / 1965 / Dropbears
“But the big point is that the use of needle-rollers enables us to have a much smaller big-end than would otherwise be the case. Less inertia!”.
Outboard of the unit, on the right, is the ignition plant – which has been variously described as “flywheel magneto” and “energy transfer”. I asked Bernard Hooper for the correct description.
“Well,” he said, “it’s an alternator – and it is energy transfer in that all leads from the unit are low tension while the separate high tension coil is mounted as close to the spark plug as possible. That way, we can have a compact unit, without restricting the size of the coil”.
“Note the light alloy flywheel, by the way, the first one of its type that we have employed”.
Greeves 24ME Starmaker / 1963
“Previous generator flywheels have been in gunmetal, but we calculated that light alloy would be more resistant to bursting at high rpm, in fact, we have tested the new flywheel to 17,000 plus rpm”.
“Besides, a heavy flywheel increases the shock loading on the shaft and could lead to twisting. The light job assists in obtaining quick acceleration, and the effect is particularly noticeable when rapid gear changes are being made”.
“Can we now discuss the cylinder?” I asked, “It is, I note, in light alloy, but with a cast-in iron liner. Any particular bonding methods employed?”
“Not ordinary iron,” Bernard corrected me, “That’s spun-cast austenitic iron, I’ll have you know! We chose it because it has an expansion rate only slightly less than that of light alloy”.
“To obtain a good bond between the iron and the alloy we rough machine the upper external surface of the liner”.
“All liner ports are machined, by the way – not cast – and we use the machined ports to locate the shell-moulded sand cores of the cylinder”.
DOT Demon Starmaker / 1963
“Widely spaced fins, to resist the clogging effect of mud, and with slots so that the inlet and exhaust areas are isolated so distortion is inhibited”.
“The fins project well out into the airstream, and those around the transfer ports project still further to obtain maximum benefit”.
“Talking of ports,’ I broke in, “Do you calculate the resonances for all three – inlet, exhaust and transfer”.
“No”, said Bernard, “Just the inlet and exhaust. But the area and flow shape of the transfer ports are carefully worked out”.
“The transfer ports direct the charge upward and toward the rear of the cylinder, so that the two incoming streams meet and fill the cylinder from the rear”.
“Theoretically, rectangular port openings would be better – though, in practice, oval openings ease the passage of the piston rings. So we have to compromise, though it does make calculation rather more complicated”.
“Port size and timing is tied up with the primary compression ratio which, in the Starmaker, stands at the very high figure of 1.69 to 1”.
Scorpion Type 4 Starmaker Special / 1964
Combustion-chamber shape is unorthodox, comprising a mushroom recess in the cylinder head, with central spark plug position, and very wide squish bands”.
Villiers, explained Bernard Hooper, had conducted numerous experiments before deciding on the final shape. Advantages? Increased turbulence, giving better resistance to detonation and therefore, smoother running at the high (12 to 1) compression ration employed.
“The twin-carburettor layout”, I commented, “Is one of the most notable features of the design. Why was it adopted?”
“Look at it this way”, says Bernard. “For given engine speed there is only one correct size of carburetor. To get maximum performance at high speeds you need a larger-bore carburetor than for good torque at low speeds, by using two carburetors we should get the best of both worlds”.
“There is no gain in peak power over say, a single Amal GP carburetor – but it does mean that we can use conventional carburetors of the type with which riders are already familiar. Easier starting is another point in favour of the dual arrangement”.
“Why the rubber-tube connections between the carbs and the engine?”
“Mainly so that the unit can be mounted readily in various makes of frame”.
James Cotswold Starmaker / 1965
“All right,” I conceded. “Well, let’s move onto the gearbox. But first, why no shock absorber on the engine shaft, or clutch centre?”
“Manufacturers asked us for a small diameter engine sprocket, which leaves no room for incorporating a shock absorber. And the clutch, as you will see, is of unconventional pattern in which it would have been difficult to make suitable shock-absorbing provision”.
“However, it is an easy enough matter to mount a shock absorber in the rear hub, and some makers are already doing this”.
New throughout, the gearbox employs a multitude of needle-roller bearings, John Favill explains, “Free running, and a better engineering job for heavy-duty work, while another interesting feature is the adoption of involute splines, resulting in a stronger shaft with more even, stress distribution”.
“Sliding dogs are undercut, and angled faces ensure really positive engagement”.
“This type of dog,” said Bernard, “Requires less actual gear movement than before, easing the load on the operating mechanism and giving us a sweeter gearbox all round. Note too, that the sleeve gear does not take clutch thrust; instead, there is a needle-roller thrust race at the remote end of the main shaft. Again, the result is sweeter operation”.
Royal Enfield Starmaker / 1965
“All metal clutches were used on some vintage car models”, I recalled, “but they went out of favour years ago”.
“Yes”, agreed Bernard, “Mainly because it was difficult to provide a sufficiently high spring pressure without making the clutch extremely heavy to operate”.
“But there are big advantages, notably in durability and resistance to abuse. We use sintered-bronze surfaces, with spiral grooves so that oil is flung clear of the working surfaces. Another good point – very little movement is necessary, just a few thou’, and the plates are free”.
“The secret lies in the diaphragm spring, which offers very high pressure without increasing the operating load. The spring plate pivots on a ridge on the pressure plate, and there is a leverage ratio of about 5 to 1”.
“That means that a pressure of, for instance 150 lb at the centre of the spring is equivalent to 750 lb at the pressure line”.
Sprite Monza Starmaker / 1966