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V-Twin

76 V-Twin Triumph

 

They said he was crazy to do it, but Lee Kernich has built the world's first 'vee-twin' Triumph. it doesn't look at all like a Ducati, or a Moto Guzzi, or even a Harley. It looks like ... well, just like a Triumph, right down to its oil-sprayed engine cases.

"I don't worry too much about trying to keep it oil-tight," the softly spoken South Australian says. "I take it apart too often to try to get the cases sealing properly."

The cause of all this development work is a 76 crank that pushes the pistons up and down against each other. As every Triumph rider knows, Edward Turner intended his marque's pistons to rise and fall in unison, which is one of the reasons they vibrate so much. Here's where another engine designer enters the fray.

"Phil Irving used to espouse the benefits of a 76 crank," says Kernich. "He'd offered the idea to major factories but they weren't interested. In 1972 1 read an article on his theories and I ended up corresponding with him, but it took me years to get the idea out on the track."

In the meantime Kernich went classic racing with a 180 crank. "It was an old three-piece Thunderbird shaft with a special flywheel that I made up." It was a pretty trick set-up 10 years ago. Most racers would have been content, but he was haunted by the 76 theory.

"I had two reasons to build the crank," says Kernich, design manager at Adelaide's Mitsubishi car plant. "With Irving's design, a lighter flywheel is required, a saving of some 1.5kg. A 76 crank also puts the peak inertia forces, which cause vibration and stress on mechanical parts, out of phase."

Kernich used a Norton Atlas crank to start the project. He remachined it to fit the Triumph cases, and the central flywheel was replaced with a steel plate and the two halves of the Norton crank are staggered on the plate. The plate is drilled and tapped so bolts go through the outer pieces of the crank and screw into the plate, rather than the original Norton set-up that uses long through-bolts. Staggered balance weights are bolted to the plates at greater than 76 to create a rocking couple which opposes the couple generated by the main crank assembly.

"That's all there is to it. It really is that simple," says Kernich, "but do you think I could explain it to any engine balancing firm? I ended up having to make my own balancing machine to fit in the lathe at home, so I could balance it dynamically myself " just as much thought has gone into the rest of the motor. The strongest conrods money can buy lurk inside those standard-looking engine cases; American Carillos, machined from high-tensile steel. on top of them sit forged Venolia pistons, also from the States. They run a compression of 12:1 so Kernich's toy can sip methanol, Australian racers' favourite tipple. The 71mm bore and 89mm (Norton) stroke gives an engine capacity of 715cc. The whole is topped with a standard, eight-stud head. "It's been cracked for years but it doesn't seem to affect anything," Kernich says with a smile. "I've got an aluminium bridge holding everything together."

This contraption fits over the whole engine and is held down with long through-studs that screw into bracing on the crankcases. 'Me bike's terrific punch out of the corners is helped by the cams, which were made by the Melbourne firm, Wade Engineering. They are the same specification as those used in the late Kenny Blake's 1969 Australian GP-winning Triumph, however, they don't look like Triumph cams. As one crank pin has been moved 76, a lobe on each cam has to be moved 38' (The cams run at half engine speed).

The head has had the usual Triumph racing mods ... shrunk-in spigots and inlet ports that taper from 32mm. The Concentric carbs are oversize at 32mm. Kernich is still experimenting with exhaust lengths and at the moment runs straight-through 13/8 in diameter twin pipes. The gearbox is a pre-unit Triumph with five-speed conversion. A standard single-row primary chain transmits engine power. Ignition is by Lucas distributor with battery and coils, but use of the 76 crank involved making a special breaker point cam.

"In the distributor the standard two lobes are 180 apart so one had to be relieved 38," Kernich says. "The flag on the distributor rotor had to be extended so it was pointing at the terminal for the high-tension lead when it was firing.

"Setting the timing is no harder than for a standard Triumph but it's a lot easier to get confused as things happen at different times. You have to keep your wits about you. "

Kernich describes the engine as being torquey as a Ducati. "It runs like a vee-twin because it really is one folded-up," he says. "It's strongest above 4500rpm but I don't need to take it over 6500rpm. it pulls better out of corners; it's torquey, has more mid-range and is smoother at higher revs. it also sounds just like a vee-twin. "


Basis of the engine modification is a Norton Atlas crankshaft with the pins offset on a new central flywheel  

The frame is a BSA A10 which was modified by brother Owen. it's had the steering head lowered and extra bracing welded into critical areas, but the most interesting part of the rolling chassis is the front brake.

"It's two BMW brakes off Earles fork models," Kernich explains, making it sound as if they'd just slotted straight into the Norton Roadholder forks. He's made his own alloy yokes, which are cast-up copies of Norton items, and the alloy tank is also homemade.

Rear braking is courtesy of a 7in Triumph front brake hub laced into a Triumph rear wheel. This cable-operated creation features a finned heat muff that has been shrunk on to the hub. Tyres are 18in; a Bridgestone Spitfire proddie racing job on the rear and a Metzeler ME33 on the front.

Kernich is typical of many home-builders. A pussycat in the pits, dressed in shorts and Akubra hat, he turns into a tiger on the track. With the distinctive, lazy bark of his Triumph heralding his approach he cuts an exhilarating sight, tilting his head as he wrestles the bike around South Australia's dusty short-circuits in pursuit of another trophy in the pre'62 class. 

It all began with discussions about the merits of Honda's recently adopted system of 180 firing. From that it passed to private thinking about the conventional parallel twin arrangement; and from that to a brand new plan, which involved throwing firing interval convention out of the window altogether...

But to begin at the beginning. By long tradition, parallel-twin four-stroke motor cycle engines are arranged with the cranks in line, or at 0, so that the pistons go up and down together. Twin two-strokes, on the other hand, are made with the cranks at 180, so that while one piston is going up, the other is coming down. This arrangement is a natural for the two-banger, because it simultaneously provides excellent balance and evenly spaced firing intervals.

But when the 180 layout is applied to a four-stroke the picture changes. Though the balance is just the same, one power stroke follows immediately upon the other, after which there is a period of one complete . revolution before the first cylinder fires again.

Like the single

Mechanically, there is no difference between the balancing problems of the "up and down together" type and those of a single-cylinder engine. Neither can be balanced completely, though by careful selection of the amount and disposition of the crankshaft counterweights, and by designing the frame so that no serious resonant vibrations are excited by out-of-balance forces, either in its component parts or in items such as the rear mudguard which are attached to it, a reasonably smooth-running ensemble can be achieved.

However, all that this process does is to disguise a defect without eliminating the prime cause, which grows steadily worse as the engine capacity is increased and the reciprocating masses become heavier.

The 180 twin, though balanced as a whole so far as the primary piston inertia forces are concerned, is not, strictly speaking, completely balanced. In addition to secondary unbalanced forces, which will be referred to later, it possesses what is termed a rocking couple, caused by the inertia force of one piston acting upwards while that from the other is acting downwards and at a distance of some inches.

This couple tends to oscillate the whole engine in a vertical plane, as shown in Fig 1. However, by treating each crank assembly as if it belonged to a single, and balancing it to 50%, the rocking couple can be halved, although this procedure introduces a horizontal couple of the same magnitude which tries to twist the engine about its vertical centreline.

Couples of this nature do not exist if the crankpins are in line; but in any case they are not particularly harmful and it is not difficult to construct a frame which will resist them successfully. A commendably smooth-running machine then results.

A fetish

But for the fetish which has developed in motor cycling circles about the necessity for equal firing intervals, it is quite likely that the 180' arrangement would have come into general use for four strokes as well as for two-strokes. As it was, it fell to the Japanese to take the plunge on the Honda Dream Super Sport - a parallel twin which originally had a crankshaft of conventional form - and later, on the racers.

Of course, with only 250cc capacity it is a small machine and the effects might not be so good on very much larger engines. However, let us take a look at another form of engine with unequal (though not quite so unequal) firing intervals.

This is the vee-twin which, in its usual 50 form, has firing intervals of 310 and 410.

Vee-twin example

In this engine, when one piston is at rest at tdc the other is well down the stroke and moving at not far off its maximum speed, so the total energy contained in the reciprocating parts is more nearly constant and consequently a vee-twin needs very little more flywheel weight than a single of half the capacity. it has little or no rocking couple and the primary balance, though not so good as that of a flat twin, is very much better than that of a single or 'both together' parallel twin.

That is the broad general picture, with less important matters like secondary inertia forces, carburation and ignition difficulties brushed out of the way.

But how let us see what might happen if we threw convention right over the wall and constructed a parallel twin with its crankpins set anywhere but in the same plane - neither in line nor at 180.

Naturally, there are a lot of possible choices, but the most logical scheme would be to locate the cranks with 76 between them. This angle is chosen because, with a connecting-rod length equal to twice the stroke (which is about the usual proportion) when one piston is at tdc, the other would be moving at its maximum velocity and consequently generating no inertia force.

With this arrangement, the total energy content would be substantially constant the whole time, and there would be little need for any flywheel at all except to smooth out the power impulses, which would occur at 284 and 436- a little more irregularly than in a 50 vee-twin, but better than the 180-540 intervals of the Honda, when one power stroke is almost treading on the other's heels.

Looking at the balance of the proposed engine as a whole, Fig 1 shows that when No I piston is at tdc it is generating its maximum inertia force, while No 2 is generating zero force. Then 76 later, there would be another maximum upward force of the same value arising from No 2 followed by downward forces at 180 and 256.

Better balance

However, at intermediate positions, when the pins are at 38 on each side of the centre-line, both pistons would be generating forces equal to .78 of their maximum. Therefore, in these two positions the total out-of balance forces would be 1.6 times that from one piston alone, instead of being twice that from one piston alone, as in the case of the conventional twin. (See Fig 2).

At these two points there would be no rocking couple, as the forces are equal in direction and magnitude, but there are rocking couples of half the value of those arising from the 180 crank arrangement at the four cardinal angles.

Besides, being so much smaller in magnitude, the frequency of these couples is twice that of the couples in the 180 twin - which might or might not be a good thing, depending, of course, on the natural vibration frequency of adjacent frame components. However, as the peaks of maximum value are not evenly spaced, and it is not possible for anything to vibrate at two major frequencies simultaneously, it is highly probable that the rocking couples would be less obtrusive than in the 180 type.

The secondaries

So far, only the primary out-of-balance forces have been considered, but the secondaries cannot be overlooked just because they are only one-quarter the value of the primaries. The trouble is that they act upwards - that is, away from the crank axis - when the pistons are at either top or bottom dead centres, and downwards when the cranks are at 90 or 270. (Fig 2). Thus the secondaries from both pistons act in unison if the crankpins are in line or are at 180, and the total secondary force is therefore equal to half the primary due to one piston.

This means that in the in-line layout, with a 50% balance factor, the secondaries are equal to half the unbalanced portion of the primaries from both cylinders and thus are large enough to be reckoned with, even though the frequency is twice engine speed.

180-degree case

In the 180 layout, the secondaries are exactly the same in value and frequency as in the other type and they constitute the major disturbing forces. in neither case do they set up rocking couples, but they might easily initiate a high-frequency tremor in some part of the machinery.

With the 76 crank arrangement, when No 1 piston is at tdc and generating an upward secondary of maximum value, the secondary from No 2 is acting downwards at about half its maximum value (which of course had occurred at the 90 crank position). The two thus balance each other to some extent, the result being a force of about one-quarter that arising from the two usual crank arrangements. True, a small rocking couple is set up, but as the peaks of this (and also of the resultant secondaries) occur at four times engine speed and at uneven intervals, their chances of becoming a nuisance are much reduced. All this delving into mechanics may seem very dry-as-dust and remote, but it leads to the very practical conclusion that this apparently cranky crank arrangement would be well worth a trial. (So too might be an alternative scheme with the cranks at a greater angle - say, at 130 - but I leave it to someone else to work out the details). it would not even be a very costly experiment to make in any of the existing designs which use either a cast one-piece shaft or one built-up from two pieces with the flywheel sandwiched between.

New camshafts would be required with the lobes at the appropriate angles. Two carburetors would be desirable (though not absolutely essential to prove the theory), but coil ignition would be necessary because a single magneto would not fire satisfactorily at the intervals required.

Now, I suppose, some historian will bob up and point out with pitying scorn that the thing was tried out in 1903. Never mind. Even if it was, I would like to see it tried again in 1962. Any takers?

 Post script

Phil Irving's object seems to have been to split the out of balance forces around TDC and BDC at the expense of a small rocking couple. If so, as has since been pointed out, he would have done far better to have chosen crank pins at 90 to one another. His fundamental assumption was that maximum out of balance forces occurred each time one of the pistons was at TDC. Not so - the two cannot be separated, and maximum out of balance forces arise when the two pistons are equidistant around TDC. That being so, a 90 angle is far better than 76.

Fortuitously, the secondary forces, which have a frequency twice crankshaft speed will have a phase difference of 180 and cancel each other out entirely.

With maximum primary out of balance forces exerted at 90 around the dead centres, and with the cancellation of secondaries, the balance of a 90 vertical twin should be a remarkable 43.5 % better than that of a conventional engine. There is still a rocking couple, but a very small one.

So Lee Kernich had better get out his spanners, make up a new centre plate, and bolt up his crank and camshafts to 90. His engine will still sound like a vee-twin, but it will be even smoother than it is at present.

Brian Whooley

 

Further reading: Vic Willoughby's Smoothness by Degrees