Sunday, 14 May 2017

Loose Lines: Vee Twins Rule [Issue 54]

A fairly weird sequence of events forced me to focus my mind on the delights of vee twin motorcycles. First, was a cross country trip on an aged, tired XV1000. To be honest, this was the kind of effete machine into whose petrol tank I'd normally be quite happy to throw a lighted match.

Still, the need to cover some miles but the time to amuse myself meant that I was in the right frame of mind to enjoy its relaxed engine, which apart from some shuddering below 2500 revs had enough guts to dump it into top gear for most at the time. There were so many things wrong with the overall design of the bike, though, that it was all more surprising, down purely to the nature of its engine type, that I arrived in one piece at my destination with a reasonably large grin writ across my face.

Soon after that little sortie I once again blagged a ride, just long enough to whet my appetite, on a Ducati 900SS. A lovely device but the clutch was already complaining (just as well it didn't go during my brief tenure, I would doubtless have been beaten to a pulp) and the back light worked only intermittently a sure sign that the old electrical malaise was showing up the Italians' usual lack of attention to detail. A great beast to ride but only when some favourable. 50000 mile reports reach the editorial desk will I be tempted to run around like a headless chicken attempting to raise the dosh.

Then things became seriously strange. I actually managed to convince a long-time contributor that it would be a good idea to let me have a blast on his beloved Vincent vee twin. 1000ccs of pure, brutish British engineering. It took all of my resolve to keep the manic grin hidden. The Vinnie's an incredibly compact machine, feeling much smaller than the Ducati, even my own GPz.

It doesn't feel that way once the engine's running. It sounded like someone had filled a washing machine with nuts and bolts, whilst outside half the Slav army was slaughtering each other with machine guns.

The clutch was set up to deter thieves by either stalling the engine dead or lurching the machine forwards so violently as to give the unwary a heart attack. I'm sure it's an acquired art and that my left hand would soon become accustomed to the massive force necessary. The gearchange was on the wrong side and so heavy that my lightweight boots ended up deeply scarred after a mere half-a-mile's canter.

Once in motion, the minimal suspension travel gave so much feedback from the ancient tyres that the bike felt very twitchy, although in retrospect, apart from a low sped wobble, incredible effort needed to turn the bars and a bit of back tyre hopping, it kept to a fairly true line on the deserted country road. The only place the owner would let me ride.

What excited, once main, was the nature of the power delivery, that even on a forty year old motorcycle, impressed with its immediacy and its depth. The Vincent, though, showed its age and the basic flaw in its design, with loads of vibes that buzzed through the tank so violently that I suspected sustained exposure would turn the rider infertile.

These rides came at a time when some of the journalists in the glossies were wetting themselves at the prospect of being forced to test new machines limited to a mere 100hp by mad European bureaucrats. I have to admit that the prospect of buying a bike with only 100hp worries me a hell of a lot less than having to pay extortionate insurance or being forced to ride around on nice sunny days wearing a crash helmet.

lt could come to pass that the 100hp limit will force manufacturers to design more into their bikes than mere speed; or design them so that with the same horsepower they go faster. The only way to achieve the latter is to improve the aerodynamics and lower the mass. The limitation of a mere 100 horses also means that the almost ubiquitous across the frame four can die a thankful death; it will simply be too heavy and wide to fit the bill.

Which brings us to the vee twin. Narrow, compact and naturally biased towards the production of excessive torque. Its only limitation is the speeds that its large pistons can sustain. Not the problem that it used to be with lighter and stronger alloy and steel available, these days. With a ninety degree layout combined with a forked con-rod on one cylinder to keep the bores in line. vibration and torque reaction are minimal even at huge cylinder displacements. With the emphasis on torque rather than power (as long as it makes a 100 horses) there‘s no reason why, say, 1300cc can‘t be used.

Water-cooling the engine, to stop the back cylinder overheating, allows the motor to be placed in the most convenient location for compactness, the idea being to make it no longer in wheelbase than something like a Vincent. Naturally. with such a large chunk of alloy engine taking up most of the space, and perfect primary balance taking care of the vibes, it makes good sense to use the motor as the main frame member. Yes, just like the Vincent.

Major improvements in modern casting techniques mean that rather than having lots of different crankcase and barrel castings, the whole lot can be cast in one large casting, which would include the swinging arm mounts. This is important because the frame would consist of two completely separate front and rear sections. Were engine castings lots of separate bits the tolerance build up in the frame would be large. Because one part of each frame is located on the main casting there is a direct link between the swinging arm and steering head.

The rear shock would be located under the engine, although there's no real reason why ultra traditionalists couldn't be catered to by a more conventional twin shock arrangement. It's just that the former is lighter and that lightness is next to godliness in the superbike limited to 100hp. I would guess that such a vee-twin, designed by one of the Big Four with their vast knowledge of engineering stresses (no over-building just to be on the safe side), would weigh in at 350 to 400lbs.

The traditionalists would enjoy the finned barrels and heads (with a small, high-flow radiator located below the steering head), and the single carb. The hoodlums would be equally impressed by the tyre churning torque, fantastic acceleration and ease of chuckability.

With a mere two cylinders it might even be cheap enough to incorporate something thoroughly modern like variable valve timing and with so few moving parts they could be made from the highest quality materials for longevity. The vee twin engine has long been neglected, Ducati apart, as a serious sport-bike motor.

Now where did I put those two XTZ660 engines!

Bill Fowler

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