A shaft driven bike with the crankshaft arranged along the bikes axis (a'la Guzzi etc) is faced with two major problems of torque reaction, in the early days bikes were not so powerful and neither represented a real problem. However with the V-six pushing out so much power Laverda simply had to reduce the effects of these torque reactions to make the bike rideable.
Problem One: Longitudinal Torque Reaction
Whenever a rotating object accelerates it produces a reactive force against the direction of rotation. There's nothing new about that statement as it was all worked out by Isaac Newton in the mid seventeenth century. His great work "Principia Mathematica" of 1686 contained his three laws of motion which can be used to describe the problems (and solutions) of the V6's handling thus proving the fact that Newton was in fact an avid motorcyclist.
Newton's third law states that "An action is always opposed by an equal reaction: the mutual actions of two bodies are always equal and act in opposite directions". Thus if something (like a V6 crankshaft) accelerates like buggery in one direction, there must be a reactive force in the other direction to counter it or else the whole thing will try to fall over. Blip the throttle of a Guzzi at rest and you will notice that reactive force must come from your right leg to stop the whole plot from tipping over!
Of course this effect is still present in a bike with a crankshaft running across the frame but it's effect is barely noticable except in cases requiring extreme chassis tuning. In fact this phenomenon was hypothesised to be the cause of the evil handling single crank Honda 500 GP bikes of the late 80's when the twin contra rotating crank bikes from Yamaha, Suzuki and Cagiva had the edge. Of course Honda's 'big bang' motor of 92 changed everyone's opinions and along with Michael Doohan's supreme riding and machine set up skills, changed GP racing from the brain out excitement of the Gardner/Schwantz/Rainey/Lawson days to the monotonous yawn we have today but I think I've gone off topic a little.... back to the V6.
The solution to this problem can be seen in the problem itself.
If every action has an equal and opposite re-action and another
mass is arranged to be accelerating in the opposite direction of
rotation to the first they will cancel each other out. There are
two principal ways that this can be achieved in an engine;
The first is to have two crankshafts rotating against each other (twin contra-rotating cranks), this is the principle behind the 1939 Velocette 'Roarer' 500cc twin cylinder supercharged GP racer. It employed two crankshafts running along the bikes axis, geared together to make them rotate in opposite directions. At the rear of one crank was the drive to the clutch and gearbox, the other crank drove the supercharger, and the contra-rotating side by side cranks cancelled each other out perfectly.
The other option is simpler and in some ways superior. By arranging the rest of the engine internals to rotate in the opposite direction to the crankshaft their forces are cancelled out without having to resort to the weight, complexity and friction associated with two crankshafts. This will unfortunately introduce smaller imbalanced forces in the other axes, but if the mass turning in each direction is approximately the same then their reactive forces will be cancelled out along the axis of rotation.
This latter method was employed by Laverda with the V6, they ran the clutch, jackshaft and gearbox off a gear to reverse their direction of rotation. The mass of these parts was approximately equal to that of the crankshaft and therefore most of the torque reaction was cancelled out. All reports I have read say that the V-six exhibits no torque reaction at all proving that this single crank method works beautifully.
Problem Two: Shaft Drive Induced Torque Reaction
The second problem was not fully appreciated untill the engine made it into a chassis and the bike was ridden. It proved to be more difficult to solve but Laverda approached it with typical originality. Following the theme of equal and opposite forces, a shaft drive arrangement on a motorcycle applies a certain amount of torque to the wheel through a bevel gear. For the wheel to actually turn there must be an equal and opposite torque holding the 'diff' housing from rotating backwards. Normally with any Guzzi or BMW, and the V6 prototype, this is achieved with a 'torque tube'- the housing that encases the shaft holds the diff housing from rotating and transmits this force into the frame through the pivot point. This is the force that causes the rise and fall of a shaft driven bike when going from power to a trailing throttle and vice versa- if you don't believe me go and take a Yamaha V-Max for a test ride!
Unfortunately the prototype V-six with it's short swingarm and underslung rear shock absorber suffered very badly from the tendency for the rear end to rise under power and squat on a trailing throttle. Obviously on a lesser powered bike in a touring application this effect can be tolerated however a racebike is much less forgiving and in Massimo's own words it was 'unrideable'. Clearly a solution had to be found.
Laverda's solution was to construct a much longer swingarm, pivoting near the clutch housing- the bikes measured centre of mass. The 'diff housing' was still fixed to the swingarm, unlike the 'paralellogram' type arrangements of BMW and Guzzi where the housing is floating. The long swingarm had two desirable effects:
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Copyright Stephen Battisson 1997 unless noted otherwise