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The two orange/yellow dots represent the centers of gravity for the pistons; the one floating around represents their combined CG. The black/green dot is the axis of the crankshaft. The little black/yellow dot represents the crankshaft's center of gravity. It rotates in the same direction as the red/yellow dot, but is 180 degrees out of phase and thus balances the red/yellow dot.

However, the pistons do not move in pure sinusoidal motion. When the connecting rod is offset, the piston moves at some other speed. It is this difference between the counterweight's circular motion and the near-circular motion of the combined piston center of gravity that makes the yellow/blue dot wiggle a bit. This wiggle is the source of the secondary forces in this design.

Primary balance means that the movements of pistons and counterweights balance each other out. Secondary forces, which must also be balanced, arise from the fact that a piston connected to a crankshaft does not move with pure sinusoidal motion, and the pistons' combined center of gravity does not move in a circle.

One benefit of the way the pistons move is that this engine does not need a large flywheel. The reason is that the two pistons are continuously exchanging kinetic energy. Whenever one piston is stopped, the other one is moving at maximum speed. The rate of one piston's acceleration always precisely matches the rate of the other one's deceleration. The system stores kinetic energy (though because of friction in the bearings, not as efficiently as a simple flywheel would).

More on Primary and Secondary Balance

Primary Balance Secondary Balance Source
the crankshaft counterweights offset the weight of the piston and rod the movement of one piston balances the movement of another http://www.canadiandriver.com/articles/jk/at_010424.htm
Now we come to the Primary Balance which deals with the force of inertia which are set up each time a piston changes direction; once on the upward stroke and again on its downward motion. A piston has mass and nothing can be forced to change direction without resisting this change. So this inertia force must be balanced. So far from being a  smooth harmonic motion, the speed of the piston varies: fast - slow - slow - fast for every crankshaft rotation. At the same time that two pistons are travelling fast, two others are travelling slowly and so there is a net upward force - the Secondary balance. http://www.fordscorpio.co.uk/tech2_3.htm
    http://www.captain.norton.clara.net/cnn2sec14.html
Primary
balance is being able to both balance the up and down movement of the
pistons with counterweights on the crank that don't themselves create
vibrations that cannot be offset. A 90 degree V twin can have primary
balance I believe, so can an inline four.
Secondary balance is where you can counteract the difference between the
piston motion and the sinusoid it would be if not for the connecting rods.
http://realbig.com/pipermail/detomaso/2002-January/005960.html
     

 


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