Parallel Twin." Vibrations.
This article appeared in the November 1991; edition of Easyriders magazine under, "Tech Tips."
The Solution to Engine Vibration
By Bob Pickett
The benefits of having your flywheel assembly dynamically balanced are:
1) smoother operation (less vibration)
2) longer bearing life
3) more usable horsepower
4) higher rpm (usually more important in racing applications)
Although vibration can come from tires, wheels, brake discs etc., the majority of vibration in a V-twin comes from the engine. This occurs in two areas:
Torque Vibration and Vibration from the rotating mass unbalance.
Torque vibration is caused by the engine firing on the power stroke.
The amount of vibration varies with the engine load; the greater the load, the more severe the power impulse, the greater the vibration. As the cylinder fires, the piston is forced down and at the same time the cylinder head is forced up. (Newton's law: for every action there is an equal and opposite reaction.) Since the cylinder is fastened to the crankcase, and the crankcase is fastened to the motorcycle frame, the result is vibration. For example:6,000 rpm is 100 power cycles per second = torque vibration.
The second source of vibration is the rotating mass of the flywheels.
The rotating mass is the total weight of the rods, pistons, rings, and complete flywheel assembly.
A rotating unit that is in perfect balance rotates concentrically around the center line of the axis of rotation. A rotating unit that is not in perfect balance tends to rotate in an elliptical (egg shaped) manner, around the center line of the axis axis rotation, with the elongated portion of the ellipse at the point of unbalance.The more unbalanced, the more exaggerated the ellipse. Of course, the elliptical movement caused by the unbalance is restrained by the ball bearings. It follows that the greater the elliptical motion of the flywheels, the greater the wear on the ball bearings. For example, as the speed of the engine doubles, the amount of force, per revolution, created unbalance tends to increase by an approximate factor of four.
Example: 1 ounce inch of unbalance at 2,000 rpm creates approximately 7 pounds of force per rpm. One ounce inch of unbalance at 4,000 rpm creates approximately 23 pounds of force per rpm.
One ounce inch of unbalance at 8,000 rpm creates approximately 120 pounds of force per rpm. Unbalance is typically expressed in ounce inches (oz. in.). This means 1 ounce (28.35 grams) of unbalance at 1-inch radius.If it takes "X" amount of horsepower to generate "X" amount of rpm for an engine with unbalance; it follows that it will take less horsepower to generate the same amount of rpm with a balanced engine, because your are not losing horsepower overcoming the effects of the unbalance. Reasons three and four for having the flywheel custom balanced-more horsepower and higher rpm.
So what does this really mean? Typically, the closer your flywheel assembly is to perfect balance, the less vibration you have. By balancing correctly and accurately, the vibration production from vibrating rotating mass can, in most cases, be greatly reduced.
I really don't know what the balancing tolerance (to what accuracy) the stock Harley-Davidson flywheel assembly is balanced at the factory. Rumor has it that they balance by an average weight of components, then they apply that formula to all engines. Some shops try to balance to .5 ounces inches. However our shop balances Harley flywheel assemblies to less than .5 ounces inches. To put it another way ".5" ounce inches of balancing tolerance is 3.336 grams placed on the outside diameter of the flywheels note: a $1 bill = 1 gram.
It is especially important to have the flywheel assembly custom balanced if you have cut down or lightened the flywheels, crankpin, changed pistons, put in different rods, or stroked or destroked the crank. For example: Let's say that the engine is in good balance but you want to go with a bigger piston.The new pistons, rings, etc. weigh 100 grams more, each, than the pistons you took out This would mean that the would be out of balance by 62 grams (8.2 oz. in) at the outside diameter of the flywheel assembly.
Why custom balance your modified Harley Davidson flywheel assembly? The obvious advantage is that the flywheels are balanced for the exact components you are using in your engine. You'll also have the smoothest running scoot on the block.
Hope its of some help in understanding the root cause of Vibrations in Engines.