I really like this explanation...
It was posted by grayracer513 over at thumpertalk
Balancing a crankshaft assembly for any reciprocating piston engine presents a variety of challenges and compromises. There are virtually no designs that completely cancel all the competing primary and secondary forces that produce shaft vibration, though some do come fairly close.
Understand that a single cylinder engine produces vibration primarily from two sources:
Rotating imbalance
Vibration induced by the reversion of the reciprocating mass at TDC and BDC
The reciprocating mass is the weight of the complete piston assembly, including rings, pin, and clips. It also includes a portion of the rod weight. A rough estimate of this is obtained by hanging the rod horizontally from two scales. What the small end weighs is reciprocating weight, and what the big end weighs is rotating weight.
The rotating mass is the crank and everything attached directly to it, the rod bearing, and the above mentioned part of the rod's weight.
Balancing rotating assemblies is simple, and needs no explanation here. The problem, however, is that we need to cancel the forces caused by the acceleration and deceleration of the piston. If we have a rotating assembly in perfect balance, you will still have two moments of force on the engine generated by the piston as it changes direction. To counter this, without the use of a counter balance shaft, the rotating assembly is deliberately imbalanced with a percentage of the reciprocating weight. The weight is place in the crank opposite the crank pin so that it generates a force in opposition to the piston's inertia. If the cylinder is oriented vertically, and the crank axis is Z, then the piston produces a downward force along the Y axis as it reverses at BDC, and an upward one at TDC. The counter weighted crank works in opposition to this.
But it can only be partially successful because the piston will generate foce only along the Y axis, while the imbalance in the rotating assembly will generate force radiating from Y out through the X axis, being helpful only at TDC and BDC. So it has to be a compromise, and can never be perfect.
The matter is further complicated by the fact that the forces at TDC and BDC don't match. Because of rod angularity, the piston travels faster in the top 90 degrees of crank rotation than in the bottom, and that means that forces generated by the change of direction are greater at the top. Even if we could totally cancel one force, we wouldn't match the one at the other end of the stroke.
What counter balance shafts allow you to do is to imbalance the rotating mass to the point of near total cancellation of the reciprocating forces, then canceling the rotating imbalance with another imbalanced shaft phased 180 degrees opposite the crank.
More:
http://en.wikipedia..../Engine_balanceIf bikes are for kids I'll never grow up.
