Actually what you want is a bob weight equal to 1/2 the weight of the piston, bearing, wrist pin, and the small end of the rod. If the counter weight is equal to the piston, then you eliminate the up and down vibration, but get all of the side to side vibration. I go for a 50/50 ballance. There are other ratios people use but 50/50 will give you half the up and down and half the side to side vibration. There is no way to eliminate all of both, so you have to comprimise. If you get it to 50/50 the two tend to cancel each other out some, so the all around vibration is less.