[seruzawa]

I thought the cylinders on Harley v-twins are kept in alignment by the "fork and knife" design of the connecting rods.

That's correct. No motorcycle engines are really smooth without some compensation. The V-8 engines or any higher multiple of 4, like V-12 or V-16 are pretty smooth. The V-6 is especially lousy and is used for compactness reasons not performance. An I-6 has better torque characteristics. The V-8 is better in every way. Motorcycle engines, having usually 2-4 cylinders are inherently nasty vibrationwise. The I-4 is really bad for high frequencies as witness my old CB750F and others before counterbalancing shafts came into use. It was a horrible dental drill at certain engine speeds, worse than almost any twin. The bike simply could not be ridden for more than a few minutes at those speeds (65mph unfortunately). The Harley style crank is actually a pretty good compromise as twins go.