From SABR member Alan Nathan at Baseball Prospectus on January 31, 2012:
The knuckleball is probably the most mysterious of baseball pitches, surrounded by a great deal of mystique. It is usually thrown at a speed significantly lower than that of “ordinary” pitches and with very little spin. The lack of spin means that the knuckleball does not experience the Magnus force that is responsible for the movement on ordinary pitches. Very early in the PITCHf/x era, we learned that the spin-induced movement of ordinary pitches bunches into relatively small clusters, with the size and location of the clusters—along with the release speed—serving as signatures for a given type of pitch thrown by a given pitcher.
But the lack of spin on a knuckleball does not signify a lack of movement. Indeed, there is considerable movement, as first discussed in a seminal article by John Walsh with the fanciful title “Butterflies are not Bullets.” John showed that unlike the movement for ordinary pitches, knuckleball movement does not cluster into a tight bunch but rather appears as a large and nearly featureless blob. Evidently, the movement is essentially random, both in magnitude and direction, so that the trajectory seems to be completely unpredictable by anyone—the batter, the catcher, or even the pitcher. Other analyses have been done subsequent to John’s, particularly the excellent series of articles, “Mastering the Knuckleball,” by Josh Smolow.
In this article, I want to focus on the common perception that the knuckleball does not follow a smooth trajectory between pitcher and batter but instead undergoes abrupt changes of direction. Indeed, it is not too difficult to find statements in the various media about the seemingly bizarre behavior of knuckleballs, such as claims that it “flutters” or “dances” or “zigs and zags” on its short trajectory to home plate. These anecdotal claims have some basis in science, primarily from wind tunnel studies that show significant transverse forces on a non-spinning or slowly spinning baseball, with magnitude and direction that depend critically on the orientation of the seam pattern relative to the direction of motion. If a knuckleball were thrown with zero spin, the orientation of the seam pattern would not change, and the ball would experience a constant force, leading necessarily to a smooth trajectory. On the other hand, if the ball is rotated very slowly—no more than half a revolution between pitcher and batter—then it is possible for the lateral forces to change both in magnitude and in direction during the trajectory, and such an effect might lead to the anecdotal claims of zigging and zagging.
Read the full article here: http://www.baseballprospectus.com/article.php?articleid=15919
Originally published: January 31, 2012. Last Updated: January 31, 2012.