Physicists May Have Finally Figured Out Why Knuckleballs Are So Hard to Hit

All with the help of a ball-throwing robot

R.A. Dickey
Pitcher R.A. Dickey is well-known for his knuckleball. Keith Allison via Flickr

In the history of sports, few plays have been as tricky to pull off as the knuckleball. Its erratic and unpredictable movement, however, isn't just hard to hit and catch—scientists have long puzzled over how the throw is even possible. Now, with a little help from a soccer ball-flinging robot, a group of scientists think they may have cracked the knuckleball’s secret.

The trick to the knuckleball that makes it so hard to hit is that the ball doesn’t spin much as it flies through the air. While most pitchers control their throws by putting in a touch of spin, knuckleball pitchers spin the ball as little as possible. The result is a slower-moving pitch that zig-zags erratically from left to right—the likes of which can throw off even the most experienced batters, Jennifer Ouellette reports for Gizmodo. Yet for years, pitchers have been unsure just how their fearsome throws work.

"Even science has a hard time explaining why knuckleballs do the things they do," Toronto Blue Jays pitcher and celebrated knuckleballer R.A. Dickey tells Popular Mechanics’ Wayne Coffey. "That's part of the allure of the pitch."

The knuckleball is most closely tied to baseball, and for years experts thought the odd movement might be the result of the baseball’s iconic stitches messing with the aerodynamics. But the puzzle isn’t unique to baseball: variations on the throw show up in other ball sports, including cricket, volleyball and soccer, Ouellette reports, and each of those balls look completely different from one another.

To answer this question, researchers from École Polytechnique and ESPCI ParisTech built a special robot designed to give soccer balls that knuckleball motion. By firing knuckleballs into a wind tunnel and tracking the movement with a high-speed camera, the scientists figured out that the throw comes from a basic aerodynamic phenomenon called “unsteady lift forces.” The researchers report their findings in the New Journal of Physics.

"Unsteady lift forces are inherent to balls traveling through the air in every sport, so to complete our work we needed to find out why zigzag shots are associated with just a few games, such as soccer or baseball," study co-author Baptise Darbois Texier says in a statement.

When an object flies through the air, it exerts a downward force, allowing it to fly. Some shapes, like airplane wings, are designed to take advantage of this by generating stable, controllable lift. By putting spin on the ball, a pitcher or kicker can exert some control over its lift. But since a knuckleball has very little spin, it behaves more erratically. By tracking the knuckleballs as they moved through the wind tunnel, the researchers found that the tricky play takes advantage of these unsteady lift forces, but only if the ball reaches a specific range of velocity and covers a certain distance.

"In bocce, for example, a zigzag path should occur over a length of [about 88.5 feet], but this distance is much longer than the typical shooting length and so the knuckleball effect will be incomplete," Darbois Texier says in a statement.

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