Rosin bag study: MLB balls are 20% slipperier than NPB balls


Japanese pitchers who make the jump to the major leagues have often said the baseballs there are slipperier than those in Nippon Professional Baseball.

But no one had done a scientific study on the differences in the rosin bags used by pitchers in the two leagues until now, according to Takeshi Yamaguchi, a tribology professor at Tohoku University.

Yamaguchi led a team that studied the benefits of substances—both legal and illegal—that pitchers use to improve their grip on the balls.

“Friction occurs all the time in sports, but the factor has yet to be surveyed sufficiently in the field,” Yamaguchi said. “I will be committing myself to research that can result in improved performances of athletes.”

Tribology is the study of interacting surfaces in relative motion. Yamaguchi, who is involved in the development of grippy soles and similar projects, also likes watching sports.

Rosin bags seen on the pitching mounds in both Major League Baseball and NPB contain a powdered mixture of magnesium carbonate and pine resin.

While watching a major league game on TV, Yamaguchi wondered why no qualitative assessment had been done on the impact of the adhesive materials and the friction levels concerning the balls.

“Deductions” on such issues were based only on the players’ opinions, he said.

Yamaguchi used a sensor that can simultaneously measure vertical and horizontal forces to determine the coefficient of friction.

The coefficient is calculated by dividing the force of the sliding direction by the force of the push direction.

Simply put, bigger coefficient figures represent larger friction levels that make objects more difficult to slide.

In the experiment, a piece of cowhide stripped from an MLB ball was placed on the sensor. Nine individuals took turns sliding their forefingers on the seams of the ball while pushing the fingers against the leather.

The results showed the friction coefficient ranged from 0.55 through 1.18 with no substances applied to the hands.

Those with dry fingers found the ball more than twice as slippery than among those with wet fingers, the researchers said.

When rosin was applied to the fingers, the friction coefficient rose to between 0.93 and 1.23.

While the powder improved the grips of all individuals, it seemed to narrow the difference between the friction coefficient with dry and wet hands, probably because rosin limits the influence of moisture on hands.

The team conducted the same test with an NPB ball.

The friction coefficient for the MLB one turned out to be 20 percent lower than the level for the NPB ball. That means major league balls are 20 percent slicker in a pitcher’s hands.

The findings corroborate assertions by San Diego Padres pitcher Yu Darvish and many other Japanese ballplayers that “MLB balls are slipperier.”

The powder in rosin bags is the only legal substance that pitchers can use to improve their grip on the ball.

But in the major leagues, the use of illicit materials to increase friction and the ball’s movement had become such a problem that MLB tightened its crackdown in June 2021.

Yamaguchi and his colleagues looked at one of the prohibited substances: sticky wax.

According to their experiments, using wax raised the friction coefficient to between 1.16 and 1.63.

The difference range with dry and wet hands was smaller than that of the experiment with no substances applied to the hands, but a relatively large 1.5-times increase was still found with friction coefficient data.

As the friction coefficient was up 50 percent or more on average, the team found that using wax improves the number of rotations and, as a result, makes it difficult for fastballs to lose their speed or produces more drastic breaking balls.

The results verified that rosin is better than other sticky substances from the perspective of fairness in the sport.

MLB balls are rubbed with mud while sand is applied to NPB balls to reduce gloss before games.

Japan’s Official Baseball Rules specify that the “umpire should inspect balls to confirm their gloss is eliminated by properly rubbing them with special sand.”

MLB’s criteria state that balls must be “properly rubbed so that the gloss is removed.”

The research team checked whether both procedures perform that task.

They found no significant changes in the friction coefficient after rubbing the balls with either mud or sand.

The scientists plan to have pitchers throw balls after their friction coefficients are measured to determine correlations with the balls’ speed, rotation number and accuracy.

They expect these efforts will lead to the introduction of less slippery balls.

The research results were published in Springer Nature’s online journal Communications Materials at (https://doi.org/10.1038/s43246-022-00317-4).

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