Let’s Check in on the KBO’s De-Juiced Baseballs

(Photo: Sung Min Kim/FanGraphs)

Back in April, I wrote about how the Korea Baseball Organization (KBO) had de-juiced their baseballs, and how the offensive environment in the league appeared to be reflecting that change. If you remember from the piece, in March, the KBO conducted a test on a batch of new baseballs to test their coefficient of restitution (COR). It was supposedly reduced from a range of .4123-.4374 to .4034-.4234, while the ball size was increased by 1 millimeter and 1 gram. However, the test brought out some interesting results, as they found several defective balls with a COR more in line with the 2018 baseballs mixed in with the new, less-lively ball.

It doesn’t mean that this season has looked like 2018, though. In fact, the changes became clear in a month’s worth of games, but now that it’s mid-August, I thought I’d give you an update with a bigger sample size. I also thought it would be interesting to see how the de-juiced balls interact with Korea’s warm summer weather. The 2019 KBO regular season is about two-thirds of the way through, and league-wide temperatures have risen enough to see if hotter temperatures have caused an uptick in power that might counteract the effects of the new ball.

Here is how the overall league offensive numbers look this year, compared to those of last year:

KBO League Offensive Stats
2018 .286 .352 .450 .802 .164 .349 3.03
2019 .268 .340 .388 .728 .119 .335 1.83
SOURCE: Statiz
2018 stats are for March-August 12, 2018; 2019 stats are as of August 12.

The contrast is pretty stark. When I wrote about this subject back in April, the league’s hitters were had a .733 OPS while slugging .391. Did warmer weather help? Let’s look at last year’s numbers from March to May, and June to August 12.

2018 KBO Offensive Stats
March to May .283 .348 .440 .788 .157 2.82
June to August 12 .289 .357 .459 .816 .170 3.24
SOURCE: Statiz

There’s a .280 boost in the OPS, with a .190 difference in SLG%, suggesting that warmer weather helps to boost offensive numbers. Here are this year’s such splits:

2019 KBO Offensive Stats
March to May .268 .341 .390 .732 .122 1.90
June to August 12 .269 .338 .385 .723 .116 1.76
SOURCE: Statiz

Interesting! Instead of overall offensive production increasing as the weather in Korea got hotter and more humid, it actually dropped. One theory is that the league and its manufacturer managed to reduce the number of defective balls mixed among the batch of new baseballs produced by Skyline. In May, they carried out a second test of the balls. They still found a few defects but there were reportedly far fewer than there were during the first test. It is possible the league had played with a mix of 2018 and de-juiced baseballs, but the number of 2018 balls may have decreased over time, which could explain the power dropoff despite hotter weather. According to a study, the newer baseballs shaved off four to five meters (13 to 16 feet) worth of distance in flight. The appropriately-produced baseballs appear to have done what they set out to do – curb the power production.

But there are more statistics that tell the tale. Let’s take a look.


In the 2018 season, 3.09% of all KBO plate appearances resulted in a home run. In 2019, the rate has decreased to 1.83%, a 40.8% decrease in balls leaving the yard. (Before the season, the SK Wyverns ran a simulation of batted balls with the new de-juiced baseballs and they came out with a 15% fewer home runs overall, which clearly lowballed the eventual effect.) Last year, the league produced 1756 home runs. As of August 12, they have 773 and are on pace for a measly 1041. That’s about 700 fewer projected home runs.

There are, of course, other factors that may have contributed to the power reduction. One thing I’ve observed is that pitchers are throwing more sinkers/two-seamers this season. Back in 2014, the league’s pitchers threw four-seamers 59.2% of the time while two-seamers were basically afterthoughts at 1.3%. By 2018, that had changed to 44.8% four-seamers and 7.1% two-seamers. In 2019, it’s progressed to 40.8% four-seamers and 12.0% two-seamers, making it something of a trend. There’s another interesting contrast to MLB, where four-seamers are much more in style than two-seamers because of the batted ball results, as our own Ben Clemens has written. With the amount of available KBO data (or lack thereof), it’s hard to do a deep dive on how much of an impact the two-seamers have had in curbing hitters’ power, but it’s a plausible factor. But would a 4.9% pitch usage increase be a major reason why the home run rate is down 40.8% from the previous season? I’m not so sure. As we’ve seen with MLB’s own commissioned study of the change in the home run rate, these sorts of factors have an effect at the margins, but ball remains the most significant factor in how many balls leave the yard.


In 2018, there were 28 qualified hitters in the league had slugging percentages of .500 or higher. Given that there are 10 teams in the league, that meant there were roughly three hitters per each team’s lineup who could hit for hefty power production. This year? There are only nine.

As August 12, the top slugger in the league is Jerry Sands, who’s hitting a healthy .311/.399/.555. A .555 SLG% is nothing to sneeze at, but that would have ranked in 14th last year. The top slugger in the 2018 season – ByungHo Park – hit for a massive .718 SLG%. His SLG% has dropped to .549 this season, which is second to his Kiwoom Heroes teammate Sands.

Of course, there are exceptions. For instance, the 23-year-old Kiwoom Heroes shortstop Kim Ha-Seong hit .288/.358/.474 last year with 20 home runs. This year, he’s hit .309/.395/.498 with 15 home runs in 106 games. However, that may more be a testament to his maturation or a change in approach than the de-juiced baseballs not deterring his skillset. The last time the league saw such a dearth of power hitters was 2013, when there were only five hitters who slugged .500 or higher; the league hit .268/.350/.388, which looks pretty close to how 2019 has gone (.267/.340/.388). (It was also the season before the KBO instituted the three-foreigner rule, which allowed teams to add foreign-born positional players.)


It’s not just home runs that have been affected, either. Because of slower batted balls, there are fewer base hits resulting from balls put in play as well. In 2018, the league had a .329 overall BABIP. That figure has been reduced to .312 this season.

As for the direction of batted balls, there’s no significant difference. The hitters have shown roughly similar tendencies all around. They also have posted about the same fly ball-to-groundball rate in both seasons, as well (0.96 in 2018, 0.97 in 2019). To me, this is simple: even with different baseballs, they are sticking with their usual strengths. There hasn’t been a mass exodus from their previous approaches. Power hitters will continue to aim for extra bases and singles hitters will likely hit grounders and slap line drives.

Eight Minutes

Through August 12, 542 total games had been played, with an average game length of three hours and 13 minutes, including all of the extra-inning games. If you exclude those, the length shaves down to three hours and 10 minutes.

In 2018, that average was three hours and 21 minutes (including the extra-inning games). If you go back to 2014, when the league collectively hit for a .807 OPS, the games went as long as three hours and 27 minutes on average. Three hours and 10 minutes would be the shortest average game time length since 2012 (three hours and 11 minutes, including the extra-inning games), when the league was just as pitcher-friendly as it seems to be right now. The depressed offensive environment throughout the league seems to correlates with a faster pace of play.

Let’s contrast the KBO’s situation to that of MLB. As has been covered extensively this season, researches have shown that the balls in use in the majors are wound tighter and have smoother leather, resulting in flyballs traveling farther than they have previously. The league is on pace to set records in home runs hit in a season. Is Major League Baseball going to take steps to remedy the current trend or are they going to just let things be? We don’t know for now, but if MLB were to look to neutralize the current home run-happy environment by tweaking their baseball production, they could learn some lessons from the KBO, where it has been made clear that a careful and intentional manufacturing process is important, and changes to the ball, even small ones, can have a significant effect on the game.

All KBO statistics are from Statiz unless specified

Sung-Min Kim writes for River Ave. Blues, and has written for MLB.com, The Washington Post, Baseball America and VICE Sports. Besides baseball writing, he is also passionate about photojournalism and radio broadcasting. Follow him on Twitter @sung_minkim.

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4 years ago

Manfred: “How you manipulate a human-dominated handmade manufacturing process in any consistent way, it’s a smarter human being than I.” Per the commish, there’s nothing that can be done about the MLB ball in 2019! I hope he knows what a donkey he looks like with this stance.

BTW, another great piece on the KBO! Is there any developing perspective on how the fans are reacting to this new, de-juiced game? That’s a lot less batflips per season!

Alex Trebek
4 years ago
Reply to  nickolai

OORR… a lot MORE bat flips on balls they thought were going out, but didn’t… which is awesome…

4 years ago
Reply to  Alex Trebek

I always thought it would be funny watching the players complain about the strike calls AFTER they switch to an electronic strike zone. You know it’s going to happen because some guys just can’t help it.

4 years ago
Reply to  nickolai

Agreed. Based on his past penchant for dishonesty with this issue however, don’t expect any changes. Manfred is trying to appeal to the drones. Even if he did de-juice the ball, as soon as some star hits a ball that he thinks should have been over the fence, that owner will move in the fences yet again to add 10 new seats to the stadium and pacify the complainer. Miami is considering doing this again. Multiple stadiums have already done this more than once. Half of my son’s high school fields are larger than most MLB stadiums. It’s a joke at this point.