Thread by @No_Little_Plans

MLB's report from yesterday finally explains why we got a rocket ball the last few years: lower seam height. There are a ton of interesting findings in there that I break down in this article: baseballprospectus.com/news/article/5…

The committee had to build new equipment and testing procedures to detect the difference in seam height from year to year, which is almost infinitesimal at *one-thousandth of an inch*. For reference, that's about the width of a single human skin cell.

But even a difference this small is apparently enough to substantially impact the drag coefficient of the ball, increasing home runs by a huge amount. Goes to show how tiny changes to the ball can produce major effects on the game.

Another finding is that batter behavior--hitters knocking the ball harder and on more favorable trajectories for home runs--explains some (but not the majority) of the increase in dingers. That's something I had found as well.

But hitters only started adopting these new strategies AFTER the ball became more lively. I think that some of the impetus for hitting more balls in the air was the fact that those fly balls became much more productive because of the changes to the ball.

On top of that, MLB discovered a small but significant increase in CCOR (bounciness) of the ball, causing leaguewide exit velocities to jump by 0.3 mph. This is close to the increase I observed early in 2019. baseballprospectus.com/news/article/4…

So, another aspect of "batter behavior"--how hard they hit the ball--is also tied to changes in the baseball itself, as I guessed in August. baseballprospectus.com/news/article/5…

The last major finding is that, yes indeed, drag increased in the postseason. This is a big win for me because MLB tried to deny this when it happened and Manfred attempted to cast doubt on my study. But it's real--drag did increase. baseballprospectus.com/news/article/5…

A couple of important caveats on that. The commission found that drag increased every year in the postseason. This is not exactly what I found--in my numbers, the 2019 postseason showed the largest increase.

But even though the pitch tracking drag numbers increased, according to the commission's analysis, home run numbers only decreased in this (2019) postseason. So something unique happened this October that didn't happen in prior seasons.

And the impact was fairly huge! On average, about six feet less batted ball distance than predicted, with some batted balls showing much less distance than that. That's enough to drop home run rates by a lot, like 15%. Quite a big change from one month to the next.

So, all in all, confirmation of a number of different things I reported on this season, from drag going down in 2019 to back up in the postseason to exit velocity jumping slightly to batter behavior changing as well. A good report for me :)

One angle a lot of reporters picked up on that I don't think is very important is the line that ball to ball variation drastically exceeds year to year variation. That's true, but also, I think, fairly trivial.

A little known fact is that individual balls vary in their flight distance by as much as 30 feet or so. If year to year variation were anything on that scale, we'd have home run rates jumping around by 50% every season. It would be almost catastrophic for the game.

So, yes, it's a good thing that year to year variation is small compared to the differences between balls. That doesn't mean the year to year variation isn't quite large, or that year to year variation isn't a problem for the league. (Even month to month variation is a problem.)

To me, it indicates that MLB has two big issues at hand:
1) baseballs are inconsistent from one pitch to the next. One ball might be a surefire home run and the very next one might be an easy flyout.
2) baseballs are inconsistent from month to month and season to season as well.

Now go read the article: baseballprospectus.com/news/article/5…