It is oft-referenced that the home run rate has declined this year, but I wouldn’t say that was a worry early in the season: it was notoriously cold and rainy, and home run totals increase anyway over the course of a season. Well, that home run deliverance never occurred, and in fact, the rate has continued to decrease:
This last month isn’t done yet, obviously, but you can see that a few days can’t cure the fact that there are only 710 home runs this August, down from a total of 1119 (!) last August. What if we stripped out some context and talked about the rate of home runs per fly ball, because what if we’re just talking about a drop in the fly ball revolution? Well...
...it actually looks like HR/FB has become somewhat stable after it vaulted up last year, but that drop can’t be discounted, though. That’s because... what if something was artificially dropping the HR/FB to pre-juiced levels.
Even though evidence starkly says the contrary, MLB and Rob Manfred claim that “the increase in home run hitting since the 2015 season was due, at least in part, to a change in the aerodynamic properties of the baseball,” and not because of “any change in the size, weight, seam height, or COR of the baseball.”
Let’s assume the same baseball, then, and look at the Statcast numbers from 2016, and then try to explain that without factoring in other externalities.
In terms of launch angle, to start, players have continued upward:
- 2016: 10.8 °
- 2017: 11.1 °
- 2018: 11.7 °
OK, that makes sense, so that would also mean that fewer home runs, and a lower HR/FB ratio, are because players aren’t hitting the ball as hard. Well, the average exit velocity would disagree:
- 2016: 87.7 mph
- 2017: 86.7 mph
- 2018: 87.8 mph
That means there has to be some discrepancy in xwOBA and xSLG, meaning that it should reflect more 2017-eque levels in the underlying peripherals. Here is wOBA - xwOBA...
- 2016: 0.318 - 0.316 = 0.002
- 2017: .321 - .321 = 0.000
- 2018: 0.315 - 0.327 = -0.012
...and xSLG - SLG:
- 2016: 0.406 - 0.417 = 0.011
- 2017: 0.415 - 0.426 = 0.011
- 2018: 0.429 - 0.410 = -0.019
Part of this could be luck, of course. But even if we were to consider that, then what explains the fact that the expected slugging percentage this year is higher than last year, yet the league is going to significantly under-perform in the total number of home runs?
There’s one way to control for the changes in batter approach, as well. Ben Lindbergh notes that pitcher hitting performance, in fact, is an excellent control group for pitchers vs. hitters as true talent writ large, because while pitchers improve, pitchers at bat largely stay at a similar offensive talent level. When you look at the HR/FB ratio, it becomes pretty illuminating:
This is how I would best understand the trend. For better or for worse, something changed in the ball in mid-season 2015. That change has not been corrected, or else you would see the HR/FB ratio drop to pre-2015 levels.
Instead, the rates have dropped to pre-2017 levels. Whatever that was fueling the rise in home run rate last season has been abetted. My qualitative analysis goes something like this: when the ball changed, there was a natural increase in home run rate. By the following year, teams’ analytics departments caught on to this and advised their players to boost their launch angles, further fueling the trend.
Then [something] happened. Maybe it was actually changing something about the ball, or putting it in a humidor. I honestly couldn’t say. Or, it could be random! I’m not a scientist! But I think just like the league seemingly deceived the public on the changes in the ball when it was clear there were some, I wouldn’t hold my breath for them to recognize a change in the opposite direction.
We haven’t undone the juiced ball, and I would say that trend indicators show we’re still in that era. But the upward climb has stopped for now, and whether that is some random blip or a conscious effort by the league will be borne out conclusively over the next year.