Remember when 2016 was the new Year of the Homer, featuring the second-highest home run rate in MLB history and supplanting 1987 in terms of unexpectedness? Well, 2017 laughed at that notion and bumped the homer rate by another 10 percent, setting a new record of 1.26 home runs per team per game. So now the question is whether 2018 will surpass even last year's "Year of the Homer 2: Electric Boogaloo."
The most maddening aspect of guessing where offense is going in baseball is the why. A 25 percent increase in homers over a two-year period is stunning. A similar change occurred from 1992 to 1994, and even a quarter of a century later, that shift is largely unexplained. League expansion isn't enough to account for that change, and one of the pop-science explanations -- performance-enhancing drugs -- would necessitate everybody discovering the benefits of PEDs in an 18-month period, because the home run rate stayed flat for most of the next decade. With no expansion teams, as well as drug testing since 2004, even those hole-filled theories aren't available to explain the latest home run boost.
One possible theory is that the baseballs are constructed differently, something commissioner Rob Manfred and Major League Baseball have denied, though without actually providing any rebuttal to what researchers have found. One thing will be different this year: MLB has announced that all baseballs will be stored in air-conditioned rooms in 2018, to help determine if they should subsequently be stored in humidors in 2019 to standardize the temperature and humidity they're kept in across the game. In theory, this change could ultimately result in lower exit velocities for a hit baseball; harder-hit baseballs are more likely to be home runs.
So one question that brings up is what effect this would have on the results, for both players and teams. Projections are made with certain assumptions for levels of offense around the league, and organizations are aware of those assumptions as they construct their teams. But what happens if we turn back the clock and the level of offense is more like 2015 than 2016-2017? To answer this question, I went back and ran my 2018 projections at 2015's level of offense and looked for the largest differences. I also used playing time generated from estimated playing time based on current rosters, rather than the straight-up ZiPS projections (ZiPS is agnostic on which minor leaguers will play).
Hitters most helped
Even with the benefit of moving to Miller Park, Christian Yelich gets the largest benefit from lower levels of home run hitting (it would have been +0.83 if he still played for the Marlins). Simply put, of the 50 hitters with the greatest projected OPS+ in MLB in 2018 (where Yelich ranks 36th), Yelich has the lowest projected home run rate. Even among the next 25 hitters, you get only two with a lower rate than Yelich, Buster Posey at 54th in OPS+ and Michael Brantley at 75th. Yelich is certainly not a bad home run hitter, but he gets a greater percentage of his offensive value from things other than homers than most excellent hitters.
Looking at the rest of the top 15, you see a lot of high-contact/line-drive hitters mixed in with a few fringe major leaguers like Alcides Escobar. This isn't unexpected, of course: Players who don't hit home runs are simply going to be the least impacted by a change to offense that results in a more balanced game.
Hitters most hurt
The players who take the largest hit with a lower home run environment by and large are hurt more than the helped players get a boost. The reason for this is that home run rates among major league hitters aren't that pretty little bell curve that you learned in a math class or maybe a statistics course (hi, Mr. Pitts!), and the great home run hitters are farther to the right side of the distribution than the lousy ones are to the left.
Giancarlo Stanton takes the biggest hit, not surprising given that ZiPS initially projects him at 56 homers per 600 plate appearances in 2018, more than 30 percent above the next-best homer hitters (J.D. Martinez at 42.0 and Judge at 41.9). But if we come back down to 2015 levels of offense, Stanton's best mean projection would be for 43 home runs -- still the highest projected tater tally, but much lower, obviously. Aaron Judge drops from 44 to 37, while Nolan Arenado would drop from 40 to 34 and Cody Bellinger from 39 to 33.
This is not quite as alarming as it sounds for Stanton or Judge or Bellinger, simply because they're projected at levels at which they remain stars even with the loss of a win's worth of value. Nor is Mike Trout's drop-off particularly worrisome, his drop in wins being relatively large simply because he has so many to start with. But for the lesser players on this list, the ones who aren't stars otherwise, a drop in league offense would be much more worrisome. Joey Gallo in particular is already projected for 2.2 WAR in 2018 given his expected playing time in Texas (I'm using 622 PA compared with 501 in the "normal" projection), so a dip in league offense could cost him nearly half his projected value. Similar drop-offs occur for sluggers like Khris Davis, Chris Davis and Wil Myers, who are more one-note players.
Pitchers most helped
Pitchers tend to have fewer projected changes in value with a lower-homer environment. This is because when it comes to home runs, a batter's home run rate is a much more significant determinant of the expected home run in a specific matchup than the pitcher's. Home run rates for pitchers regress toward the mean far more than they do for hitters, and in actual home run rates within a season there's a larger spread between good and bad hitters than between the good and bad pitchers.
James Shields tops the list, a pitcher who ranked 13th in MLB in fly ball rate (minimum 100 innings) with the unfortunate home of Guaranteed Rate Field, a park with a high home run park factor but an overall neutral environment. Fly ball pitchers, especially mediocre ones in hitters' parks, do the best here.
Pitchers most hurt
Here we largely see ground ball pitchers and a few who are simply elite getting the least benefit from a change in environment. After all, they were hurt the least by the bump in homers. For example, Dallas Keuchel had the highest ground ball rate in baseball in 2017, at 66.8 percent. He allowed so few fly balls that even with 21.1 percent of his allowed fly balls becoming homers, he still allowed under a home run per nine innings. There just aren't enough homers for Keuchel to lose compared with the average pitcher.
How it would affect teams' bids to win
The team that would benefit the most from an MLB-wide power outage is the Colorado Rockies. Instinctively, this would seem a little unusual, given that the Rockies play in the environment most conducive to home run hitting. While the Rockies did have a large home-field advantage in win-loss record in 2017, their offense in particular is poorly built for Coors Field, especially when it comes to home run hitting. Yes, the team led the NL in runs scored, but that was largely a park illusion, the team's OPS+ of 91 ranking 11th in the National League. That's even worse when it comes to homers, because finishing 10th in the NL in homers while playing in Coors is a bleak number. When park effects are taken into consideration, the only NL team worse at hitting homers than the Rockies was the Giants.
On the flip side, the Minnesota Twins would be the team that gets hurt the most, both because ZiPS sees a lot of the team's offensive growth in 2018 coming from homers and because it was banking on the pitching staff improving more from allowing fewer homers in 2018 than from any bumps in strikeout or walk rate. For example, of the 25 hitters most affected by going back to 2015 levels of offense, four were Twins: Miguel Sano, Byron Buxton, Brian Dozier and newly signed slugger Logan Morrison. Take away some of that growth potential, combine it with the fact that the Twins are projected at a place where every additional win or loss has the largest effect on their playoff potential, and the Twins see their relative standing drop a bit in a colder offensive environment.