I was poking around on Fangraphs, as I often do. It's a holiday here on the beaches of sunny Canada (Clone High, anyone?), so I'm not really slacking off. Anyway, I was on the Justin Smoak 2013 Splits page, and I noticed something that I can't recall ever being mentioned before. It may (does?) explain why Big, Strong Justin Smoak seems to have trouble replicating the homer output of Little, Fast Nick Franklin or others. First, some league context.
Since 2002 when the data first appears, the league average FB% on balls that are pulled is about 22%. This year it's at its lowest (19.9%), but most of the past 12 seasons it has hovered near the 22% mark. On these pulled fly balls, the HR/FB% has averaged about 28.5, with the past couple seasons up a bit higher (near 32%). As you move to center and opposite-field hits, FB% rises and HR/FB% drops, as you would expect. Basically, if you want to hit home runs, pull the ball in the air.
Justin Smoak does not pull the ball in the air. He used to, back when he was really a bad hitter - 23.1% in 2010, 24% in 2011, a whopping 29.9% in 2012. In 2013, only 18.6% of his pulled contact are fly balls. This may not seem too low, if the league is generally around 22%. But keep in mind that league number includes many slap-type hitters. I haven't had time to adjust for "sluggers only" type stats, but some simple filters show the number rises when you isolate first basemen (about 23%) or DHs (24%), guys who are typically not slap-hitters. For his body type, you would expect a FB% on pulled balls around 23 or 24%. That's a pretty big difference from 18.6 - about a 30% increase.
He actually has the opposite problem when going to the opposite field - an insane 78.9% fly ball rate (against a league average that's usually around 60%). Since only 3.3% of these have left the yard (league average is usually 3-4%), his HR totals aren't being helped there either. He lofts more flies than average to centre, too - 51.4% vs. a league number around 42%. His power to center has been well below average as well - 5.6% vs league averages that fluctuate from 8 to 11%.
Smoak's HR/FB on pulled-flies is actually quite solid at 36.8% this year - a career high. He's looked like a new man mostly because his pulled line drive rate is a ridiculous 34.4% (up from 17.1% last year). Personally, I'll take the reduced home runs that come from stinging line drives instead of trying to loft everything, but it's not inconceivable that, as his comfort level and confidence rises, so will his FB%. If he wants to hit more home runs, he either needs to get stronger and push out more of those center and opposite-field flies, or he needs to hit more of his pulled shots in the air.