Does closing the dome help R.A. Dickey’s Knuckleball?

There was a lot of speculation last year about the effect of the dome on R.A. Dickey’s knuckleball, to the point where the dome was closed on a perfectly lovely fall day for his last start after Dickey made some noise in the media about wanting it that way. While Dickey seems like a guy who is exceptionally in touch with his craft, and a quick look at his numbers seems to support the idea, Alex Anthopolous is right to grumble about sample size and Dickey’s early-season injury making ERA comparisons questionable at best — especially since a major fight with the MLB is looming if the Jays start closing the dome next year on sunny days in June.

It has been rumoured that knuckleballers thrive in domes since the days of Wakefield crushing the Jays, but the details have always been vague. I’ve heard humidity and the wind raised as possible factors, but never a clear explanation of why, or whether the pitch is more controllable, less predictable, or has more movement on it inside a dome. Fortunately, pitch f/x records whether the roof was open or closed, as well as the wind and direction, so it’s possible to drill down and look at the results of individual pitches to investigate some of the theories floating around.

Theory 1:  R.A. Dickey has better control over his knuckleball when the dome is closed.

Sounds plausible — if the knuckleball wasn’t quite so impossible to control it would be the best pitch ever, so maybe in calmer air the pitch is not quite as difficult to get over the plate. As Tom Candiotti said:

“It’s really a great place to pitch,” former Major Leaguer Tom Candiotti said of Rogers Centre. “I pitched with the roof open, I pitched with the roof closed, and I always preferred the movement of the ball and the consistency of the movement with the roof closed because it was the same every inning.”

However, there was absolutely no difference last year in the percentage of knuckleballs that ended up crossing the plate in the strike zone with the roof open compared to closed:

Dickey’s Knuckleball Accuracy
Conditions	In zone %
2011	49
2012	50
2013 – Away Games	48
2013 – Dome Open	49
2013 – Dome Closed	49

For most pitchers, this would not be enough to close the book on ‘control’– you really need to get into how close to the edges of the strike zone, or the spots that the pitcher is actually aiming for. But even Dickey acknowledges that at most he is starting his knuckler in slightly different regions of the strike zone and then letting the pitch do its random thing from there. You’d have a hard time convincing me that Dickey is somehow ‘hitting his spots’ within the strike zone better due to a more consistent knuckleball, but not throwing any more pitches in the zone overall.

Verdict: Busted.

Theory 2:  Dickey’s Knuckleball is harder to hit with the roof closed.

So if the movement isn’t more consistent, is it just plain nastier? I think misses per swing (with fouls taken out of the equation since there is no way to differentiate between a foul tip and a 400-foot drive that hooked just foul) is the best measure of how difficult it is to make contact with an offspeed pitch. Yes, deception is important, and getting hitters to chase pitches out of the strike zone can be as effective as throwing a pitch with incredible movement. But it all balances out very well into one number — if a major league hitter decides to swing at an offspeed pitch and comes up completely empty, that pitch had quality movement, whether it was down the middle or bounced in the dirt. And Dickey generated more misses with the dome closed last year:

Dickey’s Knuckleball
Year/Conditions	Miss/Swing %
2011	30
2012	41
2013 – Away Games	40
2013 – Dome Open	32
2013 – Dome Closed	38

Interesting, but a difference of 6% is not that significant, and Dickey’s numbers were just as good on the road in 2013. However, if we’re talking about swings and misses, remember that Dickey throws two hard knuckleballs (in addition to his 65 mph floater), one in the low 70’s and one in the high 70’s that he tends to use as a strikeout pitch. As I showed before last season started, it was throwing so many more of the high-70’s knuckler that made his strikeout rate go through the roof last year, and while pitching through injury early in the season, that pitch was largely missing from his repertoire. So here are his swing-and-miss numbers for Dickey’s knucklers that were thrown faster than 76 mph:

Dickey’s knuckleballs > 76.0 mph
Year/Conditions	In Zone %	Miss/Swing %
2011	49	30
2012	49	42
2013 – Away Games	43	31
2013 – Dome Open	47	33
2013 – Dome Closed	53	48

Now that’s something. It might not seem like it, but a 15% difference in miss rate is a TON. For example, the average miss rate for curveballs last year in the majors was 35.5%. The best curveball in the league (over 200 thrown) was 52%. So the difference in terms of swing-and-misses for Dickey’s hard knuckler with the dome closed compared to either being on the road, or with the dome open, was just about the same as between a run-of-the-mill curveball and the best one in the league.

Verdict: Very much so.

Ok, but why? Swings and misses are a lot better than ERA, but results are never quite enough to be totally sure — maybe he was more effective at home for other reasons like the mound, the home crowd, or he just happened to be facing poorer teams (correlation does not imply causation, for the statheads out there). But if there is proof of his knuckleball moving in a fundamentally different way under dome-closed conditions, that’s much more compelling and puts to rest any doubts remaining about the sample size or other factors.

So now let’s look at the average movement under different wind conditions, compared to those inside the dome. Pitch f/x movement is always measured in comparison to a spinless pitch, so the average for a knuckleball averages around 0 for both x (horizontal movement in inches, with positive values representing movement towards a left-handed batter) and y (vertical movement), since the random movement in all directions that the knuckleball takes on from there cancels out.

(Keep in mind that this is simply a measure of where the pitches tend to end up crossing the plate compared to where they would have if nothing was happening at all. This doesn’t have anything to say about when or how they flutter on the way to the plate.)

Effect of wind on Knuckleball movement and results
Wind	PFX	PFZ	In Zone %	Miss %
None (Dome)	1.18	1.61	0.49	0.39
Right to Left	0.05	0.03	0.50	0.30
Left to Right	-0.26	0.57	0.49	0.33
In	0.17	1.27	0.48	0.37
Out	0.20	0.32	0.50	0.33

So while a crosswind seems to have no particular (at least consistent) effect, on average Dickey’s knuckleball is more likely to have almost two inches more of up-and-away movement with the dome closed than in other wind conditions (the results are even more compelling if we only look at Dickey’s 76+ mph knuckleballs again: [pfx: -0.24 pfz: 1.35 with the dome open vs. pfx: 0.80 pfz: 2.15 with it closed]. As I showed in a previous article, that direction of movement is by far the most deadly: “Hitters swing through them almost twice as often as ones that sink down and in, and get fewer hits when they do make contact.”

There may be other reasons, but that makes sense considering that it’s a type of movement that is not seen in any other pitch. Sometimes Dickey’s knuckleball ends up having similar action to a changeup, or a slider, or even a rising fastball. But no other pitch in the world moves in that direction (Mariano Rivera’s cutter came the closest)

Theory 3: Higher humidity (with the dome closed) is good for the knuckleball.

Dickey has mentioned humidity as a factor several times, and In an interview with Michael Morrissey, Dickey gives his theory on how it helps the knuckle:

As expected, Dickey noted that domes and places with high humidity are good environments for his knuckleball. Domes are good because of the lack of wind. High humidity is good because “the seams grip the air better.”

But then in another interview, Dickey seems to suggest that it has an adverse effect (although he could be referring to overly humid conditions making it hard for him to get the proper grip, as most pitchers see their K rates drop at very high temperatures, most likely due to sweat):

Yes, a controlled climate is (desirable) and a dome offers you that,” Dickey said. “But if it’s been open all day and then it’s closed at game time, all that humidity (gets) stuck there and that’s not good (for a knuckleball).”

Anyway, humid air is actually less dense than normal air (seems backwards but is true), so I don’t buy the idea that it would grip the seams more. And if anything, less drag should make the knuckleball have less movement (imagine pushing a beachball through water compared to air). But another thing that would cause less resistance on the ball is having the wind at Dickey’s back — which is the other number that pops out from the above movement table as causing Dickey’s knuckleball to rise. (Wind “in” on the above table, which is a combination of in from right, left, and centre field — if we isolate down to wind coming straight in from CF, pfz rises even higher to 2.16).

This supports the idea that higher humidity inside the dome makes Dickey’s knuckleball rise and become harder to hit, in much the same way that wind at his back does — but there’s something else with the dome closed that gives it sideways movement as well, away from right-handed batters.  Or that’s my theory, anyway — for a much more rigorous look at the physics behind what makes the knuckleball do its thing, check out Alan Nathan’s site.

Verdict: Highly probable, but best left to real physicists.

In conclusion:

• While it might have some kind of subtle effect, Dickey’s knuckleball is not more ‘accurate’ in any meaningful way with the roof closed.
• Dickey’s knuckleball gets far more swings and misses when it rises/floats, especially up and away from right-handed batters.
• He can throw it harder to make that happen but also gets more lift if he has the wind at his back, and more lift + movement away from right-handed batters with the roof closed.
• The effect is much more pronounced on Dickey’s upper-70’s knuckleball than his lower-70’s offering, or his 65mph floater.
• The results are significant enough that if Dickey was starting for the Jays in a critical game there is a strong argument to be made in favour of closing the dome. (Or maybe a humidifier promotion? Hand fans for everyone in centre field? Starting the “hurricane” instead of the “wave”? The possibilities are endless…)