Postseason Baseballs and Bauer Units

With the new MLB report on baseball drag leaving far more questions than answers as far as the 2019 postseason goes, I decided to look at the mystery from a different angle.  My hypothesis was a temporary storage issue with the baseballs, and it’s not clear, without knowing more about the methodology of the MLB testing, whether or not that’s still plausible.  But what I did find in pursuit of that was absolutely fascinating.

I started off by looking at the Bauer units (RPM/MPH) on FFs thrown by starting pitchers (only games with >10 FF used) and comparing to the seasonal average of Bauer units in their starts.  Let’s just say the results stand out.  This is excess Bauer units by season week, with the postseason all grouped into week 28 each year (2015 week 27 didn’t exist and is a 0).


And the postseasons are clearly different, with the wildcard and divisional rounds in 2019 clocking in at 0.64 excess Bauer units before coming back down closer to normal.  For the 2019 postseason as a whole, +1.7% in Bauer units.  Since Bauer units are RPM/MPH, it was natural to see what drove the increase.  In the 2019 postseason, vFF was up slightly, 0.4mph over seasonal average, which isn’t unexpected given shorter hooks and important games, but the spin was up +2.2% or almost 50 RPM.  That’s insane.  Park effects on Bauer units are small, and TB was actually the biggest depressor in 2019.

In regular season games, variation in Bauer units from seasonal average was almost *entirely* spin-driven.  The correlation to spin was 0.93 and to velocity was -0.04.  Some days pitchers can spin the ball, and some days they can’t.  I couldn’t be much further from a professional baseball pitcher, but it doesn’t make any sense to me that that’s a physical talent that pitchers leave dormant until the postseason.. well, 3 out of 5 postseasons.  Spin rate is normally only very slightly lower late in games, so it’s not an effect of cutting out the last inning of a start or anything like that.

Assuming pitchers are throwing fastballs similarly, which vFF seems to indicate, what could cause the ball to spin more?  One answer is sticky stuff on the ball, as Bauer himself knows, but it’s odd to only dial it up in the postseason- all of the numbers are comparisons to the pitcher’s own seasonal averages. Crap on the ball may increase the drag coefficient (AFAIK), so it’s not a crazy explanation.

Another one is that the balls are simply smaller.  It’s easier to spin a slightly smaller ball because the same contact path along the hand is more revolutions and imparts more angular velocity to the ball.  Likewise, it should be slightly easier to spin a lighter ball because it takes less torque to accelerate it to the same angular velocity.  Neither one of these would change the ACTUAL drag coefficient, but they would both APPEAR to have a higher drag coefficient in pitch trajectory (and ball flight) measurement and fly poorly off the bat.  Taking a same-size-but-less-dense ball, the drag force on a pitch would be (almost) the same, but since F=ma, and m is smaller, the measured drag acceleration would be higher, and since the calculations don’t know the ball is lighter, they think that the bigger acceleration actually means a higher drag force and therefore a higher Cd, and it comes out the same in the end for a smaller ball.

Both of those explanations seem plausible as well, not knowing exactly what the testing protocol was with postseason balls, and they could be the result of manufacturing differences (smaller balls) or possibly temporary storage in low humidity (lighter balls).  Personal experiments with baseballs rule both of these in.  Individual balls I’ve measured come with more than enough weight/cross-section variation for a bad batch to put up those results.  The leather is extremely responsive to humidity changes (it can gain more than 100% of its dry weight in high-humidity conditions), and losing maybe 2 grams of moisture from the exterior parts of the ball is enough to spike the imputed drag coefficient the observed amount without changing the CoR much, and that’s well within the range of temporary crappy storage.  It’s possible that they’re both ruled out by the MLB-sponsored analysis, but they didn’t report a detailed enough methodology for me to know.

The early-season “spike” is also strange.  Pitchers throw FFs a bit more slowly than seasonal average but also with more spin, which makes absolutely no sense without the baseballs being physically different then as well, just to a lesser degree (or another short-lived pine tar outbreak, I guess).

It could be something entirely different from my suggestions, maybe something with the surface of the baseball causing it to be easier to spin and also higher drag, but whatever it is, given the giant spin rate jump at the start of the postseason, and the similarly odd behavior in other postseasons, it seems highly unlikely to me that the imputed drag coefficient spike and the spin rate spike don’t share a common cause.  Given that postseason balls presumably don’t follow the same supply chain- they’re processed differently to get the postseason stamp, they’re not sent to all parks, they may follow a different shipping path, and they’re most likely used much sooner after ballpark arrival, on average, than regular-season balls, this strongly suggests manufacture/processing or storage issues to me.

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