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The Twins, sticky stuff, and spin rate

What effect has the foreign substance enforcement had on the Twins?

Cincinnati Reds v Minnesota Twins
Umpires Jim Wolf #28, left, and Lance Barksdale #23 inspect Bailey Ober #82 of the Minnesota Twins for foreign substances
Photo by David Berding/Getty Images

It’s been about 10 days since Major League Baseball implemented a concerted effort to enforce long established rules (Rules 3.01 and 6.02(c) and (d), specifically) that ban players from damaging the baseball or applying foreign substances of any kind to it. Despite how the haphazard mid-season implementation, awkward optics, and public outcry by pitchers make it seem, this policy change did not come about overnight. The use of foreign substances by pitchers to enhance performance is an issue that has been bubbling under the surface for some time. Teams and players were put on notice via a memo from the league office back in Spring Training about this issue and the league’s intent to enforce the rules. When that warning had no tangible effect the Commissioner’s office gave a few weeks’ notice before beginning a crackdown in earnest on June 21.

The Twins played an interesting role in this point of emphasis becoming a public topic. Specifically, it was the Twins’ highest-paid player who forced this issue into the open near the end of May. Shortly after veteran umpire Joe West made a St. Louis pitcher switch hats during a game after finding a foreign substance there, Josh Donaldson engaged in this exchange on Twitter:

What followed were numerous articles from many great outlets detailing baseball’s “dirty little secret.” I won’t rehash that which was so well covered previously. Check those links out if you want to know more background and you somehow have managed to avoid the daily content and online discussion about “sticky stuff” in recent weeks.

The main things you need to know to go on with my piece here today are that “sticky stuff” — substances ranging from rosin, sunscreen, and pine tar to industrial grade grip enhancers primarily used in strong men competitions — has been weaponized to increase pitch spin. Its use was widespread, with some estimates suggesting 70% or more of pitchers were taking advantage. Why it matters is that more pitch spin yields greater pitch movement (something I explained in depth in my offseason look at how the Twins were making better use of spin). More movement makes pitches harder to hit. When you combine that with faster than ever velocity, hitting has never been harder. Strikeouts are way up, contact and base hits are way down, and MLB saw a way to make a change (by enforcing its own rules on foreign substances) that might help to make the playing field more level between pitchers and hitters.

League Level Impacts

Now, teams have had a sufficient number of games (working through their rotations about twice) since the enforcement of the rules went into effect. It has been several more weeks since they first got wind that things would be changing. That means we have some early data that lets us assess the impacts.

A simple way to assess if spin has changed is to look at the average spin rate over time on four seam fastballs. This animated graphic from Codify Baseball plots 7-day rolling averages of four-seam fastball spin rates (in revolutions per minute, RPM) and velocity at the league level.

It clearly shows that something happened in June. The red spin rate line decreased significantly starting in early June — roughly coinciding with when this topic started to get a lot of attention in the media and MLB started signalling that it was serious about beginning to enforce the rules.

But, it’s not quite as simple as just looking at changes in average spin rate and concluding there is a meaningful change. We know there is a linear relationship between spin rate and velocity — higher velocity tends to lead to higher spin rates. So we have to consider velocity, too. The fact that the two lines above diverge so significantly — spin rate dropping while the velocity mostly holds steady (or even increases) — is another good indication that spin is actually changing and it’s not being driven by changes in velocity.

You might also notice in the animation that the decline began well before June 21st. That detail aligns with a much more statistically rigorous analysis conducted at Baseball Prospectus by Rob Arthur and Johnathan Judge, who concluded that the spin rate change began much earlier than enforcement day and is substantial enough that it could not have occurred by chance.

Naturally, then, the league level change bears out in more granular individual pitcher level data:

Twins Impacts

So, where do the Twins come out in all of this?

It’s convenient to use June 3 and June 21 as lines of demarcation to compare before and after. Prior to June 3, most of the league was still going about business as usual, somewhat oblivious to the idea that MLB might actually enforce its own rules. After June 21, umpires had started conducting the required in-game checks for foreign substances. Therefore, data before June 3 and after June 21 should be pretty clean for analyzing. The data in between those two dates is likely to be too noisy to trust.

Using before June 3 and after June 21 as cut points, I pulled the team level four-seam fastball average spin rates from Savant so we can put this into numeric terms and get a sense of where the Twins fall in the context of the 30 MLB teams:

Data sourced from

The table above is ordered by the before and after difference in spin rate. That shows the team level declines clearly. They are very widespread. Only San Diego did not decline. Toronto barely did and it’s difference is small enough that we can likely just chalk it up to random variation.

The Twins are there near the bottom of the list (tied for 24th largest change), but they do show a potentially significant decline in average spin rate of 39 RPM. Is 39 RPM a small enough delta that we could consider it random variation? Perhaps. Maybe the Minnesota boys were following the rules? To determine that, we’ll have to do some more digging.

Next, I pulled the Twins game by game average spin and velocity data for four-seam fastballs from Savant through June 30. With that data, I calculated 7-game rolling averages for both average four-seam spin rate and velocity and plotted them similarly to the chart in the Codify Baseball tweet above:

Data sourced from

In the plot, the two lines track together until mid-May, when there is a several game spin rate spike that outpaces any changes in velocity. The lines come back together late in May and early in June, before separating again throughout June due to the spin rates coming down.

You probably also noticed in the chart that the velocity line decreases throughout June. As was mentioned before, that could have had an effect on spin rates. To know if it’s really the spin rate that is changing, we need to control them for velocity. The easiest way to do that is by dividing the spin rates by the velocity to come up with a spin to velocity ratio. I made those calculations and plotted the ratio over time:

Data sourced from

I added a dashed line on this chart that includes the overall season average spin to velocity ratio. A slight declining trend in the ratio shows up, starting late in May. While you can see it in the decline in the line, it’s also supported by the fact that 21 of the 25 points in June are below the season average line. Only 8 of the 47 data points in April and May were below the season average. This suggests the Twins have seen a decline in spin rate at the team level that is not driven by a decline in velocity.

Having some confidence that velocity was not driving any changes, I then isolated the 7-game rolling average spin rate data and plotted it along with the overall season average and two additional lines representing one standard deviation above and below the average. These additional lines can help give us some context about how much change has occurred:

Data sourced from

Here, you can see the data points occasionally exceed both the plus one and minus one standard deviation lines through May and early June. However, the data steadily declines below the average line and clusters around the minus one standard deviation line through the rest of June. Starting with the June 12 data point, the remaining 15 points in June were all below the season average (easily the longest consecutive stretch of data points to be below the average). Of those, 7 were below the minus-one standard deviation mark. Before June 12, only 6 of the prior 57 points were lower than the minus-one standard deviation mark.

To take it one level deeper, I also ran these kinds of numbers on the Twins at the individual pitcher level. The Twins have eleven pitchers that have enough four-seamers thrown before June 3 and after June 21 to make a reasonable comparison. Of those, two saw spin rate declines greater than two standard deviations, six declined somewhere between one and two standard deviations, one declined less than a standard deviation, and two saw no change. Those results loosely track with the breakdown Sarris pointed out was showing up across the league.

Taken together, this set of analyses gives me confidence to say that the Twins change in fastball spin rate is probably not just random variation. The data suggests they are really spinning the ball less than they were before the rules started being enforced and the change occurred around the same time that the league started making noise about the issue.

That said, it also seems clear that the Twins weren’t in the crowd that was using industrial grade grip enhancers (at least not in a systemic, entire team kind of way). If that had been the case, the team level deviation from the mean would likely have been much larger than a single standard deviation and more of the individual pitchers would have experienced drops of 2+ standard deviations.

John is a contributor to Twinkie Town with an emphasis on analytics. He is a lifelong Twins fan and former college pitcher. You can follow him on Twitter @JohnFoley_21.