Spin Efficiency, More Than Just a Number (Part 2)

In part 2 of this very interesting discovery - contributor, Adam Schuck, dives deeper into the question "Why does spin efficiency change?" In doing so, providing great insight into the process he uses when digging into high level problems such as this one.

In Part One of this series we drew attention to the fact that Spin Efficiency isn’t a constant throughout ball flight. In fact, Spin Efficiency sometimes is gained and sometimes it is lost. We observed, very generally, that often pitches that move armside lose efficiency while pitches that move gloveside gain efficiency.

For Part Two, I hope to dive a bit deeper looking a bit more at what contributes to the change in efficiency for individual pitch types and touching on a few prior comments from Part One. Similar to before, this piece will be pretty informal as I take you all through my process and where I’m still uncertain on things.

For a quick refresher, here’s an important graphic from Part One.

For those of you who haven’t read Part One, I’d recommend glancing over it before continuing. As a quick reminder, this is a distribution of the change of Spin Efficiency between release and when the ball reaches the plate.

Fastballs: Previously, we noted that Two-Seam Fastballs almost always lose efficiency while occasionally Four-Seam Fastballs gain efficiency. But why? What’s the difference between the two pitches? Well, it turns out that it’s not so much about the seam orientation of the pitch as it is the Spin Direction, initial Spin Efficiency, and initial Gyro Degree.

A large determining factor of whether a Fastball gains or loses efficiency is the initial Gyro Degree of the pitch. Gyro Degree is a measure of how closely the Spin Axis is in line with the initial trajectory of ball flight. For RHP, Fastballs almost always have a positive Gyro Degree which causes the pitch to lose efficiency. In the rare cases where there is a negative Gyro Degree, these pitches gain efficiency.

Imagine you are looking at the baseball on the left from on top, or a birdseye view. The ball is traveling up to the top of the page with a 12:00 Spin Direction (pure backspin). If you tilt the axis the ball is rotating around, the red arrow, counter clockwise you would have a positive Gyro Degree. This is usually referred to as “cutting the ball” and very common for RHP. If you tilted the arrow clockwise, you would create a negative Gyro Degree. This is relatively uncommon and fairly difficult to achieve for most RHP on Fastballs.

So, how can you influence whether a Fastball gains or loses efficiency? You can manipulate the Gyro Degree to be positive to lose efficiency or negative to gain efficiency (for RHP).

Okay so we know how to gain or lose efficiency. But now I’m wondering if we can influence how much efficiency is gained or lost. The answer? Kind of.

In the graph on the left we can see that the initial Spin Efficiency certainly plays a part in the amount of efficiency gained, but may not quite tell the entire story. Pitches with a lower initial Spin Efficiency tend to lose more efficiency while pitches with higher efficiency tend to remain the same or lose less, but the relationship isn’t quite linear. The same is true with Spin Direction, displayed in the graph on the right. Pitches with a more horizontal Spin Direction seem to lose more efficiency during ball flight but the relationship isn’t quite linear.

So, what else influences the change in Spin Efficiency for Fastballs? Honestly, I’m not sure and that’s part of the fun in this process. We aren’t 100% sure why this happens, but the goal is just to dive down different alleys and test multiple hypotheses. Sometimes they come true and sometimes they don’t. I brought up the idea of location influencing the amount of efficiency gained but that hasn’t quite proved to be true so far. My best educated guess at the moment is that seam effects have something to do with the variation or environmental factors affecting drag. In the meantime, I think we can feel pretty confident that the contributions from the initial Spin Efficiency, Gyro Degree, and Spin Direction play a significant role in determining the change in efficiency.

Breaking Balls:

In Part One, we covered mostly Sliders and discussed influencing whether you gain or lose efficiency and if it’s possible to control the amount of efficiency change. Similar to Fastballs, we recognized that the largest influencer of gaining vs losing efficiency was the gyro degree. For RHP, sliders with a positive gyro degree lost efficiency while sliders with a negative gyro degree usually gained efficiency.

This remains true, but similar to Fastballs the Spin Direction is playing a significant role in the amount of efficiency gained. The graphs below are for RHP only to simplify things, we found the same results for LHP as well.

Much like Fastballs, pitches with a more horizontal Spin Direction have the potential to gain or lose the most efficiency while those closer to 12:00 or 6:00 usually stay at about 0. Additionally, there is a moderate relationship between the amount of side spin and the amount of efficiency gained which makes sense as those pitches with a more horizontal Spin Direction will have a larger % of the spin geared towards side spin.

For Fastballs, we noted that the initial Spin Efficiency played a significant role in how much efficiency was gained or lost. For Breaking Balls, this isn’t necessarily true. While breaking balls with a lower Spin Efficiency (< 10) typically gain an average or above amount of efficiency, this isn’t true for other values of Spin Efficiency as it appears much more random.


Lucky for us, this is an easy one and much less ambiguous than other pitch types. Previously, Gyro Degree was indicative for whether efficiency was gained or lost for previous pitch types but didn’t provide much insight into how much efficiency could be gained or lost. For Changeups this isn’t quite the case. Again, the plot below is only RHP.

Interestingly, there is a near 1:1 relationship between Gyro Degree and the change in efficiency.

You may notice the larger amount of pitches that gain efficiency in this sample. Other than low efficiency Sliders, negative Gyro Degree values occur most commonly with Changeups due to the nature of how the pitch is thrown. As most know, there are dozens of ways to throw a Changeup. Those pitchers that throw “Straight Changes” will likely have a positive Gyro Degree that mimics their Fastball. Pitchers who struggle to pronate and end up killing Vertical Break by holding supination and throwing what is almost an opposite handed Cutter/Slider will likely have a larger Gyro Degree and hence a larger loss of efficiency. Finally, pitchers who have the ability to really pronate on the Changeup will sometimes induce a negative Gyro Degree and will gain efficiency.


In Part One, I discussed the potential role of location in determining the amount of efficiency change on any given pitch. Unfortunately, I’m at a bit of a dead end here. Intuitively, it would make sense that the more space you give a pitch to break (BB gloveside, FB/CH arm side) the more time it would have to gain or lose efficiency.

Given the plots above, this doesn’t necessarily seem to be true. I think a deeper dive can be done here, but looking at the entire sample there doesn’t appear to be any stand out trends.

Wrapping Up

We covered a lot of information in today’s blog so let’s take a chance to bring it all together.

What is Gyro Degree? The orientation of the axis the ball is spinning around relative to it’s target.

How do some pitches gain or lose efficiency? A positive or negative Gyro Degree determines whether pitches will gain or lose efficiency.

Do some pitches gain or lose more efficiency than others?

Yes, this is influenced by Spin Direction. Pitches with Spin Directions closer to 3:00 or 9:00 will typically gain or lose the most efficiency. For Fastballs, pitches with a lower initial efficiency typically lose more than those with higher initial efficiencies. For Changeups, the amount of efficiency gained or lost is directly tied to the Gyro Degree of the pitch. The higher your Gyro Degree, the more efficiency you will gain or lose depending on if that value is positive or negative.

These last two Blogs have been a lot of fun to write (and hopefully for you to read) as I got to really dive into something I didn't know much about and find some interesting relationships. I will be taking a break from writing until the 2021 season is over to focus on our athletes, but if anyone wants to talk shop please feel free to reach out to me via Twitter!

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