Bauer Units, Pitch Movement, and Evaluating Pitches

| Pitch Design
Reading Time: 13 minutes

At Driveline, Bauer Units have been used as a reference during pitch design since the pitch tracking technology such as the Rapsodo started to appear in 2016. We introduced Bauer Units in our original piece on spin rate and followed up with another article to define them more clearly. In this piece, we’ll explain Bauer Units even more specifically because the metric can be misunderstood in the context where it is most valuable.

Bauer Units = Spin Rate (RPM) / Velocity (MPH)

As a quick refresher, Bauer Units are intended to control for velocity when looking at spin rate. The equation was intended to be simple enough for coaches and players to use quickly in practice to compare pitchers to MLB average. In analyzing per-player data sets, we’ve seen that spin rate increases with velocity in a linear manner.

Knowing this, we can hopefully help put some MLB data into better context, as two pitchers with the same spin rate but different velocities will have different Bauer Units. While the MLB average may move slightly from year to year,the average Bauer Unit we’ve seen for fastballs—the pitch best used for Bauer Units—is right around 24.

To best explain when we use Bauer Units, we’re going to dive into the history of why we created the metric, how we use it for fastballs and offspeed pitches, and how it can be a useful tool for amateur athletes.

History of Spin and Bauer Units

When public data on spin rate first became available, it opened up a whole new level of analysis of pitchers. While having access to more complete pitch-level information answered several preliminary questions, it also helped the industry generate new and more complex questions.

If we go all the way back to 2013, we can start to see some of the first findings from Trackman. We started to see a relationship between spin on a fastball and both swinging strike percentage and ground ball percentage. We noticed that on average the lower the spin, the lower the swinging strike percentage and the higher the ground ball percentage. Vice versa, higher spin shows the reverse effect, as high spin rates correlate with a higher swinging strike percentage and lower ground ball percentage. This information is worth having because it turns out that different pitchers have different spin rates, even if they throw at a similar velocity. The piece linked above highlights Kenji Uehara and Jake Westbrook as examples of two such pitchers..

Up until 2016, this data was only available to professional teams and some public analysts via Statcast. As a result, amateur coaches and players couldn’t utilize the data before Rapsodo pitching unit became available.

After helping validate the first Rapsodo unit (and recently validating their second unit) while developing our pitch design process, we started to look more closely at the numbers. Fairly quickly, we realized that there were some limitations to using spin rate alone to evaluate a pitcher, particularly those with relatively high or low velocities. In working with high school, college, and professional pitchers, we realized that the high and low spin benchmarks of 2000 RPM’s and 2400 RPM’s respectively weren’t necessarily representative of high or low spin at different velocity ranges, so Bauer Units were created to control for this.

We still believe there were good reasons to throw high-spin fastballs up in the zone and low-spin fastballs down in the zone due to analysis of public data and other research, though this is a simple explanation of a complex problem. This knowledge was presented in blog posts from late 2016 and in a piece that looked at spin axis and useful spin in the context of fastballs and offspeed pitches.

Since then, we’ve experimented with grip changes on fastballs, seeing how spin axis changes on offspeed pitches with different intent, and looked further into how spin is generated in the first place.

All of that, along with deeper dives into the nuance of fastball spin rate and classification of offspeed pitches, means we know much more than what we did three years ago about pitch movement, but still have things to learn.

This brings us to today’s post, in which we will detail how we currently use Bauer Units. They give us some information about a pitcher’s capacity to apply spin to a pitch. That said, it’s worth pointing out that pitch movement is likely a more effective way to evaluate the quality of a pitch.

Fastball Bauer Units and Pitch Movement

We mentioned in our original piece on Bauer Units that they were best used to compare a pitcher’s fastball to the MLB average—the average Bauer Unit, floats around 24 BUs (92 MPH and 2200 RPM). However, this step is just one of a few steps to learning about a pitcher’s fastball, and is mostly used as an initial reference before looking further. The actual movement profile of a fastball is much more important than what a Bauer Unit can tell us.

Let’s look at two pitchers with similar Bauer Units to see how the metric can be used as a point of departure for deeper analysis. In this case, we can compare Justin Verlander to Twins pitcher Devin Smeltzer. Sample sizes are quite different but pitch spin and velocity stabilize relatively quickly so we can perform this analysis with some confidence. 

Savant:Smetzler Verlander

In looking above, you can see that even though these two throw at different velocities and spin rates, their Bauer Units are nearly identical. However, the fact that the pitches’ have similar BUs doesn’t mean that the pitches are equally effective. To get more context, you would want to look at the movement profile of the pitches, which can be done by looking at BrooksBaseball data. Let’s look at the profiles of their fastballs, especially the vertical movement from one of their most recent games.

Brooks: Smetzler Verlander

Two numbers are listed from this analysis, movement profiles from BrooksBaseball and adjustments to how those numbers would look on a Rapsodo. Different systems calculate movement using different equipment and calculations which is why the numbers are different.

Even though the Bauer Units are the same, we would evaluate each pitch based on the movement profiles, which are quite different. As you can see, Verlander gets significantly more vertical movement.

This brings us to another important point: while much has been said about spin rate, what really matters is how that spin changes the movement on the pitch. While others have noted  that Verlander did see a small increase in spin rate after being traded from Detroit to Houston in 2017, you can also see big changes in the vertical movement of the pitch. This likely came from making changes to the axis of the pitch.

Brooks Baseball link

To sum up, we’ve seen that Bauer Units can offer some insight into the behavior of a pitch, but the more valuable data are found in a deep analysis of pitch profile..

In some cases, a pitcher’s fastballs spin at a high rate but also have a good degree of cut. Carl Edwards is a good example of that if we compare him to Verlander.

Statcast: Verlander, Edwards 

Brooks: Verlander, Edwards

Edwards is shown on the left and Verlander on the right

 These visuals were created with DrivelineEDGE, Driveline’s pitch design visualization tool. Find out more here https://plus.drivelinebaseball.com/product/drivelineedge/

The different movement profiles will mean that the plan of attack for these pitches will likely be different, and pitch strategy will change depending on what other pitches the pitcher has in their arsenal.

This means that pitch movement is also very sensitive to axis. Bauer Units can be useful as an easily determined reference point to learn how fast the ball spins relative to its velocity; however, there is more to evaluating a pitch than that alone.

The fastballs of Sean Dootlitte and recently called-up Colin Poche’s show that even pitches with average Bauer Units can have exceptionally high vertical movement. Even though their velocities, spin, and Bauer Units are all close to league average, both Doolittle and Poche throw fastballs that are in the higher ends of vertical movement, likely because of axis.

When discussing fastballs, two-seam fastballs become trickier to nail down because we still aren’t sure exactly how or why certain two-seam fastballs move as they do. Some of this is because of limitations of technology that we discuss in our blog on laminar flow. At the same time, it’s very likely that two-seamers can move arm side depending on the grip and exact release point, due to resulting laminar flow or air movement across the wake of the ball.

We’ve demonstrated that controlling for velocity when discussing spin can give you some useful information, and Bauer Units can be a helpful metric for that. That said, Bauer Units aren’t a unit to maximize, because while they can give you some information, it is limited. A deeper analysis of pitch profile is necessary to truly understand the effectiveness of a pitch.. Measuring Bauer Units gives coaches a players a place to start. You could see a high Bauer Unit and cut on a fastball and think it’s worth trying to change the axis to get more vertical movement.

Offspeed Pitches, Bauer Units and Pitch Movement

For offspeed pitches, Bauer Unit’s can signal the potential to spin the ball well. However, as we’ve covered in previous blogs, the relationship between total spin and movement of a slider or curveball is very complex. So, whether the pitch is effective or not depends on the movement profile. This also gets deep into the nuances of spin axis and spin efficiency.

Measuring the Bauer Units of offspeed pitches can tell you if a pitcher spins the ball well, but even more so than with fastballs. You’ll need to look at the axis and spin efficiency, along with the movement profile, to determine how effective a pitch is.

This is especially true in the context of adjusting a pitch. You may improve a movement profile by tweaking a large number of variables. Grip changes, minor adjustments in release off the fingers, and trying to throw harder can all have effects on the spin of the pitch and the axis. For example, you can see evidence of this by comparing the curveballs of Seth Lugo, Ryan Pressly and Trevor Bauer. The first two pitchers are known for their high-spin curveballs, while Bauer throws a knuckle curve.

Spin rate numbers taken from Statcast and movement profiles taken from Brooks baseball.

Lugo and Pressly have the two highest curveball or knuckle curve spin rates on Baseball Savant, and their Bauer Units are incredibly high. If we look at the movement profile, we can see that Lugo and Pressly have more horizontal movement than Bauer. In contrast, Bauer has more vertical drop, less horizontal, and a lower spin rate. So, again, you can see that Bauer Units can identify a pitch that spins at a high rate relative to its velocity, but movement profiles can give you more information about what a pitch actually does and how it may be best used.

Amateur Pitching

While there is a plethora of data available on MLB pitchers, that amount of data is lacking for amateurs.. You may hear, for example, that an MLB pitcher has a fastball spin rate of 2450 with a velocity somewhere between 88-100. Of course, this doesn’t hold true for younger pitchers.

Because amateur/youth players (under the age of 18) can throw at a much wider range of velocities, say 70-90 MPH for a fastball, their spin rates can also be much more varied.

Similar to the example above, a spin rate of 1900 may have a below-average Bauer Unit at 90 mph but a relatively high Bauer Unit at 70 mph.

So, Bauer Units can help quickly identify how an amateur’s ability to spin the ball compares to the MLB average. Therefore, we can help give some context to amateur spin rate and pitch movement compared to the major leagues.

Besides the obvious fact that pitch movement will be different, we don’t know how the ability to spin a ball changes as a player ages. It is an interesting question that if pursued might help play a role in what pitchers focus on in development as they age.

A Quick Equation Before Diving Deeper

You’ll see that we tend to discuss Bauer Units as “high” or “low” and not in specific technical terms. This is not because we can’t calculate standard deviations of average Bauer Units. The reason is that we feel that would be unnecessary when we also have access to pitch movement and ball axis measurements, which will give us a much better representation of pitch effectiveness. This suggests the use of Bauer Units as a casual starting point players and coaches can use to evaluate pitches before moving on to other, more specific metrics.

A more robust metric that would be similar to Bauer Units but more precise would be a calculation of the revolutions a ball takes on the way to the plate. This was mentioned previously in a Baseball Prospectus piece, but in order to calculate revolutions you would also need to know the time the pitch took to get from the pitcher’s hand to the plate.

Pitching is very complex and we’re learning more about it everyday. That said, there is still a use for a simple calculation to quickly evaluate a pitch before diving further into analysis. Bauer Units most valuable aspect is that it’s simple to calculate to get an idea if a pitcher spins a ball at a high rate relative to their velocity. We will undoubtedly learn more about pitching and ball movement in the future. We still believe that Bauer Units have a place when analyzing a pitcher’s movement, even if it’s smaller than we originally may have thought.

This article was written by Senior Operations Manager Michael O’Connell

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