A Beginners Guide to Evaluating Four-Seam Fastballs

Four-Seam Fastball Grip; Photo via Toto-artist

Pitch design has become more and more prevalent, and four-seam fastballs are seemingly falling out of vogue. Thus, the landscape for how one is to judge a fastball has entirely changed. Generally, many people have only considered speed. And while that is an essential factor, it is only a fraction of the pitch’s worth. A slow fastball can still be effective - it just needs to be used and thrown in the right way. Using several criteria, this article will showcase some of the best methods a person can use to differentiate between a great and horrendous four-seam fastball.


Pitch Shape

As you may know from watching a game or playing one yourself, all fastballs look slightly different. Some will appear to rise as the pitch is thrown, and others will have a regular downward path or break away. Generally, a hitter will expect a fastball to follow a certain path from the hand, making room for a pitcher to succeed or fail based on their pitch's differentiation from the batter's expectation. Easier pitches to hit will follow an expected fastball path - not sinking, rising, or breaking much above the average. The hardest pitches to hit deviate from that path - rising, sinking, and horizontally breaking at a rate that is much different than the average pitch. Hence, the fastball shape is introduced. Pictured below is a graph of the 4-Seam pitch movement versus the average, explaining which pitches are deadly and which are not.


2022 4-Seamer Movement versus League Average.

When looking at the qualified pitches, you will see a cluster around the 0,0 mark. As the majority of a sample is closer to the average, it makes sense that most pitches fall near that area. But those fastballs are not the makings of a great pitcher. As each point moves farther away from the center, the deadlier that fastball arguably becomes. A four-seamer that drops away much sooner than expected will be difficult to hit. The same line of thinking goes for horizontal break - the farther away it is from average, the better. In general, though, the most effective four-seam fastball shape appears to be rising to the top of the zone, being much flatter than a hitter expects. All of these shapes are influenced by spin efficiency, which is the percentage of movement that is caused by top-spin, back-spin, and side-spin. When a pitcher has a higher rate of spin efficiency, he tends to have a better shape - inducing whiffs and called strikes at a higher rate, which benefits a pitcher tremendously.


Vertical Approach Angle


Like Fastball Shape, the concept of Vertical Approach Angle thrives on being different from the average. If you’re not familiar with the concept whatsoever, I will give a brief but detailed rundown. Let’s start with a fact - all types of fastballs enter the zone at certain angles. These angles can be determined by Vertical Approach Angle, or a combination metric that considers release speed, pitch height, and release point. The calculation is linked here for those that are interested, but in summation - the higher the measure, the more difficult that a fastball should be to hit. When the ball is at the top of the zone, the way that a four-seamer enters the plate is much more important. As raw VAA data is somewhat confusing, VAA AA (vertical approach angle above average) was created. After all, most of this data is useless without context. Pictured below is the 20-80 scale of VAA AA, which was developed by Alex Chamberlain.


VAA AA Scouting Grades created by Alex Chamberlain via Fangraphs.

The higher the grade, the better the four-seamer. When a pitcher can throw what appears as a flatter fastball, he also has a wider margin of error to throw low in the zone. As previously mentioned, fastballs are most effective in the upper part of the strike zone. But when the pitch appears to be rising at a high rate, it can still garner a lot of whiffs - no matter the location. With a wider margin of error, pitchers have a much higher probability of success. A good example of this would be Devin Williams. Thus far in 2022, Williams has a +0.57 VAA AA, which puts him at about a 60 Grade (above average). While Williams' VAA grade is not elite and his fastball would not be very effective low in the zone, the star reliever manages to always throw his fastballs at the top of the zone, making the pitch extremely useful. It has the highest run value per 100 pitches (-1.3) of any in his arsenal, elicits whiffs at a 33.6% clip, and allows an incredibly low rate of hard hits at 23.6%. VAA plays into all of these factors, with a high grade at the top of the zone leading to a great rate of success.


Velocity


While evaluating a fastball solely on velocity is flawed, velocity is still somewhat important. The harder pitches are thrown, the harder the pitches should be to hit (or at least in theory). However, some of my findings suggest that is not the case. When looking at pitcher's four-seamers with a) at least a 500-pitch sample size and b) going at least 84 MPH since 2015 (the start of the Statcast era), pitch velocity had no effect on wOBA produced on fastballs in the aggregate. It's worth noting that even though no finding was apparent in the long array of fastballs, a player still has less time to hit a harder thrown fastball, which makes the feat more difficult. This is especially true at the amateur level in scouting evaluations.


A good four-seamer based on velocity is fairly easy to point out - a plus pitch for a starting pitcher lands at 95 MPH or above. The 80 Scout Grade is given to anything 99 MPH or above. For a reliever, the threshold is a bit higher. A reliever with a quality fastball generally throws one at or above 97 MPH, with the 80 threshold hitting at about 100 MPH. Just remember - velocity is not the end-all-be-all (none of these listed evaluation metrics are). In fact, alone, velocity can almost be useless. But paired with a number of other factors, such as a great approach angle or a nice horizontal movement, it can be the icing on the cake of pitch quality.


Spin-Based vs Observed Axis


Seam-shifted wake is the phenomenon taking baseball by storm. I’ve previously written about what seam-shifted wake is (linked here). Simply put, seam-shifted wake is when a baseball moves in an unexpected direction that is not based on the Magnus force (which is when a spinning baseball moves vertically through regular airflow) that a regular pitch would exhibit. In that article, I primarily focused on sliders - but four-seam fastballs can undergo the same force. By looking at an observed vs actual spin axis chart, which compares what the ball should’ve done to what it did, you can see if a pitch experienced seam-shifted wake. To demonstrate this, I’ll look at San Francisco Giants pitcher Tyler Rogers. Shown below in red is the expected vs actual spin axis of Rogers in 2022 for his four-seam fastball.


Tyler Rogers Spin-Based vs Observed Movement Charts via Baseball Savant.

Based on Rogers' spin-based movement, his pitch should arrive on the 7:15 axis (on a clock-like scale). It actually comes into the plate on the 6:15 axis. This major differentiation causes the ball to move in a completely unexpected way, with the given air pressure surrounding the seams sticking suddenly around parts of the ball as it travels through the air. Hence, the hitters predicted movement of a given pitch will be wrong, making any guesses in their heads less accurate. A great fastball will deviate from the expected spin movement, with higher deviation equating to a more effective pitch. Turning the expected into the unexpected remains a common theme of a good four-seamer.


Spin Mirroring


This criterion goes hand-in-hand with the observed vs actual spin axis, although I believe it deserves a separate discussion due to the nature of the matter. This goes along with the human eye, which, while very capable of many things, serves to hurt hitters in this instance. When each pitch is thrown, it generally spins around a certain axis. Each pitch is different, with different axises needed to make the ball move a certain way. But, what if you could make two different pitches look like they are spinning the same way? This is spin mirroring. Due to the human eye being unable to differentiate between two directly opposite spin directions, a pitcher that can make two pitches in the exact opposite spin directions (180 Degrees) can create a load of deception for a hitter. A fastball that generally goes straight and a breaking pitch that won't reach the strike zone appears to be the same pitch for the majority of the path to the hitter. After all, the seam movement does look the same. Only when each pitch is about to break does it become obvious to the hitter that he is not seeing the pitch he expected, leaving them either whiffing or in a daze of confusion.


The deception on these pitches is mesmerizing - it is generally not surprising that hitters have struggled so much in recent years. Once they break away in their appropriate manners, a batter has less time to react, making these pitches incredibly difficult to hit. An effectively mirrored fastball needs a breaking ball to pair with - done alone, this concept is not in effect. But if a pitcher can successfully use both a four-seamer and off-speed to fool hitters, it is so much more deadly, proving to be another point of value-added via a four-seam fastball.


 

While seemingly a simple pitch, so much is needed to correctly assess the original fastball. These types of pitches should rarely be taken for their surface value, and I believe the showcase of different evaluation facets shows this. The above criteria I consider to be the main facets necessary to judge a four-seam fastball - although, I encourage the reader to seek out more ways to look at a four-seamer. For now, Pitch Shape, Vertical Approach Angle, Velocity, Spin Based vs Observed Axis, and Spin Mirroring should do. As studies of baseball grow, I can only imagine how much this basic list will expand.


Sources:


Alex Chamberlain's Pitch Leaderboard

BaseballSavant.com

DrivelineBaseball.com

Fangraphs.com

TheAthletic.com