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How Jack Flaherty turned the corner (Part Two)

Flaherty’s fastball has become a more effective weapon against lefties

MLB: St. Louis Cardinals at Milwaukee Brewers Jeff Hanisch-USA TODAY Sports

I left you last week with a nihilistic commentary on baseball blogging, but more importantly a promise that my analysis of Jack Flaherty’s slider and curveball success against lefties would be followed by a companion piece discussing his fastball. In the two starts he has made since, Flaherty contained the surging Rockies and Pirates to a 3.18 ERA, 2.90 FIP, 3.35 xFIP, and 3.09 SIERA.

The young righty throws two different variations of the fastball: a straighter, faster fourseamer and a slower pitch with more ‘fade,’ or horizontal movement. Pitch Info (the pitch tracking technology displayed on classifies the latter as a sinker, whereas it registers as a two-seam fastball according to Statcast’s TrackMan data. For the sake of this article, I will refer to it as a two-seam fastball.


Flaherty yielded a .417 wOBA and .429 xwOBA to opposing lefties on the four-seam fastball in 2017. While his .338 xwOBA this season doesn’t quite match an actual wOBA of .268, the fourseamer has evidently been more effective this year.

Flaherty doesn’t appear to have tapped into the magic (pine tar?) fountain of spin rate powers–his average four-seam spin rate is just three rotations per minute (RPM) lower than last year–and his average velocity is actually down a half-tick.

It’s no secret that high spin rate numbers correlate with more whiffs...and that Flaherty’s 2,214 mark is 49 RPM below average. The fourseamer’s 11.4% whiff/swing rate in 2018 isn’t anything to write home about, but it is a marked improvement upon his 2017 rate That’s right. Jack Flaherty generated exactly zero swings and misses from left-handed hitters when throwing a four-seam fastball last season. I completed the Statcast search query at least five times to assure myself that there wasn’t an error on my part.

Even when Flaherty isn’t missing bats–which remains the vast majority of the time, at least as his fastball is concerned–he has induced poorer contact. Statcast categorizes all batted balls into one of six buckets: barrels, solid contact, flares & burners, topped, weak contact, and hit under. For context, I included the average wOBA on each type of batted ball in the table below.

Jack Flaherty: Batted Balls on Fourseamer vs. Lefties

Batted Ball Type 2017 MLB Avg. wOBA 2017 2018 Change
Batted Ball Type 2017 MLB Avg. wOBA 2017 2018 Change
Barrels 1.471 7.1% 1.9% -5.3%
Solid Contact 0.697 7.1% 7.5% 0.4%
Flares & Burners 0.621 21.4% 30.2% 8.8%
Topped 0.172 42.9% 30.2% -12.7%
Weak Contact 0.149 7.1% 0.0% -7.1%
Hit Under 0.088 14.3% 30.2% 15.9%

The two key factors working in Flaherty’s favor here are the reduction in barrels, the crown jewel of batted balls, and a nearly doubled rate of fourseamers that lefties ‘got underneath.’ Balls that are ‘hit under’ consist primarily of pitches that are skied at a high launch angle, but hit weakly enough to where they don’t pose much of a threat. 78.1% of pitches in the ‘hit under’ category leaguewide are either pop-ups or fly balls with an exit velocity no greater than 95 mph. Flaherty had no such fourseamers meet those criterion in 2017, but fast forward to this season and they make up 22.6% of all four-seam fastballs that lefties have put into play against him. The average exit velocity of balls hit in the air (i.e fly balls and pop-ups) versus Flaherty on these pitches was 99.4 mph last season, which he has quelled all the way down to 85.8 mph in 2018.

So how exactly did Flaherty pull this off? More precise command likely contributes. The ratings on QOP Baseball vaulted Flaherty’s fourseamer from the 35th to 69th percentile in location optimization. Take, for example, his 2017 strike zone map on fourseamers against lefties per Brooks Baseball.

More than a quarter of all the fourseamers Flaherty threw to lefties were up and away in the two topmost boxes in the far left column. Just one of these 19 pitches outside of the strike zone resulted in a swing, essentially gifting the opponent a ball. Before Brandon Crawford lined a two-run homer off a Flaherty slider in his MLB debut, the rookie fell behind in the count, 2-0, after Crawford took a pair of fastballs high and outside to begin the at-bat.

The percentage of fourseamers thrown on the innermost column rose from 3.9% to 18.9% this year. It’s crucial that Flaherty jams lefties inside with the fastball, as the pitch was tagged for a clean .500 slugging percentage when it crept into the middle or second-from-the-right column.

In the video below from September 13 of last season, Yadier Molina sets up further inside, but Jesse Winker is about to make Flaherty pay when his fourseamer catches too much of the plate.

Juxtapose that with what proved to be one of the higher leverage spots in the Cardinals’ 5-0 victory over the Cubs on June 17. With two aboard in a tie game, Ben Zobrist lifts a shallow fly ball that Matt Carpenter hauls in with ease to record the second out.

But before you see that fastball, check out the slider Flaherty used to pick up the first-pitch strike.

This fourseamer rides up and in on Zobrist’s hands, rendering him incapable of driving the ball.

Note how the pitch’s heat map against lefties on Baseball Savant has evolved (the top image is 2017; 2018 is bottom) to center around a core located on the inside edge of the strike zone rather than out over the plate.

Baseball Savant
Baseball Savant


Let’s start by breaking down how the two-seamer’s statistical production has changed since Flaherty’s brief big league stint last year.

Jack Flaherty: Two-Seamer Results vs. Lefties

Metric 2017 2018 Change
Metric 2017 2018 Change
wOBA 0.362 0.326 -0.036
xwOBA 0.335 0.323 -0.012
Ball% 47.7% 42.3% -5.4%
GB/FB Ratio 1.0 3.3 2.3
Avg. Launch Angle 19.6 6.2 -13.4
Avg. Velocity 92.6 90.9 -1.7
Avg. Spin Rate 2,107 1,981 -126

Flaherty is doing a better job keeping the two-seamer–or sinker, whichever you prefer to call it–on the ground in 2018. The sudden drop in velocity and spin rate is concerning on the surface, but there is a chance that this is an intentional decision by Flaherty to ‘take a little bit off’ the pitch.

Driveline Baseball devised the metric Bauer Units (BU), which is simply a pitch’s spin rate divided by its velocity. The BU figure on Flaherty’s two-seamer dipped from 22.8 to 21.8, well below the league average of 23.4. That said, a BU mark significantly deviating from average (particularly if the spin rate is lower than average) can be beneficial for churning out ground balls and limiting quality contact.

Of the 156 pitchers who have logged at least 200 two-seamers this year, 11 have a BU of 21.4 or less, or a full 2.0 lower than average. Their collective wOBA and xwOBA on two-seamers are .314 and .333, respectively. As for the 39 pitchers whose BU is within 0.6 of average, their wOBA and xwOBA are .353 and .362, respectively. This is likely due in some part to the correlation between lower spin rates and higher ground ball rates.

I compared two-seam fastball spin rate with ground ball rate for the 76 pitchers with 125 batted balls on two-seamers between this year and last.

The R-squared value of 8.6% doesn’t signify overwhelming correlation, but the hurlers with extremely high or low spin rates tended to observe more extreme ground ball rates. This would indeed help to explain the changes in Flaherty’s two-seam batted ball profile.

The two-seamer’s location rating from QOP Baseball jumped from the 32nd to 53rd percentile. The core of the 2018 two-seamer is squarely in the down-and-away corner, while a greater share of the 2017 offerings were so far outside that hitters weren’t enticed to swing.

I’m not sure that I have a verbose conclusion in mind for today’s post. I don’t think it comes as any surprise that I enjoy analyzing player development and the adjustments these professional athletes make to get the most out of their bodies, and Jack Flaherty is no exception. He doesn’t turn 23 until October, but has presumably entrenched himself in the Cardinals’ long-term core with the forward steps he has taken this season.