Michael WachaZach Gifford | THE INTREPID STL

Photo Credit: @cardinalsgifs

As of this writing, the St. Louis Cardinals sit three games behind the Rockies and have a 21.3% chance of making the playoffs, according to FanGraphs. Three games is a lot to make up with only 21 left to play, so every game takes on even more significance.

Making things tougher, the Cardinals have suffered a rash of injuries. Dexter Fowler is struggling with a hip injury, Tommy Pham is battling a swollen thumb, Matt Carpenter is out with shoulder bursitis, Kolten Wong is out with an injured calf, and Jedd Gyorko is missing to a strained hamstring. In other words, five of the Cardinals top seven run producers are dealing with a physical ailment to some extent.

So, as always seems the case, pitching has taken on a heightened importance in September. Michael Wacha is expected to make four more starts, two of which are slated to be against the Cubs in pivotal matchups for the division race. If his 6.45 career ERA against Chicago isn’t enough to worry about, we now have clear visual evidence that Michael Wacha is tipping his changeup. Two bad matchups paired with pitch tipping is not a recipe for success.

The logical follow-up question is whether hitters can actually notice a difference that subtle and precise while live in an at-bat. The tip only matters if it gives the hitter an information advantage, and it’s fair to wonder if slower arm action is even noticeable when you have to prepare for 95 mph heat.

The best way to conclude whether the hitter is picking up on the tip is to isolate a decision they make at the plate. We can do this using plate discipline data – specifically, whether the batter swings at pitches in or out of the zone. If a hitter has better information than they’ve had previously (e.g., a tip), we’d expect them to be “correct” more often. Put another way, we’d expect them to swing more at pitches in the zone (Z-Swing%), swing less at pitches out of the zone (O-Swing%), and widen the gap between their zone swing and chase rates.

Using Baseball Savant, I pulled the data for every single tracked changeup Michael Wacha has thrown in the Major Leagues. I then calculated the Z-Swing% and O-Swing% against the pitch by season, shown below. The larger the gap between Z-Swing% and O-Swing%, the less hitters are being fooled.

Michael WachaZach Gifford | THE INTREPID STL

Obviously, something is different about 2017. His Z-Swing% against is at a career high and his O-Swing% against is at a career low by a significant margin. Michael Wacha is allowing a career worst Z-O Swing% by almost 10%. A 10% jump doesn’t just happen by accident, and there was previously no trend suggesting hitters were gradually picking up more on the changeup over the years.

We can look at the Z-O Swing% gap another way. I calculated a 58-pitch rolling average Z-O Swing% for each year of Wacha’s career. This allows us to see the movements and trends throughout Wacha’s career and seasons.

Michael WachaZach Gifford | THE INTREPID STL

At the start of the year, Michael Wacha was allowing a normal Z-O Swing% against his changeup. Hitters were seeing the ball the same as they always have. Then something changed around pitch 1750, the lowest point in 2017 and which corresponds with his start on May 25th. Suddenly, his changeup stopped fooling hitters. Maybe that’s when he started tipping.

Over his last three starts and following a brief period where it looked like Michael Wacha may have corrected his tip, hitters have taken Wacha’s changeup into unprecedented territory. His Z-O Swing% against reached a high of 69% on August 25th and currently sits at 65%.

Maybe it’s easier to think about this way: over his last 58 changeups thrown, hitters have swung at 92% of the ones in the zone and have chased 27% of the ones out of the zone. For context, the 2017 MLB average is a swing rate of 76% at changeups in the zone and 34% on changeups out of the zone. Combine the two, and Michael Wacha has posted a Z-O Swing% that is 55% worse than league average. That’s terrible.


On Thursday, Joe and @cardinalsgifs gave us great evidence that Michael Wacha was tipping his changeup. Today, we answered our follow up question. Hitters are swinging at more of Wacha’s changeups in the zone, less at changeups out of the zone, widening their Z-O Swing gap and getting fooled less often.

Michael Wacha is tipping his changeup, and his opponents are noticing and adjusting.