A Pitchf/x Primer, and why Trevor Cahill should start 2010 in Sacramento.

I don’t know how familiar the AN population is with the specifics of the wonderful Pitchf/x system, so I’ll start from the beginning. For those that do…I’m sorry. Skip ahead if you’d like.

In 2006, Sportsvision (the company behind the original yellow TV first down line and pretty much every piece of TV sports graphic wizardry out there) launched Pitchf/x.  They installed multiple high-speed cameras in every MLB stadium that track every pitch, computing the ball’s trajectory and location to within a half an inch. Over time, the power of the system grew, and today, the cameras allow the Pitchf/x system to calculate a huge collection of variables about the trajectory of every pitch, definitely not limited to what you see on MLB Gameday.

Thankfully, data for every pitch is freely available from It’s easier to access data from a source like FanGraphs or Brooks Baseball, where they have tools and graphs already set up, but if anyone wanted to work with the raw data, you can download the spreadsheets too. I used FanGraphs for this FanPost, but it’s really up to you. In either case, the data generated by the Pitchf/x system gives us the greatest tool we’ve ever had for analyzing a pitcher’s pitches. For example, here’s a movement graph from Justin Verlander’s start on 8/8.


Keep in mind that this isn’t a location graph. That is, the dots don’t correspond to where they crossed the plate. Rather, this is a movement graph. The vertical axis corresponds to vertical movement—from the graph it looks like his fastball (the green dots) averaged about seven or eight inches of backspin. In other words, his fastball crosses the plate roughly eight inches higher than it would have if it was thrown without spin. The horizontal axis is very similar, showing horizontal movement. In these graphs, all movement numbers are seen from the batter’s point of view, so Verlander’s fastball comes in to right-handed batters by approximately six inches.  It’s counterintuitive, but fastballs break just as much as curveballs do. The very best curveballs can break downwards by around ten inches (think Zito), but almost everyone throws a fastball that breaks upward anywhere from 6-11 inches. Upward vertical movement has to fight against gravity, however, so a hard fastball will fly far straighter than a spinless ball will, where downward breaking curveballs fall with gravity working for it.

The reason I used Verlander as an example is because his pitches are well-defined. He throws them repeatedly, with consistency. Take a look at this.


This is from Trevor Cahill’s 5/7 start, which, by all accounts, was a good one. He went seven innings, generated loads of groundballs, and only allowed one run. But yet, his movement chart is troubling. Unlike Verlander’s, Cahill’s pitches are not well-defined. Instead, the graph looks more like a smear of pitches, as opposed to tight bunches. And this is one of his good starts. When he’s bad…


Gio Gonzalez has, to put it lightly, control problems. He has trouble locating his pitches. But when he throws his fastball, the same pitch comes out every time. Cahill has trouble throwing the same pitch start after start. Here are all of Cahill's starts this year, from the beginning to 9/2. The date text is in green if he had a quality start, and it appears in red if he didn’t. (I changed the definition of "quality start" from 6.0 IP/3ER to 5.0 IP/3ER because he’s a rookie, and, well, he wouldn’t have too many quality starts if I left it as is.)


I see no consistency. I also don’t see much of a pattern. In an attempt to find one, I sorted his starts again, but instead of using earned runs as the deciding factor, I sorted them by the number of groundballs. He’s a sinkerballer, so I considered any start where he generated 50% groundballs or more as good. Anything under 50%, again, appears in red.


Now there’s some semblance of a pattern. I came to the rather obvious result that Cahill tends to generate more groundballs when his fastball has more sink. To illustrate the difference, I pooled all of his starts together, sorted by groundball rate.


In the starts where he generates less than 50% groundballs, his fastball has a nasty tendency to rise, losing it's characteristic sink. When his fastball is working, it tends to lose some of that positive vertical movement and causes batters to get poor contact. It looks like Cahill tends to overthrow his fastball at times, causing it to get more backspin than it should, which leads to the rise.

Cahill needs to get time in Sacramento to work on his consistency. If his fastball can become that consistent sinking force it needs be, he can succeed on the highest stage.

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