What is bat ‘Exit Velocity’ all about?

When StatCast™ began publishing exit velocities on batted balls, the baseball world had a new stat to talk about. The baseball world loves ‘velo’ whether it is measuring a pitcher’s fastball velocity or a hitter’s velocity of how fast a ball leaves the bat.

Greg Rybarczyk of Hit Tracker Online had been measuring distance on home runs and angles and speeds even before they became a property of ESPN. His site defined Speed Off Bat  as the calculated speed of the baseball as it left the bat, in miles per hour (mph).

Some call it exit speed and some call it batted ball speed while others call it speed off the bat, and StatCast™ calls it Exit Velocity. That term is now the official stat category of MLB for the speed of a batted ball off of the bat. The final stats are in for 2016, and Ryan Zimmerman was 14th in the Majors with a 94.1 average Exit Velocity. The full list is here. Those numbers are just part of the puzzle as launch angle and direction and spin are all part of the results. In fact, rookie Ryan Schimpf who had a 92.5 average Exit Velocity for his 2016 season also had an average distance of more than 55 feet per batted ball than Zimmerman. That is part of the complications of just looking at Exit Velocity to determine if a batter is good as you have a myriad of factors such as what their contact % is and their K rate, and their groundball ratio.

The MLB average for 2016 which includes pitchers and even bunts was 89.57 MPH in exit velocity with an average distance of 218.08 feet, and Ryan Zimmerman’s average distance was 222.50 feet which was only 4 feet over the MLB average, and he did not have any bunts for 2016. Trea Turner on the other hand did have some bunts, and his average distance on batted balls was 228.26 feet with an average exit velocity of 90.92. Yes, bunts will skew the numbers, but Turner had a HR/PA of 4.0% while Zimmerman had a HR/PA of 3.2% in 2016. How can those differences occur?

There are several determining factors of physics and the energy created for Exit Velocity, and not all of it is created equally.

1. Bat speed is the number one factor
2. Pitch speed from the pitcher
3. Bat impact point given the part of the bat the ball has at a given connection point (ex. bat Sweetspot on the barrel)
4. Bat composition (ex. BESR rating of the bat which is Ball Exit Speed Ratio )
5. Ball composition (ex. COR rating which is coefficient of restitution which determines ball compression) and should be almost identical with each official MLB baseball
6. Ball impact point given spin on ball

Bat speed is very dependent on the path of the swing much like golfers have with their different clubs and position to the ball and contact point and angle. The longer the swing, the greater you should generate bat speed. This is why many power hitters have longer swings. The shorter the swing the more controlled the contact is supposed to be by theory. Bat speed is still enhanced by strength and the torque and inertia.

We spoke to bat manufacturer David Chandler of Chandler Bats who made many of the bats Bryce Harper and Trea Turner swung this year for home runs.

“Bat density and the sweetspot on the barrels of our bats along with the quality of the wood certainly makes a difference,” Chandler said. “We feel a player also has to have confidence in the bat they swing. When you look at true distance, a ball usually goes further when a bat stays intact. Broken bats can cut down on distance. There is certainly a science to bat making, and many of our players are involved in the process, and I enjoy personally talking to them about their bats.”

Baseballs go further generally with the backspin created and Fangraphs recorded the following quote from Alan Nathan, a professor of physics at the University of Illinois and the creator of The Physics of Baseball:

“The spin of a batted ball affects its trajectory. For example, when a ball is hit at a moderate launch angle typical of long fly balls, say 25 to 35 degrees, backspin keeps the ball in the air longer so it can carry farther and improve the chances for a home run. When a ball is hit at a low launch angle — typical of line drives — say 10 to 15 degrees, topspin makes the ball take a nosedive and reduces the chance that an outfielder can catch up with it before it hits the ground.”

Backspin could explain how Trea Turner’s home runs just seemingly kept going for greater distances. Here is a summary (below) from ESPN of home run data for Harper, Turner and Zimmerman. Note the JE initials in the table is what they consider ‘Just Enough’ to make it over the fence for a home run and L is ‘Lucky’ considering the factors.

What we learned is Trea Turner’s home runs on average traveled 6 feet further than Ryan Zimmerman’s home runs and Bryce Harper’s went 3 feet further on average than Turner’s. Maybe that data will surprise you and maybe it won’t.

The most curious part is that the Exit Velocity between Turner and Zimmerman was less than 1 mph on average for their home runs yet the distance on average was sizable when comparing just home runs.

Factors on distance could of course be impacted by wind, humidity, temperature, and altitude, but for teammates playing in the same conditions, back spin created has to be considered as a factor aiding true distance.

The conclusion here is that while Exit Velocity is measurable, it has to be taken into context, and launch angle and spin along with altitude and weather conditions and air movement are also major factors in distance.

Distance is just one part of the success of a player, but as mentioned, you then have to factor in bunts into the equation as that will skew the values.

Everything seems to go back to the original stats which is batting average and slugging and now we analyze the full slash line. Hitting the ball hard with a long swing does not guarantee success, and you can scroll down the list to see that. In golf, they say “Drive for show and putt for dough” and baseball has rewarded empty home runs for far too long. Columbia University professor Mark Broadie wrote a book Every Shot Counts which is about golf but could be about almost any sport. New age baseball is much more about every shot which is really every plate appearance and what the end result is over a large enough sample size. The slash line results over a full season is what matters whereby if you want to brag about velo, it better show up in the results category.