1. Deconstructing the Home Run Surge: A Physicist’s Approach
Alan M. Nathan, University of Illinois
@pobguy baseball.physics.illinois.edu
From Ben Lindbergh, The Ringer

2. Possible Reasons for Surge
Increased COR of baseball
Ball carries better
Batters alter swing

3. 1. Increased COR (“bounciness”)
Increased CORhigher exit speeds-> more HR
I discussed this topic in my 2016 Saberseminar presentation
See Lindbergh/MGL Ringer article, May 2017
I won’t further discuss here

4. 2. Better “carry” Physics 101 (vacuum!):
Fly ball distance completely determined by EV, LA
Real life: drag and lift…
Properties of air
Properties of ball
Ball carries better when travels farther for identical initial conditions
i.e., EV, LA, direction, spin, spin axis
lift
gravity
drag

5. Reasons for different carry
Atmospheric conditions changed
Temperature, altitude, wind, …
Can control for this w/covered stadium
Properties of ball changed
Drag or lift coefficients CD, CL
Size of ball A
Want to control the first to study the second

6. CD Variation from PITCHf/x (controlled for atmospheric effects)
Lots of variation of drag coefficient

7. MMP Experiment (Saberseminar 2015)
EV=96 mph
LA=280
80-ft spread!

8. Physics Interlude CD is largest on smooth ball (“laminar flow”)
CD is smaller on rough surface (“turbulant flow”)
Plumbing: corregated pipes improve flow
Golf: dimples reduce drag
Baseball: seams reduce drag
But….
But if seams too high, CD increases
Where is crossover?

9. Drag and Seam Height
EV=96 mph
LA=280
MLB
NCAA
MiLB

10. Rob Arthur’s Analysis
Use PITCHf/x or Trackman pitch-tracking data to extract average CD values
Look at correlation with HR/FB

12. My Approach Use Trackman batted-ball trajectories from TBA
atmospheric effects constant
Use 2016-b data to fix model for CD and CL
“training” data
5 parameters for dependence on spin, speed
With fixed model & given initial conditions, calculate trajectory for
2015-E, 2015-L, 2016-a, 2017-E
Compare calculated with actual landing point
Speed, angles, spin rate, spin axis

13. Fitting to Training Data
153 batted balls
EV>90 mph, LA=
rms=2 ft

14. The Results: Actual-Calculated
~ 5 ft

15. Some Comments ~5 ft There is considerable ball-to-ball variation in CD
Only makes sense to compare averages
Each data set has trajectories
Data shows ~5 ft increase from 2015-E to 2016, then constant
Estimate: 5 ft~15% more HR

16. Into the weeds…. CD = 0.02  (distance) ~8.5 ft  ~25% more HRs
optional
CD = 0.02 
(distance) ~8.5 ft  ~25% more HRs

17. Conclusion
There is evidence suggesting some (~15%) of increase in HR between 2015 and 2016 is due to reduced drag
Reminder: atmospheric effects held constant at TBA
Better carry must be due to properties of ball
Data are consistent with no change in drag from 2016 to 2017

18. Afterthought
Original motivation for this analysis was to build a new 3D Trajectory Calculator
Spreadsheet to calculate trajectories, given EV,LA,SA,spin rate,spin axis
Beta version is ready
baseball.physics.illinois.edu/TrajectoryCalculator-new-3D.xlsx
Feedback welcome

19. 3. Batters alter swing

20. wOBA vs. EV-LA (actually, wOBAcon)
Launch Angle
(deg)
Exit Speed
(mph)

21. Possible Hitting Strategies
To get 1B
hit hard and
To hit xBH
hit hard and
To minimize timing errors
swing “level”
Question:
How does batter adjust swing to optimize outcome?

22. Issues for swinging the bat (*things I will consider)
*Timing
Getting bat in right place at right time
Swing speed
High!
*Aim—where on bat impact occurs
Along axis of bat (“sweet spot”)
*Perpendicular to axis of bat (“offset”)
*Swing plane

23. Ball-Bat Collision Model (2D version)
Batter controls:
 = swing plane (attack angle)
E= offset (“aim”)
~Max EV when =CL
Physics Model: (,E)EV,LA

24. Swing Plane + Offset  EV+LAHR 1B

25. Max wOBA:
~240 swing plane
~1.1” offset

26. Ex 1: Kris Davis wOBAcon=0.488
200
~200 swing plane

27. Ex 2: Ryan Zimmerman wOBAcon=0.34670
~100 swing plane

28. Timing & Swing Plane Suppose swing mistimed by ~±3 ms, or ~±4”
“Level” swing: E does not change
240 swing:
E changes by ~ ±0.8” (!)
“level”
weak grounder
uppercut
optional
popup

29. Timing & Swing Plane
grounder
popup
±0.8”
optional

30. Summary LA for max EV related to swing plane
In general, for given max EV, wOBA increases as swing plane increases
I have not done a complete statistical analysis
Work in progress

31. And finally…. The beat goes on with the COR story
Some evidence for reduced drag starting in 2016
New Trajectory Calculator a by-product
EV-LA a potentially useful tool
My view:
The question of why the HR surge is still not fully answered