Is Justin Wilson’s Increased Ground Ball Rate Actually Good?

According to Fangraphs, last year, Justin Wilson increased his ground ball rate from 35.1% to 51%. Among pitchers with at least 50 innings in 2018 and 30 innings in 2019, his 15.9% increase in groundball rate is tied with Amir Garrett for the greatest increase in ground ball rate for pitchers from 2018 to 2019. Groundballs are usually good for pitchers and Justin Wilson’s lower ERA suggests that this change made him better however, his higher FIP and SIERA suggest otherwise. 

Wilson’s increased ground ball rate is partly caused by an increase in cutter usage. According to Baseball Savant, in 2019, Wilson had a 59.2 GB% on cutters and a 41.9 GB% on four-seam fastballs. He increased his cutter usage from 15.1% in 2018 to 39.2% in 2019 and consequently his four-seam fastball usage decreased from 75.2% in 2018 to 52.5% in 2019. His cutter also did a much better job limiting hard contact in 2019. 

From 2016 to 2018, the average exit velocity on Wilson’s cutter has been around 85 mph, however in 2019 that dropped to 81.8 mph. Hitters’ xwOBA on the pitch decreased from .360 to .258. Wilson lost one mph on his cutter and it became more of a horizontal pitch. Wilson’s cutter, which had good rise in 2018, dropped 2.6 inches more in 2019 and broke 1.1 more inches horizontally. His cutter already had good horizontal movement in 2018 however, in 2019 it had 5.3 inches of horizontal break, 3.4 inches more than the average pitch at that velocity. 

Wilson did this by changing the spin axis on his cutter. Spin axis measures how the ball is spinning by measuring where it is spinning from. It uses a 360-degree scale where 0 is a pure topspin curve ball and 180 is pure backspin. A pure horizontal “frisbee” slider from a lefty will be at 270. Wilson’s slider is around 290 giving it vertical movement as well as horizontal movement. Most lefty four-seamers are less than 180 which gives four-seamers the arm side run that they usually have. Wilson’s was 167 last year giving it 3.2 inches of arm-side run. Wilson changed his cutter’s spin axis from 198.29 in 2018 to 208.34 in 2019 giving it more horizontal movement but essentially eliminating its rise.

Surprisingly, the pitch performed better this way. Vertical movement or rise is usually better than horizontal movement because vertical movement generates more whiffs. However, Wilson’s cutter got more whiffs and lower exit velocity in increased usage during 2019. The reason for this is where he was throwing it. Wilson throws his cutter low and in, to a righty, a location that didn’t make use of the old cutter’s rise. If he had thrown it up in the zone more he probably would have gotten more whiffs on the pitch and better utilized its rise. Given where Wilson throws his cutter the new horizontal cutter performs better. Wilson’s new cutter also plays off of his four-seamer better because they break in opposite directions horizontally.

Now that we know how Wilson increased his ground ball rate we can look at the results. The table below shows what changed between Wilson’s 2018 and 2019 season

Season	Team	G	IP	K/9	BB/9	HR/9	BABIP	LOB%	GB%	HR/FB	ERA	FIP	xFIP	SIERRA	pCRA	DRA	xwOBA
2018	Cubs	71	54.2	11.36	5.43	0.82	0.31	0.789	0.351	0.094	3.46	3.64	4.05	3.85	4.16	4.65	0.316
2019	Mets	45	39	10.15	4.38	0.92	0.299	0.868	0.51	0.148	2.54	3.91	3.95	4.06	4.18	3.51	0.281
Total	- - -	457	409.2	9.91	4.06	0.68	0.293	0.773	0.469	0.089	3.25	3.36	3.71	3.52		3.88	0.288

Wilson’s ERA outperformed his FIP due to an 86.8 LOB% which will likely regress and bring up his ERA. He got less strikeouts as would be expected but he also got fewe walks which evened it out. So if his K%-BB% didn’t really change and his K/BB increased then he should be better because of his increased ground ball rate. However, it isn’t that simple and the weirdest part of Wilson’s 2019 was his 0.92 HR/9, .1 more than 2018. This is because of a 5.4% increase in HR/FB rate. We could chalk that all up to luck and decide that it will come down however this isn’t accurate. Wilson’s infield fly ball rate was at 0% last year which likely caused the increase in HR/FB rate. Baseball Savant which calculates pop up rate using launch angle instead of gameday data has Justin Wilson at a 3% pop up rate which is less than previous years. Here is a graph of Justin Wilson’s pop up rate.

Wilson probably should have had more than zero infield flies, but his pop up rate was still very low.  Since his cutter typically produces a very low pop up rate it won’t go away unless he changes his usage back to where it was in 2018. Last year, Justin Wilson’s homerun the fly ball rate was around league average and although the low infield fly ball rate would lead us to believe it should be higher but that isn’t exactly true. For most of his career, his HR/FB rate has been lower than league average. 

One of the ways we can evaluate how Wilson should have done is using HR/(oFB+LD)%. HR/(oFB+LD)% is an adjusted HR/FB rate developed by Alex Chamberlain and previously used on this site in “Brad Wieck: The Cubs’s True Southpaw Solution.” This will help us understand if Wilson was luckier or unluckier while accounting for his low infield fly ball rate.

Season	Name	Team	HR	LD	FB	IFFB%	HR/(oFB+LD)%	League Average HR/(oFB+LD)%
2016	Justin Wilson	Tigers	6	24	49	12.20%	9%	9%
2017	Justin Wilson	- - -	5	24	58	10.30%	7%	9%
2018	Justin Wilson	Cubs	5	32	53	11.30%	6%	8%
2019	Justin Wilson	Mets	4	21	27	0.00%	8%	10%

Considering the increase in home run rate around the league, Wilson’s 2019 HR/FB rate seems in line with his career numbers. His HR/(oFB+LD)% was 2% lower than the league average just like 2017, 2018, and 2019. Since Wilson gave up fewer line drives and outfield flyballs in 2019 and was able to give up fewer home runs on those balls than league average, his lower groundball rate actually does make him a better pitcher.

Justin Wilson’s change wasn’t as good as his ERA would suggest, but it wasn’t bad. His old cutter had more potential if he used it up in the zone, however for where he pitches this new cutter is better. The new arsenal gave him a lower barrel rate, exit velocity, and xwOBA and was an overall improvement. One of the things to follow early in the season will be which cutter Wilson uses and how often he uses it.