How Jake Arrieta became an ace

It's clear that the long rebuilding process the Cubs have been mired in is finally paying off, with the team recently reaching nine games over .500 for the first time since 2009. Unsurprisingly, most of the focus has been on the impressive crop of young Chicago hitters, with Kris Bryant, Addison Russell, Jorge Soler and Kyle Schwarber joining Anthony Rizzo and Starlin Castro.

But for all the names, the offense has only been just OK -- 19th in runs scored -- and it's actually been the unexpectedly good rotation which has kept the team afloat. The Cubs' starters rank sixth in baseball in ERA and seventh in Fielding Independent Pitching (FIP), and while you'd have expected improvement given the $155 million invested in Jon Lester, he actually hasn't been one of Chicago's top two starters so far. Instead, it's been a pair of former Orioles, Jake Arrieta and Jason Hammel, who have led the Cubs' rotation to success.

We've focused on Hammel's outstanding slider earlier this month, so let's use Statcast™ to delve into how Arrieta -- who faces John Lackey and the Cardinals on MLB Network's Showcase game on Friday night -- went from frustrating Baltimore project to dominating Chicago ace. In 63 starts as an Oriole, Arrieta had a 5.46 ERA and a 4.72 FIP. In 48 starts for the Cubs since coming over in 2013, those numbers are 2.89 and 2.92. This … this is better.

Video: Forecasting a player's worth and health with Statcast™

So what's changed? Arrieta's pitch mixture, for one. He has always thrown five different pitches, but as an Oriole, he was primarily fastball-sinker, which accounted for about 60 percent of his pitches. Upon moving to the Cubs, Arrieta de-emphasized his four-seam fastball (down from 35 percent in 2011 to just over 10 percent this year) in favor of his very effective slider, which he's used about 30 percent of the time over the last two years.

Considering that Arrieta got to the big leagues on the strength of that fastball, that's a big deal. Take a look at his 2009 scouting report right here on MLB.com, which immediately notes that Arrieta "goes right after hitters with a good fastball that touches 97 mph with good command." The problem with that in the Majors is that while he's got velocity, the heater has been generally unremarkable. Among the 336 pitchers to throw the four-seamer at least 100 times, Arrieta's spin rate of 2,284 rpm is 105th. His average velocity of 94.52 mph is 85th. It just doesn't stand out.

That's been proven by years of hitters pounding it around the park. In Arrieta's first four seasons in the Majors, not once did he have above-average rate results (per wRC+) on the fastball, and twice he was at least 50 percent below average on it. It's all well and good to be able to throw hard, but big league hitters feast on a heater that's not elite.

But what saved Arrieta is the improvement he made on his secondary pitches, which are all fascinating in their own way. He ranks highly on the spin rate and velocity leaderboards (minimum 100 pitches, except for the change, which is 50) on all of them:

Sinker (49 qualifiers) -- 2,243 rpm (6th), 94.65 mph (3rd)
Curve (75) -- 2,651 rpm (8th), 80.77 mph (18th)
Change (147) -- 2,128 rpm (4th), 87.42 mph (5th)
Slider (154) -- 2,176 rpm (47th), 90.29 mph (3rd)

Arrieta's slider is the only one without elite spin, but it's thrown hard, even harder than Chris Archer's famously deadly slider. What's interesting here, though, is that he's getting high spin rate on two pitches that you wouldn't expect -- the change and the sinker.

It's great to get high spin on a curveball, as Arrieta does, and getting high spin on a four-seam fastball generally bodes well for missing bats. (As we've noted, his fastball doesn't have high spin.) But for a pitch like the change, which doesn't rely on speed so much as it does movement, you'd expect a lower spin rate. Generally, pitches with higher spin can defy gravity slightly longer, letting them stay up; lower-spin pitches like the change and sinker get down, which is where you want them.

That's not what Arrieta's done, and he's not getting particularly outstanding swinging-strike rates on either pitch. But that might not be his plan. Arrieta's change and sinker come in harder and with higher spin than batters are used to seeing, and they're having trouble compensating, likely because it's a different look.

No, really: Only one starting pitcher has managed to induce softer exit velocity on the sinker than Arrieta's 85.54 mph. His change hasn't been put into play enough to make the leaderboards, but if it had, his 82.00 mph would rank in the top 15.

The fastball is still getting hit, because it's not a standout pitch. But now Arrieta's got two pitches with which he can get strikeouts -- the curve and slider both have double-digit strikeout percentages -- and two others on which he can induce weak contact. Arrieta, young flamethrower, is gone. Arrieta, multipitch weapon, is here. This ... this is so much better.