Which contenders have built strongest bullpens?

We already know what this year's postseason is going to be about: Relievers. So, so many relievers. Over the past few years, we've seen how much the game changes in October -- with short series and extra days off -- allowing bullpen arms like Andrew Miller to exert outsized influence on a series. With the "opener" and "bullpenning" becoming more prominent than ever in 2018, you can expect to see more and more relievers in big spots than you could have ever imagined.
Teams know that, which is why most contenders have loaded up on as many short-relief arms as they can find. Now that the deadline for adding postseason-eligible players has passed, we can look ahead to ask an important question: Which team is best positioned in relief as we finish off the final month and get to the playoffs?
It's actually a more difficult question than you'd think. It's tempting to just look at season-long stats, but that's also a little misleading. For example, the A's had an unimpressive 4.22 relief ERA in April, and that counts towards their season mark. But how much should it matter now that Liam Hendriks (9.00 ERA in April) or Chris Hatcher (9.83) -- neither of whom is likely to pitch in the playoffs -- were ineffective way back when? Who cares that Wilmer Font (14.85 ERA for the A's) got lit up when he was still there? 
"Not much," is the answer, so we need to account for that. The way we'll do it is with some simple 5-4-3-2-1 weighting, where August (including the first few days of September, through Sunday) counts the most, then July, then June and so on. We'll look only at the 13 teams who still have playoff odds of at least 10 percent (sorry, Seattle), and we'll express team performance in wOBA, which is just like on-base percentage, except more credit is given for extra-base hits than treating all hits the same. (The Major League reliever average wOBA is .312.)
From 1-13, the best bullpens from contending teams are... 
1. Astros (.273 weighted on-base average)
It never feels like the Astros have an elite bullpen, in part due to the high-profile failures of Ken Giles, who's since been exiled to Toronto, but they do. Houston has baseball's lowest relief ERA, lowest relief wOBA, lowest relief walk rate and second-highest relief strikeout rate. (The Astros have also thrown baseball's fewest relief innings. Thanks, rotation!)

That's partially because no one has noticed how good Héctor Rondón, Joe Smith, Collin McHugh, Tony Sipp, Brad Peacock and Will Harris have been this year, but also because trade acquisition Ryan Pressly has been fantastic, striking out 19 without a walk allowed since coming to Houston. Oh, and there's more: Injured starter Lance McCullers Jr. will probably return as a reliever. You might remember how that worked out last October.
2. Athletics (.281)
Oakland is probably the biggest beneficiary of our recency weighting, because in April, its bullpen was the fourth worst in baseball. Now, we're looking at the A's as the second best. It's not hard to see why: the Opening Day roster included Hendricks, Hatcher, Santiago Casilla and Danny Coulombe. Now, they have Fernando Rodney, Jeurys Familia, Lou Trivino and Shawn Kelley. Oh, and Blake Treinen has a 0.91 ERA this year, and Ryan Buchter, Emilio Pagán and Yusmeiro Petit (all added last offseason) have all been good. This group is so deep and so talented. They're actually built better for October than for the regular season.
3. Yankees (.288)
Back in January, we suggested that the Yankees might have one of the best bullpens ever, and while that hasn't quite happened, they've still been very good, in part because they're incredibly deep. While Aroldis Chapman's status is uncertain due to a knee injury, Dellin Betances, David Robertson, Zach Britton and Chad Green are quite the quartet anyway, and Jonathan Holder (2.53 ERA) has added solid work as well. It would be nice to have the 2017 version of Tommy Kahnle back, though.

4. Dodgers (.290)
It probably feels stunning that the Dodgers are ranked this highly, because Kenley Jansen has had some high-profile meltdowns since returning from the disabled list, yet this collection of relatively unknown arms has had some strong performances. Did you know Caleb Ferguson has a 41/6 strikeout/walk ratio (and a 2.23 ERA) in 32 1/3 innings as a reliever? Did you know Ferguson was a Major League pitcher?
They've also received good work from Dylan Floro (1.59 ERA) and Pedro Báez (.175/.230/.316 since returning from injury in late July), plus they now have the benefit of starters Kenta Maeda and All-Star Ross Stripling, at least when he returns from a back injury later this week, and they just traded for Ryan Madson. But really, this all comes down to Jansen. They aren't the same without him.
5. Cubs (.291)
The Cubs are still hoping for closer Brandon Morrow to return, but in the meantime, they have received solid work from the likes of Steve Cishek (2.02 ERA), Carl Edwards Jr. (2.36), Pedro Strop (2.45) and Justin Wilson (2.90). Perhaps most surprising have been the performances of midseason additions Jorge De La Rosa, who joined the Cubs last month after the D-backs released him (0.82 ERA in 11 innings) and Jesse Chavez, acquired from the Rangers in July (1.05 ERA in 25 2/3 innings). They could use Morrow back, and they need to figure out what's wrong with Brandon Kintzler (8.44 ERA since being traded from the Nationals), but this is a good, solid group.

6. D-backs (.298)
Arizona has the National League's lowest bullpen ERA, but the D-backs suffer from our recency weighting, because they're headed in the wrong direction -- they had a 2.85 relief ERA in the first half, and carried a 4.09 mark into Sunday's games. That's in large part due to the recent struggles of Brad Boxberger (5.19 ERA from Aug. 1 entering Sunday) and Archie Bradley (5.84), as you may have noticed over the weekend, when Matt Kemp kept walking off wins. The unheralded trio of T.J. McFarland, Andrew Chafin and Yoshihisa Hirano have been fantastic, carrying a 1.92 ERA into Sunday, but they really need the "big two" to rebound.
7. Braves (.302)
Somewhat quietly, Atlanta has put together a solid bullpen, with trade acquisitions Brad Brach and Jonny Venters combining to allow only two earned runs in 27 1/3 innings since arriving. It's not just them, though; has anyone noticed that Dan Winkler and Jesse Biddle have combined for 127 whiffs in 111 2/3 innings, with a 2.34 ERA? Or that A.J. Minter has struck out more than a man per inning? This group doesn't have the name value of some other clubs, but it's been a productive collection of relief arms. It would help if closer Arodys Vizcaíno could make it back from shoulder trouble this month.

8. Red Sox (.304)
Ryan Brasier (1.50 ERA in 23 games) may be the best reliever you don't know, while Joe Kelly and Brandon Workman have quietly been very good in the second half, and Drew Pomeranz has been interesting since moving to relief. While it's easy to worry about Craig Kimbrel's career-high home run rate, he's still piling up strikeouts (18 in nine innings since Aug. 1), and the larger concern here may Matt Barnes. After a 2.36 ERA in the first half, he entered Sunday with a 6.46 second-half mark.
9. Cardinals (.313)
In July, St. Louis had a 5.98 bullpen ERA, fourth worst in the Majors. In August, the Cardinals had a 2.82 ERA, the fourth best in the Majors. That makes them a little difficult to evaluate, though big changes in personnel came in between; gone are Greg Holland, Tyler Lyons, Sam Tuivailala and Matt Bowman, while in came Tyler Webb, Tyson Ross, Chasen Shreve, Dominic Leone, Dakota Hudson and, yes, Carlos Martínez. (Plus Bud Norris and the flame-throwing Jordan Hicks, of course.) While we can't simply ignore the performance over the first few months, this version is probably better. It's definitely different.

10. Phillies (.319)
There's a good news/bad news situation happening here. The good news is that Héctor Neris has been unbelievable since returning from the Minors, striking out 20 of the 32 batters he's faced while walking only two. Pat Neshek is finally healthy, while Tommy Hunter and Adam Morgan have each been solid. The problem is that Seranthony Domínguez, Luis García and Víctor Arano have all been hit hard lately. That said, there's probably more talent here than this; some of their numbers are inflated thanks to some massive disasters, like the 24 runs they allowed to the Mets.
11. Brewers (.321)
This is surprising, right? Early in the year, the Brewers overcame a weak rotation thanks to Josh Hader leading a breakout bullpen, but things haven't been quite so rosy lately. Part of that is that closer Corey Knebel struggled so badly he found himself back in Triple-A, but even Hader hasn't quite been the same, dropping from a 50 percent strikeout rate in the first half to 33 percent in the second. (Forced into a third inning on Sunday, he allowed a game-tying home run to Anthony Rizzo.)
While Jeremy Jeffress remains solid, Matt Albers has an ERA north of 7, and Dan Jennings is having a bizarre year: In the second half, he's striking out fewer batters, walking more and allowing more homers ... but his ERA has gone down. That's unlikely to last.

12. Rockies (.324)
While Colorado's pitching has received some much-deserved credit thanks to starters like Kyle Freeland and Germán Márquez, the bullpen is still problematic, in part because Wade Davis had a 7.02 ERA since the All-Star break entering Sunday and Chris Rusin has more walks than strikeouts in the second half, along with a 7.14 ERA on the season. The good news is that Seunghwan Oh has been a nice addition, Bryan Shaw has improved, and Adam Ottavino can still dominate, though even he's walked over five per nine in the second half.
That .324 number is not park-adjusted, by the way, so if you want to mentally give this group a little extra credit, that would be fair; even so, it's not exactly controversial to say the Rockies' relievers have been a weakness, not a strength.
13. Indians (.332)
The bullpen has been a season-long issue for Cleveland, one the Indians had hoped they'd fixed by acquiring Brad Hand and Adam Cimber from the Padres. It hasn't helped. Hand has been fine, but Cimber has been a disaster (two strikeouts in 10 2/3 innings for the Tribe), Cody Allen has a 5.68 ERA since the start of July, Dan Otero had a 7.45 ERA in August, and now Miller is on the disabled list for the third time this year. The Indians' most reliable reliever might actually be 37-year-old Oliver Pérez, of all people. He was released by the Reds in the spring and opted out of a Minor League deal with the Yankees before landing in Cleveland in June. It's not what you want.