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49ers ranked fifth in Net Gen Stats offensive line metric ‘Expected rushing yards per carry’

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The 49ers were eighth in the NFL in adjusted line yards last year

Atlanta Falcons v San Francisco 49ers Photo by Michael Zagaris/San Francisco 49ers/Getty Images

Next Gen Stats has been rolling out a few new advanced metrics tis week. Their most recent metric tries to capture offensive line performance. They call the metric, “expected rushing yards per carry, or xYPC.”

Here is NFL.com explaining their metric:

Offensive line play is just one factor that determines xYPC, but ranking the top 10 offensive lines from 2019 based on xYPC, as we’ve done below, gives us an idea of how effective these units were at creating opportunities for running backs on a per-play basis. What will be interesting is the juxtaposition of rushing yards over expectation (RYOE) alongside xYPC, which can tell us a lot about whose rushing struggles were more the fault of the offensive line and whose were more the fault of the team’s running backs.

Over in Philadelphia, meanwhile, Miles Sanders delighted Eagles fans with his 818 rushing yards on 179 carries (4.6 YPC). Those numbers obviously look better than Bell’s (789 rushing yards on 245 carries, with 3.2 yards per carry), but we can also see that Sanders was working with a personal xYPC that was a full yard higher (4.7) than Bell’s with the Jets — and, in fact, Sanders finished with a YPC that was actually below expectation (-0.16 RYOE per attempt).

Other factors, including offensive scheme and situation, can impact xYPC, as mentioned above. Defenders likely keyed on Bell while knowing they wouldn’t have to worry too much about someone else beating them. Sanders, as a rookie, was focused on much less (at least early) and ran behind a significantly better offensive line. Even so, we can use xYPC to illuminate how a bad line can make a good running back look horrible, while a good line will help a rookie get out to a fast start.

How will the San Francisco 49ers offensive line differ in this metric compared to Football Outsiders “adjusted line yards,” which attempts to capture how many yards the offensive line created for the running back. The 49ers were eighth in the NFL in adjusted line yards last year. In xYPC, the Niners were fifth:

2019 stats: 4.42 xYPC, 1,878 ERY, 167 RYOE, +0.39 RYOE per attempt

I was a bit surprised to see the 49ers weren’t rated higher, considering their effectiveness on the ground. Kyle Shanahan’s scheme, his use of fullback Kyle Juszczyk and the blocking effectiveness of guys like tight end George Kittle, tackle Joe Staley (enjoy retirement!) and the rest of the bunch created a monster on the ground in 2019. No team rushed for more yards strictly with running backs (2,045) than the 49ers. The 4.42 xYPC puts the Niners in the top five of the league, which is very surprising, considering defenses loaded the box against them (eight-plus defenders) at the highest rate in the NFL (34.4 percent). Their positive RYOE per attempt hammers home what we already knew: The 49ers ran to the Super Bowl with both an effective offensive line and a stable of running backs with excellent vision and burst. That combination was deadly against everyone but the team displaying the Lombardi Trophy in Kansas City.

San Francisco faced the highest men in the box in 2019 in the NFL. Only 12% of the 49ers runs came against a “lightbox,” which is considered six or fewer players. That’s the lowest rate in the NFL, while 41%, yes, 41%, came against a heavy box against eight or more. That’s a staggering number, but when you run your offense through a fullback, defenses are going to load the box.

San Francisco had plenty of success against “lightboxes,” where they averaged 6.3 yards per carry. Even with seven men in the box, the 49ers averaged 5.2 yards per carry, per Football Outsiders Almanac. Whether it’s on Kyle or Jimmy for not changing the plays, it’s running into an eight-man box or more that doomed the 49ers last year. Their DVOA went from 20.6% against lightboxes to -16.4% with eight or more.

Next-Gen Stats is throwing a lot of numbers at us, and there wasn’t a great description in the article of what all the metric tries to capture. The Cardinals are ranked ahead of the 49ers, but their adjusted line yards were significantly worse, and Arizona was ranked 24th. I’m all for rolling out new stats, and there’s no better time to do so than the offseason, but we need to be able to understand them on the most basic level. Give us a definition that way we can interrogate down the line. Neither can happen without an explanation, though.