On sustainability of 49ers' defensive line

The San Francisco 49ers have the oldest projected starting defensive linemen in the NFL.

The group also happens to be one of the most dominant, but the age factor leads naturally to questions about the future.

Three-and-out Percentage

Vic Fangio, the 49ers' defensive coordinator, has been refreshingly informative in addressing such matters this offseason.

Fangio has been honest in setting defensive expectations overall, noting that some of the statistical success from 2011 owed to favorable field position, which he traced to few offensive turnovers and strong play by the special teams.

More recently, Fangio said he wasn't worried about older defensive linemen such as Justin Smith logging extended snaps, contingent on an interesting variable: opponents' drive length.

"To me, a lot of it depends upon what type of game it is, how you’re playing on defense -- if you’re getting a fair amount of three-and-outs, or you give up one first down and get off the field," Fangio said. "If the series last five or six plays or less, they can play a lot. If the series starts becoming an eight-, nine-, 10-play series, you’ll see more subbing than you saw last year."

The chart, courtesy of Marty Callinan at ESPN Stats & Information, shows the 49ers' defense ranking fifth last season in percentage of three-and-out drives. I've included rankings for the other NFC West teams as well.

About 37 percent of the 49ers' 2011 opponent drives lasted three or fewer plays. Another 38.9 percent lasted four to seven plays. That left 24.3 percent in the eight-and-longer danger zone Fangio referenced. We'll want to revisit these percentages as the 2012 season progresses to see how the 49ers adjust their rotation.

Smith, who turns 33 next month, played 91 percent of the 49ers' defensive snaps last season, up from about 75 percent the previous season.