Big challenge ahead for Bears' Fangio

Vic Fangio, the Bears' new defensive coordinator, built a beast in San Francisco over the past four years using a 3-4 scheme, an alignment the Bears have never played.

The Bears always have been a 4-3 defense, utilizing four down linemen and three linebackers. That's expected to change under Fangio, although the transformation could be gradual.

The 3-4 utilizes one fewer lineman and one more linebacker. But the bigger difference between the two schemes is the kind of players needed to make the each system work - mostly the linemen and linebackers.

The problem for the Bears is that few of their players have the skills to fit ideally into a 3-4.

Critics would argue that most Bears defenders don't fit ideally into any scheme, and they would make a good point. In the past two seasons, the Bears have allowed the most points and the second-most points in franchise history.

That might actually make the transition easier because the defense can't get much worse.

At least in the beginning, the Bears might choose to slowly incorporate the 3-4 while still playing some 4-3, especially on passing downs.

That hybrid style has become more popular in recent years as very few teams play exclusively 3-4.

The front seven players in a 3-4 generally are bigger and stronger, especially the linemen, who must be able to defend two gaps instead of just one as 4-3 linemen usually are required to do.

The key players in any effective 3-4 alignment are the two outside linebackers and the nose tackle.

The 3-4 nose tackle usually lines up directly over the center and is responsible for the gaps on either side of him.

The New England Patriots' Vince Wilfork is the ideal nose tackle in a 3-4: a huge, wide-bodied player who can absorb double-team blocks, which frees up linebackers to run unblocked to the ball. The 6-foot-2 Wilfork is listed at 325 pounds but appears much closer to 375.

Players such as Wilfork and the Baltimore Ravens' 6-4, 340-pound Haloti Ngata, another prototypical nose, also can hold their ground and stuff run plays up the middle.

Unfortunately for the Bears, there are only a handful of people on the planet with Ngata's and Wilfork's combination of size and agility, and none of them are on the current roster.

The good news is that Fangio's 49ers defenses played an effective 3-4 without the benefit of a massive nose tackle.

The Bears' Stephen Paea (6-1, 300) would be an undersized nose tackle in a 3-4, as would 2014 third-round pick Will Sutton at an even 6 feet and 303 pounds. Ego Ferguson (6-2, 315), last year's second-round pick, seems to be a more natural fit.

Since all the linemen in traditional 3-4 defenses have two-gap responsibility, the ends, who line up over the offensive tackles, also usually are bigger than ends in a 4-3. The 49ers' 6-4, 285-pound Pro Bowler Justin Smith is perfectly suited to play end in a 3-4; he most likely would be a tackle in a 4-3.

Unlike a 4-3 end, who have primary pass-rush duties, a 3-4 end must be able to stuff the run in the gaps on either side of the offensive tackle.

The Bears' 6-3 Lamarr Houston played end in their 4-3 but has the size to transition to remain at end in a 3-4. He came to the Bears with the reputation of being an elite run-down player, although he didn't demonstrate that in the eight games he started in 2014 before suffering a season-ending knee injury.

Houston was listed last season at 300 pounds but played at about 270. He will need to be closer to 300 as a 3-4 end.

Jared Allen, the Bears' other opening-day starting defensive end, does not fit as an every-down 3-4 lineman and might not be able to transition to linebacker. He played at less than 270 pounds last year and would seem to fit best as a situational pass rusher.

Willie Young, probably the Bears' defensive MVP in 2014, is most certainly not a lineman in a 3-4, since he weighs 251. But the 6-foot-4 Young's skill set seems perfect for a 3-4 outside linebacker.

The outside linebackers in a 3-4 are pass rushers with the athleticism to drop into coverage while also having the strength to set the edge in run defense.

Young led the Bears with 10 sacks and was second among linemen with 55 tackles, even though he didn't become a starter until the ninth game of the season.

Christian Jones, a 6-3, 240-pound undrafted rookie last year, finished third on the Bears with 98 tackles. He also could project as an impact 3-4 outside linebacker because of his athleticism.

The 49ers' 3-4 was most effective when freak athlete Aldon Smith (6-4, 265) was on the field with his elite pass-rush skills and ability to play the run and also drop into coverage.

A 3-4 might be the best opportunity yet for the Bears' 2012 first-round pick Shea McClellin to make an impact. Rushing the passer and making plays in space are supposed to be his strongest attributes, and he also has the agility to drop into coverage.

The inside linebackers in a 3-4 are primarily run defenders, a role in which Lance Briggs has excelled in the past. But at 34 and with his performance and health declining, Briggs isn't expected back.

Overall, linebacker was a Bears weakness last season, and a 3-4 naturally requires more depth at the position.

That means the Bears must acquire more linebackers, while hoping that 2013 second-rounder Jon Bostic can take another couple of steps in what has been a slow progression. The 6-1, 245-pound Bostic projects inside in a 3-4 after playing mostly on the outside.

Under Fangio, almost everyone will have new responsibilities, and the 2015 Bears will be different on defense. But it could take some time before they're better.

• Jay Rodgers has joined the Bears as defensive line coach. He spent the last six seasons on the Denver Broncos' staff, including overseeing the defensive line over the last three years.

Over the last three seasons, the Broncos led the NFL in rushing defense (90.8 yards per game), allowing the fewest rushes of 10 or more yards (99), and were fourth in the league with 134 sacks.

• Follow Bob's Bears and NFL reports on Twitter@BobLeGere.