Lake Forest, IL (SportsNetwork.com) - Former San Francisco 49ers defensive coordinator Vic Fangio is joining the Chicago Bears in the same role, the team announced Tuesday.

Fangio, the architect of one of the NFL's premier defenses during a four-year run in San Francisco, bolted the 49ers after being passed over in favor of defensive line coach Jim Tomsula to become Jim Harbaugh's successor as the team's head coach.

The longtime NFL assistant instead joins the staff of newly named Bears head coach John Fox, where Fangio will be in charge of rebuilding a defense that was one of the league's most porous units

Chicago has ranked 30th overall in total defense in each of the past two seasons, while the 442 points the Bears allowed in 2014 were surpassed only by Oakland for the most in the league. Chicago finished 30th in scoring defense the previous year.

In contrast, the 49ers ranked in the top five in total defense in each of Fangio's four years at the helm, and were one of only three teams to record 25 or more takeaways in each of the last three seasons. San Francisco also finished second, second and third in scoring defense from 2011-13 before ending 10th in that category this past year.

Those numbers helped the Niners reach the NFC Championship Game in three straight seasons from 2011-13, as well as an appearance in Super Bowl XLVII.

The 56-year-old Fangio also served as the defensive coordinator for Indianapolis (1999-2001) and Houston (2002-05) and will entering his 29th season as an NFL assistant. Additionally, he was the defensive coordinator under Harbaugh at Stanford in 2010, helping the Cardinal to a 12-1 record and a final ranking of fourth in the Associated Press Top 25.