The reason why the Bears 46 Defense isn't ranked as high at least by a lot of NFL historians as lets say the Steel Curtain Pittsburgh Steelers of the 1970s or even the Ravens of the last decade or so, is because the Steelers and Ravens were great on defense for a decade and not just one season or a few seasons. After the the Bears lost to the Redskins at home in the 1986 Divisional Playoff, the dominance and fear of the Bears I wouldn't say was gone, but teams knew how to beat them now. Which is what I'm going to get into.
I have a friend on Facebook who I met on YouTube who actually who disagrees with this. But Don Shula and the Miami Dolphins figured out how to beat the Bears defense in 1985 and not just figured it out, but had the offensive personal to get it done. And the Dolphins were the only team that beat the Bears in the 1985 season. Most NFL teams believed the way you beat the Bears was establishing the run so they can't kill you with the blitz. Well there are a couple of problems with that. The Bears were excellent against the run. Second with always having seven guys if not eight in the box, it makes it very difficult to run against that formation especially with the size and strength the Bears had on defense.
The Dolphins approached the 46 differently. One for strategic reasons that they had a great QB in Dan Marino with a very quick and accurate release who got rid of the ball quickly. They also always had at least three very good wide receivers in the 1980s with Marino when you are talking about Mark Duper, Mark Clayton and Nat More. The other reason being practical for the Dolphins. Lets be real here, the Dolphins didn't have a running game other than maybe Tony Nathan. Who was better suited as a change up back and a wing back, hybrid between a halfback and wide receiver. Someone who catches a lot of screens and passes out of the backfield and runs draws.
So what the Dolphins did is what they did better than anyone else in the NFL back then except for maybe the San Francisco 49ers was throw the football and throw it a lot. And just didn't throw the football a lot, but threw it very well. They spread out the Bears and threw a lot quick short routes to the guys I've already mentioned and forced the Bears with all of their big men to play a lot of pass coverage. So their horses upfront didn't have the time to get to Marino who was getting rid of the ball on quick drops anyway. The Dolphins turned their Monday Night game against the Bears into a shootout. Thinking the Bears couldn't keep up on offense and they were right.
I'm not saying the Bears secondary was a weak link and their linebackers sure as hell weren't. At least one more of their linebackers should be in the Hall of Fame, Wilbur Marshal comes to mind real fast. But they didn't have that one strong cover corner like a Rod Woodson or a Mel Blount. And they didn't have lot of speed and range with their safety's. Which meant they played their coverages and their assignments, but when something broke down, or they had to cover one-on-one against a very good or great WR, their defense became exposed.
The Dolphins beat the Bears in 85 by spreading the 46 out and throwing a lot of quick passes. The Redskins beat them in the 86 and 87 defenses by going maximum protection and taking shots downfield against single coverage with WR's Art Monk and Gary Clark. The Bears linebackers were hitters and stuffers first, but could get exposed when it came to pass coverage because of a lack of running speed and the fact they didn't play a lot pass coverage. There job was to hit and to destroy, not cover.
Now the positive aspect of the 85 Bears. If I had to take one defense for one season as far as how dominant they were, I take the 85 Bears over everyone else. Because of how dominant they were not just in the regular season, but in the NFC Playoffs not giving a single point against two good teams in the LA Rams and New York Giants. I think the 86 Giants had better overall personal on defense especially in the secondary and that their linebackers could play pass coverage as well. But nobody was more dominant on defense for one season than the 85 Bears.