How does a caliper bracket just snap clean in half? 98 Crown Vic with bigger wheels

I had almost the exact same failure on a Grand Marquis years ago, same platform as yours. In my case the caliper bracket bolts had backed off, probly never torqued right at some shop years before. Once they loosened, the bracket could shift a few thousandths under braking load, and that metal-on-metal hammering eventually fatigued the casting until it let go. The grinding noise for a couple days beforehand matches that pattern too, because the loose piece starts kissing the rotor before it fully breaks.

I don't think it was just the bolts shifting and then slowly cracking the metal. Even if the bracket was holding perfectly tight, putting that much unsprung weight and torque on a casting that wasn't designed for that setup is asking for trouble. The forces generated by the weight of those new wheels and the aggressive braking puts a shear stress on the mounting points that can just cause a clean fracture instantly. I've seen those types of failures where the casting just isn't robust enought for the application, even without any shift. It's more of a sudden structural failure than a gradual fatigue from a few thousandths of an inch of movement.

I agree with the idea about bolts backing off, that is a huge factor. I had a similar issue on a prior Crown Vic, and even when the bolts looked tight, the casting itself had some pre-existing stress fractures from heavy use. The bracket finally gave way after a lot of hard driving, and it wasn't just the shift that broke it, the metal was just too brittle from years of heavy loading. The shifting of the caliper, though, really seems to be the catalyst here. Always check the torque spec before doing anything on those brackets.

Saw this happen once on a similar setup, and I think it might be more about the heat cycle than just the weight. When those large wheels spin up fast, they dump a huge amount of heat right into the caliper bracket. If the metal has any microscopic weakness, that rapid thermal expansion and contraction during braking can actually fatigue the casting and make it snap, even if the bolts are tight. It's not just the load, it's the temperture swings too.

I suspect you should consider the possibility that the failure wasn't purely tension or thermal, but rather a shear failure originating at the mounting points where the bracket meets the suspension. Even if the main body of the caliper bracket remained intact, excessive lateral load applied through the caliper assembly could have induced a critical stress concentration right at the attachment lug, causing a sudden fracture. Given the nature of the clean break and the gouges, it suggests a violent, sudden slippage rather than a slow fatigue, which makes an inadequate shear pin or mounting bolt the most likely initiator.

Update: pulled the other side apart today and sure enough one of the bracket bolts had backed off a bit, so the loose bolts theory checks out at least. Snugged everything back to spec but the broken bracket still needs replacing, no saving that casting. Question for the guy who mentioned stress fractures in old ones, did you reuse the stock brackets or swap to new when you fixed yours?