I swap pads so often for track/summer/winter that I don't have any correlation between pad wear and disc wear. At 50K miles, I knew my discs were getting worn, but the faces still looked good.
Obviously, the way to determine disc wear to is measure the thickness. I've been using my standard digital caliper, and last fall I measured 29.02 and 28.96 on my front discs. That was down from 29.4mm the year before. Definitely some wear, but well above the 28.4mm minimum thickness. Considering the new thickness of 30.1mm, I still had at least a third of the thickness left. Not great for track, but no real worries.
The problem is that I was only measuring the outer edge of the disc. (Not the wear ridge, but just inside that on the worn surface.) My standard caliper isn't really the right tool, and since I'm a tool nerd, I decided to buy a cheap digital disc caliper. For $40, I got a tool that isn't super accurate, but easily measures to 0.1 mm accuracy, and more importantly can read multiple places across the disc face. And the longer arms make it WAY easier to use the caliper and read the values.
So, measured the discs with my fancy new tool, and I got a 28.9+mm reading on the outer edge. About the same as my good caliper. Than I measured the inner edge of the wear face.
27.9mm. Uh-oh. I just went from having some comfortable meat on the discs, to being WAY under minimum spec.
Both discs were about the same, and definitely needed immediate replacement.
So, I thought I would post this here to remind everyone that the disc wear may not be even from the inside to outside edge. And that maybe a cheapish tool would be a good investment to learn how much wear your discs really have.
Got the new discs in the mail today. On the car, I've never really noticed how big these things are, but handling them as separate pieces, wow, they are MASSIVE. Way bigger and heavier than any other discs I've ever replaced on my family sedans/wagons. They are weighing in at about
22.8 lbs. And these are the original BMW aluminum hat models. Hate to see what the cheaper full iron discs weigh.