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My daughter's car's front disk brakes grind loudly, only when stopping. The noise is a low rumble, not a squeal. Roadside visual inspection through the spokes of the wheel showed a smooth, mirror-like non-grooved rotor on both passenger and driver sides.

After sleeping on it, I now wonder if perhaps a pebble is lodged in the kerf of one of the inboard pads, grinding at the inboard surface of the rotor. I'll check it this weekend.

But, I'm wondering how serious that could be. Under what circumstances could we just pry the pebble out, reassemble, and drive on? And at what point would we be forced to replace the rotor? They are slotted & drilled, so I'm not sure resurfacing is an option. (They are less than 2 years old.)

If a rotor has been grooved by a rock stuck in the pad, then the pad will have a complementary groove that mates the rotor perfectly. As long as the rotor isn't gouged so deeply that it is out-of-spec, then I'm hoping the answer is that we can save the expense. (Although, safety is the most important thing.)

UPDATE: no rock grooves... both rotors look brand new on both sides. Still hunting down the source of my noise, however I still think this question has value for future visitors (if someone cares to answer it for the general case.)

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    Difficult to answer hypothetical scenarios. Without actual details, we can only speculate, and that is not a good idea when it comes to brakes. We would be remiss in giving a false sense of security that it would be OK to drive on. When you get it apart, post some pictures and maybe we can give you sound advice. – CharlieRB Jun 14 '17 at 18:37
  • I'm not going to make a real-world decision based on the answer to a hypothetical. However, this hypothetical isn't uncommon, and a general answer like "never accept a rock groove deeper than the resurfacing limit of the rotor, or more than 2mm wide" would be valuable to others, even without a photo of a specific instance (which would only be valuable to me.) – Ryan V. Bissell Jun 14 '17 at 20:37
  • That said: if this does turn out to be a rock groove, I'll add a photo. – Ryan V. Bissell Jun 14 '17 at 20:43
  • So this is a theoretical question? – Moab Jun 18 '17 at 2:47
  • @Moab I would say hypothetical, not theoretical. At the time I asked it I didn't know if there was a rock groove or not, and now I know there isn't. However, while I was suspecting a rock groove, I decided to change m.se.com, and didn't see this question asked yet. It seemed valuable to ask, regardless if it ended up helping me or not. – Ryan V. Bissell Jun 19 '17 at 4:05
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I suspect that one of the inboard pads is worn down to the point that it is metal to metal against the rotor. Please stop driving the vehicle until the source of the noise is identified and repaired.

  • I cant rule that out yet-- rotor was too hot for me to feel the backside with my finger-- and I'll certainly check when I visually inspect this weekend. But, your answer doesn't really address the actual question,which is what is the industry standard for assessing rock grooves on rotors. I think people are spooked because I framed my question personally rather than purely a hypothetical curiosity question. I'm not going to make a real-world decision based on the answer to a hypothetical. – Ryan V. Bissell Jun 14 '17 at 20:41
  • Both rotors are beautiful, in-board and out. Pads might be 70% worn, though. Indicators are not yet rubbing on the rotors. I'll replace the pads, but (1) still hunting down the cause of the noise, and (2) my hypothetical question still stands. – Ryan V. Bissell Jun 19 '17 at 4:09

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