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In one of Paulster2's helpful answers, he mentions:

Usually when the brake pads get shallow, they start digging into the rotor and will cause all kinds of valleys and ridges. They may be large or small, but you'll definitely start seeing relief on the rotor surface (not always, but in a VAST majority of the cases).

Why is this?

Obviously, if brake pads get really, really low, the brake calipers will start to hit the disc rotors, causing extensive grooving. But I don't think that's what Paulster2 is talking about. I think he's talking about the actual brake pads themselves causing valleys and ridges (AKA grooves) when they get shallow.

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To answer your question, it isn't something which I have a definite "this is why it happens", but rather experience has shown me this is what happens when you get further into the brake pad.

I do have a guess as to why, well, maybe even a couple.

Theory 1: Over time, road debris and such contaminate the brake pads and make them have hard places in them, which causes wear in certain spots of the rotor.

Theory 2: As the pads get thinner and get closer to the metal part of the pad, the bonding agents or the pad itself forms hard spots on them due to heat curing or whatever, thus causing greater wear in certain spots.

It could be something else entirely, or possibly a combination of these two things as well. I really don't know for sure, but this is what my educated guess is giving me.

One other thing which will cause wear spots in the rotors are riveted pads. As the friction surface gets thinner and these come in contact with the rotor, since they are harder (or at least of a different hardness then the friction material), grooving will occur in the surface of the rotor.

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    I subscribe to Theory 1, road debris, grit, dirt, etc. But what that means is that the "shallow" or thin brake pads are not the cause of the grooves. It's strictly related to time or distance in-use of the brake discs since new or since the last resurfacing. One usually coincides with the other, but it's not cause-and-effect. Like if I notice that when my brake pads get thin, my tires need to be replaced. That doesn't mean that worn brake pads caused my tires to wear out.
    – MTA
    Oct 16, 2023 at 21:15
  • @MTA - I'd tend to agree with you. I think it is a buildup of "stuff" overtime and the associated wear the "stuff" causes at the point of incursion. Oct 16, 2023 at 21:35
  • Thanks for your ideas. Another possibility, and this is just a hypothesis, is that the manufacturer smooths out the outer surface of the pads real good, but under that visually smooth surface, the material is really too chunky (I'm not using quite the correct word... but imagine a smooth concrete surface with chunks of rocks under it, if you will). Oct 17, 2023 at 5:54

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