Why would shallow brake pads cause valleys & ridges (grooving) in disc brake rotors?

Question

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.

Answer 1

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.