What are these dark spots on my front rotor?

[The motor guy]

Looks like this all the way around. Just replaced these rotors about 6 months ago. Pads were a out 50 percent when I replaced the rotors and I reused the pads. Not sure if that was a good idea or not.

 

[Hermann]

Could not even garner a guess. Maybe you have discovered "Dark Matter"

 

[The motor guy]

Looks like heat to me.

 

[Chris142]

They been real hot.

 

[Tifosi]

I think that is from the pad material being transferred to the rotor.

With most cars, you can replace the pads and just resurface the rotors but normally, not the other way around as in replacing the rotors and keep the old pads on.

 

[The motor guy]

I think that is from the pad material being transferred to the rotor.

With most cars, you can replace the pads and just resurface the rotors but normally, not the other way around as in replacing the rotors and keep the old pads on.

I wonder if the rotors werent even true straight out of the box. I've heard that a lot of times they're warped straight out of the box. They were good for a little while before I developed a shimmy when braking. Not all the time though

 

[The motor guy]

I think that is from the pad material being transferred to the rotor.

With most cars, you can replace the pads and just resurface the rotors but normally, not the other way around as in replacing the rotors and keep the old pads on.

The AI internet answer said improper bedding. I wonder if I can bed in new pads.

 

[shortyb]

The AI internet answer said improper bedding. I wonder if I can bed in new pads.

You can, but you have to cut all the old pad material off the rotor (skim cut) first and start with fresh metal.

Nine times out of ten, "shimmy", or judder is caused by uneven deposits of pad material due to an insufficient/incomplete transfer of pad material during bedding. Or the total lack of any bedding whatsoever. Best practice is to make sure you start with either a new rotor or fresh surface, and bed pads in accordingly as soon as possible.

 

[shortyb]

Looks like heat to me.

It is, sort of. Just not enough .

 

[Tifosi]

I wonder if the rotors werent even true straight out of the box. I've heard that a lot of times they're warped straight out of the box. They were good for a little while before I developed a shimmy when braking. Not all the time though

Brake rotors don't actually "warp", that's a misnomer (https://alconkits.com/blogs/news/the-real-truth-about-warped-brake-rotors). The vibration is caused by uneven deposit of brake material onto the rotor which leads to uneven contact between the pad and rotor when braking. I think your issue is because you changed the rotors and not the pads. I recommend that you purchase new pads, have the rotors resurfaced and start over. "Bedding" isn't really necessary, just drive normally and avoid hard braking for a few hundred miles and the bedding process will occur naturally. "Bedding" procedure just speeds up this process but not necessary.

 

[shortyb]

Brake rotors don't actually "warp", that's a misnomer (https://alconkits.com/blogs/news/the-real-truth-about-warped-brake-rotors). The vibration is caused by uneven deposit of brake material onto the rotor which leads to uneven contact between the pad and rotor when braking. I think your issue is because you changed the rotors and not the pads. I recommend that you purchase new pads, have the rotors resurfaced and start over. "Bedding" isn't really necessary, just drive normally and avoid hard braking for a few hundred miles and the bedding process will occur naturally. "Bedding" procedure just speeds up this process but not necessary.

Sort of. Pad/rotor mating will occur without any hard braking after a few hundred miles of normal driving. However, for pad transfer to occur, both surfaces need heat. The kind that happens during proper bedding. If bedding transfer doesn't happen, the first time you have to brake hard will cause that heat. If the heat does not last (as in incidental hard braking) the pad material won't transfer evenly. This causes the deposits to create "hot spots" where the pad will ride the high spots of uneven material deposits. The increased friction in these areas causes a structural change in the iron material called cementite, which is very hard and no longer porous. At that point, the rotors become doorstops.

 

[The motor guy]

Sort of. Pad/rotor mating will occur without any hard braking after a few hundred miles of normal driving. However, for pad transfer to occur, both surfaces need heat. The kind that happens during proper bedding. If bedding transfer doesn't happen, the first time you have to brake hard will cause that heat. If the heat does not last (as in incidental hard braking) the pad material won't transfer evenly. This causes the deposits to create "hot spots" where the pad will ride the high spots of uneven material deposits. The increased friction in these areas causes a structural change in the iron material called cementite, which is very hard and no longer porous. At that point, the rotors become doorstops.

I always thought you were supposed to take like 3 or 4 good hard stops to bed the brakes. This whole brake easy for 200 miles is news to me.

 

[shortyb]

I always thought you were supposed to take like 3 or 4 good hard stops to bed the brakes. This whole brake easy for 200 miles is news to me.

Yes, you should generally follow the guide that @Tifosi posted in his link soon after you change pads/rotors. What I was saying is absent of proper bedding, the pads and rotors will "mate", meaning that the rotor and pads conform to each others imperfections after several hundred miles of normal braking. This is NOT the same as bedding, where you need the heat of repeated stops to create the transfer of pad material. This results in the properties of preferred adherent friction over more abrasive friction. Like surfaces (pad material) create and break bonds several times over during braking which is frictionally stronger than merely a pad rubbing against a rotor to scrub speed.