Just because the aftermarket application guide says it's the right fit …

[FindingForester]

The rotor on the right is an aftermarket rotor that I had installed on my wife's 2005 Forester a couple years ago. On the left is one of the original rotors, after resurfacing at a local shop.

 

 

You might be able to see in the photo that the aftermarket rotor has a raised area, about a half millimeter or so, just inside the braking surface. The original rotor is actually recessed slightly inside of the braking surface. That raised area apparently was just enough to cause a moaning, slightly grinding sound as the car came to a stop after a couple applications of the brakes.

 

This noise was mystifying. Some friends suggested that the rotors were glazed, so I tried roughing up the rotor surfaces a bit a couple times with sandpaper. The noise came back. I disassembled the brakes and made sure everything was in place, no bent clips or anything. Nothing seemed amiss. Added some anti-seize lube to the backs of the pads. The noise came back again anyway.

 

Finally in frustration I decided to replace the rotors with the originals. One of the original rotors had developed a significant warp (which is why I had decided originally to save time by replacing the rotors when I did the front brake job a couple years ago, instead of leaving the car jacked up for a few hours to have the rotors done) so I had them turned at a local shop to prepare them for reuse. When I compared the original and aftermarket rotors side by side, that raised area was a pretty obvious and visible difference.

 

The original, resurfaced rotors are back on the car. So far, no noise.

 

The lesson I learned is that just because an auto parts store lists an aftermarket product for a given year, make, and model, it might not actually be a good fit.

[Fairtax4me]

That slightly raised area shouldn't cause any issues unless it was actually making contact with the brake caliper or bracket. I don't see any markings or evidence of rubbing.

 

The quality of the steel used to make the rotors can make a big difference.

Just by looking at the rust on the aftermarket rotor compared to the Original, you can tell the aftermarket is of lower quality steel.

 

Low quality brake pads can cause noise as well.

[FindingForester]

 

unless it was actually making contact with the brake caliper or bracket.

 

Or more likely, with the pads.

 

Since the noise wouldn't appear until after the car had been driven at least a few minutes with several applications of the brakes, it was probably related to thermal expansion of the steel rotors (coefficient of about 0.000011 to 0.000013 per C°), which would increase—though slightly—the diameter of both the raised portion and the narrow ring between it and the braking surface. Perhaps enough to scape against the edge of the brake pads.

 

You can't really tell anything by the rust in this photo. The original rotors are cleaned up pretty thoroughly. They were very scaled, especially after sitting in my garage for a couple years. Both the original and aftermarket rotors develop visible rust markings on the braking surfaces even as the car sits overnight.

[1 Lucky Texan]

what brand is the aftermarket?

[brus brother]

FYI I recently replaced genuine Subaru pads with Autozone lifetime replacement pads. There is a marked difference between the two. Autozone pads have a much smaller surface aea contacting the rotor. While they are both the same width, Autozone is more than a 1/2 inch shorter in the length (along the arc).

I verified several different boxes of the pads to make sure it wasn't miss-packaged.

[NorthWet]

...

Just by looking at the rust on the aftermarket rotor compared to the Original, you can tell the aftermarket is of lower quality steel.

...

Present or absence of rust is not an indication of the quality of the metal used in these rotors.

Edited August 12, 2013 by NorthWet

[Fairtax4me]

Or more likely, with the pads.

 

Since the noise wouldn't appear until after the car had been driven at least a few minutes with several applications of the brakes, it was probably related to thermal expansion of the steel rotors (coefficient of about 0.000011 to 0.000013 per C°), which would increase—though slightly—the diameter of both the raised portion and the narrow ring between it and the braking surface. Perhaps enough to scape against the edge of the brake pads.

Then there should be evidence on the pads and rotors of something making metal to metal contact. I see a wear line that stops well short of the ridge in the center, and no grooves on the ridge to indicate that the pads were making contact with the rotor.

The noise was most likely caused by the material of the pads. As they get hot the material emits gasses that can cause the pad to lose contact with the rotor momentarily and "skip" along the surface. It can lead to squealing, or groaning similar to the brakes on a large truck (though on a much smaller scale since your brakes are less than 1/4 the size of those on a big rig).

Mix the right pad material with the right rotors and you can end up with some pretty interesting sounds.

 

You can't really tell anything by the rust in this photo. The original rotors are cleaned up pretty thoroughly. They were very scaled, especially after sitting in my garage for a couple years. Both the original and aftermarket rotors develop visible rust markings on the braking surfaces even as the car sits overnight.

Of course you can. You can tell by the color, by the scaling on the edges, and by the way the original rotors cleaned up. The braking surface is "self cleaning" so by itself it doesn't really tell you anything unless you mic it occasionally to determine average wear. But Id be willing to bet if you did, the aftermarket rotors would be wearing at a faster rate than the originals. The rust in the hub of the rotor, and in the vanes in between the discs tells all of the story.

[FindingForester]

what brand is the aftermarket?

I wish I could remember; it was a Japanese brand, and I ordered them online from either Kragen (now O'Reilly) or Pep Boys a couple months in advance of doing the brake job.

 

The pads are Raybestos.

Edited August 15, 2013 by FindingForester

[FindingForester]

 

The noise was most likely caused by the material of the pads. As they get hot the material emits gasses that can cause the pad to lose contact with the rotor momentarily and "skip" along the surface. It can lead to squealing, or groaning similar to the brakes on a large truck (though on a much smaller scale since your brakes are less than 1/4 the size of those on a big rig).
Mix the right pad material with the right rotors and you can end up with some pretty interesting sounds.

 

That seems to not be the case. With the same pads but with the original rotors back on the car, my wife reports that the noise is gone. I'd say it most likely has something to do with the aftermarket rotors having a different profile from the originals.

Of course you can. You can tell by the color, by the scaling on the edges, and by the way the original rotors cleaned up.

 

I would repeat that the original rotors were cleaned up quite substantially (I gave them a naval jelly and wire brush treatment before I took them to be turned on a lathe).