Ring of rust on rear rotors

[bimmer414]

Hi guys,

Nice weather in Wisconsin meant the M3 got to stretch it's legs today. Rotors were all rusty when I pulled it out as usual, but the rotors didn't clean up after 50+ miles of driving including several very hard stops. Anyone seen this before? Took off a wheel and pads are looking a little tapered.

[pbonsalb]

I’d keep driving it and going fast and braking hard.

[roastbeef]

mine are starting to do the same thing. i'm not really investigating further because i have a bbk that is going on.

[Saandiaago]

I've got similar on my BBK front rotors switching from track pads to street pads, albeit a smaller ring of rust after a car wash followed by attempted bedding in. As long as the car brakes well I would not be too concerned but clearly there is probably some diminished contact between the pad and rotor in that spot. Whether it be due to the rotor surface not being perfectly flat, the pad not being flat or both.

[roastbeef]

I have a hunch something is worn on my rear brakes that are doing the same thing... I don't know what exactly is wrong, but they recently started doing this after a pad swap.

[Spoolin1]

Anyone know why the rotors on most cars are just steel that will rust instead of a stainless that has less corrosive properties? I never understood this.

[roastbeef]

Originally Posted by Spoolin1

Anyone know why the rotors on most cars are just steel that will rust instead of a stainless that has less corrosive properties? I never understood this.

it is considered to be less brittle than stainless. by how much, or if it is significant in this application, i don't know. the small amount of rust is merely surface rust and doesn't do any harm. in the end, it all comes down to costs.

[Scharbag]

Originally Posted by Spoolin1

Anyone know why the rotors on most cars are just steel that will rust instead of a stainless that has less corrosive properties? I never understood this.

Cost is an issue as SS alloys are pricey to buy and expensive to machine. Plus SS is a difficult thing to talk about as there are 5 types: ferritic, austenitic, martensitic, duplex and precipitation hardened. Each type has specific advantages and disadvantages, including brittleness, hardness, malleability, thermal conductivity, chemical resistance and even magnetic properties. It is a really cool set of alloys.

Also, "stainless" is not always stainless. If we use 304 SS in our brine process, it will disappear due to a chemical reaction. Whereas 316L SS lasts pretty good. Duplex is the longest lasting, but it is crazy expensive and almost impossible to work with (our welders would revolt...).

Second reason is the thermal conductivity of SS. Mild steel conducts heat better than SS, and especially better than some SS types. Getting heat away from the pad surfaces is one of the most important jobs of the rotor.

Third is that the harder steels typically have a lower coefficient of friction. Not what you want for good brake performance.

Cheers,

[pbonsalb]

Some of the coated or treated rotors are good if you are ina rust area. I drive year round in NH and my rotor vanes usually fill up with rust before I wear out the rotor since I don’t track and drive two cars each about 7-8k per year. I don’t worry about the surface rust since that is merely cosmetic. And on my car it wears off evenly. Not sure what the issue is with the OP’s car.