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Rotors changing color

Old 03-09-13, 01:41 AM
  #1  
jsdavis
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Rotors changing color

Some parts of my rotor are starting to turn into a copper color whereas the rotor is mostly silver color. I'm pretty sure these are not rust spots because they're not distinct dots, but rather large patches and they are in the area that gets scrubbed by the pads. It's more of a copper color like a new shiny penny rather than the darker rust color.

Are my rotors worn out? I've used up three sets of pads on the rotor but that's only about 5000 miles of commuting.
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Old 03-09-13, 02:20 AM
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Might be scorched from over heating. hard and long braking ?
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Old 03-09-13, 08:36 AM
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Probably from heat build up.

If you want to measure your rotors to see if they are worn (very unlikely with only commuting) you can take a set of calipers and measure rotor thickness, if it is 1.5mm they should be replaced, new they should be right at 2mm.
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Old 03-09-13, 09:13 AM
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Is it safe to continue using a scorched rotor? I have to descend about 500ft every day on the way home, average 7% grade but the steepest section is around 17%, and there are stop signs every 500ft or so.
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Old 03-09-13, 09:29 AM
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Originally Posted by jsdavis View Post
Is it safe to continue using a scorched rotor? I have to descend about 500ft every day on the way home, average 7% grade but the steepest section is around 17%, and there are stop signs every 500ft or so.
It shouldn't be a problem. It may even temper the metal and make it harder which will make the rotors last longer but squeal more.
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Old 03-09-13, 09:43 AM
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Originally Posted by cyccommute View Post
It may even temper the metal and make it harder.
I doubt it. Tempering actually reduces the hardness of hardened steel so that it is less brittle. If the rotors got too hot, they might be softened. To harden the rotors, you'd have to cool them suddenly, typically by plunging into water.
But I doubt the heating is enough to affect the temper.
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Old 03-09-13, 10:08 AM
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When I first saw this topic, I thought the answer was easy: over heating and/or rapid cooling by pouring water on the rotor while it's hot. But now that you say the rotor is turning a copper color... my only guess is that it's something from the pads maybe rubbing off onto the rotor. Try taking apart the caliper, take out the pads, and give them a good wipe down with an alcohol pad and see what color the towel is afterwards. Also clean the rotors with an alcohol pad as well. Maybe the color will go away after you wipe it but also check to see what the color of the towel is after that. Do this and let us know the results.

Josh
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Old 03-09-13, 10:19 AM
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Hi,

It could be heating to the bottom of the steel tempering range.
Bottom of the range (light straw) is just over 200 degrees C.

rgds, sreten.

Last edited by sreten; 03-09-13 at 11:52 AM.
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Old 03-09-13, 11:33 AM
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This is assuming that the rotors are made of stainless steel. They are commonly made of SS because of the anti-rusting properties, but can also be made of aluminum or titanium. The OP never said what disc brakes he has so it's not completely safe to say it's the heating of steel.

The picture posted by sreten above is very helpful. The copper color the OP is reporting is exactly what you see in the left side of the picture.

Josh

Last edited by jowilson; 03-09-13 at 11:44 AM.
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Old 03-09-13, 12:08 PM
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Originally Posted by jowilson View Post
This is assuming that the rotors are made of stainless steel. They are commonly made of SS because of the anti-rusting properties, but can also be made of aluminum or titanium. The OP never said what disc brakes he has so it's not completely safe to say it's the heating of steel.

The picture posted by sreten above is very helpful. The copper color the OP is reporting is exactly what you see in the left side of the picture.

Josh
I'm not sure there are any brake rotors currently available with an aluminium or titanium braking surface, although I believe there are some with a ceramic one and quite a few with aluminium structural elements.
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Old 03-09-13, 01:23 PM
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Titanium brake rotors are available for sale and are very light weight when compared to SS rotors, weighing in at just 88 grams. The Avid G2 Clean Sweep rotors, made of Stainless Steel, weigh 111 grams. A popular disc brake among bikers is the Avid X0's which are aluminum and those weigh in at 108 grams. There are many other alloys out there being used in rotors but those 3 are the most popular.

Josh
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Old 03-09-13, 01:28 PM
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Yes, and that very large 20% mass reduction acts on a very long moment!

Last edited by AnkleWork; 03-09-13 at 01:30 PM. Reason: typo
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Old 03-09-13, 02:26 PM
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Originally Posted by jowilson View Post
Titanium brake rotors are available for sale and are very light weight when compared to SS rotors, weighing in at just 88 grams. The Avid G2 Clean Sweep rotors, made of Stainless Steel, weigh 111 grams. A popular disc brake among bikers is the Avid X0's which are aluminum and those weigh in at 108 grams. There are many other alloys out there being used in rotors but those 3 are the most popular.

Josh
Ah, my knowledge of high-end MTB stuff seems to be a little out of date, my bad.
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Old 03-09-13, 05:35 PM
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Originally Posted by jowilson View Post
This is assuming that the rotors are made of stainless steel. They are commonly made of SS because of the anti-rusting properties, but can also be made of aluminum or titanium. The OP never said what disc brakes he has so it's not completely safe to say it's the heating of steel.

The picture posted by sreten above is very helpful. The copper color the OP is reporting is exactly what you see in the left side of the picture.

Josh
Titanium and aluminum rotors are pretty exotic. It's a pretty safe bet that the rotors are steel.
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Old 03-09-13, 06:20 PM
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The rotors are some sort of steel because there are some small rust spots where the pads don't scrub the rotors. The rotors are original Hayes rotors installed by Marin.
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Old 03-09-13, 06:36 PM
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Can you post a pic of the rotors?
Thanks.

Josh
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Old 03-09-13, 07:30 PM
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Originally Posted by jsdavis View Post
The rotors are some sort of steel because there are some small rust spots where the
pads don't scrub the rotors. The rotors are original Hayes rotors installed by Marin.
Hi,

Then everything is very probably hunky-dory.
The coloured oxide layer helps prevent rust.

rgds, sreten.
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Old 03-10-13, 10:46 AM
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Originally Posted by jsdavis View Post
The rotors are some sort of steel because there are some small rust spots where the pads don't scrub the rotors. The rotors are original Hayes rotors installed by Marin.
OEM rotors are very likely to be steel. Probably 302 stainless. Given that you use this for commuting, your rust spots are likely due to salt. 'Stainless' will rust if exposed to road salt, specifically the chloride ion in the salt. It probably won't cause too many problems but you may want to remove the salt from time to time before it gets too out of hand.
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Old 03-10-13, 02:10 PM
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IME, stainless steel turns various colors as it heats and will stay that way under various conditions.

i would ignore it.
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Old 03-10-13, 02:46 PM
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Originally Posted by storckm View Post
I doubt it. Tempering actually reduces the hardness of hardened steel so that it is less brittle. If the rotors got too hot, they might be softened. To harden the rotors, you'd have to cool them suddenly, typically by plunging into water.
But I doubt the heating is enough to affect the temper.
Sounds like there's some confusion here between tempering and annealing.

- Wil
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Old 03-10-13, 07:39 PM
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Originally Posted by cyccommute View Post
OEM rotors are very likely to be steel. Probably 302 stainless. Given that you use this for commuting, your rust spots are likely due to salt. 'Stainless' will rust if exposed to road salt, specifically the chloride ion in the salt. It probably won't cause too many problems but you may want to remove the salt from time to time before it gets too out of hand.
I'm in San Francisco, so no road salt here. If anything it's due to higher humidity. I don't know if salt air is a real thing or not but I'm about 1 mi from the Pacific Ocean so we get lots of fog too.

I'm well aware that stainless is just more resistant to rust rather than rust proof.

When I posted this I thought that the rotors might have multiple layers of different metals and I wore through whatever the top layer is. If it's just due to heating of the metal, then I'm not too concerned about it. I did not realize that I am heating the rotors to 200C/400F though
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Old 03-11-13, 06:40 AM
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Originally Posted by Wil Davis View Post
Sounds like there's some confusion here between tempering and annealing.

- Wil
I agree. Annealing is a process of controlled temperatures and usually long heat soaks. Tempering is a faster quench. Brake rotors are designed to dissipate heat quickly to avoid temperature related problems like brake fade. The temperature quench isn't as fast as many tempering processes but it's not as slow nor as controlled as a annealing process.

Originally Posted by jsdavis View Post
I'm in San Francisco, so no road salt here. If anything it's due to higher humidity. I don't know if salt air is a real thing or not but I'm about 1 mi from the Pacific Ocean so we get lots of fog too.

I'm well aware that stainless is just more resistant to rust rather than rust proof.

When I posted this I thought that the rotors might have multiple layers of different metals and I wore through whatever the top layer is. If it's just due to heating of the metal, then I'm not too concerned about it. I did not realize that I am heating the rotors to 200C/400F though
You have salt sources so you can corrode stainless. It's not as much as what is used with active road salt applications in snowy climates but it could cause some corrosion. You could probably remove the surface corrosion with a Scotch-brite pad.

Heavy braking can generated a lot of heat on a rotor. The rotor is smaller than a rim brake and isn't as large a heat sink as a rim is so it can heat to higher temperatures. You are correct to not be concerned about wear for now. Watch the thickness over time however.
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Old 03-11-13, 07:42 AM
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Originally Posted by jsdavis View Post
I did not realize that I am heating the rotors to 200C/400F though
Hi,

Apparently the colours are for a one off heating cycle. If you bake or
keep repeating the heating cycle, the colour will go up the heating
scale and no longer indicate the actual maximum temperature.

Hard to say what temperature your brakes are really reaching.

rgds, sreten.
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Old 03-11-13, 08:13 AM
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Your brake rotors can handle temperatures of up to 425 C and still work fine. Since you haven't reported rainbow coloring or brake fading, which would indicate possible rotor glazing - just ride.

A larger rotor will disipate heat more effectively, as will an aluminum core rotor or a rotor with an aluminum spider. Doesn't sound like you need any of those.
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Old 03-11-13, 12:11 PM
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Originally Posted by cyccommute View Post
Heavy braking can generated a lot of heat on a rotor. The rotor is smaller than a rim brake and isn't as large a heat sink as a rim is so it can heat to higher temperatures.
I'd like to add to this.

Say you're (your weight is 68kgs) riding along a road at a constant speed of 16 kph and you're trying to stay at that speed for training or whatever reason have you. Using your speed and weight, you can calculate your energy to about 69 joules. You start going down a hill and your speed starts to increase, but for training purposes, you must stay at a speed of 16 kph and so you start to use your brake. In this case, you have rim brakes on a 700c wheel. Convert all that energy into a 700c wheel and you don't raise the temperature a whole lot unless you are braking to not lock up the wheel and on a hill like the example above.

Imagine the same scenario but you have disc brakes. You are converting the same amount of energy to heat but on a smaller "wheel". The rotor is bound to heat up more than the rim because if you focus heat on a larger area, it will heat up slower than if you focus heat on a point. It's like cooking a fat steak with a grill versus cooking it with a soldering iron.

Josh
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