why are my brakes scorched up? picture inside

[another Todd]

I agree with the engine compression braking to help slow or maintain speed on a downhill run.

I disagree with pulsing the brakes on and off as a better method than slight constant pressure to maintaqin reasonable speed. I am a 30 year Fire Department Veteran, back in the day brake fade was a huge problem with our fire engines and many exhaustive studies were done on the subject. Pulsing or stabbing the brakes causes a very rapid rise in temperature over a short amount of time and will overheat brakes during the braking period, whereas slight pressure absorbs the same amount of energy (to keep the same speed) over a longer period of time which allows more time for the brakes shed heat as they are generating the heat. If you absorb x amount of energy over a two minute period or the same x amount of energy over a 1 minute period (on and off equal times)your brakes may become overheated either way, but the rapid rise of the pulsing or stabbing method reaches higher peak temperatures.

 

[yonyon]

It seems to me that with extended light braking the surface of the pads will reach temperatures much higher than the average temperature of the rotors' surfaces. The rotors have a built in sink and have air cooling them constantly. The pads have proper cooling only when the brake is fully released. With brakes applied even slightly the pads can't give up much of any heat except to the already-hot rotors.

The same amount of heat is produced whether the braking is steady or pulsed, but with pulsed braking the pads have some chance to cool directly to air and won't reach as high of temperatures. An extremely hot pad will cause the surface of the rotor to reach nearly the same temperature for a moment as it passes under the pad causing discoloration and even checking. The highest temperature should be lower with the pulsing (provided that you get completely off the brake) and this problem should be easier to avoid.

Of course, with engine braking the tremendously larger surface of the radiator and additional airflow through the engine will help dissipate some of the heat....

 

[Texan4Life]

Originally Posted By: garlicbreadman
i checked the brakes this morning and those 'cracks' are actually brake dust marks.

also some of the blue discoloration wore off after some light driving yesterday and the brakes feel fine with no pulsation. im going to drive 200 miles home today with about 70 miles of hills, so ill see how the brakes work in heavier duty driving


Thats good to hear.. personally I wouldn't worry. If you feel that breaking performance isn't as good as before, you could sand the pads some (they are probably glazed) or better yet get a new set of semi-metallics.

my .02

 

[another Todd]

Let me clarify one thing in my post above, the testing I referred to was done on drum brakes not disc, and the issue there was the brake drums "growing" in diameter due to high heat, and having the result be that the shoes could no longer apply sufficient pressure to have effective braking. Overheated brakes acted similar to out of adjustment brakes. Disc brakes are different indeed, I think the higher heat build up with pulsing the brakes would still be relevant (same energy absorbed in a shorter amount of time), but you (yonyon) have interesting if not compelling thoughts on brake pad temperature and their cooling mechanism.

 

[GMBoy]

If you change pads, no need to go semi-metallic. They would NOT have made any difference. The ceramic pads are the way to go.

 

[Kestas]

Originally Posted By: Astro14
The blue discoloration is from the metal overheating. Wearing off the color doesn't change the fact those rotors were overheated and need to be replaced.

I disagree that these rotors need to be replaced. Why would (over)heating the rotors ruin them? (I'm looking for a technical explanation).

 

[GMBoy]

Originally Posted By: Kestas
Originally Posted By: Astro14
The blue discoloration is from the metal overheating. Wearing off the color doesn't change the fact those rotors were overheated and need to be replaced.

I disagree that these rotors need to be replaced. Why would (over)heating the rotors ruin them? (I'm looking for a technical explanation).


It has something to do with the metalurgy (spelled wrong I know)....the steel is now weaker. This is what I was once told by a brake systems engineer. How weak and if it really matters under regular use is unknown to me....but they are probably warped by now.

 

[SteveSRT8]

Sorry but rotors are NOT made of steel. The difference is critical.

No damage from turning solid red in operation as long as the pad doesn't weld into the surface.

Happens at the track all the time.

 

[garlicbreadman]

drove home yesterday and the brakes performed fine. There was no pulsation at any speed and the blue discoloration is all gone. Brakes are grabbing hard, so it seems to be fine. If i have time, i'll take off the pads and inspect it for any signs of glazing.

 

[another Todd]

You should flush your brake fluid too, as I suspect that "mushy" feel you had was from boiled moisture in the fluid.

 

[GMBoy]

Originally Posted By: SteveSRT8
Sorry but rotors are NOT made of steel. The difference is critical.

No damage from turning solid red in operation as long as the pad doesn't weld into the surface.

Happens at the track all the time.


Yeah, I knew I was wrong there but the time to edit had expired. You bring up valid points, too.

 

[Gotch]

Originally Posted By: GMBoy
Originally Posted By: Kestas
Originally Posted By: Astro14
The blue discoloration is from the metal overheating. Wearing off the color doesn't change the fact those rotors were overheated and need to be replaced.

I disagree that these rotors need to be replaced. Why would (over)heating the rotors ruin them? (I'm looking for a technical explanation).

It has something to do with the metalurgy (spelled wrong I know)....the steel is now weaker. This is what I was once told by a brake systems engineer. How weak and if it really matters under regular use is unknown to me....but they are probably warped by now.

...they (rotors) are not weaker... They may have surface hardening which makes the pads harder to bed. This is what can cause a pulsing brake pedal, they are not "warped".

I'd try a re-bed procedure before just replacing everything. Might be ok, at least you'll get a couple of more miles out of them.

...and yes, they overheated...

 

[PandaBear]

First of all, is this only on one side or both of the front rotor?

I've done something silly in my car before and install the caliper back with the brake line twisted, and smoke comes out very soon when I overheat the pad/rotor. Usually you see this very soon after the install.

I've also had bad pad/rotor that isn't making good contact and get overheated before. If it is only one side of one rotor that has discolored, it is likely a stuck caliper.

If both side have this discoloration, it is likely the rotor made with the wrong material or process.

Why are you using low end pads like Monroe Ceramic with a slotted rotor? I hope your slotted rotor is at least reputable, because the pads certainly are not (at least its braking power is not). I have Monroe Dynamic on right now and while it is ok for daily driving, it is nothing good compare to all the other pads I've used before (Axxis Ultimate, Hawks HPS, OEM) and certainly doesn't warranty a high end slotted rotor.

 

[HerrStig]

Originally Posted By: GMBoy
If you change pads, no need to go semi-metallic. They would NOT have made any difference. The ceramic pads are the way to go.
I've found ceramics to be clean, long wearing, and quiet, but I don't like the feel as opposed to EBC "Greenstuff:. I recently upgraded a Camry to ES 300 front calipers and rotors, easy job, and I used EBC pads. What a difference.

 

[rslifkin]

Plain and simple, you overheated the brakes. You either need much more aggressive pads, bigger brakes, or suck it up and downshift (or some combination of the above).

As long as you keep it below redline and the engine is mechanically healthy, spinning 5k rpm for a few minutes down a hill won't hurt anything. Heck, I almost never get on the highway without winding the Jeep past 5k in 1st and often 2nd. Making an uphill passing run is another WOT in 2nd at 3500 - 5400 rpm ordeal. Getting off the highway involves rev-matched downshifts from 4 - 3 - 2, usually exceeding 3000 rpm, and sometimes hitting 4000.

 

[Kestas]

As long as we're splitting hairs on metallurgy, let me point out the the color blue indicates around 600°F. This is below the temperature needed to harden cast iron. Unless the original poster detects pulsation in his braking now or in the near future, the problem is only cosmetic and he has no need to replace the rotors.

 

[Gotch]

^^^ Exactly!