Glowing rotor dark rings

For example: cast iron is dull gray ("colorless") at 1000 F but bright tomato red by 1500 F.

That is great information but irrelevant to the pictures in question because the rotors pictured are almost certainly carbon/ceramics. Only the GT2 class of ALMS requires steel brakes.

And in variable, uncontrolled lighting conditions, judging color with true accuracy is very difficult if not impossible.

True, color rendering is extremely hard and changes drastically with variations in light, however photographers can control the color rendering in their pictures through controlling the white balance. Also, as an fyi unless you have carefully calibrated your monitor you are not seeing the colors as they are meant to be displayed.

We know that disc braking sytems are carefully designed to apply large amounts of hydraulic pressure uniformly across the faces of the rotors. And since brake engineers are pretty damned good at what they do, they largely accomplish this.

That is quite a broad assumption that you are making there. I think these pictures challenge your assumption that the hydraulic pressure is uniform across the face of the rotor. The reason why this pressure and therefore temperature and in turn color varies is what we are trying to understand.

So when we see the rotors glowing in a uniform pattern across the face, or perhaps even center-hot, we can safely presume that the driver is applying moderate to maximum pedal force. This is a thoroughly understood, carefully and reasonably controlled process. However, when the caliper pressure is decreased -- as when the driver trail-brakes or steps off the pedal altogether, for example, this controlled process ceases, and a far less controlled (at times even seemingly randomized) process takes its place. It can look very dramatic and variable, but the actual variations in temperature are relatively small, and thus their causes are relatively subtle.

I cannot speak for the other photographers but I can guarantee that the pictures I have taken and uploaded were taken in heavy brake zones at Sebring (either turn 3, turn 7 or turn 10) which means that the driver was almost definitely not varying their pedal pressure in a way to cause this effect. I also took some video with my camera this year and it clearly shows these color variations and the differences in the variations car to car are also obvious.

While this is a quite ridiculous video I found, it shows how the brake disc can glow at different temperatures at the same time:

While this is a quite ridiculous video I found, it shows how the brake disc can glow at different temperatures at the same time:

Useful video. Doesn't illustrate the phenomenon we are discussing but does demonstrate a few interesting concepts:

- Duller inner ring on the visible face due to conduction of heat into the "hat" area.
- Bright surface fades very quickly when the disc is stopped (at 1:33) as heat soaks into the substrate.
- Immediately after, the fins become "visible" as darker/cooler sections on the disc surface

It does illustrate how quickly the surface brightness can fade where the substrate is cooler. To my mind it confirms at least that the ring effect is due to differential heat-input, not cooling.

Edited by gruntguru, 13 August 2009 - 01:47.

As one of the photographers of the images that you all are discussing, (and thanks to all that have been giving them a look) I would like to give my input since I can discuss things like camera settings fairly knowledgeably as well as being an engineer.

Hey, welcome! Thanks for chiming in. Your input is both interesting and greatly appreciated.

To my mind it confirms at least that the ring effect is due to differential heat-input, not cooling.

Do you say that because the colour bands are concentric, whereas you would expect the variations to be radial if it was due to cooling? That makes sense to me - I'm just surprised that the effect, a darker (cooler) stripe seems to be in more or less the same position in every example, slap bang in the middle of the swept area. Odd - and interesting.

Do you say that because the colour bands are concentric, whereas you would expect the variations to be radial if it was due to cooling? That makes sense to me - I'm just surprised that the effect, a darker (cooler) stripe seems to be in more or less the same position in every example, slap bang in the middle of the swept area. Odd - and interesting.

No, I say that because the colour fades so slowy (after the initial cooling of the surface down to the substrate temperature) it is unlikely that one band could be cooled quickly enough to cause the temperature differential observed. Secondly, the photos seem to be taken while still braking (or very shortly after) since the initial surface glow fades to the substrate temperature so quickly. Thirdly, the rapid initial fading indicates that conduction dominates convection and would therefore maintain more even temperatures across the disc in the event of uneven cooling.

No, I say that because the colour fades so slowy (after the initial cooling of the surface down to the substrate temperature) it is unlikely that one band could be cooled quickly enough to cause the temperature differential observed. Secondly, the photos seem to be taken while still braking (or very shortly after) since the initial surface glow fades to the substrate temperature so quickly. Thirdly, the rapid initial fading indicates that conduction dominates convection and would therefore maintain more even temperatures across the disc in the event of uneven cooling.

I misunderstood your comments, gg, sorry. Well, we don't seem to be any nearer to an explanation for this visible phenomenon.

That is great information but irrelevant to the pictures in question because the rotors pictured are almost certainly carbon/ceramics. Only the GT2 class of ALMS requires steel brakes.

Their subjective color range is no wider, which is the issue.

That is quite a broad assumption that you are making there. I think these pictures challenge your assumption that the hydraulic pressure is uniform across the face of the rotor. The reason why this pressure and therefore temperature and in turn color varies is what we are trying to understand.

No, I am only challenging your first-glance assumption as to when/how/why it happens. Perhaps you could understand it better this way: Until the caliper itself yields, the pad force will be distributed most evenly at maximum caliper pressure -- and naturally the caliper is rated beyond that pressure. As a thought experiment, consider a pair of brake pads clamped in a bench vise. As you bear down on the screw handle, the pressure distribution will proceed to maximum/uniform until the vise gives up -- or you back off the handle.

While this is a quite ridiculous video I found, it shows how the brake disc can glow at different temperatures at the same time:

What you have there is a production car floating caliper and rotor setup being abused beyond the point of failure. By the time the rotor has developed any color, the pads have already long since failed and the brake is no longer functioning normally. (Note the smoke.) I don't doubt that rotors can glow at different temperatures at the same time. That is the subject of this thread.

However, what is the actual difference in temperature between color/no color on localized areas of a rotor? That depends on many things, the key one being ambient lighting. In direct sunlight, brake rotors usually have no color. But their temperature hasn't changed, only their subjective color.

Useful video. Doesn't illustrate the phenomenon we are discussing but does demonstrate a few interesting concepts:

- Duller inner ring on the visible face due to conduction of heat into the "hat" area.

Production-type, one-piece brake rotors incorporate a heat dam. This is an area of reduced cross-sectional thickness at the radius between the hat and friction face of the rotor, and is readily visible on the component. It serves an identical function as the floating attachment system used on two-piece racing rotors: it prevents the rotor from belling, i.e. taking a conical shape, and isolates the bearing/seal package from brake heat. Thus brake rotors are specifically designed NOT to conduct heat from the friction surfaces to the hat.

I think I found a clue as to why the Corvettes have a circle in the center of their pad swept area.

This is a picture of the C6R's braking system: http://image.vettewe..._C6R brakes.jpg
In a picture (and to the visible eye) any dark spot such as this will blur together due to the velocity that the wheels are rotating at.

Now to ask why those holes are placed there? Perhaps the holes are there as a way for pad material to escape the rotor surface?

I think I found a clue as to why the Corvettes have a circle in the center of their pad swept area.


In a picture (and to the visible eye) any dark spot such as this will blur together due to the velocity that the wheels are rotating at.

Interesting - it would be the answer if all discs were indented like that, but the darker band seems to occur with different layout of cross-drilling...

Around 2:20 on this video I think shows the development and decay of the phenomenom under discussion with a Ferrari caliper. I don't pretend to understand it but others might.

Around 2:20 on this video I think shows the development and decay of the phenomenom under discussion with a Ferrari caliper. I don't pretend to understand it but others might.

Well there you go. In application the rotor heats in a uniform to center-hot pattern, while in release the apparently contrasting bands occur. Which only stands to reason: application is a uniform, controlled process -- the positive application of controlled hydraulic pressure -- while cooling upon release is a less uniform, less controlled process -- conduction and convection.

Interesting - it would be the answer if all discs were indented like that, but the darker band seems to occur with different layout of cross-drilling...

The variously-placed divots on carbon rotors are gas ports or wear/directional indicators. It certainly seems plausible that they would have some effect on the color pattern, though they don't necessarily align.

Another interesting thing you will see in some photos: two contrasting dark rings, not quite concentric but overlapping. I take that to mean the wheel was in bump while the shutter was open.

I take that to mean the wheel was in bump while the shutter was open.

I imagine that quite a few of the photographs taken of these glowing rotors were taken with a long-ish shutter speed relative to the movement of the subject.

Has anyone taken infrared images of this phenomena?

Sure, all types of thermal imaging including video. However, all of it has a similar limitation to the visible-light spectrum -- you've still got to look at it. It's significantly less accurate than say, contact measurement because what it really measures is emissivity. Which is not to say it's not useful... sometimes the most inaccurate tools can be the most valuable. Before FEA, lots of space frames were tested as 1:12 scale balsa stick models.

Somewhat OT, but the wildest stuff you will ever see in re brake rotors is scanning laser vibrometer imaging.

Sure, all types of thermal imaging including video. However, all of it has a similar limitation to the visible-light spectrum -- you've still got to look at it. It's significantly less accurate than say, contact measurement because what it really measures is emissivity. Which is not to say it's not useful... sometimes the most inaccurate tools can be the most valuable. Before FEA, lots of space frames were tested as 1:12 scale balsa stick models.

Somewhat OT, but the wildest stuff you will ever see in re brake rotors is scanning laser vibrometer imaging.

One thought I had was that NASA space weenies routinely use CCD devices to meaure emissivity of stars and other celestial objects, and routinely make a variety of conclusions accordingly. I was thinking a similar process could be followed to study this phenomena.

OTOH, I agree with your statement about emissivity. In my last job we used to use an induction heater in the assembly of our product so as to insert one component into another component. When the assembly cooled, the inserted part was locked in place as a result of an interference fit. Anyways, on one particular job we were working with the biggest part we'd ever made in the history of the company. The part to be heated was titanium, and we attempted to use a non-contact pyrometer to measure the temperature of the part as it was being heated. We'd heat the part, use the pyrometer to measure temperature, and attempt to install the other piece at a prescribed temperature. After 3 failed attempts with the pyrometer, we simply heated the part until it was cherry red and then installed the other piece. Using this "TLAR" method we had 100% success. Undoubtedly the emissivity of the titanium was the issue, and I'm pretty sure we had our pyrometer incorrectly calibrated (despite following the manufacturer's instructions). Rather than get to the bottom of the problem, we made a few test parts, sectioned the bits that got hot, and did a series of metallographic studies to determine if we were screwing up the titanium.

Production-type, one-piece brake rotors incorporate a heat dam. This is an area of reduced cross-sectional thickness at the radius between the hat and friction face of the rotor, and is readily visible on the component. It serves an identical function as the floating attachment system used on two-piece racing rotors: it prevents the rotor from belling, i.e. taking a conical shape, and isolates the bearing/seal package from brake heat. Thus brake rotors are specifically designed NOT to conduct heat from the friction surfaces to the hat.

I'm sure to some extent they succeed too. Obviously not enough to avoid the colour difference seen in the video.

Around 2:20 on this video I think shows the development and decay of the phenomenom under discussion with a Ferrari caliper. I don't pretend to understand it but others might.

Very interesting. Important to note, the disc in this video is quite "cold", so the glow you see is very much surface only. The cooling is extremely fast because the primary mechanism is conduction into the cooler substrate.

Here's a theory. Because the pad is clamped in the centre it will deflect like a centre loaded beam and the centre region will wear a little more, so that under heavy braking the pad is flattened onto the disc and pressures are higher towards the centre of the face. Then as the pedal pressure is modulated back (as Aero decreases or cornering commences) the "beam" is deflected less and the pressure favours the outer - less worn area of the pad. So the glowing disc starts cooling in the centre of the friction area first.

Edited by gruntguru, 13 August 2009 - 23:56.

I'm sure to some extent they succeed too. Obviously not enough to avoid the colour difference seen in the video.

Actually, the cause of the color difference is blatantly obvious, and appears on the rotor long before any color is visible, even before the pads start smoking.

I have been asked to explain this. The brighter area on the outer radius of this rotor is a transferance or deposition band. Just like tires, brake pads generate friction in multiple ways, including abrasion and adhesion. In the latter, brake pad material fuses with with the rotor surface material, is torn from the pad and deposited on the rotor.

But in the case of this video, the brakes and throttle are applied at the same time, which among other things cocks the floating caliper on its mount, applying an uneven clamping force that is biased to the outer radius of the outboard face, as you can see. In only a few seconds, the pads will be smoking badly, and very soon they will be on fire. Needless to say, this is not a very useful video in analyzing the normal operation of racing brakes. These aren't racing brakes, and this sure isn't normal operation.

More on material transfer.... when you bed-in a new set of brake pads, you are not bedding in the pads so much as the rotors -- applying a thin layer of identically pad-compatible material to the rotor faces. Much like rubbering-in a race track surface. Also worth mentioning, "warped rotors" due to heat are actually pretty rare. Far more commonly, the pad deposition builds up unevenly on the rotor faces, eventually forming a material thickness variation (MTV) on the rotor faces. The rotor is no longer of uniform thickness and its faces are no longer sufficiently parallel, so now you have pedal pulsation and vibration (official industry slang term "judder") on brake application. Further, the cast iron under a region of excessively deposited pad material will not cool adequately, which causes zones of cementite aka iron carbides to form on the rotor -- commonly known as "hard spots" or "hot spots." But in properly designed brake systems, rotors don't commonly "warp." That's sort of a myth which survives because the diagnosis, while wrong, at least points to the correct component.

I have been asked to explain this. The brighter area on the outer radius of this rotor is a transferance or deposition band. Just like tires, brake pads generate friction in multiple ways, including abrasion and adhesion. In the latter, brake pad material fuses with with the rotor surface material, is torn from the pad and deposited on the rotor.

But in the case of this video, the brakes and throttle are applied at the same time, which among other things cocks the floating caliper on its mount, applying an uneven clamping force that is biased to the outer radius of the outboard face, as you can see. In only a few seconds, the pads will be smoking badly, and very soon they will be on fire. Needless to say, this is not a very useful video in analyzing the normal operation of racing brakes. These aren't racing brakes, and this sure isn't normal operation.

More on material transfer.... when you bed-in a new set of brake pads, you are not bedding in the pads so much as the rotors -- applying a thin layer of identically pad-compatible material to the rotor faces. Much like rubbering-in a race track surface. Also worth mentioning, "warped rotors" due to heat are actually pretty rare. Far more commonly, the pad deposition builds up unevenly on the rotor faces, eventually forming a material thickness variation (MTV) on the rotor faces. The rotor is no longer of uniform thickness and its faces are no longer sufficiently parallel, so now you have pedal pulsation and vibration (official industry slang term "judder") on brake application. Further, the cast iron under a region of excessively deposited pad material will not cool adequately, which causes zones of cementite aka iron carbides to form on the rotor -- commonly known as "hard spots" or "hot spots." But in properly designed brake systems, rotors don't commonly "warp." That's sort of a myth which survives because the diagnosis, while wrong, at least points to the correct component.

Yes - Blatantly obvious.

the brakes and throttle are applied at the same time, which among other things cocks the floating caliper on its mount

Why wouldn't the caliper be cocked on its mount under normal braking?

Why wouldn't the caliper be cocked on its mount under normal braking?

The straight and simple answer is they do, just not to the absurd and obscene extent depicted in the video. This is why race cars typically do not use single-piston, floating calipers, not if they can possibly help it. Proper racing brakes generally use multi-piston calipers solidly attached to the upright. Floating/sliding calipers have a host of advantages for production cars including cost, service life, simplicity of manufacture, NVH, etc, but one thing they do not provide is superior uniformity of application. They are "self-aligning," and the more they wear the sloppier they get.

...when you buy a set of quality brake pads for your road car, often it includes a little plastic sack of associated bushings and bobbins that comprise the caliper mounting hardware. Often the mechanic will throw it away or into the bottom of his toobox, as the originals "look fine." That's a mistake. That hardware is included not because automotive suppliers like to include free prizes in their boxes, but because the service engineers have determined that the life of the mounting hardware is approximately that of the pads. Replacing the pads alone will allow the brakes to continue operating another n thousand miles without grinding up the rotors, but it will not restore the braking system's original capability.

One thought I had was that NASA space weenies routinely use CCD devices to meaure emissivity of stars and other celestial objects, and routinely make a variety of conclusions accordingly. I was thinking a similar process could be followed to study this phenomena.

Well, needless to say all that is well above my pay grade but from what I know of it, that is not photography at all but a form of photometry -- brain-numbingly sophisticated electronic spectrography. Far beyond anything the auto industry needs. While these ephemeral color patterns are interesting they are not terribly critical in brake system design.

OTOH, I agree with your statement about emissivity. In my last job we used to use an induction heater in the assembly of our product so as to insert one component into another component. When the assembly cooled, the inserted part was locked in place as a result of an interference fit. Anyways, on one particular job we were working with the biggest part we'd ever made in the history of the company. The part to be heated was titanium, and we attempted to use a non-contact pyrometer to measure the temperature of the part as it was being heated. We'd heat the part, use the pyrometer to measure temperature, and attempt to install the other piece at a prescribed temperature. After 3 failed attempts with the pyrometer, we simply heated the part until it was cherry red and then installed the other piece. Using this "TLAR" method we had 100% success. Undoubtedly the emissivity of the titanium was the issue, and I'm pretty sure we had our pyrometer incorrectly calibrated (despite following the manufacturer's instructions). Rather than get to the bottom of the problem, we made a few test parts, sectioned the bits that got hot, and did a series of metallographic studies to determine if we were screwing up the titanium.

Good work, very clever.

Of course you realize you went about this all wrong. You are supposed to throw your hands up in the air and demand a half-million dollar piece of equipment that will sit in the corner collecting dust forever afterward. This increases your department's budget, and more importantly, your personal power within the organization.

Or, how General Motors was the #1 volume producer of automobiles in the world and filed Chapter 11.

Or, how a smart, agile subcontractor can charge hilariously exorbitant rates for its services and get filthy rich while still saving its clients a ton of money. Just do it. They won't ask how if they don't have to.

QUOTE (gruntguru @ Aug 15 2009, 20:23)
Why wouldn't the caliper be cocked on its mount under normal braking?

The straight and simple answer is they do, just not to the absurd and obscene extent depicted in the video. This is why race cars typically do not use single-piston, floating calipers, not if they can possibly help it. Proper racing brakes generally use multi-piston calipers solidly attached to the upright. Floating/sliding calipers have a host of advantages for production cars including cost, service life, simplicity of manufacture, NVH, etc, but one thing they do not provide is superior uniformity of application. They are "self-aligning," and the more they wear the sloppier they get.

...when you buy a set of quality brake pads for your road car, often it includes a little plastic sack of associated bushings and bobbins that comprise the caliper mounting hardware. Often the mechanic will throw it away or into the bottom of his toobox, as the originals "look fine." That's a mistake. That hardware is included not because automotive suppliers like to include free prizes in their boxes, but because the service engineers have determined that the life of the mounting hardware is approximately that of the pads. Replacing the pads alone will allow the brakes to continue operating another n thousand miles without grinding up the rotors, but it will not restore the braking system's original capability.

QUOTE (McGuire @ Aug 15 2009, 22:05) the brakes and throttle are applied at the same time, which among other things cocks the floating caliper on its mount

I only asked because your original quote states that applying brakes and throttle simultaneously is what cocks the caliper.

Edited by gruntguru, 15 August 2009 - 22:20.

Here's a theory. Because the pad is clamped in the centre it will deflect like a centre loaded beam and the centre region will wear a little more, so that under heavy braking the pad is flattened onto the disc and pressures are higher towards the centre of the face. Then as the pedal pressure is modulated back (as Aero decreases or cornering commences) the "beam" is deflected less and the pressure favours the outer - less worn area of the pad. So the glowing disc starts cooling in the centre of the friction area first.

You beat me to posting this. I had that exact same thought some days ago but didn't post it in time. IMO this is the most whole explanation given the range of what we see in the photos and using some logic.

Thanks everyone

You beat me to posting this. I had that exact same thought some days ago but didn't post it in time. IMO this is the most whole explanation given the range of what we see in the photos and using some logic.
Thanks everyone

The theory is at odds with the typical case.

The theory is at odds with the typical case.

The fact that new or "near new" pads and disc creates the reverse banding, actually supports the theory. After a few applications at 3 or 4 G the pads and the disc will wear a little more in the centre - creating the dark centre effect when braking effort is only say 1 or 2 G.

Edited by gruntguru, 16 August 2009 - 20:53.

I only asked because your original quote states that applying brakes and throttle simultaneously is what cocks the caliper.

There is no discrepancy; you have simply constructed a faulty disjunctive. See also black-or-white fallacy aka false dilemna. Floating calipers are prone to rock on their saddles. When significant brake and throttle are applied simultaneously, it's gauranteed. That's simple enough, isn't it?

Very interesting. Important to note, the disc in this video is quite "cold", so the glow you see is very much surface only. The cooling is extremely fast because the primary mechanism is conduction into the cooler substrate.

Actually, you have no idea if the rotor is cold at the start of the video sequence. All you know is that the rotor is showing no apparent color. But at the end of this sequence the rotor is not showing any color either, when it was orange hot just a moment earlier. In fact the rotor can show no color at all and still be over 1000 F. There is no way that rotor is cold, relatively cold, moderately cold, or less than really, really hot. A rotor likely will not heat from room temperature to orange-hot in one brief braking cycle, hmm.

The fact that new or "near new" pads and disc creates the reverse banding, actually supports the theory. After a few applications at 3 or 4 G the pads and the disc will wear a little more in the centre - creating the dark centre effect when braking effort is only say 1 or 2 G.

LOL what new pads? You just made that up to fit your theory.

If you look at the video you can clearly see old pads (~50 percent) and a used rotor:

Edited by McGuire, 17 August 2009 - 15:52.

How long before we have electro-magnetic brakes? The orange is pretty but it sure is wasteful.

There is no discrepancy; you have simply constructed a faulty disjunctive. See also black-or-white fallacy aka false dilemna. Floating calipers are prone to rock on their saddles. When significant brake and throttle are applied simultaneously, it's gauranteed. That's simple enough, isn't it?

Dilema, dilemma and now dilemna. (Sorry couldn't resist - I know its only a typo - I make plenty of them myself)
No I haven't misinterpreted your statement. Your original statement clearly has the meaning I ascribed "the brakes and throttle are applied at the same time, which among other things cocks the floating caliper on its mount"

Actually, you have no idea if the rotor is cold at the start of the video sequence. All you know is that the rotor is showing no apparent color. But at the end of this sequence the rotor is not showing any color either, when it was orange hot just a moment earlier. In fact the rotor can show no color at all and still be over 1000 F. There is no way that rotor is cold, relatively cold, moderately cold, or less than really, really hot. A rotor likely will not heat from room temperature to orange-hot in one brief braking cycle, hmm.

I didn't say the rotor was cold - I said the rotor was "cold" - I suppose its only a hotrod magazine you edit - no need to be aware of nuance.

I can categorically assert that the disc is "cold". A technical person that "understands" such phenomena can recognise that the rapid dissipation of the "glow" could only be caused by CONDUCTION to a substrate that is hundreds of degrees cooler than the glowing surface.

LOL what new pads? You just made that up to fit your theory.
If you look at the video you can clearly see old pads (~50 percent) and a used rotor:

Perhaps the pads are worn - I will rephrase. The pads and the disc are relatively "flat". This could be because this design does not produce the phenomenon in question or perhaps all recent braking events have been at low or moderate pressure.

My theory predicts appearance of the phenomenon in question after extreme braking events (perhaps several) to produce a slight "cupping" of pads and/or rotor.

My theory predicts appearance of the phenomenon in question after extreme braking events (perhaps several) to produce a slight "cupping" of pads and/or rotor.

Since your theory does not predict the color pattern in your own example, you can see why some might find it disappointing.

Since your theory does not predict the color pattern in your own example, you can see why some might find it disappointing.

Sorry not my example.

Sorry not my example.

Hmm... so your theory does not fit the example you were discussing at the time, nor does it fit the typical example. It would appear that you have constructed your theory, and now all you need to find is an event or phemenonon which it describes. What a novel approach. Please keep us posted.

Hmm... so your theory does not fit the example you were discussing at the time, nor does it fit the typical example. It would appear that you have constructed your theory, and now all you need to find is an event or phemenonon which it describes. What a novel approach. Please keep us posted.

No the theory is one of many proposed in this thread, to explain the phenomenon of a dark ring in the centre of the friction area of a glowing disc. There has been no evidence to disprove it so far.

McGuire stop reading here - what follows is logic.

1. The most impressive photos of glowing discs will be obtained after sustained, heavy braking.
2. This will occur at the end of long straights.
3. For aero cars at top speed the braking will initially be as high as 4G.
4. This extreme pressure is likely to create distortion of the pad such that the centre of the friction area is doing a higher proportion of the work (and wear).
5. At the end of this braking event, braking may be as low as 1.5G (even less if trail braking occurs)
6. During this period the distortion will be reduced (clamping force is more than halved) and the less-worn, outer friction area will be doing a higher proportion of the work.
7. The photograph is taken at this point because the disc is at its hottest and the car is travelling more slowly.

The Brembo video served to show how quickly the glow can fade when it is only a surface temperature effect and so supports the plausability of the above explanation.

The rotors have a thicker cross-section near the middle where the fins are. It's made for the air flow. Rotors are like sliced toasted bagel sandwiches turned the buttered side out. And the fins have an airfoil profile too. The air channels between the fins form little venturis. The rotors heat up across the whole outer surface, but the middle of the rotor has thicker walls, thicker fins, and therefore the middle is a bigger heat sink. If you keep the rotor braking like that for longer then the color will be more uniform. Blah-blah-blah.... this all applies to cast iron rotors only. Carbon rotors just have stupid round holes drilled radially. No venturis. So I do not know for sure.

The REAL answer is that there are RIVETS in the middle of the pads. The pads do not rub the rotors as much where the rivets are.

Great discussion

It's something I have noticed for years too and never really questioned profoundly but always assumed it was because of the concept forthcoming. I'd just like to add that I too saw the phenomenon at LM24 08 at night without any photographic equipment, again especially on the Corvettes. I think the great photo of the notches on the C6R disc explains why it's so pronounced on them, compared to other class competitors etc.

Let me draw your attention to this image of the production Audi R8 cutaway (with a brake system not totally dissimilar from the cars in question):
http://pictures.tops...8-81_800x0w.jpg

It is my belief that the phenomenon is, as I think someone suggested, more a function of cooling than heating on application of brakes. I think it has more to do with the caliper, (in particular the pistons thereof) than the disc. We agree I'm sure on the basic principles that force will be applied by the driver, transferred to the fluid, transferred to the pistons, transferred then to the pad and on to the disc. This energy is transformed into heat energy, which is then dissipated by radiation (ie. converted into light energy, attributed to glowing), convection (to free air stream) and conduction to all parts that were put into contact initially by the force of the driver's foot. Naturally the ability to diffuse heat energy by any three mechanisms will affect the behaviour of the other two. Keep that in mind.

Focusing on the last item, conduction, the heat energy in the pad will escape to the pad backing, which is in contact only with the pistons. Heat would be transferred to them, and also to the fluid and caliper body. My assumption is then that there are 3 cooler spots on the pad area where the pistons' interface is, and as such, slightly more heat will be drawn out of the disc at the same areal interface with the pad. Because the disc is spinning, the 3 piston 'spots' have a radial effect on the entire heat system and visibly we can see the effect on reduced radiation emission along that circumference.

Looking at the radial positioning of this slightly cooler dark ring, it seems to support the theory. It could also explain why the darker line is at a different radial distance on different cars, ie. the cool line isn't always exactly at mid-radius of the disc.

Thoughts?

Great discussion

It's something I have noticed for years too and never really questioned profoundly but always assumed it was because of the concept forthcoming. I'd just like to add that I too saw the phenomenon at LM24 08 at night without any photographic equipment, again especially on the Corvettes. I think the great photo of the notches on the C6R disc explains why it's so pronounced on them, compared to other class competitors etc.

Let me draw your attention to this image of the production Audi R8 cutaway (with a brake system not totally dissimilar from the cars in question):
http://pictures.tops...8-81_800x0w.jpg

It is my belief that the phenomenon is, as I think someone suggested, more a function of cooling than heating on application of brakes. I think it has more to do with the caliper, (in particular the pistons thereof) than the disc. We agree I'm sure on the basic principles that force will be applied by the driver, transferred to the fluid, transferred to the pistons, transferred then to the pad and on to the disc. This energy is transformed into heat energy, which is then dissipated by radiation (ie. converted into light energy, attributed to glowing), convection (to free air stream) and conduction to all parts that were put into contact initially by the force of the driver's foot. Naturally the ability to diffuse heat energy by any three mechanisms will affect the behaviour of the other two. Keep that in mind.

Focusing on the last item, conduction, the heat energy in the pad will escape to the pad backing, which is in contact only with the pistons. Heat would be transferred to them, and also to the fluid and caliper body. My assumption is then that there are 3 cooler spots on the pad area where the pistons' interface is, and as such, slightly more heat will be drawn out of the disc at the same areal interface with the pad. Because the disc is spinning, the 3 piston 'spots' have a radial effect on the entire heat system and visibly we can see the effect on reduced radiation emission along that circumference.

Looking at the radial positioning of this slightly cooler dark ring, it seems to support the theory. It could also explain why the darker line is at a different radial distance on different cars, ie. the cool line isn't always exactly at mid-radius of the disc.

Thoughts?

This sounds feasible. If this is the mechanism, the dark rings will appear whenever the braking glow appears. If on the other hand, the rings appear mainly when braking is reduced (eg end of straight), it would tend to favour the theory in post #85.

Edited by gruntguru, 22 August 2009 - 11:14.

this heat transfer trough pistons theory is not feasible IMHO, as every step is taken to minimize the heat transfer to the calipers and fluid... pistons are usualy titanium and/or have some form of insulation or a block for heat transfer (take a look at wilwood's two piece pistons) so calipers will have little effect on disc surface temp as far as heat dissipation is concerned..
Mcguire is right in saying that we are in effect talking about very small temp differences on the disc and it would be smart to consider disc heat dissipation dinamics as a cause. For example I'd like to see how a non vented disc will glow... my guess is that we would not see such a pattern..

this heat transfer trough pistons theory is not feasible IMHO, as every step is taken to minimize the heat transfer to the calipers and fluid... pistons are usualy titanium and/or have some form of insulation or a block for heat transfer (take a look at wilwood's two piece pistons) so calipers will have little effect on disc surface temp as far as heat dissipation is concerned..
Mcguire is right in saying that we are in effect talking about very small temp differences on the disc and it would be smart to consider disc heat dissipation dinamics as a cause. For example I'd like to see how a non vented disc will glow... my guess is that we would not see such a pattern..

I hear what you're saying, and I do agree that fundamentally pistons are designed not to conduct heat to the fluid. In reality it does happen to some degree, whether it's significant enough to contribute to these small temp differences, I do not know.

However, I remembered that I had this thermal image (courtesy University of West Bohemia, Czech Republic) - it's for a ventilated steel disc, not slotted, cross-drilled or notched, but I think contributes to your last statement and might visually help us understand the topic, although it might show that the temp variation is not as small as some might think (order of ~150 deg C). Of particular interest, 2nd and 3rd-last images:
http://i29.tinypic.com/28iat86.jpg

I would think that a more homogenous carbon disc would be less affected by this random spacial heat distribution though.

Edited by Jezztor, 23 August 2009 - 09:43.

Two questions about the above link:

1. By "steel disc" you do mean cast iron, corrrect? Not a quibble, this is important.

2. Where is the caliper?

The lack of a caliper indicates these are not photographic images but scanning images, obtained by noncontact thermal sensors (probably fiber-optic infared judging by the colors) placed adjacent to the rotor, which record the temperature as it sweeps past. Hence no caliper -- the psuedoimage of a complete rotor is assembled in software. This is also reflected in the color patterns, which are radial or apparently "static" in character owing to the manner in which the images are collected and assembled: At all 360 degrees of the rotor as it rotates, but always from the same point in space. Which is not to say this form of testing is invalid, only of limited use in this discussion. We see an entire disc more or less that is rotating at significant speed. In other words, something completely different. That's why these images appear nothing like the photographs or what you will see at the track.

this heat transfer trough pistons theory is not feasible IMHO, as every step is taken to minimize the heat transfer to the calipers and fluid... pistons are usualy titanium and/or have some form of insulation or a block for heat transfer (take a look at wilwood's two piece pistons) so calipers will have little effect on disc surface temp as far as heat dissipation is concerned..
Mcguire is right in saying that we are in effect talking about very small temp differences on the disc and it would be smart to consider disc heat dissipation dinamics as a cause. For example I'd like to see how a non vented disc will glow... my guess is that we would not see such a pattern..

I think we would. The effect is not related to the location of the rear-of-disc convective cooling. Even if we only air cooled the region behind he centre of the friction area it would not create the effect because conduction is the principal driver and would tend to equalise the temperatures across the face of the disc.

I am certain that the effect is due to uneven heat input and not uneven cooling.

Two questions about the above link:

1. By "steel disc" you do mean cast iron, corrrect? Not a quibble, this is important.

2. Where is the caliper?

The lack of a caliper indicates these are not photographic images but scanning images, obtained by noncontact thermal sensors (probably fiber-optic infra-red judging by the colours) placed adjacent to the rotor, which record the temperature as it sweeps past. Hence no calliper -- the psuedoimage of a complete rotor is assembled in software. This is also reflected in the color patterns, which are radial or apparently "static" in character owing to the manner in which the images are collected and assembled: At all 360 degrees of the rotor as it rotates, but always from the same point in space. Which is not to say this form of testing is invalid, only of limited use in this discussion. We see an entire disc more or less that is rotating at significant speed. In other words, something completely different. That's why these images appear nothing like the photographs or what you will see at the track.

I think the key point to note on these images is that every phase of braking - heating through to cooling shows a HIGHER average temperature in the region of interest ie the ring in the centre of the friction area. This indicates that the pads do deform such that pressure is greatest at the centre of the pads (where the piston(s) are) which is pretty obvious.

I think the difference here is that braking probably did not follow the cycle required to produce the rings ie. extreme braking, tapering back to moderate braking.

Edited by gruntguru, 30 August 2009 - 23:28.

I think we would. The effect is not related to the location of the rear-of-disc convective cooling. Even if we only air cooled the region behind he centre of the friction area it would not create the effect because conduction is the principal driver and would tend to equalise the temperatures across the face of the disc.

I am certain that the effect is due to uneven heat input and not uneven cooling.

I am sure that we could easily confirm/disprove the "concave" pad theory by just measuring it's thickness across the face?

Two questions about the above link:

1. By "steel disc" you do mean cast iron, corrrect? Not a quibble, this is important.

2. Where is the caliper?

The lack of a caliper indicates these are not photographic images but scanning images, obtained by noncontact thermal sensors (probably fiber-optic infared judging by the colors) placed adjacent to the rotor, which record the temperature as it sweeps past. Hence no caliper -- the psuedoimage of a complete rotor is assembled in software. This is also reflected in the color patterns, which are radial or apparently "static" in character owing to the manner in which the images are collected and assembled: At all 360 degrees of the rotor as it rotates, but always from the same point in space. Which is not to say this form of testing is invalid, only of limited use in this discussion. We see an entire disc more or less that is rotating at significant speed. In other words, something completely different. That's why these images appear nothing like the photographs or what you will see at the track.

1. Unfortunately no further info given, I would assume cast iron. The paper focused more on the software and rig than any specific empirical results.

2. Situated at 9 o' clock. It is a fibre-optic IR system measuring radiation intensity with sensors located at 3 o'clock, disc spinning clockwise.

Gotta disagree that they're of limited use, and nothing like what we see in practice. The imaging clearly shows the "origination, progression and disappearance" of hotspots over a period of 120 seconds. Each image represents 1 revolution of the disc being braked from 1500 rpm and respective instantaneous temperatures measured at the 3 o' clock. Not divinely accurate as an instantaneous representation of the whole body as a snapshot, I agree, but at these speeds pretty close, and helpful IMO to debunk this mystery.

What I've done below is take the 3rd last image (I imagine just after heavy braking, start of cooling), and put a radial blur on it to see what it might look like at high speed. It's a little misleading because the hottest temperature is the dark red/maroon, which doesn't show up too well on the image's outer periphery, but regardless, it is clear to me that there is a cooler 'ring' around mid-radius.



I tried to put it into a gif as per below but couldn't get it to 'rotate' fast enough with my el-cheapo software
http://i32.tinypic.com/29l22yb.gif

Edited by Jezztor, 24 August 2009 - 16:10.

Another one using the 3rd image, representing the intial heat-up of the disc:



Reminder that the darker the blue, the cooler the surface. The outer periphery again is misleading because the colour starts transforming into oranges and reds which doesn't blend nicely with the blue, but it is obvious from image #3 that the hottest spot is on the other surface.

I am sure that we could easily confirm/disprove the "concave" pad theory by just measuring it's thickness across the face?

Yes. To be 100% sure, you would need to execute an extreme braking event in the relevant vehicle - without the lower G braking that normally follows, since the outer section of the pads is being preferentially worn during this phase (while the rings are apparent.)

Edited by gruntguru, 25 August 2009 - 11:55.

What I've done below is take the 3rd last image (I imagine just after heavy braking, start of cooling), and put a radial blur on it to see what it might look like at high speed. It's a little misleading because the hottest temperature is the dark red/maroon, which doesn't show up too well on the image's outer periphery, but regardless, it is clear to me that there is a cooler 'ring' around mid-radius.

Nice work. I didn't notice originally, but that particular image does have a cooler ring on a second inspection. Likewise for the one prior to it but the ones immediately before and after those two are hotter at mid-radius again. I think this confirms that the effect occurs immediately after the maximum braking event - in the early part of the cooldown.

It would be great to know if there is any caliper force still applied at that point (images 9 and 10) - does the paper elaborate?

Nice work. I didn't notice originally, but that particular image does have a cooler ring on a second inspection. Likewise for the one prior to it but the ones immediately before and after those two are hotter at mid-radius again. I think this confirms that the effect occurs immediately after the maximum braking event - in the early part of the cooldown.

It would be great to know if there is any caliper force still applied at that point (images 9 and 10) - does the paper elaborate?

Sadly not... Assuming there is a 10-second gap between each image (120 second timespan from first to last image), I'd say particularly for #10 that there is no significant braking force present.

Found a thermal realtime image of the same rig which reinforces the blurred images. It shows that the particular steel in question exhibits the same phenomenon as on the carbon discs. Again, not sure too sure at what point in the braking cycle the snap was taken.

Sadly not... Assuming there is a 10-second gap between each image (120 second timespan from first to last image), I'd say particularly for #10 that there is no significant braking force present.

Found a thermal realtime image of the same rig which reinforces the blurred images. It shows that the particular steel in question exhibits the same phenomenon as on the carbon discs. Again, not sure too sure at what point in the braking cycle the snap was taken.

An interesting image.

I would point out that the emissivity of the rotor should be a point of discussion. The emissivity is what affects the photographic images that started this debate, and the image you posted is evidently some sort of infrared image (no units depicted in the piccie).

What I learned from my prior experience with non contact pyrometers is that the temperature reading given by the pyrometer is a function of the emissivity of the material and the atmosphere between the emitter and the pyrometer. As I recall from the pyromter's instruction manual, the surface finish was a big deal. When the surface finish of the material changes, the pyromtere's readings will change accordingly and the accuracy of the measurement becomes questionable.

As you can read on the wikipedia page on emissivity ( http://en.wikipedia....wiki/Emissivity ), a number of issues affect the emissivity of the rotor (in addition to surface finish).

As such, although I find the image interesting, it is difficult for me to think that it is conclusive in any way. It is IMO basically an infrared version of the visible light photographs that started the discussion.

Edited by dosco, 25 August 2009 - 13:49.