Example here.. http://gallery.autosport.com/picture.php/d..._329011_HiRes-4

One would think that the center of the rotor swept area where the pads/pistons are centered would glow the brightest because that's where the friction is, but why is there a dark ring instead, which suggests a cooler state than the other areas of the rotor?

Two pads/rows of pads with a slight space between?

Groove in the pad for dust/gases?

don't think so cos the grooves in pads run radially, plus no groove is ever that wide, and the twin pad systems I've seen are also arranged inline with the rotor motion so wouldn't create that pattern. I've seen many photos like this across a large range of car types and am finally curious enough about it to ask.

New pads on badly worn rotors?

Could you post a picture that doesn't require an Autosport login?

the autosport protected image in the original post was the clearest example in high resolution form, but here are others that show roughly the same..

http://farm1.static.flickr.com/222/4749311...7dacba81d_o.jpg

http://farm4.static.flickr.com/3213/247608...e1434d38a_b.jpg

http://farm4.static.flickr.com/3569/355651...4f90e3967_o.jpg

http://www.flickr.com/photos/linkf1/2377836097/


And then there's this one where the front brakes seem to what you'd imagine, with only the rear showing the dark ring..

http://farm2.static.flickr.com/1248/104189...cb2edb173_o.jpg


Intuition says the rotor should normally look like this.. hotter around the middle

http://farm3.static.flickr.com/2246/238209...d4d214c9f_o.jpg

Drilled rotors?

no, if you look at the types of cars, they've never been known to run drilled rotors

and even then, holes wouldn't explain the temp distribution

[quote name='shaun979' date='Aug 3 2009, 06:25' post='3781683']

http://www.flickr.com/photos/linkf1/2377836097/


Even this one shows a darker central band, but much fainter. The number of times I've seen this effect and not questioned it! I can't think what causes it, but I would have thought that the glowing band should be slightly brighter as it increases in diameter, as the surface speed increases it should get hotter. I assume the pads are sufficiently rigid not to produce hot-spots directly under the pistons - my experience is that they are very rigid, but it wouldn't take much flex to do it. However, the dark band is a puzzle.

But holes might produce a band of reduced luminosity.

grunt, these cars simply don't run cross drilled rotors.. and even if they did, cross drilled rotor hole distribution is not concentrated in any one area of the rotor but evenly distributed.

I can't see the photo (I'm not a subscriber, for shame) but from the description it sounds to me like the brakes are released or releasing at the moment the shutter was open. Assuming the clamping force is reasonably uniform across the rotor face, the temperature should be reasonably uniform as well, and thus the color across the rotor face, too. However, there is no particular reason the heat must be dissipated in a nice, uniform pattern. Heat will flow out of the rotor via any path available, producing one or more rings of apparently darker color.

Non-subscribers should be able to view the low-res shot.

http://www.autosport.com/gallery/photo.php/id/1314378/

Surely the "Hot spots" are directly behind the pistons on the caliper? I know we like to think of carbon discs and pads as rigid items, and relatively speaking they are, but there will definitely be flex at the sort of pressures applied hydraulically and the points of highest contact pressure will be directly behind the pistons.

So hotter, not colder...?

hmm, it definitely seems to indicate that the pressure across the pad is not even despite thinking that it would be?

To whatever extent the caliper and pads are sufficiently rigid, the clamping force will be reasonably distributed across the rotor face. However, as the clamping force is released, heat will escape from the rotor through whatever thermal conduits are available. So what part of the rotor may tend to cool faster?

Mmm, something like this...



Excuse hasty scribble.

This may not be true of carbon/carbon but conventional pads create gas as they heat up. The gas would possibly be at its most intense at the pad centre since the distance to the edge and pressure release is longest there. The pupose of holes/slots etc. is to vent off the gas but maybe it builds up enough in the centre to locally reduce the friction level so that part of the rotor is slightly cooler?

No idea if that is possible but it is a suggestion.

Hasty scribble excused, but I've never seen a piston arrangement like that. Doesn't mean to say, of course...

It's pure conjecture. I've no idea how the pistons are arranged, or if there are even 4, I'm trying to hypothesise as to where an apparent cold spot might come from.

That's all.

Me too!

You guys are over-thinking it.

At least on the Corvette example, it's a carbon disc. There's a shallow groove in the center of the disk that is a wear indicator. The pad doesn't touch the disc there, so it runs cool.

This is embarrassing to admit, but I have a carbon disc sat on my coffee table and I've never really noticed before but the surface is not flat in cross-section. It's not so much a groove as a shallow wave across the surface with two distinct high spots.

I'm a dumbass, I could've cleared this up ages ago.

Now you tell us!

Are you kidding? What would be the point of having a rotor with a wave in it? I have a set right here and they are dead flat, but they have been run. I've never seen rotors with anything other than a flat surface.

===

Fat Boy's point about the wear indicators makes sense, but the combined area of the two sets of wear indicators is only about 1/10th of the swept area inline with it so it is still surprising that the ring is so dark and not more torwards just a darker orange. It's like if you took a white disc, added 2 patches of black paint totaling 1/10th the circumference and then spun it up..wouldn't you see more of a very light gray band (much more towards white than it is to black) rather than a band that is near black?

Don't ask me, I'm no brakes engineer. This one has been run too, on an F1 car no less, and there's a definite wave to it by touch. I could try and take a picture, but I'm not sure how well it will highlight it.

Answer, not at all...

It _used_ to be. Them-thar brakes wear, matey.

Also, sometimes the wear indicators are round depressions about the size of a small coin. At high-speed they can appear to be a line.

Believe whatever you want but I am telling you guys straight.

Also, the effect is exaggerated by underexposing the photo by one or more stops to increase the contrast. That's why the rest of the photo is typically darker than real. The variation in color and temperature across the rotor face is not as great as the photos portray.

Crap photo all right, but it looks to me like a normal, worn disc ie cupped in the centre of the friction area. If you throw a new set of pads at this disc, the first few stops will produce less heat in that area (as I suggested in post #5). Once the new pads bed-in however, they will assume the shape of the disc and the friction will be equal at all radii.

I thought technically the out rotor has the highest friction because that's where the highest speed of the rotor is and also the outer rotor is compressed by the strongest part of the caliper, closer to the bracing, subject to less flexing.

Sure - higher energy but greater circumference to absorb and dissipate the energy so temp should be the same

isnt that what holes are for? to introduce passing air for cooling?

Overall, but it doesn't really explain the dark ring., and while I think about it, the cross-drilling may be more for allowing gas release from between the pad and disc.

The color/temperature pattern on the face of the rotor is much-ly a function of when the photo was snapped. If the image is captured when the driver is standing on the pedal at the top of the braking zone, the color pattern will be reasonably uniform or if anything, perhaps center-hot. However, there is no particular reason the rotor must dissipate heat in a uniform pattern. The presumption of this thread is if the rotor is glowing, it must be heating. Not at all, maybe it's cooling. The rotors may be glowing on corner exit, but that doesn't mean the driver is on the brakes. Above we have different photos of the same model race car (Corvette ALMS) showing very different color patterns on the rotors. Well there you go. The photos were taken at different points.

no one is saying the rotor must take on or shed heat in a uniform pattern, but the question is what is the cause of the center dissipating heat quicker than the inner and outer edge?

+1

Convection cooling fluxes are low and it is highly unlikely that localised air cooling will significantly overcome conduction towards the cooler parts of the rotor - and certainly not in the brief time between braking events. The phenomenon is due to unequal heating - not unequal cooling.

I'd also like to add that in the original autosport photo in the first post, you can see that the rotor is being heated by the pads because both the 'hot' rings are glowing brightest immediately after the caliper and then get progressively dimmer till it again passes the caliper. It is more obvious on the inner hot ring, but you can see it in the outer ring as well.

The dark ring is very clearly defined and in extremely stark contrast with the hot rings... like there's no temperature gradient across the dark area in any direction.. just dead dark

Good point. The cooling ocurring in that very brief time is mostly conduction. As the disc departs the caliper, the heat energy has just been applied to the suface, but on its way back to the pads, some of that heat soaks into the disc - cooling the surface slightly. There is obviously much less heat being applied at the "dark band" radius.

Look at the rest of the photo -- it's underexposed by several stops. This is done deliberately by the photographer to heighten the brake glow.

As its a nightime shot, I think the effect is probably close to what you would experience with the naked eye.

Suit yourselves.

OK, we get it. The photo is underexposed. What does that have to do with the observation that the brake glow immediately after the caliper is more intense and fades gradually around the circumference of the rotor?

I'm not sure what you mean by this. My comment about this being a night-time shot in no way contradicts your post - nor was it intended to. If you look at the photo the background is darker than the car, the headlights are on and the road in front is illuminated by the headlights - so it is night-time or dusk.

http://www.autosport.com/gallery/photo.php/id/1314378/

My point is - glowing brake rotors show up best in low light - whether its a photo or live at the track.

The quote below obviously refers to me, and once again - you are obviously wrong (not even a technical issue this time, I won't need to find a research paper.) Even if you had been correct, you jumped to an unnecessary conclusion (that I was contradicting you) and then produced the unnecessary piece of sarcasm below. Pity its not funny - it certainly qualifies as a "Quote of Outstanding Brilliance" on its foot-in-mouth rating alone.

Quote McGuire "Indeed, if that. Brake cooling is an interesting subject, but since we have people here who will argue that day is night (literally, they go that far) there is no point trying to discuss it. After a while you learn which subjects (and members) tend to generate the most useless and tiresome arguments. Many "quotes of outstanding brilliance" can be found right here in this forum."

That things are not as they appear -- we are looking at smaller variations in color and temperature than the images indicate. For example, the "black stripe" is not really there. It's certainly not black, anyway. It's really just a darker cherry than the area on either side. In underexposure the bright colors are saturated and the dark colors are blackened. But even in a normal exposure or viewed live with the naked eye, the rotor is still under-illuminated by orders of magnitude. It's in shadow (wheelwell) and is viewed through the spokes of a spinning wheel. However, if you are viewing in the afternoon with a bright sun at your back, you will see no glowing at all.

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. Anyone who has been to a night race will be able to tell you that the brake glow is both as vibrant and impressive as seen in these pictures. As to the cause of the variance in the glow patterns I am convinced that it has to do with the differences in contact patches between different caliper and rotor designs. I say this because in my experience different makes of cars have different glow patterns, with the Corvette C6.R being the most vibrant.

Secondly, the fact that the images are underexposed has little effect on the presence of brake glow. Take this properly exposed image for example: http://www.flickr.com/photos/linkf1/3409115783/
Here is a 100% crop of the front wheel and you can very faintly see the glow coming off of the rotor at a shutter speed of 1/250th of a second: Rotor Closeup (As an aside, the jpeg compression on the image causes the colors to become less vibrant than the original colors)

The original image was taken at dusk which is why it appears 'underexposed' however it is probably properly exposed for what the photographer was actually seeing.

Last, there is a group on flickr that was made by me especially for collecting pictures of brake glow, it can be found here: http://www.flickr.com/groups/brakeglow/pool/

That is an entirely subjective statement -- truly, one hundred percent in the eye of the beholder -- and more importantly, irrelevant. As an engineering or scientific statement, it's air. We are more interested in what these gradations of color represent in temperature so that we may assess brake heating/cooling. For example: cast iron is dull gray ("colorless") at 1000 F but bright tomato red by 1500 F.

And meanwhile, as a photographer you know that if you take a bright red sports car, fire engine red, Pantone 485, and park it in the sun on a summer afternoon, it will appear tomato red on the sunlit side and cranberry on the other. Park the same car in the shade and now it's deep maroon. Obviously, color is a very imprecise means of judging variations in incandescant temperature even in the most controlled lighting conditions, because a relatively small variation in temperature can cause a large change in color, as we saw above. And in variable, uncontrolled lighting conditions, judging color with true accuracy is very difficult if not impossible. As you know, the color of an object in human eyes is solely a function of the frequency of its reflected and emitted light.



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. 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. Thus categorical assertions are clearly out of order, but we can make some generalized statements, including these and others:

- the inner radius of the rotor face is thermally isolated from the hub to prevent belling and protect the bearings.
- rotor airflow generally proceeds from the inner radius of the rotor outward, varying with speed.
- air tends to tumble and recirculate around the roots of the vanes at rotor entry.
- the outboard face of the rotor tends to run hotter than the inboard face.
- the caliper's mass tends to be centered over the rotor face.

Has anyone taken infrared images of this phenomena?

http://en.wikipedia.org/wiki/Thermal_radiation

A subjective colour able is mentioned in theabove link:

T, K oC Subjective colour

750 480 faint red glow
850 580 dark red
1000 730 bright red, slightly orange
1200 930 bright orange
1400 1100 pale yellowish orange
1600 1300 yellowish white
> 1700 > 1400 white (yellowish if seen from a distance)