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      11-25-2008, 06:14 PM   #23
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A great driver does not make a great engineer (not vice versa!) that is really all I have to say about it. Pure speculation from a driver is as reasonable of an explanation as stupidity or an agenda is it not?
The point is that you don't know if it is pure speculation or not. You don't know if there is a team of engineers behind this guy who are telling him to say what he is saying, or if the guy is an engineer himself, or if he simply learned certain things via lots of experience, which is possible.

Anyway, can you explain to me why exactly there would not be any performance difference between a steel rotor and a carbon ceramic rotor in their ability to resist fade? Assume that the same amount of heat is being generated at the pad to disc contact surfaces due to friction in both cases.
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      11-26-2008, 07:58 AM   #24
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The point is that you don't know if it is pure speculation or not. You don't know if there is a team of engineers behind this guy who are telling him to say what he is saying, or if the guy is an engineer himself, or if he simply learned certain things via lots of experience, which is possible.
What is most likely? Doesn't mean we can know for sure, but I definitely know what is most likely.

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Anyway, can you explain to me why exactly there would not be any performance difference between a steel rotor and a carbon ceramic rotor in their ability to resist fade? Assume that the same amount of heat is being generated at the pad to disc contact surfaces due to friction in both cases.
I'm pretty sure it depends on which non composite system you are comparing to. All brake rotor/pad material combinations will exhibit a knee in their friction vs. temperature curve. Pass the knee and you will get fade. When properly matched, CC pads/rotors can operate in the 800-1000 °C range (yes °C) whereas alloy steel rotors and race pads will only get up to about 700-800 °C (normal cast iron rotors will actually become so weak they will fail around 700 °C). Unfortunately, I don't know exactly where the knee points are in the curves for each combination. I'd like to. But this also begs the question of the COMBINATION. The knee point is highly pad dependent. Is it fair to compare a system that works great for street use and driving events to one that is so racy it only works at the track?

Next on to temperature rise. For a given heat energy (a given bleed of speed) CC rotors will actually get much hotter than a comparable iron or steel rotor. CC has a much higher specific heat but the equivalent rotor is so light it more than makes up for the higher specific heat. Doing the arithmetic you find the temperature gain for any given heat energy is typically about 40% higher in the CC setup (assuming all heat energy goes into the rotor, which isn't a terrible first order approximation).

So does a CC setup have a knee point 40% higher than a traditional non composite metallic rotor set up? I'm not quite sure. I would guess that in fact it is more than 40% higher compared to an OEM quality cast iron rotor/sporty pad combo. So that means it would take more energy to reach the fade point with the CC system. But it is highly dependent on exactly which system you are comparing it to, the system above vs. say a much stronger and heat resistant alloy steel rotor and high temp track or race pad combination. These latter systems may be about on par with the CC system or maybe even exceed it.

But, as I mentioned, the wear/longevity and weight reduction issues are absolutely clear cut and significant. Many CC systems are rated for "lifetime" or about 300k km and the rotors can weigh as little as 1/3rd of an iron/steel rotor. These are the real advantages.
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      11-26-2008, 10:58 AM   #25
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At a guess I would say some of the problems being had with ceramic disc in road cars is the lack of temperature. Even track days don't give anything like the temperature required to function at their best.

So like MickB says, they aren't the B all to end all and there is a sound argument for sticking with a high quality steel disc setup. Though one area they do make a world of difference is in handling, turn-in and steering feel.
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      11-26-2008, 02:03 PM   #26
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What is most likely? Doesn't mean we can know for sure, but I definitely know what is most likely.
Learning through lots of experience is not as unlikely as you put it. If you are a professional racecar driver, you must have experienced many many permutations of pads and discs for instance, and if you are intelligent, you can make some highly relevant observations. Not to mention that you must be working very closely to a team of competent engineers, who are constantly exposing you to how things work. You might not be able to explain the why part without theoretical exposure, but that doesn't mean the observed relationships are incorrect. I think you are too quick to dismiss anything coming from someone without some kind of technical education.

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Next on to temperature rise. For a given heat energy (a given bleed of speed) CC rotors will actually get much hotter than a comparable iron or steel rotor. CC has a much higher specific heat but the equivalent rotor is so light it more than makes up for the higher specific heat. Doing the arithmetic you find the temperature gain for any given heat energy is typically about 40% higher in the CC setup (assuming all heat energy goes into the rotor, which isn't a terrible first order approximation).
The above is an over-generalization and is inaccurate two reasons:

1. The units for specific heat capacity are J/(kg K). So a material with a higher c will experience less of a temperature increase for the same amount of energy transferred per unit mass.

c of a cast iron rotor is about 450 to 500 J/(kg K)
c of a carbon ceramic composite material can range from 600 to 1700 J/(kg K)

I must assume the wide range of the CC rotor is due to the design parameters associated with its composite construction.

So, even if you chose to make the CC rotors half the mass of the iron rotors, you can easily end up with the same temperature behavior in this respect. Meaning, you could be transferring the same amount of energy into both rotors, and seeing a similar temperature increase at steady state.

Also, keep in mind that mass is simply a design parameter. One can easily chose to make heavier CC rotors if one wanted, which would actually yield less temperature gain in the above scenario.

2. Another key material characteristic that governs heat transfer is thermal conductivity, k.

k of a cast iron is about 51 W/(m K)
k of a carbon ceramic composite material can range from 20 to 150 W/(m K)

I must assume the conductivity has to do with the carbon content, which is highly conductive.

As you must know conductive heat flow at steady state is directly proportional to deltaT and k.

Qdot = k x deltaT x S, where S is the shape factor.

In order to maintain consistent and sustainable heat flow away from the contact area to the rest of the disc so that it can be transferred to the environment via convection, you want high k and also high deltaT—the highest Tcontact the pads and the discs will endure for maximum fadeless stopping power will result in the highest deltaT. Meaning, it might be to your advantage to adjust the mass and composition of the CC rotor for operation at an intended higher Tcontact at the friction surface since that will yield a higher deltaT and more heat flow.

Another way to think about this is to consider thermal diffusivity, alpha, which is simply k/(c density). The larger the diffusivity, the faster the temperature change, thus the units of m^2/s. You want the material to experience whatever temperature change it will experience as fast as possible, so a high value is desirable. Based on k and c data from a manufacturer's site for a specific product, I calculated the following:

alpha of cast iron rotor 1.49 x 10^-5 m^2/s
alpha of that specific CC rotor 2.04 x 10^-5 m^2/s

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So does a CC setup have a knee point 40% higher than a traditional non composite metallic rotor set up? I'm not quite sure. I would guess that in fact it is more than 40% higher compared to an OEM quality cast iron rotor/sporty pad combo. So that means it would take more energy to reach the fade point with the CC system.
As I explained above, you have more control over Tcontact with a CC setup than a cast Iron setup. There is no such single rule as Tcontact will be 40% higher for the CC setup. It is obvious what you need to do if you want to lower that. If Tcontact is indeed higher for a CC setup, then the chances are that is by design because that results in higher heat flow as explained above. The designers of these systems are not stupid. For a performance application, they will not shoot for an operating temperature that will exceed the optimal operating temperature of the pads and/or the pads can’t deal with.

The street car vs. performance car discussion is not particularly meaningful in that regard. You have been making general remarks about the fade resistence characteristics of the CC braking systems, and I am responding to that. Of course, one does not need a CC system for street use. Fade is not an issue with most systems on the street to begin with.

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But, as I mentioned, the wear/longevity and weight reduction issues are absolutely clear cut and significant. Many CC systems are rated for "lifetime" or about 300k km and the rotors can weigh as little as 1/3rd of an iron/steel rotor. These are the real advantages.
The little research I’ve done indicates that CC brake systems indeed offer superior fade characteristics to the cast iron systems (assuming both systems are properly optimized) due to superior (and adjustable) thermal properties. I obviously disagree with your claims about there being no difference in fade performance. In my mind, this reinforces the validity of what they experienced and reported in those videos during their supercar test at the Ring. Cost, manufacturability, and operational issues such as cracking are not included in that consideration however.
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      11-26-2008, 05:08 PM   #27
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And if you think about it, this all started from the speculation that those brakes are CC, probably because of the caliper color, which is the same as standard PCCB (Porsche Ceramic).
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      11-26-2008, 05:19 PM   #28
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Lucid,
that was an awesome post, but, I'll bet you just killed this thread.
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      11-27-2008, 05:51 AM   #29
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Quote:
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I think you are too quick to dismiss anything coming from someone without some kind of technical education.
Perhaps, but I also think I use a filtering type of system to trust different kinds of things from folks with different backgrounds.

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1. The units for specific heat capacity are J/(kg K). So a material with a higher c will experience less of a temperature increase for the same amount of energy transferred per unit mass.
Yes for a given mass but this is countered by the much lower mass. That is what I stated.

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c of a cast iron rotor is about 450 to 500 J/(kg K)
c of a carbon ceramic composite material can range from 600 to 1700 J/(kg K)

I must assume the wide range of the CC rotor is due to the design parameters associated with its composite construction.
Source? Sure composites can have a widely varying c but the sources I found had only single values for the material used in rotors.

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So, even if you chose to make the CC rotors half the mass of the iron rotors, you can easily end up with the same temperature behavior in this respect. Meaning, you could be transferring the same amount of energy into both rotors, and seeing a similar temperature increase at steady state.
I don't think so. The volumes are very close to work with existing wheels and calipers, etc. The massively lower density gives you a typical rotor weight of about 1/3rd.

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Also, keep in mind that mass is simply a design parameter. One can easily chose to make heavier CC rotors if one wanted, which would actually yield less temperature gain in the above scenario.
Not really, see above.

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I must assume the conductivity has to do with the carbon content, which is highly conductive.
The conductivity of carbon varies drastically depending on what form it is in. Pure amorphous carbon is highly NON conductive.

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In order to maintain consistent and sustainable heat flow away from the contact area to the rest of the disc so that it can be transferred to the environment via convection, you want high k and also high deltaT—the highest Tcontact the pads and the discs will endure for maximum fadeless stopping power will result in the highest deltaT. Meaning, it might be to your advantage to adjust the mass and composition of the CC rotor for operation at an intended higher Tcontact at the friction surface since that will yield a higher deltaT and more heat flow.
Sounds like you are mixing up the delta T between pad and rotor and between rotor and environment. Either way I don't agree with your conclusion at all, you want the temperature of the pads and rotor to be such that you have no chance of permanent damage to either, are in the sweet spot for friction vs. temperature and have some safety margin for higher temperatures. Getting a higher temperature difference between various components just is not part of the design goal or process.

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Another way to think about this is to consider thermal diffusivity
Really not a factor, you are thinking way too much about this. The only relevance here is that carbon-carbon rotors in F1 (not carbon ceramic) actually have really high heating rates and cooling rates and don't have much friction when cold. This causes a split second of fade as the brakes jump in temperature very rapidly. The diffusivity is a factor here but you don't design for it, you deal with it.

Again it is a great first approximation to just say all of a vehicle scrubbed kinetic enegry goes straight to the rotors. Why you you think you see glowing rotors but not glowing pads or glowing calipers? One the physics of the heat transfer does not work that way and even more obviously you would not see a system that behaves this way because the caliper would fail, material, seals, fluids, everything.

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The little research I’ve done indicates that CC brake systems indeed offer superior fade characteristics to the cast iron systems (assuming both systems are properly optimized) due to superior (and adjustable) thermal properties. I obviously disagree with your claims about there being no difference in fade performance. In my mind, this reinforces the validity of what they experienced and reported in those videos during their supercar test at the Ring. Cost, manufacturability, and operational issues such as cracking are not included in that consideration however.
Again perhaps superior to a low end cast iron rotor and crappy pads but perhaps not superior to a higher end BBK type of system with rotors and pads that can take the heat and are tailored for it. I don't have enough data to rigorously prove this, neither do you. But I can tell you it is not simple fact across the board. F1 brakes before the advent of carbon ceramic or carbon-carbon rotors were able to work just fine with no fade problems. They did so through proper materials, proper cooling and proper mass. The biggest advantage of the current systems is WEIGHT and WEIGHT.
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      11-27-2008, 06:50 AM   #30
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They did so through proper materials, proper cooling and proper mass. The biggest advantage of the current systems is WEIGHT and WEIGHT.
Amen. +10000. Nail on head hit. Ceramic disks are total overkill for road cars, and even track cars.

Lots of the physics above went completely over my head, I am sure you two know what you are talking about. But looking at actual usage tells all.

Swamp you asked earlier whether the issues I referred to was the norm or an isolated issue. All I can say is that (at least over in the UK) I have heard loads of people switching to alcon aftermarket steel rotors on the pccb's and numerous stories of people having to get replacements. The longevity argument for pccb's does not stack up in practice imo.

ALso, what disks do most private/club racing teams use? Mostly steel afaik. Yes the ceramics give a weight advantage but this is offset by the fact that they are too expensive, crack too often, and don't offer any real stopping advantage compared to a good steel setup.

I was at the Porsche Club Ireland track day recently which was a mixture of road cars, plus a few racing teams with various RS, RSR's & Cup cars running full slicks. The guys came with 40ft containers and teams of people with computers and telemetry and other stuff.

Not one car had ceramic disks as far as I could tell. Why is that?

Mick
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      11-27-2008, 07:41 AM   #31
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I was at the Porsche Club Ireland track day recently which was a mixture of road cars, plus a few racing teams with various RS, RSR's & Cup cars running full slicks. The guys came with 40ft containers and teams of people with computers and telemetry and other stuff.

Not one car had ceramic disks as far as I could tell. Why is that?

Mick
When and where was this held, I know nothing about it. Also is a said earlier a Porsche mechanic felt it's the fact they are not working at optimum temperature the majority of the time is the cause of the continuing problems they (Porsche) are having, I doubt other manufacturers are any different.

A good aftermarket setup gives equally good braking performance and are as good for most club/track day events at half the price. Weight is the sole advantage I see when placed on a road car, oh yeah and the bragging rights of course.
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      11-27-2008, 08:29 AM   #32
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When and where was this held, I know nothing about it. Also is a said earlier a Porsche mechanic felt it's the fact they are not working at optimum temperature the majority of the time is the cause of the continuing problems they (Porsche) are having, I doubt other manufacturers are any different.

A good aftermarket setup gives equally good braking performance and are as good for most club/track day events at half the price. Weight is the sole advantage I see when placed on a road car, oh yeah and the bragging rights of course.
http://content.us.porsche-clubs.pors...2574BB004F5C6B

As for your mechanics assertion, I don't know why operating at lower than optimum temps would cause disks to crack more. I can understand if operating at lower than optimum temps could cause them not to operate to their best possible standards but to cause more failures? That seems unlikely to me, and also, a few hard laps at any track should have disks up to their proper operating temperatures. Its not as if thye only come in after 50 laps or something.
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      11-27-2008, 09:05 AM   #33
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Source? Sure composites can have a widely varying c but the sources I found had only single values for the material used in rotors.
Some of the values are from the a heat transfer textbook, and CC specific ones are from a reputable manufacturer's website ($1b+ in sales):

http://www.sglgroup.com/cms/internat...ml?__locale=en

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I don't think so. The volumes are very close to work with existing wheels and calipers, etc. The massively lower density gives you a typical rotor weight of about 1/3rd.
Any you can't design a different caliper if you wanted to? The point is you don't need to make it heavier to begin with as you want the higher delta T. I stated why clearly in my previous post.

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The conductivity of carbon varies drastically depending on what form it is in. Pure amorphous carbon is highly NON conductive.
I am aware of that. I've seen many different values, most of them higher.

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Sounds like you are mixing up the delta T between pad and rotor and between rotor and environment. Either way I don't agree with your conclusion at all, you want the temperature of the pads and rotor to be such that you have no chance of permanent damage to either, are in the sweet spot for friction vs. temperature and have some safety margin for higher temperatures. Getting a higher temperature difference between various components just is not part of the design goal or process.
I am not mixing up anything, but it sounds like you don't understand the series of interactions that dissipate the heat. Heat is created at the contact surface. It is then conducted to the rest of the disc material, including parts of the disc that are not directly below the contact surface. Then it is finally transferred to the environment via convection. A higher Tcontact would result in a higher temperature distribution throughout the disc. You saw the conductive heat transfer equation above. The convective transfer rate will also have a delta T, and result in a higher transfer rate. More energy transfered per unit time. No point in arguing against that.

You are all caught up in pad temperatures and friction. Sure that matters, but as I said above you want Tcontact to be as high as possible to remove max amount of possible energy per unit time. If they can make a rotor to operate at high temps, they can make pads to do the same.

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Really not a factor, you are thinking way too much about this. The only relevance here is that carbon-carbon rotors in F1 (not carbon ceramic) actually have really high heating rates and cooling rates and don't have much friction when cold. This causes a split second of fade as the brakes jump in temperature very rapidly. The diffusivity is a factor here but you don't design for it, you deal with it.
Diffusivity puts the two relevant thermal variables in perspective.

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Again it is a great first approximation to just say all of a vehicle scrubbed kinetic enegry goes straight to the rotors. Why you you think you see glowing rotors but not glowing pads or glowing calipers? One the physics of the heat transfer does not work that way and even more obviously you would not see a system that behaves this way because the caliper would fail, material, seals, fluids, everything.
Not sure what you are saying here. You are arguing against the basics of conductive heat transfer? That's like saying f is not equal to ma. There is the geometry and the thermal properties of the other components, and energy will flow down the path of least resistence. The discs are designed such that they are the path of least resistence. A heat sink. That doesn't mean the pad surface does not get hot or anything. As we both know it does, and manufacturers design different pads to operate at different temps. But energy flow through the pads is substantially less than the energy flow through the disc by design. That's the whole point.


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Again perhaps superior to a low end cast iron rotor and crappy pads but perhaps not superior to a higher end BBK type of system with rotors and pads that can take the heat and are tailored for it. I don't have enough data to rigorously prove this, neither do you. But I can tell you it is not simple fact across the board. F1 brakes before the advent of carbon ceramic or carbon-carbon rotors were able to work just fine with no fade problems. They did so through proper materials, proper cooling and proper mass. The biggest advantage of the current systems is WEIGHT and WEIGHT.
So, they shipped out an exotic MC12 will crappy pads? Right. Those are both supercars that must be safe to driven on the street. So it is unlikely that one has high temp pads and the other doesn't. There is a possibility that the pads do have different charactersitics though. What happened in the video is not even the issue here.

I am not saying you can't design a cast iron system that will be fade free even in a demanding situation. That is not the point of this discussion. The discussion is about if a CC will be less likely to fade than an iron setup in general. I am telling you why it will be less likely to fade when things are pushed to the extreme. If you really want to, you can make any brake system fade by putting it an operational scenario that is demanding all the time, not allowing it to cool, etc.

Saving weight is not always the primary consideration, in high speed trains for instance. But fade resistence and stable braking is. To the best of knowledge, CC systems were initialy developed for the TGV.

I'm off for the weekend. Have a good Thanksgiving guys...
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      11-28-2008, 02:27 AM   #34
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Some of the values are from the a heat transfer textbook, and CC specific ones are from a reputable manufacturer's website ($1b+ in sales):

http://www.sglgroup.com/cms/internat...ml?__locale=en
Good site, the specific page is this one and it lists a single specific value for the specific heat of CSiC used in their brakes, which I'd guess is the norm - a single value manufacturer by manufacturer. But the source I used indeed has a different value, 1350 J/kgK. My source is here. Either way the lower value listed in your source only furthers my point that CSiC brakes, for a given stop, will get significantly hotter than a iron/steel set up. That could be hotter and into a better part of the friction-temperature curve or hotter into the fade domain.

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Any you can't design a different caliper if you wanted to? The point is you don't need to make it heavier to begin with as you want the higher delta T. I stated why clearly in my previous post.

...

I am not mixing up anything, but it sounds like you don't understand the series of interactions that dissipate the heat. Heat is created at the contact surface. It is then conducted to the rest of the disc material, including parts of the disc that are not directly below the contact surface. Then it is finally transferred to the environment via convection. A higher Tcontact would result in a higher temperature distribution throughout the disc. You saw the conductive heat transfer equation above. The convective transfer rate will also have a delta T, and result in a higher transfer rate. More energy transfered per unit time. No point in arguing against that.

You are all caught up in pad temperatures and friction. Sure that matters, but as I said above you want Tcontact to be as high as possible to remove max amount of possible energy per unit time. If they can make a rotor to operate at high temps, they can make pads to do the same.

Diffusivity puts the two relevant thermal variables in perspective.

Not sure what you are saying here. You are arguing against the basics of conductive heat transfer? That's like saying f is not equal to ma. There is the geometry and the thermal properties of the other components, and energy will flow down the path of least resistence. The discs are designed such that they are the path of least resistence. A heat sink. That doesn't mean the pad surface does not get hot or anything. As we both know it does, and manufacturers design different pads to operate at different temps. But energy flow through the pads is substantially less than the energy flow through the disc by design. That's the whole point.
Sure you could design a different caliper but that is out of the scope of this discussion as standard calipers are the ones used in these systems (albeit sometimes they are a bit longer to reduce pad pressures). As well the points about conductivity and diffusivity are mostly diversions for the following reason. For a given energy transferred in a stop it is a darn good approximation that all of that energy goes into heat increase of the rotor. You can ignore short term convective cooling and you can ignore the temperature rise of the pad, caliper and fluid, hub, etc. It is just conservation of energy. For simplicity, assuming the end speed is zero and there is a single rotor the basic equation is:

ΔT = m(car)*V^2 / 2*m(rotor)*c

The denominator is 1.4 - 2.4 times LARGER for an iron/steel system compared to a CSiC system (using my c value or the one in your source). Thus the changes in temperature are correspondingly that much lower. It is really significant, not just 10% or 20% lower, as much as 2.4 times less. The reason I keep focusing on temperature and friction is because this is the essence of brake fade, it ultimately has very little to do with heating rates nor dissipation. You have conservation of energy, which determines the temperature rise and then you are either below the fade temperature or above it. Simple.

(Note: conductivity does become very important for the designers of the rotors themselves as the materials low conductivity can have cause a situation where their surface overheats compared to the inner material resulting in a strong gradient in expansion and the problem it presents. I believe improper regard for these effects in the design and engineering of some rotors was a factor responsible for some failures.)

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So, they shipped out an exotic MC12 will crappy pads? Right. Those are both supercars that must be safe to driven on the street. So it is unlikely that one has high temp pads and the other doesn't. There is a possibility that the pads do have different charactersitics though. What happened in the video is not even the issue here.

I am not saying you can't design a cast iron system that will be fade free even in a demanding situation. That is not the point of this discussion. The discussion is about if a CC will be less likely to fade than an iron setup in general. I am telling you why it will be less likely to fade when things are pushed to the extreme. If you really want to, you can make any brake system fade by putting it an operational scenario that is demanding all the time, not allowing it to cool, etc.
In never said anything like the MC12 has crappy pads but the question here is are CSiC rotor based braking systems more resistance to fade than traditional system? It obviously begs the question; which traditional system? My argument is not definitive that a high end traditional system is for sure more fade resistant, nor is it that a CSiC system is not clearly superior in fade compared to a low end OEM system with iron rotors. My argument is the following: the rotor material of a CSiC brake system due to its drastically lower weight will gain more temperature for an equivalent stop than an iron/steel rotor of the same size. Repeating this stop after stop could produce temperatures high enough to pass the knee point of the temperature -friction curve EARLIER than a high end steel rotor with a high performance/high temperature pad. In short the CSiC system could fade sooner/easier. The CSiC systems are not magic nor immune from fade

Another factor that one needs to keep in mind in such a comparison is the "apples vs. apples" factor. CSiC rotors being typical on very high end cars often are accompanied by many other things that help braking like lower weight, better pads, better calipers, better hoses and fluid and most importantly better brake cooling. So sure, this brake SYSTEM will almost surely be more fade resistant than most low end OEM SYSTEMS, but it is not just due to the rotor material.

Cheers man, Happy Thanksgiving to you also.

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      11-28-2008, 04:10 AM   #35
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And if you think about it, this all started from the speculation that those brakes are CC, probably because of the caliper color, which is the same as standard PCCB (Porsche Ceramic).
Well yes, but the video quality wasn't clear enough for me to come to a certain conclusion.

Once we got the pics, it was easy to tell. Regardless, eventually we will see BMW carbon ceramic brakes. When? Who knows.
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      11-30-2008, 03:21 PM   #36
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this one[/url] and it lists a single specific value for the specific heat of CSiC used in their brakes, which I'd guess is the norm - a single value manufacturer by manufacturer. But the source I used indeed has a different value, 1350 J/kgK.
Yes, that is what they are referencing for that specific product. There is no reason to guess that is the "norm". It is a composite material, and its properties can be manipulated depending on what the design requirements are. There surely will be strength trade-offs, but there is no reason to assume that all CC brake systems do/will have similar material properties. Actually, they simply don’t.

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Either way the lower value listed in your source only furthers my point that CSiC brakes, for a given stop, will get significantly hotter than a iron/steel set up. That could be hotter and into a better part of the friction-temperature curve or hotter into the fade domain.
Sure, it will be hotter if the volumes are the same. I never said it wouldn't be. I said it doesn't have to be if you don't want it to be as you can change the volume or manipulate the composition. But the higher temps in the CC system are actually desirable for reasons I explained previously as long as they don’t exceed the operational ranges.

You keep on referencing some kind of absolute a friction-temp curve. What friction-temp curve exactly? For what pad and rotor combination?

I've said this several times now: pad material can be engineered the same way rotor material can be engineered to perform at higher temperature so that peak friction will be produced within the intended temperature range. The point is to match the operational temp range of the pad with the operational temp range of the disc. You’ll clearly not use the pads you use with a cast iron rotor with a CC rotor. You’d have to use a different pad that will yield peak CoF within the higher operational temp range of the CC system. The higher that operational temp range, the higher the heat transfer. Period.

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Sure you could design a different caliper but that is out of the scope of this discussion as standard calipers are the ones used in these systems (albeit sometimes they are a bit longer to reduce pad pressures).
That is within the scope of the discussion. You have been making very general statements about the fade characteristics of CC systems, not just in this thread, but in other threads as well. As I said earlier, that is what I am responding to. Plus, there is no actual product that is being referenced in this thread to begin with.

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As well the points about conductivity and diffusivity are mostly diversions for the following reason. For a given energy transferred in a stop it is a darn good approximation that all of that energy goes into heat increase of the rotor. You can ignore short term convective cooling and you can ignore the temperature rise of the pad, caliper and fluid, hub, etc. It is just conservation of energy.
Swamp, buddy, I don't even know where to begin here. You refuted Fourier's Law of Heat Conduction in your post #29, and you keep on arguing against well known heat transfer principles for reasons I can't decipher. Why argue against well established and utilized theory? Have you actually studied heat transfer as a distinct discipline? If you have, I am perplexed. This stuff would be pretty transparent to anyone who has received any formal training on the topic. I'll try to explain what you are missing once more below.

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Originally Posted by swamp2 View Post
For simplicity, assuming the end speed is zero and there is a single rotor the basic equation is:

ΔT = m(car)*V^2 / 2*m(rotor)*c

The denominator is 1.4 - 2.4 times LARGER for an iron/steel system compared to a CSiC system (using my c value or the one in your source). Thus the changes in temperature are correspondingly that much lower. It is really significant, not just 10% or 20% lower, as much as 2.4 times less. The reason I keep focusing on temperature and friction is because this is the essence of brake fade, it ultimately has very little to do with heating rates nor dissipation. You have conservation of energy, which determines the temperature rise and then you are either below the fade temperature or above it. Simple.
Your consideration is rather inadequate. The conservation of energy principle you outlined above says absolutely nothing about how the thermal system behaves in the time domain. Nothing. Zip. The heat transfer equation provides that insight. Units of Qdot are obviously Watts. Energy per unit time. That's braking power.

Imagine that a rotor has very low thermal conductivity. The equation you outlined above still applies. However, what it does not tell you is that the temperature change would take a very long time to occur. But you don't have a very long time to brake—to have that energy conversion process to take place--and slow down. That is why thermal conductivity is NOT a diversion of any kind. Why do you think all of these sources are quoting thermal conductivity figures?

Again, this is a direct implication of Fourier's Law. That is why you have to consider heat transfer rates. You have to describe and analyze the whole system as a dynamic thermal circuit with transient and stead-state thermal response. (The conductive heat transfer equation I outlined above is for steady-state only by the way.)

As a note, it seems that the thermal conductivities of CC materials used in the current CC systems are either slightly higher or lower than cast iron systems although the potential range for CC systems is clearly significantly higher based on the properties of CC materials.

Just to beat the relevance of thermal conductivity to death, I did a little literature review today and found a relevant journal article. Here is a quote from the section, “C/C-SiC composites for advanced friction systems: Tribological Properties”:

“High transverse heat fluxes from the outer region (friction surface) to the center of the composite are necessary to avoid an overheating of the friction surface, i.e. high transverse conductivities must be adjusted. Standard C/C-SiC composites with bi-directional reinforcements used in hot structures of spacecraft have been modified in their composition and microstructure and led to essential improvements. Particularly, by increasing the silicon carbide content of the composite the transverse thermal conductivity was doubled in comparison to the original space materials.”

Krenkel, W., Berndt, F. “C/C-SiC composites for space applications and advanced friction systems,” Material Science and Engineering, A 412, p. 177-181, 2005.

This is exactly what I have been saying all along. If you want to disagree with a domain expert as well, and call the discussion on thermal conductivity a diversion, that’s your choice I guess, but that would be neither scientific nor scholarly.

Building on that, the equation you outlined does not say anything about what happens when you brake often. Where does that thermal energy stored in the rotor go? If it doesn't go somewhere else quickly, then what will happen when you brake again soon, and again? The temperature will rise above whatever the operational range for the system in question, and things will melt/vaporize. That is where convection and heat transfer into the environment comes in.

Again, the faster you cool the rotor in between brake application, the less likely temps will exceed the operational range for the system. And according to Newton's Law of Cooling, you will transfer more energy per unit time if (Trotor-Tair) is higher. Actually, forced conduction simply steepens the temperature gradient at the surface, and increases the convective heat transfer coefficient, which is why vented discs and cooling ducts, etc, make a huge difference in fade characteristics.

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Originally Posted by swamp2 View Post
In never said anything like the MC12 has crappy pads
Fine. I thought you were still talking about the video.

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Originally Posted by swamp2 View Post
but the question here is are CSiC rotor based braking systems more resistance to fade than traditional system? It obviously begs the question; which traditional system?
Compare high end CC system to high end cast iron system.

Quote:
Originally Posted by swamp2 View Post
My argument is the following: the rotor material of a CSiC brake system due to its drastically lower weight will gain more temperature for an equivalent stop than an iron/steel rotor of the same size. Repeating this stop after stop could produce temperatures high enough to pass the knee point of the temperature -friction curve EARLIER than a high end steel rotor with a high performance/high temperature pad. In short the CSiC system could fade sooner/easier. The CSiC systems are not magic nor immune from fade
Who said CC systems are magic? They just have superior thermal properties, that's all. Are they affordable, easy to manufacture, failure free at this time? No. Heck, for 99.99% of the people who deal with cars, they are not even necessary even if they were cheap and problem free. But they can offer better fade performance. However, the cost, manufacturing, and failure issues will most likely be resolved over time.

By the way, thanks for the reference to the SGL Carbon presentation.

You should check out the other parts of that presentation. For instance, they also suggest making larger discs and improving convective heat transfer to cope with the lower volumetric heat capacity of CC materials (note that I said volumetric heat capacity and not heat capacity).

http://www.sae.org/events/bce/presen...6wuellner3.pdf

Just make them bigger. And since their density is 4 times less than cast iron, weight is not an issue, and you end up with a rotor that can absorb more energy for the same temperature gain, and conducts heat at a higher rate for better thermal response and dissipation at higher operational temps. Those are facts. Not magic.

The SGL carbon presentation clearly lists the fade performance as an advantage CC systems here although the wording is exaggerated (remember, that is your source):

http://www.sae.org/events/bce/presen...6wuellner2.pdf

A more neutral party, a scientist, the domain expert referenced earlier, also identifies fade performance as an advantage:

“After seven years of development, C/C-SiC braking materials are now entering the market in high performance cars, trains, and lifts. Their main advantages lie in the reduced mass, their non-fading characteristics, their improved coefficient of friction, and their extremely low wear rates.” (Krenkel, 2003; from conference paper abstract).
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      11-30-2008, 04:48 PM   #37
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      12-01-2008, 05:29 AM   #38
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For ALL CSiC thermal conductivities I have seen and all rotor thicknesses I have examined, for a given stop, due to the material properties, the CSiC system will indeed be hotter. And you can't argue that at some point hotter is closer to fading. This is more so from their low weight than their thermal conductivity. You just can not find a conductivity that will change this basic physics. There are no CSiC rotors that are three times the thickness nor three times the area of a traditional system to provide the equivalent thermal mass. The reason that high temperatures are the ultimate evil is that the pads will eventually simply give out, fade is first, accompanied or followed by excessive outgassing, material transfer, delamination, melting, ignition, etc.

We could settle this debate pretty well if we had the "magic" friction-temperature curve for a typical CSiC rotor and pad combination but for the life of me I can not find one. I actually expect that information is fairly proprietary. I have seen some fade data for CSiC systems, but not in this useful form and they were so far from fade (i.e. at such low temperartures) it was not really very interesting. A typical temperature-friction plot for a variety of Pagid brake pads is shown below. As you can see the lesser ones fade around 400 °C and the more high performance ones not until about 650 °C (note the C and F reversal/typo in the plot...). Since I figured you had seen on of these plots previously I did not feel the need to post it, but it sounds like you haven't, so here it is. You can see here, as I have been referring to, the "knee point" in each curve and it represents the onset of fade. Lastly on this point these are definitely not the highest performance/highest temperature pads available. Turner Motorsports sells a pad that can cope with over 760 °C!

Now here is a kicker: As far as I can tell, CSiC rotor brake systems actually do use standard issue pads, albeit standard "ceramic" pads with added ceramic for both wear and high temperature capability. Of course such pads are available for non CSiC rotors as well! Pads are the more critical factor here, much more critical than the rotors.

You keep talking about how this can be made heavier or this can be thicker or this can be engineered, etc., etc. That is not what this discussion is about. It is simply about is a CSiC braking system more fade resistant (with the clarification you have added that we should discuss a high end version of each system - do note that that would imply an alloy steel rotor not a cast iron one...)? But we are not talking about what CAN be done, we are restricted here to what IS available now, today. Agree or disagree?

I never denied any fundamental equations of heat transfer and yes I have studied the subject thank you very much. Although typically my interest was a notch or two above the basics of heat transfer - more on the statistical mechanics side of things - the first principles from which thermodynamics comes.

The equation I provided is a darn good approximation, based on sound principles and real world experience. The other effects are truly secondary as I pointed out. Think about it this way: just at the end of a very hard but very intense and rapid braking event, when everything begins at ambient temperatures, where is the vehicles kinetic energy? It has to go somewhere and very little is lost to the structures other than the rotor (and pad) and very little has had time to dissipate by convective cooling. The rotor may still have a thermal gradient and indeed in a CSiC rotor it will probably have a higher gradient than in a very conductive iron/steel rotor which will reach a uniform temperature quite quickly (this is certainly a factor in saying the equation ignores 2nd order effects, of which non uniform temperatures is one indeed). Then when you combine multiple stops you certainly get some convective cooling and conduction which provides the temperature rise of other components (caliper, pistons, fluid, hub, rim, etc) accompanied by the temperature fall of the rotor. Indeed all of this is exactly as per some of the equations you mentioned. But once you slam on the stoppers again the temperature rise is, to first order, governed by the equation I provided. I know it is fairly accurate not only as it is from first principles, but because I have actually tested it. It works, plain and simple. If you can provide another simple expression that provides the temperature gain of a rotor under a given braking scenario please do so. It is a matter of drastically different time scales, energy production can be extremely rapid but both conduction and dissipation are much slower due to a poor path (by design - we can't have brake fluid at the temperature of the disc!) and basic material properties respectively. Again the other equations you mention would be used to rigorously prove this for a typical, real world brake system. But this is why the parameters and equations you mentioned are not critical to the question of fade. It is quite simply - more simple than that for an actual real world system.

Now we all know that a particular pad may operate better at a higher temperature than another, but you also keep sticking to this rather absurd idea that you always want a hotter rotor so it will cool faster. This is pretty nonsensical. Its just like saying that if the goal is to empty a container you should keep filling it so you can empty more from it. Hotter is fine, as long as your typical operating temperature is far enough away from the knee for some margin.

I read the "...Tribological" paper as well. It was the source for my comments above about the importance of conductivity in the design of CSiC rotors. BUT conductivity is important to prevent thermal stress from making a rotor fail - NOT TO PREVENT IT FROM FADING. These are fundamentally different concerns.

Until we have the magic temperature-friction curve for a typical high end CSiC pad-rotor combination we can not really settle this debate. If we can find such data we may have the answer for one particular system but even then not an answer in general. Lastly if we did have the data, I would simply go right back to the conservation of energy equation and see which system would reach its fade temperature first. What would you do with the temperature-friction curve?

My beliefs/suspicions/educated guesses/calculations support the following:

1. Some high end racing brake systems based on a steel rotor and very high temperature pads will show better fade resistance than a typical CSiC rotor based system. The reason being is the simple fact that a CSiC systems rotors are so lightweight they will get to their fade temperature sooner under an identical total/cumulative energy input.

2. Most CSiC systems (being that they are intrinsically very high end) will show improved fade resistance to a mid range/sports brake system such as say our very own E92 M3 braking system. The primary weak link in a system like ours is NOT the rotor material but first pad material, followed by sufficient brake cooling.

3. The advantages of a CSiC rotor based braking system are, in order of most benefit to least benefit:
a. Weight reduction
b. Longevity
c. Fade resistance (which comes more from the pad choice rather than the rotor material).


P.S. Eagle: This is not a thread jack. We are right on topic on carbon ceramic rotors and this forum tends to get pretty techie/nerdy (especially when lucid and I are involved...). If you don't like it you don't have to read.
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      12-01-2008, 08:53 PM   #39
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This is more so from their low weight than their thermal conductivity. You just can not find a conductivity that will change this basic physics. There are no CSiC rotors that are three times the thickness nor three times the area of a traditional system to provide the equivalent thermal mass.
You’ve got that one wrong. For a CC rotor to match the heat capacity of a cast iron rotor, it needs to be exactly 1.7 times larger in volume. At that increased volume, the CC rotor is still 2.4 times lighter than the cast iron rotor. What is that if not a superior thermal property?

The 1.7 ratio is coming from your source, and is likely to be reduced in the future.

I don’t know the thickness of the Veyron’s rotors, but SGL carbon indicates it was designed with CC brake system in mind, so that would be interesting to find out. Apart from that, what is or is not available as a “swap in” part is rather irrelevant. What’s the point of trying to plug in new technology into old infrastructure?

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We could settle this debate pretty well if we had the "magic" friction-temperature curve for a typical CSiC rotor and pad combination but for the life of me I can not find one. I actually expect that information is fairly proprietary.
That info seems to be proprietary at this time as indicated in the Krenkel article. There is nothing magical about it though. If it was indeed impossible to produce pads that would provide peak friction characteristics in the operational temperature range cited for CC systems, there would be absolutely no point in developing the rotors, or designing for such operational ranges. Do you really think the developers of such systems are that stupid?

Since you seem to have not read my previous post, I’ll re-post the other quote from Krenkel, this time focusing on CoF:

““After seven years of development, C/C-SiC braking materials are now entering the market in high performance cars, trains, and lifts. Their main advantages lie in the reduced mass, their non-fading characteristics, their improved coefficient of friction, and their extremely low wear rates.” (Krenkel, 2003; from conference paper abstract).”

That language may not be very specific—probably because I am quoting from the abstract as I don’t have access to the full paper yet—but you think this guy is just making this stuff up?

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Originally Posted by swamp2 View Post
Since I figured you had seen on of these plots previously I did not feel the need to post it, but it sounds like you haven't, so here it is. You can see here, as I have been referring to, the "knee point" in each curve and it represents the onset of fade.
I saw that plot. If I remember correctly, it was created by a vendor. Regardless, so what? What does that tell me that I don’t already know? Please read what I wrote in my previous post about peak friction and operational temperature range. Do I have to repeat everything several times in this discussion for you to understand what I am saying?

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Now here is a kicker: As far as I can tell, CSiC rotor brake systems actually do use standard issue pads, albeit standard "ceramic" pads with added ceramic for both wear and high temperature capability.
Probably for street drivability, etc. Who cares about those systems since they will never be really driven hard enough and fade will never be an issue anyway like I said several times before?

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Originally Posted by swamp2 View Post
You keep talking about how this can be made heavier or this can be thicker or this can be engineered, etc., etc. That is not what this discussion is about. It is simply about is a CSiC braking system more fade resistant (with the clarification you have added that we should discuss a high end version of each system - do note that that would imply an alloy steel rotor not a cast iron one...)? But we are not talking about what CAN be done, we are restricted here to what IS available now, today. Agree or disagree?
That’s your interpretation, not mine. So, following your logic, people shouldn’t have discussed DFI 5-7 years ago? This technology is in its first iteration as a commercial application. It will see significant change/improvement in 5-7 years.

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I never denied any fundamental equations of heat transfer
You dismissed the role of conductivity in heat transfer and said the following:

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Originally Posted by swamp2 View Post
Again it is a great first approximation to just say all of a vehicle scrubbed kinetic enegry goes straight to the rotors. Why you you think you see glowing rotors but not glowing pads or glowing calipers? One the physics of the heat transfer does not work that way and even more obviously you would not see a system that behaves this way because the caliper would fail, material, seals, fluids, everything.
The heat conduction equation I used explains all that. The physics of heat transfer works exactly like that. The reason you don’t see glowing calipers is explained by that principle as follows:

1. Most high end pads will have insulating (low conductivity) materials between themselves and the pistons as that is the surface area through which conductive heat transfer to the calipers and the rest of the brake system takes place. Low conductivity means low heat transfer.
2. Piston/pad contact surface area is much smaller than the rotor/pad contact surface area. This again results in low heat transfer.

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Originally Posted by swamp2 View Post
The equation I provided is a darn good approximation, based on sound principles and real world experience.
What real world experience? Have you tested brake systems with significantly different conductivity values for the rotors in high energy systems? If not, your experience is irrelevant. If yes, please share any relevant data.

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Originally Posted by swamp2 View Post
Think about it this way: just at the end of a very hard but very intense and rapid braking event, when everything begins at ambient temperatures, where is the vehicles kinetic energy? It has to go somewhere and very little is lost to the structures other than the rotor (and pad) and very little has had time to dissipate by convective cooling.
The rotor may still have a thermal gradient and indeed in a CSiC rotor it will probably have a higher gradient than in a very conductive iron/steel rotor which will reach a uniform temperature quite quickly (this is certainly a factor in saying the equation ignores 2nd order effects, of which non uniform temperatures is one indeed). Then when you combine multiple stops you certainly get some convective cooling and conduction which provides the temperature rise of other components (caliper, pistons, fluid, hub, rim, etc) accompanied by the temperature fall of the rotor. Indeed all of this is exactly as per some of the equations you mentioned. But once you slam on the stoppers again the temperature rise is, to first order, governed by the equation I provided. I know it is fairly accurate not only as it is from first principles, but because I have actually tested it. It works, plain and simple. If you can provide another simple expression that provides the temperature gain of a rotor under a given braking scenario please do so.
How does the kinetic energy of the vehicle end up in the rotor? How does the rotor experience the temperature change the conservation of energy equation dictates? Through what mechanism exactly? Magic? Actually, through conduction. Heat is generated at the surface and is conducted to the rest of the rotor. This is what you are missing. That is what I’ve been saying, and the Krenkel quote states as well. Since you clearly did not read my previous post, here is the relevant quote again:

“High transverse heat fluxes from the outer region (friction surface) to the center of the composite are necessary to avoid an overheating of the friction surface, i.e. high transverse conductivities must be adjusted. Standard C/C-SiC composites with bi-directional reinforcements used in hot structures of spacecraft have been modified in their composition and microstructure and led to essential improvements. Particularly, by increasing the silicon carbide content of the composite the transverse thermal conductivity was doubled in comparison to the original space materials.”

Now, what does this quote say Swamp? It has been written in clear English. It is saying you need high conductivity, and otherwise, the friction surface will overheat. How clearer can this be made exactly?

And, what happens when the friction surface overheats? You exceed the operational temperature range of the pad and the rotor. The rotor might start melting. But before that you will most likely see reduction in friction unless you are using a pad that has a flat or positively sloped CoF curve at that temperature range. If you see reduction in friction, you will experience fade. So, to control for fade, you need to control friction surface temperature by increasing conductivity as well as heat capacity. Convection is also a factor, but probably to a much lesser degree.

Quote:
Originally Posted by swamp2 View Post
Now we all know that a particular pad may operate better at a higher temperature than another, but you also keep sticking to this rather absurd idea that you always want a hotter rotor so it will cool faster. This is pretty nonsensical. Its just like saying that if the goal is to empty a container you should keep filling it so you can empty more from it. Hotter is fine, as long as your typical operating temperature is far enough away from the knee for some margin.
You continue to miss the basics here. You are repeatedly charging and discharging a thermal reservoir by transferring energy in and out of it. That’s what braking is. The faster you can charge and discharge that reservoir, the better, which means the faster you transfer the energy, the better. Qdot is a measure of that rate, and it is higher when the deltaTs are higher. What is so hard to understand here?

Finally, you completely ignored the expert opinion I referenced in my previous post on the improved fade performance of CC systems as an advantage.
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      12-01-2008, 11:14 PM   #40
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Lucid: I suppose I am about done here. But your style and tone will probably keep the discussion going.

Despite our reasoably good history and most often seening eye to eye, we are clearly not seeing eye to eye on anything on this topic. We can not agree on the physics, we can not agree on the question that is being asked and we can not agree on what information is missing or what is needed to answer the question. I will offer a few more selected comments though.

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Originally Posted by lucid View Post
I don’t know the thickness of the Veyron’s rotors, but SGL carbon indicates it was designed with CC brake system in mind, so that would be interesting to find out. Apart from that, what is or is not available as a “swap in” part is rather irrelevant. What’s the point of trying to plug in new technology into old infrastructure?
The factor depends on the specific heat used and thus there is a range of factors. But again any way you slice it there are no CC rotors I have seen with a volume required to offset their reduced mass. Sure the lower mass is good but not good from a thermal mass point of view - that is the whole point.

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Originally Posted by lucid View Post
I don’t know the thickness of the Veyron’s rotors, but SGL carbon indicates it was designed with CC brake system in mind, so that would be interesting to find out. Apart from that, what is or is not available as a “swap in” part is rather irrelevant. What’s the point of trying to plug in new technology into old infrastructure?
On a car where almost all parts are ultra high end, totally custom and made from exotic materials I think the Veyrons brakes are a bit outside of the scope of our debate. But then again we can't agree on much so maybe it is fair game.

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Originally Posted by lucid View Post
That language may not be very specific—probably because I am quoting from the abstract as I don’t have access to the full paper yet—but you think this guy is just making this stuff up?
I've read all of your (their) wonderful quotes about the improved fade resistance of CSiC systems. That does not make it so. I have seen NO test data, no component data nor any basic calculations that support such claims. Neither have you. Scientists and engineers have been known to stretch the truth and even to lie outright in the name of promoting their own products. Do note how most such statements begin with the first benefit being reduced mass, inline with what I have said all along (again making an assumption that folks tend to list the most important things first). Lastly have any of these folks offered one miniscule piece of evidence as to the CSiC systems behaving better COMPARED TO WHAT? They haven't and this is a big part of my point. Your skepticism here is very curiously lacking.

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Originally Posted by lucid View Post
I saw that plot. If I remember correctly, it was created by a vendor. Regardless, so what? What does that tell me that I don’t already know? Please read what I wrote in my previous post about peak friction and operational temperature range.
What is tells you is that when you exceed a certain tempertaure you get fade, period. This is why the temperature-friction curve FOR THE PADS (in combination with a rotor) is the ESSENSE of this question.

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Who cares about those systems since they will never be really driven hard enough and fade will never be an issue anyway like I said several times before?
You have completely missed the point. I suspect that CSiC rotor based brake systems are using, more or less, the "same old pads" which means they will be PAD fade limited not rotor material limited.

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Originally Posted by lucid View Post
1. Most high end pads will have insulating (low conductivity) materials between themselves and the pistons as that is the surface area through which conductive heat transfer to the calipers and the rest of the brake system takes place. Low conductivity means low heat transfer.
2. Piston/pad contact surface area is much smaller than the rotor/pad contact surface area. This again results in low heat transfer.
Exactly - as I stated more simply the path is non conductive (or is more resistant to the heat flow). Which is exactly why my equation is valid! A further reference to this equation comes from the "Brake Handbook" by Fred Puhn, ISBN 0-89586-232-8, which provides the exact equation and reasoning I have on pp 8-11.

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What real world experience? Have you tested brake systems with significantly different conductivity values for the rotors in high energy systems? If not, your experience is irrelevant. If yes, please share any relevant data.
In my first mechanical engineering position I did design, CAD, analysis, testing and fabrication of the first cable actuated hydraulic disc brake system for high end mountain bikes. I built static and dynamic test fixtures, performed thermal studies in the lab and in the field, tested for hand pressure vs. brake torque and studied a wide variety of organic and sintered metal pads for friction vs. temperature characteristics. I think the two highlights of my achievements on this project was the design of the anti noise/drag spring which has been copied on many bicycle brake systems through the present day and the design and testing of high end hollow ceramic brake pistons that drastically reduced brake fluid temperatures. As part of this effort I did testing with various riders and from various speeds to verify the conservaton equation I keep focusing on. Of course I don't have that data but I was satisfied with its accuracy.

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But before that you will most likely see reduction in friction unless you are using a pad that has a flat or positively sloped CoF curve at that temperature range. If you see reduction in friction, you will experience fade. So, to control for fade, you need to control friction surface temperature by increasing conductivity as well as heat capacity.
We basically agree here, amazing, it is just that surface temps are always initially higher, in all materials but it can be further exaggerated (i.e. worsened, i.e. made closer to a fade situation) in CSiC. After a braking event, to observe conservation of energy you need to let the rotor reach equilibrium or estimate the internal vs. external temperature and integrate or average. But again you will still observe conservation of energy. The point is not only that the CSiC rotors are hurt in fade performance by being so much lighter they are also hurt by having higher surface temps from less transverse conductivity. No matter how high the conductivity gets you can not have an average temperature lower than that predicted by the conservation of energy. Now if you add the effect of constant heating and cooling from repetitive braking events you end up with the same situation, a bit hotter surface temps and a bit cooler internal temps and each braking event increases both at a slightly different rate but governed to first order by conservation of energy.

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You continue to miss the basics here.
Well I hold the exact same position.

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You are repeatedly charging and discharging a thermal reservoir by transferring energy in and out of it. That’s what braking is. The faster you can charge and discharge that reservoir, the better, which means the faster you transfer the energy, the better. Qdot is a measure of that rate, and it is higher when the deltaTs are higher. What is so hard to understand here?
Conservation of energy, pad compound and friction-temperature curves are the keys to which will fade first or easier. Conduction through the rotor as well as all heating and cooling rates, regardless of source are secondary effects for both cast iron and CSiC systems. It is so simple - that system which first gets to the fade temperature will fade first. This equation works, is simple, is accurate enough and can answer the question along with some basic friction-temperature curves. I note that curiously you have not proposed a single conceptual calculation method nor stated what data is missing for determining an answer to the fundamental question. Before rattling on and on I would suggest that you do propose both a method and what is missing. Again a FIRST order type of calculation, not exact in all of its gory details.

I still hold very firmly to my numbered points 1->3 at the end of my previous post.
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      12-02-2008, 12:09 AM   #41
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I don't see the need to respond to the details of your last post. And to try to keep things collegial, I won't refer to your language as "rattle", but do go over your posts to see the tone you used in this exchange.

You have once again avoided the issue, the clear question I posed, and actually answered, which is simply: How does the kinetic energy of the vehicle end up in the rotor? How does the rotor experience the temperature change the conservation of energy equation dictates? Through what mechanism exactly?

The question has been answered by a domain expert, a scientist, as well in a well respected peer reviewed journal. The man has written a dissertation and a book on the topic and who knows what else, and you question the validity of his explanation--that one needs to increase conductivity to control friction surface temperatures. You think this explanation would have been published in a peer reviewed journal if it was false? Something as basic as that, simply wrong?

Let's see. Who is more credible here? A well-respected and published scientist, who specializes in this specific area, or Swamp, who has worked on mountain bike brakes for two years (not to discredit your experience in that domain, but that is a different domain, a different ball game).

As to your question, I have already answered it. The ultimate temperature gain of the entire rotor will be governed by the conservation of energy equation (which should also account for whatever energy that is transfered to the environment via convection during braking as well). However, the friction surface temperatures will be dictated by how fast you can transfer the energy that is being generated at the surface elsewhere, which is dictated by the conductive heat transfer principles I've been outlining all along, deltaT (temperature difference between the friction surface and the vented surface of the disc) and conductivity. The temperature distribution throughout the rotor is the key issue here. You want that to be uniform as possible during braking (and it clearly will not be uniform) to achieve the lowest friction surface temperatures.
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      12-02-2008, 05:25 AM   #42
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I don't see the need to respond to the details of your last post. And to try to keep things collegial, I won't refer to your language as "rattle", but do go over your posts to see the tone you used in this exchange.
I chose my tone clearly based on your jabs big guy, tit for tat.

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You have once again avoided the issue, the clear question I posed, and actually answered, which is simply: How does the kinetic energy of the vehicle end up in the rotor? How does the rotor experience the temperature change the conservation of energy equation dictates? Through what mechanism exactly?
Wait a second friction generating heat, isn't that absolutely common grade school knowledge??? The conversion of energy into heat. Again we must not be communicating, as this is the answer to the question you are asking. I really did not think it warranted a reply.

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The question has been answered by a domain expert, a scientist, as well in a well respected peer reviewed journal. The man has written a dissertation and a book on the topic and who knows what else, and you question the validity of his explanation--that one needs to increase conductivity to control friction surface temperatures. You think this explanation would have been published in a peer reviewed journal if it was false? Something as basic as that, simply wrong?

Let's see. Who is more credible here? A well-respected and published scientist, who specializes in this specific area, or Swamp, who has worked on mountain bike brakes for two years (not to discredit your experience in that domain, but that is a different domain, a different ball game).
Nope I don't question this particular point. Any effort in any material to increase lateral conductivity will help limit peak temperatures and hence reduce fade. BUT... THIS IS NOT THE QUESTION WE ARE ASKING IT IS SIMPLY ONE SECONDARY EFFECT IN DETERMINING THE PAD TEMPERATURE FOR THE SYSTEMS AND TO THEN COMPARE THOSE TO THEIR RESPECTIVE LIMIT TEMPERATURES. It is again nearly common knowledge and something that was part of my very first explanation saying that you needed to "average" and that there are "second order effects". As you might know the source of all of this emphasis on conductivity of CSiC materials is because many initial materials and perhaps even initial PRODUCTS had a very poor lateral conductivity which only FURTHERS the higher surface temperatures and hence causes the onset of fade EARLIER! You can think of this as part of the rotor - the interior - what should be very "valuable" thermal mass being so insulated from the surface that is does not even participate in the conservation equation. What happens - EARLIER FADE!!

But back the THE expert:

-Who is stating a fact without providing any evidence whatsoever?

-Who is stating a scenario while being terribly (inexcusably IMO) vague with regards to which exact systems are being compared? We've already been through this ourselves and you know you can not make a blanket statement here that all CSiC rotor based systems are superior in terms of fade. Again they are not magic materials. IT IS THE PAD THAT IS ALMOST FOR SURE THE LIMITING FACTOR.

-These massive shortcomings point out excessive marketing and either being light on the science or seriously heavy on the confidentiality.

-I am not calling the experts wrong I am simply saying they may not be universally correct.

-I figured you would pull the punch on bikes being different than cars. But you know what, you are categorically incorrect. That is the good thing about physics as compared to engineering. Physics is universal and conservation of energy is universal as well. And finally disc brakes are disc brakes are disc brakes. They convert kinetic energy to heat by friction, period. Grab a copy of the brake handbook and argue with this "domain expert" if you like.

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Originally Posted by lucid View Post
As to your question, I have already answered it. The ultimate temperature gain of the entire rotor will be governed by the conservation of energy equation (which should also account for whatever energy that is transfered to the environment via convection during braking as well). However, the friction surface temperatures will be dictated by how fast you can transfer the energy that is being generated at the surface elsewhere, which is dictated by the conductive heat transfer principles I've been outlining all along, deltaT (temperature difference between the friction surface and the vented surface of the disc) and conductivity. The temperature distribution throughout the rotor is the key issue here. You want that to be uniform as possible during braking (and it clearly will not be uniform) to achieve the lowest friction surface temperatures.
Well we almost have some agreement. But you keep missing the forest for the trees. We agree surface temps are higher during braking and before equilibration, however this is a second order effect and it is not unreasonable to approximate "the" rotor temperature by the spatial average of it. Either way, with any rotor that has a higher surface temperature will simply cause such a system to fade a bit earlier than the conservation equation predits. Finally, to truly compare two systems we must have the friction-temperature curve for a typical CSiC rotor-pad matched set and it seems you may be coming around to this idea. As well it seems you are coming around the the validity of the conservation equation. As you first called it an "over generalization" and "inaccurate" but now state is it yourself just above as a definitive part of a calculation based approach to answering this question.

I'm still sticking firmly to my points #1-#3 from many posts back. I haven't heard any specific disagreement to those yet. If you believe identically as your infallable expert that categorically all CSiC systems provide superior fade resistance you should be disputing my item #1.
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      12-02-2008, 08:56 PM   #43
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Wait a second friction generating heat, isn't that absolutely common grade school knowledge??? The conversion of energy into heat. Again we must not be communicating, as this is the answer to the question you are asking.
Sure friction generates the heat. The question is about how the heat that is generated at the surface of the rotor ends up in the rest of the rotor. Via conduction. We clearly are not communicating.

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Nope I don't question this particular point. Any effort in any material to increase lateral conductivity will help limit peak temperatures and hence reduce fade.
So, you finally acknowledge the relevance of conduction after initially dismissing it as a "diversion"…

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It is again nearly common knowledge and something that was part of my very first explanation saying that you needed to "average" and that there are "second order effects". As you might know the source of all of this emphasis on conductivity of CSiC materials is because many initial materials and perhaps even initial PRODUCTS had a very poor lateral conductivity which only FURTHERS the higher surface temperatures and hence causes the onset of fade EARLIER! You can think of this as part of the rotor - the interior - what should be very "valuable" thermal mass being so insulated from the surface that is does not even participate in the conservation equation. What happens - EARLIER FADE!!
Great, you acknowledge the relationship between conductivity and fade in detail here. Then:

1. Fade performance of any brake system will be improved by increasing rotor’s thermal conductivity, but this will be ultimately limited by the rotor’s ability to absorb the energy without heating too much, which is dictated by mass x specific heat. (Again, that is why I quoted BOTH specific heat capacity AND conductivity ranges in my very first post, #26.)

2. Conductivities of current CC rotors are either slightly above or below the conductivity of iron rotors. Conductivity of CC rotors will most likely increase even further given the potential of CSiC materials and the developmental history of their application to CC brake systems (I realize there are some trade-offs there). However, as I said above and as we agree, increasing conductivity might not be enough as the rotor’s ability to absorb energy without heating too much should be increased as well, so you might need to increase mass x specific heat as well (there is another consideration there around what is too much exactly and is that different for the two systems). Specific heat of CC rotors is already much higher than iron rotors (and it might increase even slightly more in the future). So, you need to increase their volume by about 37% to match the specific heat x mass value of the iron rotor (I had this as 70% in my previous post as I mistakenly took down the carbon-carbon values). That combination will yield a CC system that has better fade characteristics than iron systems.

3. Moreover, CC rotors can actually operate in a stable manner at higher temperatures to begin with, so increasing their mass by making them larger might not be as big of an issue to begin with (this is assuming proper pads are being used).

4. The conservation of energy equation does not explain how temperature is distributed across the rotor. It does not provide any information as to the specifics of the thermal response across the rotor cross section. And we both agree that temperature at the friction surface is the critical issue. That’s why you need to consider conduction principles in conjunction with the conservation of energy principle, and that is why I said your consideration is inadequate. When you look at those principles in detail, it is clear that temperature distribution across the rotor will be a function of conductivity AND specific heat capacity as outlined above.

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But back the THE expert:
-Who is stating a fact without providing any evidence whatsoever?
-Who is stating a scenario while being terribly (inexcusably IMO) vague with regards to which exact systems are being compared? We've already been through this ourselves and you know you can not make a blanket statement here that all CSiC rotor based systems are superior in terms of fade. Again they are not magic materials. IT IS THE PAD THAT IS ALMOST FOR SURE THE LIMITING FACTOR.
-These massive shortcomings point out excessive marketing and either being light on the science or seriously heavy on the confidentiality.
I said that I took that quote on fade performance from the abstract of a paper I don’t have access to. I am trying to get access to the full text for the details. The man is apparently the Chair of Ceramic Materials in a well-respected German university, and has a AA engineering background. Questioning his scholarship by saying “he is stating facts by not providing evidence” and/or “being light on science” seems rather inappropriate. The real issue is most likely confidentiality as you state. The guy must have seen tons of data/experiments, but probably can’t talk about them in detail.

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-I figured you would pull the punch on bikes being different than cars.
I am not pulling punches on anything. No need to be defensive. I told you I am not trying to discredit your experience in designing mountain bike brake systems. However, how exactly did you end up being an expert on CC brakes in high energy systems and elevate yourself to the expertise level of someone with a Ph.D. and professional career focused specifically on that topic?

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We agree surface temps are higher during braking and before equilibration, however this is a second order effect and it is not unreasonable to approximate "the" rotor temperature by the spatial average of it.
As I said above, approximating the rotor temperature by using that method is not precise enough if what you are really interested in is the friction surface temperature. This becomes an even more significant issue in unsteady conduction, which is what is really happening to the rotor (the conduction equation I outlined is really a gross simplification as it is for steady state conduction only and makes some other assumptions that make it not directly applicable). If you really want to be precise about the temperature response, you need to solve Fourier’s equation in 3D, which is a function of thermal diffusivity. But again, the point is that higher conductivity will yield lower friction surface temperatures as it will result in more uniform temperature distribution/response during braking.

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Finally, to truly compare two systems we must have the friction-temperature curve for a typical CSiC rotor-pad matched set and it seems you may be coming around to this idea.
I have acknowledged the relevance of pad material starting with my first post (#26). Quote from that post below:

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For a performance application, they will not shoot for an operating temperature that will exceed the optimal operating temperature of the pads and/or the pads can’t deal with.
The example you gave, the Turner pad is supposed to deliver performance at 760C+ temps. They say, “coefficient of friction remains stable across the temperature range of the pad”.
http://www.turnermotorsport.com/catalog/coolcarbon.htm

Then, there are other existing high temp pad applications such as F1 (I am referencing the pads and not the rotors). Yes, the contact surfaces are not CC, but the point is that pads that can deliver high CoF at high temps with other types of contact surfaces exist. The SGL group website says, they provide “matching” pads although they don’t specify the composition.

I repeated the same opinion several times in various posts, saying pads must be matched to the intended operational temperatures of the rotors. So a well designed CC system will either have pads that deliver optimal CoF at high temps (if the system is designed such that it will indeed run hotter than the iron system), or can use the same type of pad the iron system uses if the intended temperature range is comparable to the iron systems’ (by increasing the CC system volume by 37%).

Conductivity is not a diversion or secondary effect at all if you are using pads that are designed to operate at higher temperatures, or if the CC system is designed to operate at temps comparable to the operational temps of iron rotors.

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As well it seems you are coming around the the validity of the conservation equation. As you first called it an "over generalization" and "inaccurate" but now state is it yourself just above as a definitive part of a calculation based approach to answering this question.
I did not call the conservation of energy equation an "over generalization" and "inaccurate". I was referring to your analysis and conclusion that CC systems have to run 40% hotter. They clearly don’t have to as one can increase rotor volume with the current technology, and specific heat capacitise might be increased even more in the future. All heat transfer assumes conservation of energy. How else would you derive any of these equations?

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I'm still sticking firmly to my points #1-#3 from many posts back. I haven't heard any specific disagreement to those yet. If you believe identically as your infallable expert that categorically all CSiC systems provide superior fade resistance you should be disputing my item #1.
I don’t think anyone has claimed that all CC systems will be superior in fade resistance. I certainly didn’t make that claim, and that’s not how I interpret the publication. That’s why I said compare high end systems. The best designs the two technologies can offer with the only constraint being weight.

I don’t agree with the rationale of our 3rd point for the following reasons:

1. I am treating volume as a design parameter. Again, a CC rotor that has the same specific heat capacity x mass still weighs 2.25 times less than an iron rotor.
2. Even if volumes are similar and the rotor surfaces are hotter, that doesn’t mean the fade performance increase is caused by just the pad. The superior specific heat capacity and temperature endurance of the CC rotor are also causes for that outcome. (You realize iron goes through a phase transformation slightly above 700C, right?)
3. Continued improvements in specific heat capacity and conductivity of CC rotors are likely.

What this indeed comes down to is that I have all along assumed pads that deliver optimal CoF at the intended operational surface temperatures for CC rotors do/will exist, and therefore, are not the determining factor in a CC vs. iron rotor fade performance comparison. You seem to be saying they don’t/won’t exist. We don’t have access to that information. Yet…
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      12-03-2008, 05:08 AM   #44
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So, you finally acknowledge the relevance of conduction after initially dismissing it as a "diversion"…
Yes and no. To FIRST ORDER conduction is irrelevant. Conservation of energy alone will give a reasonable prediction of rotor temperature in order to do a YES-NO calculation for the question - was the fade temperature reached. Another reason this is so is because the peak temperature is maintained for a relatively short amount of time as the rotor itself and pad both equilibrate. So sure if you want a very precise calculation of whether the fad temperature is exceeded by even the slightest amount for even a short period of time, indeed you will need to worry about conduction. Otherwise, again, you don't. I have never said anything different.

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2. Conductivities of current CC rotors are either slightly above or below the conductivity of iron rotors. Conductivity of CC rotors will most likely increase even further given the potential of CSiC materials and the developmental history of their application to CC brake systems (I realize there are some trade-offs there). However, as I said above and as we agree, increasing conductivity might not be enough as the rotor’s ability to absorb energy without heating too much should be increased as well, so you might need to increase mass x specific heat as well (there is another consideration there around what is too much exactly and is that different for the two systems). Specific heat of CC rotors is already much higher than iron rotors (and it might increase even slightly more in the future). So, you need to increase their volume by about 37% to match the specific heat x mass value of the iron rotor (I had this as 70% in my previous post as I mistakenly took down the carbon-carbon values). That combination will yield a CC system that has better fade characteristics than iron systems.
Sorry but I have to call diversion on this as well. We are not talking about a future improved design, we are talking about currently available systems and none of the ones I have seen have a 37% greater volume compared to a similar high end iron/steel set up.

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I said that I took that quote on fade performance from the abstract of a paper I don’t have access to. I am trying to get access to the full text for the details. The man is apparently the Chair of Ceramic Materials in a well-respected German university, and has a AA engineering background. Questioning his scholarship by saying “he is stating facts by not providing evidence” and/or “being light on science” seems rather inappropriate. The real issue is most likely confidentiality as you state. The guy must have seen tons of data/experiments, but probably can’t talk about them in detail.
I am specifically questioning the quotations you have provided, not the full context of all work, public or confidential that a particular source has conducted. Whether or not the conclusions are light on science, I still firmly believe that such statements which attempt to universally rank the performance of a material against another for a specific capability such as fade resistance are woefully imprecise and inadequate. As we both surely can agree it is a system question and even more so, system by system. Science, not marketing...

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I am not pulling punches on anything. No need to be defensive. I told you I am not trying to discredit your experience in designing mountain bike brake systems. However, how exactly did you end up being an expert on CC brakes in high energy systems and elevate yourself to the expertise level of someone with a Ph.D. and professional career focused specifically on that topic?

As I said above, approximating the rotor temperature by using that method is not precise enough if what you are really interested in is the friction surface temperature. This becomes an even more significant issue in unsteady conduction, which is what is really happening to the rotor (the conduction equation I outlined is really a gross simplification as it is for steady state conduction only and makes some other assumptions that make it not directly applicable). If you really want to be precise about the temperature response, you need to solve Fourier’s equation in 3D, which is a function of thermal diffusivity. But again, the point is that higher conductivity will yield lower friction surface temperatures as it will result in more uniform temperature distribution/response during braking.
I certainly saw it that way and I think anyone reading would agree. I never said I was an expert in CSiC nor CC brake technology. Nor did I make any comparisons between myself and any experts in the field. However, I certainly have done enough work to find that conservation of energy is a darn good approximation and have enough insight to know it will work reasonably well for bikes, cars, motorcycles, wheelbarrows or whatever.

The simple ratio of (mass1 x specific heat1)/(mass2 x specific heat2) will be a great factor to determine the ratio of peak OR average rotor temperature increases for any combination of unsteady braking conditions (again given identical scrubbed energies and identical cooling).

You still have not suggested a simple formula and procedure to get a first order accurate calculation of which rotor will reach its pad specific fade temperature first. To settle this we might consider testing. But finding two brake systems identical enough and to have them in identical enough vehicles and under identical enough test conditions is going to be next to impossible. Hence we need a simple physics/engineering approach to answer "the question". Again, I think that we can absolutely predict/calculate the amount of energy that will cause fade and answer the debate. All we need is the missing pad data for a pad for a representative CSiC system.

If you really wanted to get down to "brass tacks" with regards to material properties, non uniform temperature distributions, geometry, cooling, etc. A finite element model could readily answer most of these concerns without tremendous effort. I'll consider some work on this.

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I was referring to your analysis and conclusion that CC systems have to run 40% hotter. They clearly don’t have to as one can increase rotor volume with the current technology, and specific heat capacitise might be increased even more in the future. All heat transfer assumes conservation of energy. How else would you derive any of these equations?
Come on lucid, this in itself is a diversion. I did not say they HAD to I am saying in todays real world systems where the rotor sizes are very close (iron/steel vs. CSiC) they WILL. We are not discussing which system CAN BE better in the future but which IS BETTER today.

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I don’t think anyone has claimed that all CC systems will be superior in fade resistance. I certainly didn’t make that claim, and that’s not how I interpret the publication. That’s why I said compare high end systems. The best designs the two technologies can offer with the only constraint being weight.
It very much sounded to me like this was your point and that the point was confirmed by the literature/quotes/references. Further on that I would scrap the idea of requiring the systems to weigh the same. Occupying the same volume is a much more fair comparison as each system would need to fit a given wheel/hub/suspension set up and fixing the rotor overall diameter and choosing and existing caliper is the most fair way to evaluate the rotor material change alone.

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What this indeed comes down to is that I have all along assumed pads that deliver optimal CoF at the intended operational surface temperatures for CC rotors do/will exist, and therefore, are not the determining factor in a CC vs. iron rotor fade performance comparison. You seem to be saying they don’t/won’t exist. We don’t have access to that information. Yet…
Might be a good assumption Regardless of if the "ideal" pads exist or not the onset of fade will still be determined by the temperature of the pad! And, I just found some very useful data on this.... See my next post.
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