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11-25-2008, 05:14 PM | #23 | |
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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.
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11-26-2008, 06:58 AM | #24 | ||
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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). 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, 09:58 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, 01:03 PM | #26 | ||||
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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. 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 Quote:
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. Quote:
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11-26-2008, 04:08 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, 04:19 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, 04:51 AM | #29 | |||||||||
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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. Quote:
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11-27-2008, 05:50 AM | #30 | |
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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, 06:41 AM | #31 | |
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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, 07:29 AM | #32 | |
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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, 08:05 AM | #33 | |||||||
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http://www.sglgroup.com/cms/internat...ml?__locale=en Quote:
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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. Quote:
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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, 01:27 AM | #34 | |||
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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.) Quote:
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. Last edited by swamp2; 11-28-2008 at 01:43 AM.. |
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11-28-2008, 03:10 AM | #35 | |
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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, 02:21 PM | #36 | |||||||
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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. Quote:
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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. Fine. I thought you were still talking about the video. Quote:
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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, 03:48 PM | #37 |
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Thread Jack!!
Take it to the PM !!!
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12-01-2008, 04:29 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 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, 07:53 PM | #39 | |||||||||
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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? Quote:
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? Quote:
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You dismissed the role of conductivity in heat transfer and said the following: Quote:
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. Quote:
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“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:
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, 10:14 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. Quote:
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Well I hold the exact same position. Quote:
I still hold very firmly to my numbered points 1->3 at the end of my previous post. |
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12-01-2008, 11:09 PM | #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, 04:25 AM | #42 | ||||
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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 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. Quote:
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, 07:56 PM | #43 | ||||||||||
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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. Quote:
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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. Quote:
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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, 04:08 AM | #44 | |||||||
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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. Quote:
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