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      12-02-2008, 08:56 PM   #43
lucid
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Quote:
Originally Posted by swamp2 View Post
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.

Quote:
Originally Posted by swamp2 View Post
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"…

Quote:
Originally Posted by swamp2 View Post
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.

Quote:
Originally Posted by swamp2 View Post
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.

Quote:
Originally Posted by swamp2 View Post
-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?

Quote:
Originally Posted by swamp2 View Post
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.

Quote:
Originally Posted by swamp2 View Post
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:

Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by swamp2 View Post
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?

Quote:
Originally Posted by swamp2 View Post
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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