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      11-27-2008, 05:51 AM   #29
swamp2
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Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by lucid View Post
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 deltaTóthe 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.

Quote:
Originally Posted by lucid View Post
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.

Quote:
Originally Posted by lucid View Post
The little research Iíve 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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