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      11-26-2008, 02:03 PM   #26
lucid
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Drives: E30 M3; Expedition
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Quote:
Originally Posted by swamp2 View Post
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.

Quote:
Originally Posted by swamp2 View Post
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

Quote:
Originally Posted by swamp2 View Post
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.

Quote:
Originally Posted by swamp2 View Post
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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