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      11-28-2008, 03:27 AM   #34
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Drives: E92 M3
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Originally Posted by lucid View Post
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):
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.

Originally Posted by lucid View Post
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.)

Originally Posted by lucid View Post
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.

Last edited by swamp2; 11-28-2008 at 03:43 AM.