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      11-27-2008, 09:05 AM   #33
lucid
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Drives: E30 M3; Expedition
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Quote:
Originally Posted by swamp2 View Post
Source? Sure composites can have a widely varying c but the sources I found had only single values for the material used in rotors.
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):

http://www.sglgroup.com/cms/internat...ml?__locale=en

Quote:
Originally Posted by swamp2 View Post
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.
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.

Quote:
Originally Posted by swamp2 View Post
The conductivity of carbon varies drastically depending on what form it is in. Pure amorphous carbon is highly NON conductive.
I am aware of that. I've seen many different values, most of them higher.

Quote:
Originally Posted by swamp2 View Post
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.
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.

Quote:
Originally Posted by swamp2 View Post
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.
Diffusivity puts the two relevant thermal variables in perspective.

Quote:
Originally Posted by swamp2 View Post
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.
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.


Quote:
Originally Posted by swamp2 View Post
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.
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.

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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