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      12-01-2008, 08:53 PM   #39
lucid
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Quote:
Originally Posted by swamp2 View Post
This is more so from their low weight than their thermal conductivity. You just can not find a conductivity that will change this basic physics. There are no CSiC rotors that are three times the thickness nor three times the area of a traditional system to provide the equivalent thermal mass.
You’ve got that one wrong. For a CC rotor to match the heat capacity of a cast iron rotor, it needs to be exactly 1.7 times larger in volume. At that increased volume, the CC rotor is still 2.4 times lighter than the cast iron rotor. What is that if not a superior thermal property?

The 1.7 ratio is coming from your source, and is likely to be reduced in the future.

I don’t know the thickness of the Veyron’s rotors, but SGL carbon indicates it was designed with CC brake system in mind, so that would be interesting to find out. Apart from that, what is or is not available as a “swap in” part is rather irrelevant. What’s the point of trying to plug in new technology into old infrastructure?

Quote:
Originally Posted by swamp2 View Post
We could settle this debate pretty well if we had the "magic" friction-temperature curve for a typical CSiC rotor and pad combination but for the life of me I can not find one. I actually expect that information is fairly proprietary.
That info seems to be proprietary at this time as indicated in the Krenkel article. There is nothing magical about it though. If it was indeed impossible to produce pads that would provide peak friction characteristics in the operational temperature range cited for CC systems, there would be absolutely no point in developing the rotors, or designing for such operational ranges. Do you really think the developers of such systems are that stupid?

Since you seem to have not read my previous post, I’ll re-post the other quote from Krenkel, this time focusing on CoF:

““After seven years of development, C/C-SiC braking materials are now entering the market in high performance cars, trains, and lifts. Their main advantages lie in the reduced mass, their non-fading characteristics, their improved coefficient of friction, and their extremely low wear rates.” (Krenkel, 2003; from conference paper abstract).”

That language may not be very specific—probably because I am quoting from the abstract as I don’t have access to the full paper yet—but you think this guy is just making this stuff up?

Quote:
Originally Posted by swamp2 View Post
Since I figured you had seen on of these plots previously I did not feel the need to post it, but it sounds like you haven't, so here it is. You can see here, as I have been referring to, the "knee point" in each curve and it represents the onset of fade.
I saw that plot. If I remember correctly, it was created by a vendor. Regardless, so what? What does that tell me that I don’t already know? Please read what I wrote in my previous post about peak friction and operational temperature range. Do I have to repeat everything several times in this discussion for you to understand what I am saying?

Quote:
Originally Posted by swamp2 View Post
Now here is a kicker: As far as I can tell, CSiC rotor brake systems actually do use standard issue pads, albeit standard "ceramic" pads with added ceramic for both wear and high temperature capability.
Probably for street drivability, etc. Who cares about those systems since they will never be really driven hard enough and fade will never be an issue anyway like I said several times before?

Quote:
Originally Posted by swamp2 View Post
You keep talking about how this can be made heavier or this can be thicker or this can be engineered, etc., etc. That is not what this discussion is about. It is simply about is a CSiC braking system more fade resistant (with the clarification you have added that we should discuss a high end version of each system - do note that that would imply an alloy steel rotor not a cast iron one...)? But we are not talking about what CAN be done, we are restricted here to what IS available now, today. Agree or disagree?
That’s your interpretation, not mine. So, following your logic, people shouldn’t have discussed DFI 5-7 years ago? This technology is in its first iteration as a commercial application. It will see significant change/improvement in 5-7 years.

Quote:
Originally Posted by swamp2 View Post
I never denied any fundamental equations of heat transfer
You dismissed the role of conductivity in heat transfer and said the following:

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.
The heat conduction equation I used explains all that. The physics of heat transfer works exactly like that. The reason you don’t see glowing calipers is explained by that principle as follows:

1. Most high end pads will have insulating (low conductivity) materials between themselves and the pistons as that is the surface area through which conductive heat transfer to the calipers and the rest of the brake system takes place. Low conductivity means low heat transfer.
2. Piston/pad contact surface area is much smaller than the rotor/pad contact surface area. This again results in low heat transfer.

Quote:
Originally Posted by swamp2 View Post
The equation I provided is a darn good approximation, based on sound principles and real world experience.
What real world experience? Have you tested brake systems with significantly different conductivity values for the rotors in high energy systems? If not, your experience is irrelevant. If yes, please share any relevant data.

Quote:
Originally Posted by swamp2 View Post
Think about it this way: just at the end of a very hard but very intense and rapid braking event, when everything begins at ambient temperatures, where is the vehicles kinetic energy? It has to go somewhere and very little is lost to the structures other than the rotor (and pad) and very little has had time to dissipate by convective cooling.
The rotor may still have a thermal gradient and indeed in a CSiC rotor it will probably have a higher gradient than in a very conductive iron/steel rotor which will reach a uniform temperature quite quickly (this is certainly a factor in saying the equation ignores 2nd order effects, of which non uniform temperatures is one indeed). Then when you combine multiple stops you certainly get some convective cooling and conduction which provides the temperature rise of other components (caliper, pistons, fluid, hub, rim, etc) accompanied by the temperature fall of the rotor. Indeed all of this is exactly as per some of the equations you mentioned. But once you slam on the stoppers again the temperature rise is, to first order, governed by the equation I provided. I know it is fairly accurate not only as it is from first principles, but because I have actually tested it. It works, plain and simple. If you can provide another simple expression that provides the temperature gain of a rotor under a given braking scenario please do so.
How does the kinetic energy of the vehicle end up in the rotor? How does the rotor experience the temperature change the conservation of energy equation dictates? Through what mechanism exactly? Magic? Actually, through conduction. Heat is generated at the surface and is conducted to the rest of the rotor. This is what you are missing. That is what I’ve been saying, and the Krenkel quote states as well. Since you clearly did not read my previous post, here is the relevant quote again:

“High transverse heat fluxes from the outer region (friction surface) to the center of the composite are necessary to avoid an overheating of the friction surface, i.e. high transverse conductivities must be adjusted. Standard C/C-SiC composites with bi-directional reinforcements used in hot structures of spacecraft have been modified in their composition and microstructure and led to essential improvements. Particularly, by increasing the silicon carbide content of the composite the transverse thermal conductivity was doubled in comparison to the original space materials.”

Now, what does this quote say Swamp? It has been written in clear English. It is saying you need high conductivity, and otherwise, the friction surface will overheat. How clearer can this be made exactly?

And, what happens when the friction surface overheats? You exceed the operational temperature range of the pad and the rotor. The rotor might start melting. But before that you will most likely see reduction in friction unless you are using a pad that has a flat or positively sloped CoF curve at that temperature range. If you see reduction in friction, you will experience fade. So, to control for fade, you need to control friction surface temperature by increasing conductivity as well as heat capacity. Convection is also a factor, but probably to a much lesser degree.

Quote:
Originally Posted by swamp2 View Post
Now we all know that a particular pad may operate better at a higher temperature than another, but you also keep sticking to this rather absurd idea that you always want a hotter rotor so it will cool faster. This is pretty nonsensical. Its just like saying that if the goal is to empty a container you should keep filling it so you can empty more from it. Hotter is fine, as long as your typical operating temperature is far enough away from the knee for some margin.
You continue to miss the basics here. You are repeatedly charging and discharging a thermal reservoir by transferring energy in and out of it. That’s what braking is. The faster you can charge and discharge that reservoir, the better, which means the faster you transfer the energy, the better. Qdot is a measure of that rate, and it is higher when the deltaTs are higher. What is so hard to understand here?

Finally, you completely ignored the expert opinion I referenced in my previous post on the improved fade performance of CC systems as an advantage.
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