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      11-30-2008, 03:21 PM   #36
lucid
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Quote:
Originally Posted by swamp2 View Post
this one[/url] 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.
Yes, that is what they are referencing for that specific product. There is no reason to guess that is the "norm". It is a composite material, and its properties can be manipulated depending on what the design requirements are. There surely will be strength trade-offs, but there is no reason to assume that all CC brake systems do/will have similar material properties. Actually, they simply don’t.

Quote:
Originally Posted by swamp2 View Post
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.
Sure, it will be hotter if the volumes are the same. I never said it wouldn't be. I said it doesn't have to be if you don't want it to be as you can change the volume or manipulate the composition. But the higher temps in the CC system are actually desirable for reasons I explained previously as long as they don’t exceed the operational ranges.

You keep on referencing some kind of absolute a friction-temp curve. What friction-temp curve exactly? For what pad and rotor combination?

I've said this several times now: pad material can be engineered the same way rotor material can be engineered to perform at higher temperature so that peak friction will be produced within the intended temperature range. The point is to match the operational temp range of the pad with the operational temp range of the disc. You’ll clearly not use the pads you use with a cast iron rotor with a CC rotor. You’d have to use a different pad that will yield peak CoF within the higher operational temp range of the CC system. The higher that operational temp range, the higher the heat transfer. Period.

Quote:
Originally Posted by swamp2 View Post
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).
That is within the scope of the discussion. You have been making very general statements about the fade characteristics of CC systems, not just in this thread, but in other threads as well. As I said earlier, that is what I am responding to. Plus, there is no actual product that is being referenced in this thread to begin with.

Quote:
Originally Posted by swamp2 View Post
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.
Swamp, buddy, I don't even know where to begin here. You refuted Fourier's Law of Heat Conduction in your post #29, and you keep on arguing against well known heat transfer principles for reasons I can't decipher. Why argue against well established and utilized theory? Have you actually studied heat transfer as a distinct discipline? If you have, I am perplexed. This stuff would be pretty transparent to anyone who has received any formal training on the topic. I'll try to explain what you are missing once more below.

Quote:
Originally Posted by swamp2 View Post
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.
Your consideration is rather inadequate. The conservation of energy principle you outlined above says absolutely nothing about how the thermal system behaves in the time domain. Nothing. Zip. The heat transfer equation provides that insight. Units of Qdot are obviously Watts. Energy per unit time. That's braking power.

Imagine that a rotor has very low thermal conductivity. The equation you outlined above still applies. However, what it does not tell you is that the temperature change would take a very long time to occur. But you don't have a very long time to brake—to have that energy conversion process to take place--and slow down. That is why thermal conductivity is NOT a diversion of any kind. Why do you think all of these sources are quoting thermal conductivity figures?

Again, this is a direct implication of Fourier's Law. That is why you have to consider heat transfer rates. You have to describe and analyze the whole system as a dynamic thermal circuit with transient and stead-state thermal response. (The conductive heat transfer equation I outlined above is for steady-state only by the way.)

As a note, it seems that the thermal conductivities of CC materials used in the current CC systems are either slightly higher or lower than cast iron systems although the potential range for CC systems is clearly significantly higher based on the properties of CC materials.

Just to beat the relevance of thermal conductivity to death, I did a little literature review today and found a relevant journal article. Here is a quote from the section, “C/C-SiC composites for advanced friction systems: Tribological Properties”:

“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.”

Krenkel, W., Berndt, F. “C/C-SiC composites for space applications and advanced friction systems,” Material Science and Engineering, A 412, p. 177-181, 2005.

This is exactly what I have been saying all along. If you want to disagree with a domain expert as well, and call the discussion on thermal conductivity a diversion, that’s your choice I guess, but that would be neither scientific nor scholarly.

Building on that, the equation you outlined does not say anything about what happens when you brake often. Where does that thermal energy stored in the rotor go? If it doesn't go somewhere else quickly, then what will happen when you brake again soon, and again? The temperature will rise above whatever the operational range for the system in question, and things will melt/vaporize. That is where convection and heat transfer into the environment comes in.

Again, the faster you cool the rotor in between brake application, the less likely temps will exceed the operational range for the system. And according to Newton's Law of Cooling, you will transfer more energy per unit time if (Trotor-Tair) is higher. Actually, forced conduction simply steepens the temperature gradient at the surface, and increases the convective heat transfer coefficient, which is why vented discs and cooling ducts, etc, make a huge difference in fade characteristics.

Quote:
Originally Posted by swamp2 View Post
In never said anything like the MC12 has crappy pads
Fine. I thought you were still talking about the video.

Quote:
Originally Posted by swamp2 View Post
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?
Compare high end CC system to high end cast iron system.

Quote:
Originally Posted by swamp2 View Post
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
Who said CC systems are magic? They just have superior thermal properties, that's all. Are they affordable, easy to manufacture, failure free at this time? No. Heck, for 99.99% of the people who deal with cars, they are not even necessary even if they were cheap and problem free. But they can offer better fade performance. However, the cost, manufacturing, and failure issues will most likely be resolved over time.

By the way, thanks for the reference to the SGL Carbon presentation.

You should check out the other parts of that presentation. For instance, they also suggest making larger discs and improving convective heat transfer to cope with the lower volumetric heat capacity of CC materials (note that I said volumetric heat capacity and not heat capacity).

http://www.sae.org/events/bce/presen...6wuellner3.pdf

Just make them bigger. And since their density is 4 times less than cast iron, weight is not an issue, and you end up with a rotor that can absorb more energy for the same temperature gain, and conducts heat at a higher rate for better thermal response and dissipation at higher operational temps. Those are facts. Not magic.

The SGL carbon presentation clearly lists the fade performance as an advantage CC systems here although the wording is exaggerated (remember, that is your source):

http://www.sae.org/events/bce/presen...6wuellner2.pdf

A more neutral party, a scientist, the domain expert referenced earlier, also identifies fade performance as an advantage:

“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).
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