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      12-02-2008, 12:14 AM   #40
swamp2
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Lucid: I suppose I am about done here. But your style and tone will probably keep the discussion going.

Despite our reasoably good history and most often seening eye to eye, we are clearly not seeing eye to eye on anything on this topic. We can not agree on the physics, we can not agree on the question that is being asked and we can not agree on what information is missing or what is needed to answer the question. I will offer a few more selected comments though.

Quote:
Originally Posted by lucid View Post
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?
The factor depends on the specific heat used and thus there is a range of factors. But again any way you slice it there are no CC rotors I have seen with a volume required to offset their reduced mass. Sure the lower mass is good but not good from a thermal mass point of view - that is the whole point.

Quote:
Originally Posted by lucid View Post
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?
On a car where almost all parts are ultra high end, totally custom and made from exotic materials I think the Veyrons brakes are a bit outside of the scope of our debate. But then again we can't agree on much so maybe it is fair game.

Quote:
Originally Posted by lucid View Post
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?
I've read all of your (their) wonderful quotes about the improved fade resistance of CSiC systems. That does not make it so. I have seen NO test data, no component data nor any basic calculations that support such claims. Neither have you. Scientists and engineers have been known to stretch the truth and even to lie outright in the name of promoting their own products. Do note how most such statements begin with the first benefit being reduced mass, inline with what I have said all along (again making an assumption that folks tend to list the most important things first). Lastly have any of these folks offered one miniscule piece of evidence as to the CSiC systems behaving better COMPARED TO WHAT? They haven't and this is a big part of my point. Your skepticism here is very curiously lacking.

Quote:
Originally Posted by lucid View Post
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.
What is tells you is that when you exceed a certain tempertaure you get fade, period. This is why the temperature-friction curve FOR THE PADS (in combination with a rotor) is the ESSENSE of this question.

Quote:
Originally Posted by lucid View Post
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?
You have completely missed the point. I suspect that CSiC rotor based brake systems are using, more or less, the "same old pads" which means they will be PAD fade limited not rotor material limited.

Quote:
Originally Posted by lucid View Post
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.
Exactly - as I stated more simply the path is non conductive (or is more resistant to the heat flow). Which is exactly why my equation is valid! A further reference to this equation comes from the "Brake Handbook" by Fred Puhn, ISBN 0-89586-232-8, which provides the exact equation and reasoning I have on pp 8-11.

Quote:
Originally Posted by lucid View Post
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.
In my first mechanical engineering position I did design, CAD, analysis, testing and fabrication of the first cable actuated hydraulic disc brake system for high end mountain bikes. I built static and dynamic test fixtures, performed thermal studies in the lab and in the field, tested for hand pressure vs. brake torque and studied a wide variety of organic and sintered metal pads for friction vs. temperature characteristics. I think the two highlights of my achievements on this project was the design of the anti noise/drag spring which has been copied on many bicycle brake systems through the present day and the design and testing of high end hollow ceramic brake pistons that drastically reduced brake fluid temperatures. As part of this effort I did testing with various riders and from various speeds to verify the conservaton equation I keep focusing on. Of course I don't have that data but I was satisfied with its accuracy.

Quote:
Originally Posted by lucid View Post
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.
We basically agree here, amazing, it is just that surface temps are always initially higher, in all materials but it can be further exaggerated (i.e. worsened, i.e. made closer to a fade situation) in CSiC. After a braking event, to observe conservation of energy you need to let the rotor reach equilibrium or estimate the internal vs. external temperature and integrate or average. But again you will still observe conservation of energy. The point is not only that the CSiC rotors are hurt in fade performance by being so much lighter they are also hurt by having higher surface temps from less transverse conductivity. No matter how high the conductivity gets you can not have an average temperature lower than that predicted by the conservation of energy. Now if you add the effect of constant heating and cooling from repetitive braking events you end up with the same situation, a bit hotter surface temps and a bit cooler internal temps and each braking event increases both at a slightly different rate but governed to first order by conservation of energy.

Quote:
Originally Posted by lucid View Post
You continue to miss the basics here.
Well I hold the exact same position.

Quote:
Originally Posted by lucid View Post
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?
Conservation of energy, pad compound and friction-temperature curves are the keys to which will fade first or easier. Conduction through the rotor as well as all heating and cooling rates, regardless of source are secondary effects for both cast iron and CSiC systems. It is so simple - that system which first gets to the fade temperature will fade first. This equation works, is simple, is accurate enough and can answer the question along with some basic friction-temperature curves. I note that curiously you have not proposed a single conceptual calculation method nor stated what data is missing for determining an answer to the fundamental question. Before rattling on and on I would suggest that you do propose both a method and what is missing. Again a FIRST order type of calculation, not exact in all of its gory details.

I still hold very firmly to my numbered points 1->3 at the end of my previous post.