View Single Post
      12-03-2008, 05:08 AM   #44
swamp2
Lieutenant General
 
swamp2's Avatar
 
Drives: E92 M3
Join Date: Sep 2006
Location: San Diego, CA USA

Posts: 10,059
iTrader: (1)

Quote:
Originally Posted by lucid View Post
So, you finally acknowledge the relevance of conduction after initially dismissing it as a "diversion"Ö
Yes and no. To FIRST ORDER conduction is irrelevant. Conservation of energy alone will give a reasonable prediction of rotor temperature in order to do a YES-NO calculation for the question - was the fade temperature reached. Another reason this is so is because the peak temperature is maintained for a relatively short amount of time as the rotor itself and pad both equilibrate. So sure if you want a very precise calculation of whether the fad temperature is exceeded by even the slightest amount for even a short period of time, indeed you will need to worry about conduction. Otherwise, again, you don't. I have never said anything different.

Quote:
Originally Posted by lucid View Post
2. Conductivities of current CC rotors are either slightly above or below the conductivity of iron rotors. Conductivity of CC rotors will most likely increase even further given the potential of CSiC materials and the developmental history of their application to CC brake systems (I realize there are some trade-offs there). However, as I said above and as we agree, increasing conductivity might not be enough as the rotorís ability to absorb energy without heating too much should be increased as well, so you might need to increase mass x specific heat as well (there is another consideration there around what is too much exactly and is that different for the two systems). Specific heat of CC rotors is already much higher than iron rotors (and it might increase even slightly more in the future). So, you need to increase their volume by about 37% to match the specific heat x mass value of the iron rotor (I had this as 70% in my previous post as I mistakenly took down the carbon-carbon values). That combination will yield a CC system that has better fade characteristics than iron systems.
Sorry but I have to call diversion on this as well. We are not talking about a future improved design, we are talking about currently available systems and none of the ones I have seen have a 37% greater volume compared to a similar high end iron/steel set up.

Quote:
Originally Posted by lucid View Post
I said that I took that quote on fade performance from the abstract of a paper I donít have access to. I am trying to get access to the full text for the details. The man is apparently the Chair of Ceramic Materials in a well-respected German university, and has a AA engineering background. Questioning his scholarship by saying ďhe is stating facts by not providing evidenceĒ and/or ďbeing light on scienceĒ seems rather inappropriate. The real issue is most likely confidentiality as you state. The guy must have seen tons of data/experiments, but probably canít talk about them in detail.
I am specifically questioning the quotations you have provided, not the full context of all work, public or confidential that a particular source has conducted. Whether or not the conclusions are light on science, I still firmly believe that such statements which attempt to universally rank the performance of a material against another for a specific capability such as fade resistance are woefully imprecise and inadequate. As we both surely can agree it is a system question and even more so, system by system. Science, not marketing...

Quote:
Originally Posted by lucid View Post
I am not pulling punches on anything. No need to be defensive. I told you I am not trying to discredit your experience in designing mountain bike brake systems. However, how exactly did you end up being an expert on CC brakes in high energy systems and elevate yourself to the expertise level of someone with a Ph.D. and professional career focused specifically on that topic?

As I said above, approximating the rotor temperature by using that method is not precise enough if what you are really interested in is the friction surface temperature. This becomes an even more significant issue in unsteady conduction, which is what is really happening to the rotor (the conduction equation I outlined is really a gross simplification as it is for steady state conduction only and makes some other assumptions that make it not directly applicable). If you really want to be precise about the temperature response, you need to solve Fourierís equation in 3D, which is a function of thermal diffusivity. But again, the point is that higher conductivity will yield lower friction surface temperatures as it will result in more uniform temperature distribution/response during braking.
I certainly saw it that way and I think anyone reading would agree. I never said I was an expert in CSiC nor CC brake technology. Nor did I make any comparisons between myself and any experts in the field. However, I certainly have done enough work to find that conservation of energy is a darn good approximation and have enough insight to know it will work reasonably well for bikes, cars, motorcycles, wheelbarrows or whatever.

The simple ratio of (mass1 x specific heat1)/(mass2 x specific heat2) will be a great factor to determine the ratio of peak OR average rotor temperature increases for any combination of unsteady braking conditions (again given identical scrubbed energies and identical cooling).

You still have not suggested a simple formula and procedure to get a first order accurate calculation of which rotor will reach its pad specific fade temperature first. To settle this we might consider testing. But finding two brake systems identical enough and to have them in identical enough vehicles and under identical enough test conditions is going to be next to impossible. Hence we need a simple physics/engineering approach to answer "the question". Again, I think that we can absolutely predict/calculate the amount of energy that will cause fade and answer the debate. All we need is the missing pad data for a pad for a representative CSiC system.

If you really wanted to get down to "brass tacks" with regards to material properties, non uniform temperature distributions, geometry, cooling, etc. A finite element model could readily answer most of these concerns without tremendous effort. I'll consider some work on this.

Quote:
Originally Posted by lucid View Post
I was referring to your analysis and conclusion that CC systems have to run 40% hotter. They clearly donít have to as one can increase rotor volume with the current technology, and specific heat capacitise might be increased even more in the future. All heat transfer assumes conservation of energy. How else would you derive any of these equations?
Come on lucid, this in itself is a diversion. I did not say they HAD to I am saying in todays real world systems where the rotor sizes are very close (iron/steel vs. CSiC) they WILL. We are not discussing which system CAN BE better in the future but which IS BETTER today.

Quote:
Originally Posted by lucid View Post
I donít think anyone has claimed that all CC systems will be superior in fade resistance. I certainly didnít make that claim, and thatís not how I interpret the publication. Thatís why I said compare high end systems. The best designs the two technologies can offer with the only constraint being weight.
It very much sounded to me like this was your point and that the point was confirmed by the literature/quotes/references. Further on that I would scrap the idea of requiring the systems to weigh the same. Occupying the same volume is a much more fair comparison as each system would need to fit a given wheel/hub/suspension set up and fixing the rotor overall diameter and choosing and existing caliper is the most fair way to evaluate the rotor material change alone.

Quote:
Originally Posted by lucid View Post
What this indeed comes down to is that I have all along assumed pads that deliver optimal CoF at the intended operational surface temperatures for CC rotors do/will exist, and therefore, are not the determining factor in a CC vs. iron rotor fade performance comparison. You seem to be saying they donít/wonít exist. We donít have access to that information. YetÖ
Might be a good assumption Regardless of if the "ideal" pads exist or not the onset of fade will still be determined by the temperature of the pad! And, I just found some very useful data on this.... See my next post.
swamp2 is offline   United_States
0
Reply With Quote