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12-03-2008, 09:27 AM | #45 | |
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OK lets get to specifics. Lets look at the 2004 Porsche GT3 with and without PCCB. Perhaps not the most recent example, but a good one in which a high end brake system is fitted in both configurations. The car was available with both a cast iron rotor (perhaps alloy steel, but we will simply assume cast iron) or the PCCB system with carbon ceramic (CSiC) rotor. A key finding here is that at the OEM brake pads were either Pagid directly or used Pagid compounds and the RS19 compound was supplied for both systems. This compound, according to the chart I supplied earlier, fades at approximately 500 °C. The CSiC rotors were supplied by SGL Group so I used their specific material properties. Perhaps the properties listed presently are not the ones from 2004. I actually expect they are not, but the properties today are likely improved so again this small assumption errs on the side of giving the benefit to the PCCB system.
I am going to continue to use the conservation of energy principal but will demonstrate that the materials relative conductivity favors my conclusion even stronger. If a rotor had an extremely high conductivity it would exhibit nearly isothermal conditions in any shape and under any thermal loads. Since the conductivity of CSiC as used for PCCB system is lower than the cast iron it means that the temperature difference across a given path from the surface where heat is generated to cooler areas such as the center of the disc (laterally or radially) or the exposed surface of an internal cooling fin will always be LARGER with the CSiC rotor. Basically the CSiC rotor stores more surface heat longer creating larger peak temperatures than the cast iron system. The snapshot of a spreadsheet (at the end of the post) I made shows clearly that this rotor will always experience a larger increase in average temperature for any given stop independent of its beginning temperature distribution. Yes the friction surface of both rotors immediately after braking will be hotter than their respective spatially averaged values, but again the CSiC rotor will have AN EVEN larger difference between its cooler and warmer spots due to its lower conductivity. Now assuming there is no brake cooling (which certainly is not true in the initial braking portion from this very high speed) a single stop from 165 mph to zero will produce and average rotor temperature and hence pad temperature that exceeds the brake pads fade temperature limit with the CSiC system. However, the cast iron system remains well under its limit. The difference between the two is over a whopping 300 °F. And I would make an educated guess (again supported by the conductivity point) that this is not only the difference between their average temperature but also a lower limit between their peak temperatures. The difference is plenty large enough to make up for other approximations in the method. No cooling is not a terrible assumption either as brake heat generation rates drastically exceed both the lumped capacity and forced convective (through the internal fins/flow) cooling rates. If you still do not like the assumption of no cooling, simply consider a rapid succession of stops from 70 mph ->0 where the cooling rates are truly negligible in comparison with the heat generation rate. Each one will cause a larger temperature increase in the CSiC rotor. Lastly even if compatible with and fitted with the Pagid RS15 pads with a fade temperature of 650 °C the CSiC system would still reach this temperature BEFORE a traditional system with the RS19 compound. The conclusion here is simply that he CSiC brake system in the 2004 Porsche GT3 is more prone to fade than the conventional brake system offered in the car. This is due primarily to the reduced mass of the rotors and a pad choice that does not offer an increased fade knee point to deal with the naturally higher rotor peak and operating temperatures. I was able to find "experimental" evidence of this phenomena as well (at least on the 2002 GT2 with PCCB vs 2001 TT with conventional. This story about a very frustrated PCCB owner who clearly highlights his problems with brake fade at Watkins Glen. Link to post here. Or direct quote, The rest of the post is even more damning of the technology in terms of wear and insuitability of "alternate" Porsche recommended brake pads. Note Porsche seems to be well aware of this fundamental limitation as the current GT3 has substantially larger rotors when you get the PCCB option 15" vs. 13.8" (I believe they have been doing this for some years now) They simply need more thermal mass and a larger rotor makes up some of that ground. As well I'd bet they do not use the same pads and the PCCB pad will have a higher knee temperature. |
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12-03-2008, 10:18 AM | #46 |
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12-03-2008, 04:00 PM | #47 |
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12-04-2008, 01:19 PM | #48 |
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Hey Swamp, thanks for the comparison. I'll respond to your post(s), just busy. At a first glance, 2 clarification questions:
1. Why is the iron rotor volume less than the CC rotor volume given they seem to have the same thickness? (this will actually strengthen your argument if it is a simple data entry error). If this is not an error, does it have to do with the vent geometry? 2. Why did you pick c=800 J/kgK for the CC rotor? Were you able to verif that was the spec for the GT3 back in 2004? I assume you went with the data on the SGL website (I don't know how up to date that is). However, their 2006 SAE presentation is referencing 1350 J/kgK, and 2005 Krenkel paper is stating 1200 J/kgK for the Brembo rotors.
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12-04-2008, 01:28 PM | #49 | |
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Swamp and I had a few email exchanges as things as we weren't approaching the dialog the right way, and sorted things out. It'll be constructive from now on.
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12-04-2008, 01:38 PM | #50 | |
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1. Yup, typo and vents, great guesses. When looking at rotors I wanted to properly model the volume due to the vents. My approximation, which I think is very good is to consider the annular friction surface part to be three sections, two outer ones solid and an inner one, all of equal thickness. For the inner one I figured it had 1/2 the material removed for vents. This places a 5/6 factor in the volume equation which was then missing for the CSiC case. It raised the 534 temperature figure significantly to a whopping 637. However, the quoted weight savings spec from Porsche of 18 kg for all four rotors is the way I backsolved for the rotor ID. taking the above into account changes the ID and again worsens the case for the CSiC. A revision is included below that fixes both issues. 2. Again I used numbers from SGL website for current SGL materials. SGL was and is the supplier for Porsche. If their specific heats are better now than then this again would only show worse performance for the PCCB set up back in 2004. A value as high at 1350 would bring the temp way down to 386, just a hair higher than the iron. |
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12-04-2008, 07:12 PM | #51 | |||||||
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You are saying general claims were made in the article, and I am saying that that is most likely due to the fact that I quoted from an abstract. Regardless, I think the same criticism can apply to your statements on the fade issue as well, which have been somewhat broad. You clearly have a "constant volume" and "system must currently exist" criteria for instance when comparing these systems, and that wasn't clear to me until we started this exchange. Quote:
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Please feel free to check the math and let me know. And also feel free to solve the resulting governing equation based on the initial and boundary conditions. I am not doing that. It's been 15 years since I solved one of these suckers, and it would take time for me to remember all that. However, I suspect that the temperature response can be obtained via the seperation of variables method. Better yet, a computational model would be great as you suggested! I agree that convection during braking is not a significant factor. I think at exreme conditions and at high speeds (such as braking with an seriously overheated system), it could be ~5% of the Power into the system. In more realistic conditions, such as when you are breaking from 165mph and reach 100mph, that could be around 2.5% at that point. That's based on some very simple estimations though. P.S. I will respond to your GT3 post when I get a chance... Cheers man!
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12-04-2008, 07:21 PM | #52 |
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12-04-2008, 09:46 PM | #53 | |||||
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Finite elements are the absolute natural answer. |
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12-05-2008, 11:41 AM | #54 | ||||
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How about the ZR1? I know it has 15.5" Brembo rotors up front, but I couldn't find any data on thickness or volume. Quote:
And, again--I've repeated this several times--they don't even have to weigh the same for the thermal performance of the CC rotor to be superior. According to the specs for the CSiC materials that are being used in the rotors TODAY, you need increase the CC rotor volume/mass by just 37% for the mass x specific heat capacity ratios to be equal. Anything beyond that hands the advantage over to the CC rotors. Quote:
The point is that the lump sum thermal model will not yield that level of detail, which might very well be necessary to understand the fade performance of a braking system. The lump sum model would most likely have not predicted the surface temperature/fade issues in the early CSiC compositions that are referenced for instance. I'd be happy to work on a computational model with you on this one.
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12-05-2008, 02:57 PM | #55 | |||
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That would be fun. We still need some concrete material (pad and rotor data) to make the models truly meaningful. We'll also need conductivity as a tensor (ugh). My thoughts were just a transient thermal analysis leaving all thermal expansion and stress effects out of the model. As well I would assume a non-rotating system with a annular patch power input, initially constant then later allowing it to ramp down with vehicle speed. I'd like to have an estimate of this difference between peak and spatially averaged temperature results and a model can do that. It would also test conservation of energy. My company was recently acquired by ANSYS so I will have access to a great deal of best in class tools in the structural, thermal and fluid domains (ANSYS bought Fluent (CFD) a while back as well). Cheers. |
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12-07-2008, 06:15 PM | #56 | |
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The ZR1 seems to be using the Enzo rotors, which are 380x34mm, in the rear wheels only. In the front, the ZR1 has 394mm rotors. I couldn't find thickness info though.
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As you can see, if one uses the currently available commercial Brembo CSiC rotor, one needs just 33% more volume to obtain the same thermal mass as an iron system. If the CSiC rotor referenced in the SGL SAE presentation exists, the difference is only 13%! Now, back to your GT3 iron vs. CSiC rotor comparison. Let's shelve the relevance of conductivity and the temperature distribution, and use the lump sum thermal model you've been using. Let's also shelve where CSiC technology is going in the near-mid future, and look at what is available today for high-end production cars. I did a survey of rotor sizes in high-end performance cars, and the following are the largest rotor sizes I could find that are available today: Iron: Maserati MC12; 380x34 mm CSiC: F430 Scuderia; 398x36 mm (Brembo) It turns out your assumption about the air gap being 1/3 of the total thickness and the total vent space resulting in 1/6 reduction in volume does not hold up. I went through the Brembo catalog for racing rotors and the air gap is more like 1/2 of the total thickness, and the reduction due to vents is more like 33% for a rotor of similar size 380x35 mm (see attachement for pdf page). This makes sense if you look at some pictures of the M3 rotors. Anyway, not that this should affect the iron vs CSiC comparison unless they have drastically different vent geometries, which I doubt is the case. (The carbon racing rotors do have a drastically different vent geometry though--they seem to have less vent spacing. Check them out on the Brembo website). Anyway, using the 33% vent volume factor, I came up with a comparison. I stuck the largest available rotors of each kind in the F430 Scuderia. It obviously wouldn't have been fair to use the MC12 chassis as that is heavier than the Scuderia. If the Scuderia can accommodate the larger CSiC rotors, it can accommodate MC12's iron rotors as well, so this is an appropriate comparison. Here are the results: The first thing to notice is that the two systems have comparable heat capacity--just above 4000 J/K--so the temperature increases experienced by the two systems should be comparable as well (again, we are not considering conductivity). I am assuming that the pads can deliver peak CoF close to the respective peak operational temperatures of the two systems, and we have already agreed that that is not bad assumption. If we just run a 260kph to 50kph brake event with cold rotors, neither system will fade of course. Let's assume that we run the car through some tight parts of the track and seriously heat up the rotors without giving them a chance to cool down (we also agree that one can charge a rotor much faster than one can discharge it). Say they go up to 600C. Then let's say we accelerate to 265 kph and brake down to 50kph. Let's assume to rotors cooled down from 600C to 450C before the beginning of the braking event. As expected, the lump sum thermal model predicts a similar temperature rise for each system, which puts the temperature toward the end of the braking event slightly over 700C for both systems. Now, the catch is that this is well below the operational limits of the CSiC system, whereas the operational limit for the iron system has been exceeded. Also, note that the CSiC rotor is still 5.3 kg lighter. It is possible to make it thicker for even higher thermal mass and lower temperatures while maintaining a weight advantage.
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Last edited by lucid; 12-07-2008 at 06:57 PM.. |
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12-07-2008, 08:00 PM | #57 |
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Just a couple of comments.
-The 1/6th correction factor was again a rough 1st order approximation that I got from taking one quick look at one rotor picture and another one in a vertical cross section to see the vanes. The vanes in typical cast iron systems are clearly different than the ones in the pdf you posted and for them my 1/6th approximation is better. As you note is does not affect the comparison if both rotors are roughly the same which is very likely. I did not choose this number to bias either system. I did so because I knew the OD, a rough ID and weight differences. This value seemed to work fine based on my quick examination and the quoted weight differences. -My figures above stand of how much thickness or diameter you would need with the 800 J/kgK material in comparison to the 13.8" OD iron Porsche rotors. An equivalent sized rotor of this material is at a SIGNIFICANT thermal disadvantage hence the much larger sizes required. -In terms of the comparison of the larger CSiC rotor to the smaller iron one the results are obviously skewed in the direction favoring the CSiC. The advantage here is from the larger rotor more than from the material! Also, I would say the figures are close enough that conductivity may just start to to be the deciding factor. We know the CSiC materials have a lower thermal conductivity so they will exhibit a larger core to surface delta T than iron. We need an estimate of the difference in peak surface vs. core temperature for two different values of conductivity (again FEM would be good to get a reasonable estimate of this). -I also found some data that says in general grey cast iron, despite the type or alloy, looses considerable strength above 500 °C (and looses it rapidly with increasing temperature, source here: http://www.sae.org/events/bce/tutorial-ihm.pdf ). What this means is that yout Tinitial is likely way too high and in practice would be almost impossible to obtain on any real iron braking system under any conditions except the most deliberate fade inducing ones. A more reasonable value would be 500 as a max allowable.I think both of our analyses together shows the important conclusion. CSiC systems are not hands down better. If you run an oversize system with the absolute latest greatest CSiC material and a carefully matched pad, you may just barely outperform a cast iron system from a thermal perspective. But you will clearly out perform it on the weight side which is very important in other regards. More or less my point all along! Finally when you factor in cost, I would say you are actually taking a step backwards. |
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12-08-2008, 02:40 PM | #58 | |||||||||
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Then I checked the stoptech website here: http://www.stoptech.com/products/img...rDataSheet.pdf I punched in the numbers for the 4 rotors to the right of the chart, and the gap ratio ranged from 35-40%. Just look at the ratio of the air gap to the overall thickness. It's almost always around 1/2 (same for the Brembo rotors). You must be looking at an unusual rotor, or my calculations have an error, or your estimate is off. Quote:
For c=800 J/kgK, punch in 0.085 for thickness into the last spreadsheet you posted and check the c X m row, and you should end up with m X c of ~4700 J/K, which is significatly higher than the cast iron spec. One of us has an error in the calculations. Quote:
Any increase in volume is NOT really a penalty as long as: 1. There is no interference between the brake system and the wheel and suspension. 2. The resulting mass of the CSiC brake system is still lower than the iron system. Quote:
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Anyway, if we take 500C as the max operational temperature, then the situation is clearly even worse for the Scuderia with the cast iron rotors in the comparison even if you pull the initial T down as the Scuderia with CSiC rotors will keep on performing. Quote:
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Now, maybe the real question is this: is there a need for higher total heat capacity than what can already be achieved with a cast iron 380x35mm rotor? If there is, for whom? I surely do not have that need on the street or the track. I don't have a need to spend $30k on a brake system either, but we'll see where this all goes in 10 years in terms of pricing.
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12-08-2008, 03:28 PM | #59 | ||
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Of course I agree just by a visual examination that 66% is a better estimate for many systems. Quote:
Overall I think we are both just about on the same page, which is encouraging. Cheers. |
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12-08-2008, 04:34 PM | #60 | |||||
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Not really. I still maintain that CSiC can offer better thermal performance than cast iron rotors. It's just that most people don't need it. Quote:
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If you run the same analysis for the 2008 GT3 with 380mm PCCB and iron rotors (they were still at 350mm in 2008) and use c=1200 J/kgK for the PCCB rotors, which we know is on the street today, the results will favor PCCB by 35C. My guess is that the c=1350 J/kgK spec in the SGL presentation was a response to the Brembo specs. Meaning, SGL is most likely currently shipping rotors to Porsche with c=1350 J/kgK--if SGL is still the Porsche vendor--which would give the 2008 GT3 with PCCB an 87C advantage according to the lump sum thermal model. However, I agree that most people would not benefit from the thermal performance of a CSiC system. They don't need it. I've been saying that all along. Definitely more agreement than before.
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12-08-2008, 08:58 PM | #61 | ||||||
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Well, almost... At least we are back to smilies...
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Yes, finally, cheers! We probably need to wind this one down. Not that anyone other than Rad. Joe is reading but we have each made our points and pretty much agree. |
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12-08-2008, 09:23 PM | #62 |
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OK man. Let's shelve this one. Good points made overall. It's been a good discussion...
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12-09-2008, 05:27 AM | #65 | |
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Lucid and I might actually resort to blows if we had to do this... (sarcasm...) If you followed this mess this long but don't quite get all of the details I think you truly deserve an executive summary. Maybe we can put one together that we agree on and is concise and to the point. |
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12-09-2008, 08:23 PM | #66 | |
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