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Re: PCCB break in procedure
Quote:thought this was a good way to describe it .
These days, (unlike past) the way to bed in new brakes are as follows. AT MRT we use a simple way to describe it! Its like cooking a chicken, cook it hot and fast and it will be terrible, but slow and long and it will be sweet and enjoyable. Like chooks, brakes need time and low temps to bed in, the longer the time and more frequent the cycle of cold and warm, before the "big test" the better most pads and discs will perform. On our rally car we often get my wife to bed in the pads before we fit them to the rally car, as we have little time to bed them in, Sydney traffic is good!
Re: PCCB break in procedure
Quote:
OCEAN said:
Quote:thought this was a good way to describe it .
These days, (unlike past) the way to bed in new brakes are as follows. AT MRT we use a simple way to describe it! Its like cooking a chicken, cook it hot and fast and it will be terrible, but slow and long and it will be sweet and enjoyable. Like chooks, brakes need time and low temps to bed in, the longer the time and more frequent the cycle of cold and warm, before the "big test" the better most pads and discs will perform. On our rally car we often get my wife to bed in the pads before we fit them to the rally car, as we have little time to bed them in, Sydney traffic is good!
As many gentle cold to warm to cold cycles as possible.
Even after break-in, as with any kind of brake/friction system, try to find and keep the system temp. sweet spot. Do not drive aggressively without progressive warm-up/cool-down as the rotors may shatter (accomplished unofficially by a Rennlist member) and/or be unevenly deposited with pad material. Nobody has ever claimed ceramic-composite brakes to be indestructible.
Try to find out the maximum operating temperature (MOT) of the pads. Imho, if you are within, I'm guessing, 250 degrees of MOT, you are close to overheating the system and need to change to PCCB-approved higher MOT pads.
Porsche recommends at least 500 miles of moderate street driving to bed in the PCCB system before getting on it hard.
In a July 2004 article of GT Purely Porsche titled, "Brake Fade," regarding PCCB failure,
consensus seemed to point towards overheating and stone chips damage being the most likely culprits. Which would seem to tally with expert opinion...
Although there doesn't seem to be a factory-recommended bedding-in procedure for PCCB, the material properties of the composite requires that the discs be allowed to form a layer of protective oxidisation before they can be used in anger, which would imply that taking it easy on the brakes would be the way to go until the layer has had a chance to form. This layer would most llikely appear to be a discolouration on the disc face, but far from being a flaw, is perfectly normal -- and would start to become more manifest with time and use.
According to [Professor John Binner, an expert in ceramics at Loughborough University], although silicon carbide materials can be heated up to around 1600*C without too much oxidising, they will form a layer of oxide on the surface that will protect them from further oxidisation.
"Once you've formed that protective layer, which usually happens in the first minute or two, you don't want to damage it. If anything starts to crack just a little bit, and the interface isn't just quite right, you can get oxygen diffusing down into the material and you will get some degree of oxidisation, the rate of which will depend on how hot the discs get. Of course, the harder you brake the hotter the discs get and the more you'll be ripping into this protective layer, although it does self-heal to an extent. If Porsche's system is designed to work with this protective layer, the implication is that heavy braking could result in damage to the oxide layer, which in turn causes more oxidisation and eventual failure."
It goes without saying that an effective braking technique that doesn't overheat the discs and pads would improve the longevity of the components, which might explain why some drivers have experienced problems where others have not.
But if the oxidised layer theory is applicable to PCCB, then it would follow that scouring of the discs caused by grit getting trapped between the pad surface and disc face would be enough to open up the layer and bring about the onset of oxidisation, thus weakening the material. And by the same token, using the wrong brake pads -- which has ben attributed to at least one failure in the US -- would have a similar effect if the friction material were too abrasive and subsequently genrated too much heat.
The same 'layer' theory would also give credence to the supposition that excursions through gravel traps would be a contributing factor to disc failure. Because most of the stress runs through the surface of a ceramic, the outer faces of the brake disc would represent their weakest point, and being hit by high velocity gravel could conceivable [sp.?] cause the surace to chip and again result in the material breaking up in the worst-case scenario. This could be one possible explanation as to why some owners have experienced rotors splintering with chunks of the ceramic compound falling away from the disc surface.
One other possible explanation, or at least a contributing factor to the problem and another reason why a disc might appear to splinter, is thermal shock. If a ceramic composite is put through too many rapid heating and cooling cycles, cracks can appear as the surfaces of the discs expand and contract around the more stable inner core. Although this effect would be more pronounced on a cold day, it's unlikely that the rate of cooling would be rapid enough to cause major problems to occur.
But while there is little that can be done about stone chips and grit, specialists are working on fixes to ensure better and more consistent cooling and prevent failure. A handful now supply ducting kits, which improve the airflow to the brakes; however, it would appear that more and more owners of PCCB-equipped Porsches, who tend to use their cars mostly for track days, are simply boxing up their ceramic discs and pads and replacing them with conventional steel items. Which makes sound financial sense considering that a set of four steel discs with Pagid...pads and some high performance braided hoses costs a fraction of just one PCCB disc.
Despite Porsche's assurances that there is no problem with PCCB, and the fact that concrete information to the contrary remains thin on the ground, the outcome to the issue remains uncertain. What is certain, however, is that unless it acts to resolve the matter, Porsche will have difficulty convincing its customers that ceramic composites have a viable place in its sports cars. Which, considering the amount of time and money invested in the project, is something that needs to be done sooner rather than later.
Sources:
http://www.europeancarweb.com/tech/0406ec_install_carbon_ceramic_brakes/index.html
http://experts.about.com/q/Aeronautical-Engineering-1809/Carbon-Fiber-Aircraft-Brakes.htm
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