Carbon Brakes - walk away or worth it?
Discussion
Hellers said:
Adam B said:
IMHO the advantages are way outweighed by the cost right now when speccing new - especially as most first time owners are paying for 100k miles life and using a fraction of that. If they cost £2k I might think about it, not £5-6k.
Makes sense on a 2nd hand car as you pay little or nothing extra, provided you have them thoroughly inspected
I hope some of the irony of what you just posted isn't lost on you...?Makes sense on a 2nd hand car as you pay little or nothing extra, provided you have them thoroughly inspected
Stuff the science all I know when I went on the Porsche Experience day and drove a Boxster S with Carbons on I thought so what.
I then tried a Boxster with standard brakes and I am converted.
There is a big difference to the braking feel, it may be due to the unspung weight of the Carbon compared to the standard brakes the extra ££ is a lot to justify for the Carbons but they will be on my final spec.
I then tried a Boxster with standard brakes and I am converted.
There is a big difference to the braking feel, it may be due to the unspung weight of the Carbon compared to the standard brakes the extra ££ is a lot to justify for the Carbons but they will be on my final spec.
Bigger brakes do not mean an increase in braking torque from a design stand point.. They may well offer increased brake torque but size isn't everything so never assume that it is the case.
Clamping force is related to brake line pressure and caliper piston area. Thats it!! Force is the same irrespective of pad size so you do not gain any improvement from the larger pad in terms of clamping force. Increasing line pressure or brake piston area is the only way to increase clamping force. The bottom line is that there is really not that much of a difference between the small 4 piston and 6 piston calipers in terms of clamping force. From memory (the last time I ran through brake system calculations) there was around 10% difference between a 996 GT3 6 piston caliper and a 996 C2 4 piston caliper in terms of clamping force available. The early GT3 and 996 Turbo 4 piston calipers have more clamping force than the bigger 6 piston calipers.
When discussing brake disc sizes, It is the effective radius of the rotor that matters for braking torque, not its actual diameter. A 330mm to 350mm rotor size increase gains an increase in effective rotor radius of 5mm max and thats only if caliper spacing/design/pad shape combine to allow it to be effectively exploited. Often they are not. The 362mm alcon rotor for the 996 GT3 therefor does absolutely nothing to increase torque of the braking system. It cannot since it only increases the rotor size and doesn't change the effective radius. It does offer more material for heat absorption but thats a fade topic, not a stopping power one.
Frictional Force = clamping force x brake pad mu.
Total torque = Frictional force x effective radius.
These are fundamental equations that cover brake systems. There is no escaping them. In addition to clamping force and effective radius, the relationship at the system torque level is very dependent on brake pad mu. IIRC, the PCCB pad has a mu value of 0.35-0.4, the non pccb pad has a mu value of around 0.4-0.45. Mu changes in response to heat so the amount of torque that can be provided by the braking system is a constantly changing value.
Interestingly, the mu value of pccb pads versus non pccb pads offsets the slightly increased clamping force offered by the 6 piston caliper. The system generates less frictional force than the 4 piston caliper set up.
Assuming the 350mm rotor can gain back a full 5mm of effective radius, then the 6 piston setup using the PCCB pad ends up offering approx 3% increased torque over the 4 piston using oem pads.
PCCB has no alternative pad choice option. The steel set up changing to even RS29 pads (i.e. a low mu street able track pad) ends up offering approx 5.5% increased braking torque over the 6 piston PCCB setup under optimal conditions for both.
These all ignore system losses, nothing is 100% effective.
The PCCB system obviously gains over the smaller setup in the rear corners with effective radius increasing considerably. However rear braking contributes around 15-20% of total braking so this gain is reduced in the overall picture as a result. The fronts are the area of most focus and concern. The harder the braking, the less the rears contribute.
Since Force = mass x acceleration, mass also plays a role in deceleration just as it does acceleration.
And so to total stopping power...
All braking is governed by tyre traction and force on the tyre....always. Maximum braking force = tyre mu x load on the tyre. Force on the tyre is calculated by dividing torque on the tyre (same as at the rotor) by tyre radius. One then needs to do some dynamic calculations around weight transfer to work out the total force on the tyre at a given time.
The load on the tyre is a dynamic force that changes constantly under driving conditions. Under braking increased force is placed on to the front and weight is removed from the rear. This increases the tyre mu at the front and reduces the tyre mu at the rear. This is why its harder to lock a front wheel at higher speed but it does not mean that the traction of the tyre is not crucial to the braking equation at high speeds just as it is at low ones.
Clamping force is related to brake line pressure and caliper piston area. Thats it!! Force is the same irrespective of pad size so you do not gain any improvement from the larger pad in terms of clamping force. Increasing line pressure or brake piston area is the only way to increase clamping force. The bottom line is that there is really not that much of a difference between the small 4 piston and 6 piston calipers in terms of clamping force. From memory (the last time I ran through brake system calculations) there was around 10% difference between a 996 GT3 6 piston caliper and a 996 C2 4 piston caliper in terms of clamping force available. The early GT3 and 996 Turbo 4 piston calipers have more clamping force than the bigger 6 piston calipers.
When discussing brake disc sizes, It is the effective radius of the rotor that matters for braking torque, not its actual diameter. A 330mm to 350mm rotor size increase gains an increase in effective rotor radius of 5mm max and thats only if caliper spacing/design/pad shape combine to allow it to be effectively exploited. Often they are not. The 362mm alcon rotor for the 996 GT3 therefor does absolutely nothing to increase torque of the braking system. It cannot since it only increases the rotor size and doesn't change the effective radius. It does offer more material for heat absorption but thats a fade topic, not a stopping power one.
Frictional Force = clamping force x brake pad mu.
Total torque = Frictional force x effective radius.
These are fundamental equations that cover brake systems. There is no escaping them. In addition to clamping force and effective radius, the relationship at the system torque level is very dependent on brake pad mu. IIRC, the PCCB pad has a mu value of 0.35-0.4, the non pccb pad has a mu value of around 0.4-0.45. Mu changes in response to heat so the amount of torque that can be provided by the braking system is a constantly changing value.
Interestingly, the mu value of pccb pads versus non pccb pads offsets the slightly increased clamping force offered by the 6 piston caliper. The system generates less frictional force than the 4 piston caliper set up.
Assuming the 350mm rotor can gain back a full 5mm of effective radius, then the 6 piston setup using the PCCB pad ends up offering approx 3% increased torque over the 4 piston using oem pads.
PCCB has no alternative pad choice option. The steel set up changing to even RS29 pads (i.e. a low mu street able track pad) ends up offering approx 5.5% increased braking torque over the 6 piston PCCB setup under optimal conditions for both.
These all ignore system losses, nothing is 100% effective.
The PCCB system obviously gains over the smaller setup in the rear corners with effective radius increasing considerably. However rear braking contributes around 15-20% of total braking so this gain is reduced in the overall picture as a result. The fronts are the area of most focus and concern. The harder the braking, the less the rears contribute.
Since Force = mass x acceleration, mass also plays a role in deceleration just as it does acceleration.
And so to total stopping power...
All braking is governed by tyre traction and force on the tyre....always. Maximum braking force = tyre mu x load on the tyre. Force on the tyre is calculated by dividing torque on the tyre (same as at the rotor) by tyre radius. One then needs to do some dynamic calculations around weight transfer to work out the total force on the tyre at a given time.
The load on the tyre is a dynamic force that changes constantly under driving conditions. Under braking increased force is placed on to the front and weight is removed from the rear. This increases the tyre mu at the front and reduces the tyre mu at the rear. This is why its harder to lock a front wheel at higher speed but it does not mean that the traction of the tyre is not crucial to the braking equation at high speeds just as it is at low ones.
On paper who cares, bring your cayman S ORD and we will see who stops shorter.
And then who can do it 5 times and if PCCB can keep that distance the same on the 5th run. Vs steels.
As I have 2 cars and I am also about to fit rs29 to the steel set up car I will be able to post 3 sets of figures
Oem steel oem pads
Oem steel rs29pads
PCCB set up.
I,ll post back the results when done from 120mph to zero.
The only on paper figures I want to see is the final real results.
As we talked about mass, that also would indicate my car would easy out brake a 911 being 150kg more weight to stop.
And then who can do it 5 times and if PCCB can keep that distance the same on the 5th run. Vs steels.
As I have 2 cars and I am also about to fit rs29 to the steel set up car I will be able to post 3 sets of figures
Oem steel oem pads
Oem steel rs29pads
PCCB set up.
I,ll post back the results when done from 120mph to zero.
The only on paper figures I want to see is the final real results.
As we talked about mass, that also would indicate my car would easy out brake a 911 being 150kg more weight to stop.
I think that might well be right.
My car probably weighs about 100kg more than yours and has squidgier suspension (which, I assume makes a small difference from high speed) - the chap who knows stuff would probably expect it to need further to stop. I would be amazed if it didnt.
So no need to play willy waving games. My willy is also 100kg heavier than yours and takes a lot longer to stop.
My car probably weighs about 100kg more than yours and has squidgier suspension (which, I assume makes a small difference from high speed) - the chap who knows stuff would probably expect it to need further to stop. I would be amazed if it didnt.
So no need to play willy waving games. My willy is also 100kg heavier than yours and takes a lot longer to stop.
mrdemon said:
On paper who cares, bring your cayman S ORD and we will see who stops shorter.
And then who can do it 5 times and if PCCB can keep that distance the same on the 5th run. Vs steels.
As I have 2 cars and I am also about to fit rs29 to the steel set up car I will be able to post 3 sets of figures
Oem steel oem pads
Oem steel rs29pads
PCCB set up.
I,ll post back the results when done from 120mph to zero.
The only on paper figures I want to see is the final real results.
As we talked about mass, that also would indicate my car would easy out brake a 911 being 150kg more weight to stop.
You REALLY like PCCBs don't you And then who can do it 5 times and if PCCB can keep that distance the same on the 5th run. Vs steels.
As I have 2 cars and I am also about to fit rs29 to the steel set up car I will be able to post 3 sets of figures
Oem steel oem pads
Oem steel rs29pads
PCCB set up.
I,ll post back the results when done from 120mph to zero.
The only on paper figures I want to see is the final real results.
As we talked about mass, that also would indicate my car would easy out brake a 911 being 150kg more weight to stop.
mrdemon said:
As we talked about mass, that also would indicate my car would easy out brake a 911 being 150kg more weight to stop.
It absolutely does not indicate that in any way. For those tuning in, I actually quite like PCCB for a road car. They look cool, they don't get all rusty and you get light brown dust instead of nasty black dust. It all seems good to me. It wouldn't put me off buying a car with some unless they were about to need replaced and the price didn't reflect this looming cost.
Edited by fioran0 on Thursday 22 May 01:45
PCCBs seem to have lots of merits - superb performance, long life, no brake dust, etc - but surely the key factor is the unsprung weight saving - which, when factored up, could be as much as 100kg. This must improve ride, handling, rolling resistance, etc.
Agree that the relative additional cost new would be more of a factor on a Cayman or Boxster, (although they look good value on the £100k+ Porsches) but the relative weight saving on the 981s must be even more noticeable - so an even greater benefit.
I'd buy a Boxster with pccbs without hesitation, although worth checking the history of the car as Porsche often fitted them to 'management' cars - so you'd need to see if it had spent some time at PEC.
Agree that the relative additional cost new would be more of a factor on a Cayman or Boxster, (although they look good value on the £100k+ Porsches) but the relative weight saving on the 981s must be even more noticeable - so an even greater benefit.
I'd buy a Boxster with pccbs without hesitation, although worth checking the history of the car as Porsche often fitted them to 'management' cars - so you'd need to see if it had spent some time at PEC.
I have said it 2 times now, I am a very fair person, offered to hire out brunters and for 2 people just need to turn up and do a run side by side with me.
winner takes all.
ORD has admitted my car will already stop faster, so that's one out as fast as that although he does admit to having a huge penis.
PCCB are very expensive, the people who have posted here who have them choose them after a visit to the PEC and then spent 5k after driving 2 cars back to back, you don't do that on a wim.
I my self run two cars which for all intents are the same one with steels one with PCCB, I don't need convincing.
Others have driven my cars, every one likes the PCCB better, people with steel oem brakes are always on the upgrade path, that says it all really.
ps to the above poster the unsprung weight saving is very little as the disks and calipers are much bigger maybe 2kg in total for all 4.
The rotational mass will be less though, which in theory helps turn in, helps braking, helps accelerating all be it by a very small amount.
winner takes all.
ORD has admitted my car will already stop faster, so that's one out as fast as that although he does admit to having a huge penis.
PCCB are very expensive, the people who have posted here who have them choose them after a visit to the PEC and then spent 5k after driving 2 cars back to back, you don't do that on a wim.
I my self run two cars which for all intents are the same one with steels one with PCCB, I don't need convincing.
Others have driven my cars, every one likes the PCCB better, people with steel oem brakes are always on the upgrade path, that says it all really.
ps to the above poster the unsprung weight saving is very little as the disks and calipers are much bigger maybe 2kg in total for all 4.
The rotational mass will be less though, which in theory helps turn in, helps braking, helps accelerating all be it by a very small amount.
Edited by mrdemon on Thursday 22 May 09:14
Koln-RS said:
PCCBs seem to have lots of merits - but surely the key factor is the unsprung weight saving - which, when factored up, could be as much as 100kg.
where on earth did that number come from? 
OP - avoid that car, not because of the nice PCCB, but becasue of the vile blue/red combo - HTH
Fiorano - excellent and interesting info, thanks
interestingly, given the heat energy, Q , is expressed using the formula:
Q = m Cp dT where m=mass (i.e. density x volume), Cp = specific heat capacity and dT = change in temperature
it should be obvious that mass of a brake disc has a direct relationship to its ability to absorb heat.
Carbon ceramic brakes have a Cp of around 1,350 J/kgK, cast iron brakes have a Cp of around 600 J/KgK however the density of carbon ceramic is 2.35 g/cm^3 while for cast iron it is 7.25 g/cm^3
This leads to carbon ceramic brakes having a lower volumetric heat capacity versus iron brakes. 3,173x10^3 J/Kcm^3 versus 4,350x10^3 J/Kcm^3
This is the technical reason why carbon ceramic brakes are larger than equivalent iron brakes. They are disadvantaged in terms of their ability relative to heat storage/temperature as a direct consequence of their lack of mass.
Q = m Cp dT where m=mass (i.e. density x volume), Cp = specific heat capacity and dT = change in temperature
it should be obvious that mass of a brake disc has a direct relationship to its ability to absorb heat.
Carbon ceramic brakes have a Cp of around 1,350 J/kgK, cast iron brakes have a Cp of around 600 J/KgK however the density of carbon ceramic is 2.35 g/cm^3 while for cast iron it is 7.25 g/cm^3
This leads to carbon ceramic brakes having a lower volumetric heat capacity versus iron brakes. 3,173x10^3 J/Kcm^3 versus 4,350x10^3 J/Kcm^3
This is the technical reason why carbon ceramic brakes are larger than equivalent iron brakes. They are disadvantaged in terms of their ability relative to heat storage/temperature as a direct consequence of their lack of mass.
fioran0 said:
some stuff
just about all steel owners who drive hard upgrade the braking system, you don't find PCCB owners having to do this.why not wait and I'll do the tests myself on Monday when it might be dry and we can see the figures.
all I see on here is Alcon, RS29, endless, buying these things later is rather expensive esp a full set of Alcons and endless pads.
interesting about mass and why they make them bigger, but bigger means the caliper is out more which mean more torque, always easier to stop a spinning wheel from the outside than trying to stop it in the middle.
So too many figures get in the way of on paper physics and the only real way is to run 2 cars side by side ;-)
ideal would be a Boxster with steels which I might get a friendy owner to lend me, calling ZYP :-)
mrdemon said:
just about all steel owners who drive hard upgrade the braking system, you don't find PCCB owners having to do this.
I know you seem to have a massive hard on for PCCB, and each to their own, but most people who drive and brake hard are people who regularly track their cars. Most of those people remove PCCB and fit Alcons due to fragility and frightening replacement cost of PCCB. Then stick PCCB back on to resell.I thought the general considered consensus was PCCB good for road use but not for hard use on track.
(before you call me out to Brunters for a challenge, I actually like PCCB and would gladly choose a used car with them, just wouldn't spend £5k on them new, as hardly any do on Boxsters/Caymen)
science said you were an ape, then changed it to a bird, then they changed to a fish.
what does science have to do with any thing so simple as running two cars side by side to prove a point.
this is now over, I am not posting again (bar another reply to the above), you are not willing to take the bet and will not turn up, there is no more I can offer you.
Troll away all you like on it :-) keep enjoying googling info and reposting from other sites.
what does science have to do with any thing so simple as running two cars side by side to prove a point.
this is now over, I am not posting again (bar another reply to the above), you are not willing to take the bet and will not turn up, there is no more I can offer you.
Troll away all you like on it :-) keep enjoying googling info and reposting from other sites.
Edited by mrdemon on Thursday 22 May 10:50
Adam B said:
I know you seem to have a massive hard on for PCCB, and each to their own, but most people who drive and brake hard are people who regularly track their cars. Most of those people remove PCCB and fit Alcons due to fragility and frightening replacement cost of PCCB. Then stick PCCB back on to resell.
I thought the general considered consensus was PCCB good for road use but not for hard use on track.
(before you call me out to Brunters for a challenge, I actually like PCCB and would gladly choose a used car with them, just wouldn't spend £5k on them new, as hardly any do on Boxsters/Caymen)
that's more because of the risk of chipping a disk in the gravel not because of performance.I thought the general considered consensus was PCCB good for road use but not for hard use on track.
(before you call me out to Brunters for a challenge, I actually like PCCB and would gladly choose a used car with them, just wouldn't spend £5k on them new, as hardly any do on Boxsters/Caymen)
do don't see F1 teams switching back to steels do you ? not that they are the same, but if steels were the best F1 would use them.
they are bloody expensive, yes, hence people will not take the risk, they still wear out on track due to the hard use as people push harder with them, so heat is an issue and you will have to replace them at great cost.
change your pads more often and do real cool down laps and PCCB should be ok on track, I know may people who track PCCB inc ex racers and myself they are awesome on track vs my Cayman which leaves me cold and gives me no confidence at all, ending up with the pedal on the floor after a few laps.
Or spend 5k on a brake upgrade.
PCCB to me is a win win, esp 2nd hand, you get to use PCCB, then when they wear out you get the keep the nice large 6 pots and can swap the disks out for some hi end Alcon or Brembo units.
looks alone is a winner esp on the rear where a Cayman with a 280mm(guess) disk vs 350mm disks looks a far cooler :-).
fioran0 said:
insightful, cogent, thoughtful articulation of braking dynamics...(Thank you!)
Over 10 yrs of watching Porsche "betatest" PCCB's... I still struggle to see the benefit :: cost of them...wrt to PCCB's being superior in stopping power...here's some differing opinions:
http://forums.rennlist.com/rennforums/997-gt2-gt3-...
http://forums.rennlist.com/rennforums/997-gt2-gt3-...
http://forums.rennlist.com/rennforums/porsche-tech...
As for my experience, was a track rat for much of mid last decade, and watched PCCB's come on to the scene with 996 GT2...and then iterate with the 996.2 GT3...(as well as watch a few Challenge Stradale's stress and break - that's not a pun - something similar)...Gen.1 and 2 PCCB's were unimpressive, loads of green fade, some instances of delaminating, and the salutary first hand lesson of seeing a 996.2 GT3 heavily pit the rotors after running over gravel/stones in an off track excursion...The owner, faced with a choice of $16k to replace or going to steels...went the logical route.
Given the foregoing, when it came to speccing my 997.1 GT3, first hand experience said, if you track...no way. And there were few at the track I frequented (Texas World Speedway) who saw ticking that box as rational.
Family expansion required my GT3 to find a new home and put a halt on my tracking - distinctly bittersweet - but even with first hand experience of punting a Gen 3 PCCB equipped C2S around Barber Motor Sport....did not see the benefit::cost making sense....and hence my C2S came without the porcelain...
and even now that I've a bit more flex on car choice....my 7.2 GT3 RS did not come with chinaware....(not like I saw the cost difference as all that material either..)
if you love the initial bite of PCCB's...then upgrade the steel pads to RS29....or something equivalent...at least with steels you get some (greater) flexibility on pad choice...
PCCB = a solution in search of a problem...rather like centrelocks...(which I despise)
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