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While the 918 also uses regenerative braking it's a much harder case. With that much power and weight its arriving at corners with over twice the kinetic energy than the GT4, and all that energy needs to go someplace. And it uses the same brakes.
I will be honest, I am tempted by the PCCB, for all the wrong reasons - the biggest one being the sheer overkill of having 918 brakes on a GT4!
And after reading your wonderful explanation, I was curious how much lesser KE the brakes would have to dissipate on a GT4 compared to a 918 around a typical track. Fortunately I found this speed plot comparing 918 and 991 50th anniversary (and Jack Olsen's incredible car) around Big Willow -
So based on the speeds on that plot (and 1750kg weight), my crude calculation shows a 918 would have to dissipate 2695 KJ of kinetic energy per lap.
Few unknowns -
1. What fraction of that KE is used for charging the battery - maybe one of you with the inside connections can find out?
2. The speed plot is missing couple of braking spots, like turns 4 and 5.
Now for GT4, I used the 991 50th Anniversaries speeds and a weight of 1400kg.
The actual figures should be little more favorable for the GT4 because it would probably not hit the same max speeds as the 991 50th anniversary (less HP, more drag) but have higher cornering speeds (grippier tires, more downforce) thus requiring less braking and less energy to dissipate.
But ignoring that for now, with those figures, my crude calculation yields the GT4 has to only dissipate around 1350 KJ per lap. Almost 50% of the 918's figure!
Once again, a big unknown is how much of the braking in the 918 is done by regenerative braking vs the conventional brakes.
Also I have no idea, how all this translates to the PCCB's durability in GT4? But I am guessing it doesn't hurt?
They need to be larger both to absorb and shed heat.
A brake converts kinetic energy into heat, so it needs to get hot to function. How hot it gets depends on its thermal mass (thermal mass = specific heat of brake rotor material x amount of brake rotor material). The ceramic material in PCCBs is formulated to have a higher specific heat (60% better), but its density is 1/3 of cast iron. Thus if you simply swap cast iron material for ceramic you'll get a rotor with 1/3 the weight and just over half the thermal mass. That second bit means for any given stop it will get twice as hot.
PCCB brakes work to higher temperatures than cast iron: 900C vs 700C, so a little hotter is OK. Twice as hot it not- the rotor and pad will come apart. To prevent this you make the rotor bigger- in this case they weigh roughly 55% (without hats) of their cast iron counterparts, so they have (160% x 55% =) 88% of the thermal mass, which means if the cast iron gets to 700C the PCCB will get to ~800C, still under the safe operating limit.
There are issues with this. The hotter pads and rotors wear out faster, so running at higher temperatures can get expensive. Porsche screwed this up with early PCCBs: the 996 GT2 came with ~25% lighter PCCB rotors than the 991 GT3 but it had similar power and weight. Thus its rotors would try to get nearly 25% hotter on track, leading to extremely rapid failure and a lot of pissed off customers. Porsche has been backing the operating temperature of PCCBs down since.
The GT4's case is the most conservative we've seen. The car has 20% less power and a little less weight , call it ~25% less kinetic energy than the GT3 at the end of the average straight, but the brakes are the same. And because of their large size and surface area they shed heat rapidly, so they'll stay relatively cool. Not as cool as the cast iron, but much cooler than any other Porsche PCCB, so they should last longer. Add in the discounted price and the car's low weight and if there was ever a time to take a risk on PCCBs this would be it.
However. As mooty suggested there are still issues. Many of us, myself included, have hated the over-assisted feel of PCCBs in the past- they make it harder to heal-toe, harder to modulate, etc. They will prevent you from running 19" wheels, so no Hoosiers for you. Finally they will be more expensive, both pads and rotors, without question.
I've mentioned this before, but it bares repeating because I think it says it all. I asked Walter Röhrl how he'd order the car, and he said stripped- no AC, no radio, etc- the weight makes a difference. But for all his focus on weight loss he said he'd order it without PCCBs. Ask yourself why.
So in essence, the PCCB rotors need to be bigger to not become as hot, with the added benefit of having more surface area to dissipate the heat. Seems that PCCB rotors are just a marketing tool.
Based upon what your saying I have to ask how we have aftermarket companies offering carbon ceramic replacement for steel brakes, size for size? Have they found ways to improve the compounds in the pads and rotors to a point that they can dissipate the heat or are they assuming the people buying won't push them to their limits and realize they're inadequate?
And after reading your wonderful explanation, I was curious how much lesser KE the brakes would have to dissipate on a GT4 compared to a 918 around a typical track. Fortunately I found this speed plot comparing 918 and 991 50th anniversary (and Jack Olsen's incredible car) around Big Willow -
.......
So based on the speeds on that plot (and 1750kg weight), my crude calculation shows a 918 would have to dissipate 2695 KJ of kinetic energy per lap.
....
But ignoring that for now, with those figures, my crude calculation yields the GT4 has to only dissipate around 1350 KJ per lap. Almost 50% of the 918's figure!
Right idea, but I think your math is a little off. Here is what I get:
Kinetic energy is (1/2 Mass x Velocity^2), units in KG and meters per second.
For the 918 at the first corner we convert units to get (1/2 x 1750 kg x 74 m/s ^2 = ) 4,852,000 Joules (4,852 kJ) entering the first corner at 166 mph, while we still have 1,141 kJ leaving it, so 3,711 kJ just for that single stop. Over a full lap I get more like 16,000 kJ in braking total, though that's rough as much isn't listed.
Your conclusion is correct, however- the 50th will be dumping roughly half the energy into the brakes, largely because it has half the power.
We can use these numbers to look at brake heating. Stopping from 166 mph dumps 4,852 kJ, and we can put that into the brakes if we know rotor mass (call it 23 kg for all 4) and material specific heat (.8 kJ x kg per degree C from the link posted before). It takes (23 kg x .8 =) 18 kJ to heat the brake rotors one degree C, so stopping from 166 mph would heat them roughly (4,852 / 18 = ) 270 degrees C. So on our first stop from 166 mph the 918's brakes would get to about 300C, not too bad. Three of these stops back to back without cooling, however, would see them at nearly 900 C, which is why brake cooling is critical.
The energy to heat the brakes comes directly from the engine, so the bigger the engine the bigger the brakes we need. The 918 has 877 hp or 654 kW. If it was at full power for a 1:23 lap it would make (654 x 83 =) 54,000 kJ. We see now this isn't a power track- we counted only about (16,000 / 54,000 = ) 30% of this going into our brakes, largely because of all the sweepers that don't let the driver use full throttle. We can even condense this to a single number: the 918 is dumping 192 kW on average into its brakes over this lap, or ~257 hp.
Now we get to cooling- the brakes need to get rid of that much energy constantly if the car keeps lapping. The hotter they get the faster they cool, so it's a question of how hot the brakes get before they stabilize. They're really hot when the start to glow, but radiative cooling grows very quickly (temperature to the fourth power) so when they are that hot they can dump a huge amount of heat. The issue being that they are also eating themselves at a huge rate. PCCBs have a big advantage with cooling, however- the fact that they can operate hotter plus the additional surface area both mean they can shed far more heat before they stop working. Which means you're not going to fade PCCBs, you're going to eat them instead.
On a track that's harder on brakes the 918 is going to be dumping more than 50% of its power into the brakes on average, meaning they will need to be dumping close to 500 hp constantly. They will keep working, but they won't last long. On the same track a GT4 might be dumping over 60% of its engine power into the brakes, but that's still only ~250 hp on average. Thus the brakes will stay a couple hundred degrees cooler, extending their life dramatically. We'd need a temperature vs life curve to guess how much, but it's going to be well over the 2x you'd get simply because you're dumping half the energy.
So I wouldn't say PCCBs are only marketing. They are lighter and they will offer more absolute brake power- the 918 could probably melt the GT3's cast iron brakes. With less than half the power of the 918 the GT4 can't, so it's a question of feel, cost, preference and other factors.
Right idea, but I think your math is a little off.
Little off? haha, I was way off. Made the sillly mistake of doing M/2*(v1-v2)^2 instead of M/2(v1^2 - v2^2). .
Originally Posted by Petevb
They're really hot when the start to glow, but radiative cooling grows very quickly (temperature to the fourth power) so when they are that hot they can dump a huge amount of heat. The issue being that they are also eating themselves at a huge rate.
Mind elaborating that? Do you mean the brakes shed material at a faster rate once they are hot enough to glow?
Originally Posted by Petevb
We'd need a temperature vs life curve to guess how much, but it's going to be well over the 2x you'd get simply because you're dumping half the energy.
Yes, that would be very helpful. I am guessing there is some non-linear relation between the two. Probably why PCCBs supposedly lasts "forever" on street, because they probably never cross some threshold temperature. Wonder if that is ever practically possible on track given big enough brakes?
Again, thanks Pete for the very informative posts. Much appreciate them.
This. ^^^
Also I don't know anyone who has felt the difference in mass when driving since the weights offset each other and are therefore balanced.
Not sure what you mean by the weights offset each other. Even with the larger size, the rotors weigh 8-10 lbs less than steel and anyone who has driven a car that has dropped 40 lbs of unsprung weight would have to have pretty numb steering to not feel the difference. I have dropped 3 or 4 lbs per wheel and felt a huge difference.
They need to be larger both to absorb and shed heat.
A brake converts kinetic energy into heat, so it needs to get hot to function. How hot it gets depends on its thermal mass (thermal mass = specific heat of brake rotor material x amount of brake rotor material). The ceramic material in PCCBs is formulated to have a higher specific heat (60% better), but its density is 1/3 of cast iron. Thus if you simply swap cast iron material for ceramic you'll get a rotor with 1/3 the weight and just over half the thermal mass. That second bit means for any given stop it will get twice as hot.
PCCB brakes work to higher temperatures than cast iron: 900C vs 700C, so a little hotter is OK. Twice as hot it not- the rotor and pad will come apart. To prevent this you make the rotor bigger- in this case they weigh roughly 55% (without hats) of their cast iron counterparts, so they have (160% x 55% =) 88% of the thermal mass, which means if the cast iron gets to 700C the PCCB will get to ~800C, still under the safe operating limit.
There are issues with this. The hotter pads and rotors wear out faster, so running at higher temperatures can get expensive. Porsche screwed this up with early PCCBs: the 996 GT2 came with ~25% lighter PCCB rotors than the 991 GT3 but it had similar power and weight. Thus its rotors would try to get nearly 25% hotter on track, leading to extremely rapid failure and a lot of pissed off customers. Porsche has been backing the operating temperature of PCCBs down since.
The GT4's case is the most conservative we've seen. The car has 20% less power and a little less weight , call it ~25% less kinetic energy than the GT3 at the end of the average straight, but the brakes are the same. And because of their large size and surface area they shed heat rapidly, so they'll stay relatively cool. Not as cool as the cast iron, but much cooler than any other Porsche PCCB, so they should last longer. Add in the discounted price and the car's low weight and if there was ever a time to take a risk on PCCBs this would be it.
However. As mooty suggested there are still issues. Many of us, myself included, have hated the over-assisted feel of PCCBs in the past- they make it harder to heal-toe, harder to modulate, etc. They will prevent you from running 19" wheels, so no Hoosiers for you. Finally they will be more expensive, both pads and rotors, without question.
I've mentioned this before, but it bares repeating because I think it says it all. I asked Walter Röhrl how he'd order the car, and he said stripped- no AC, no radio, etc- the weight makes a difference. But for all his focus on weight loss he said he'd order it without PCCBs. Ask yourself why.
They're really hot when the start to glow, but radiative cooling grows very quickly (temperature to the fourth power) so when they are that hot they can dump a huge amount of heat. The issue being that they are also eating themselves at a huge rate.
Mind elaborating that? Do you mean the brakes shed material at a faster rate once they are hot enough to glow?
Yes, though we need better data to see how much. In a metal rotor the process is clear- the metal itself loses strength as it gets hot. Braking puts huge stresses on the rotor, particularly at a microscopic scale. As the rotor heats up is gets weaker and softer. When steel brakes get really hot they fade because the top layer of metal is simply sheering off. This is a temperature vs strength curve for a steel alloy with much better properties than cast iron, but you get the idea:
I don't think the same process takes place in carbon ceramic composites- yield strength stays high as temperature increases, which is why PCCBs don't fade. But the literature shows wear rate going up non-linearly with brake force and temperature, for example (Si CMC is similar to what SGL carbon uses for PCCB):
Originally Posted by johnr265
Not sure what you mean by the weights offset each other. Even with the larger size, the rotors weigh 8-10 lbs less than steel and anyone who has driven a car that has dropped 40 lbs of unsprung weight would have to have pretty numb steering to not feel the difference. I have dropped 3 or 4 lbs per wheel and felt a huge difference.
Weight dropped from a wheel/ rim is on a larger radius, and since the flywheel effect is squared with radius you'll feel that difference much more easily. The PCCB's weight is also further out, canceling out some of the theoretical advantage. Rough numbers, saving 10 lbs with PCCBs is going to feel similar to losing 4 lbs on the rim or 3 at the tire.
Weight dropped from a wheel/ rim is on a larger radius, and since the flywheel effect is squared with radius you'll feel that difference much more easily. The PCCB's weight is also further out, canceling out some of the theoretical advantage. Rough numbers, saving 10 lbs with PCCBs is going to feel similar to losing 4 lbs on the rim or 3 at the tire.
In that case, cost/benefit points to spending your money on lighter rims instead of PCCBs. Interesting reading in this thread - many thanks.
Weight dropped from a wheel/ rim is on a larger radius, and since the flywheel effect is squared with radius you'll feel that difference much more easily. The PCCB's weight is also further out, canceling out some of the theoretical advantage. Rough numbers, saving 10 lbs with PCCBs is going to feel similar to losing 4 lbs on the rim or 3 at the tire.
But the flywheel effect is not the only difference you would feel, right? That is the effect on acceleration and deceleration. On the other hand, you also have just the total effect on unsprung weight on the suspension. That's what I experienced at least in comparing the 991S with and without PCCB back to back on the street. The car with PCCB was noticeably more composed on roads around here, and that drive made me much more interested in PCCB.
For me the GT4 will be a street and track car. If it were 80% track, I'd go standard brakes probably to avoid risk of going through expensive rotors too quickly. Although, to be honest, if I were going to spend $100k on a dedicated track car it probably wouldn't be a GT4 to begin with.
For a car that will spend a majority of its miles on the street, including to/from tracks, (like mine will) then I go back to that test drive and it makes we want the PCCB. Not because of the flywheel effect on acceleration (in either direction), but just the total unsprung weight and how that felt on normal roads (at least on a 991)
02-28-2015 | 05:37 AM
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