Why Carbon-Ceramic Brakes Are Expensive. And Why They Might Be Worth It

Motor 1

11 hours ago

A couple of years ago, a Brembo engineer told me something that stuck: If you buy a car with carbon-ceramic brakes, you'll likely never need to replace the rotors. I'd heard the benefits of carbon-ceramic brakes talked up before, but this particularly bold claim seemed wild, an answer to the ultimate question: Are these fancy brakes worth their huge price tag?
On the Cadillac CT5-V Blackwing, the carbon-ceramics are a $9,000 option; BMW charges $8,500; Porsche charges more than $9,000. Carbon-ceramic brakes are routinely among the priciest options for cars that already have a lot of big-ticket extras. Is there any world in which they're worth it?
Welcome to The Rabbit Hole, a bi-weekly column where Senior Editor Chris Perkins explores his latest obsession with automotive technology. He speaks to the best in the business to understand how cars work and what the future of the automobile looks like.
Photo by: Brembo
Cast iron is a wonderful material for making brake discs. It's relatively cheap, easy to cast and machine into shape, and crucially, it has higher thermal conductivity than, say, steel. To perhaps state the obvious here, brakes convert a car's kinetic energy (forward motion) into heat via friction between the pad and rotor when the two come together. So a brake disc's thermal properties are of key importance.
"[Cast-iron discs] have a better ability of absorbing the heat," explains Emanuele Bruletti, senior engineering manager for Brembo North America. "They can absorb it at a lower rate [than other common materials], and therefore, they can help in taking some of that away from the pads."
It's the same reason cast iron makes for a great skillet, but if you cook with one, you know just how heavy it is. Weight is a car's enemy. So too is the increased demand on braking systems as cars evolve.
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"What has been driving the size increase in braking systems in the last few years is basically the performance envelope increasing," Bruletti explains. Cars are simply more powerful and heavier. Tires also play a role. Bruletti says that modern developments in tires have allowed for far greater deceleration rates, further increasing the demand on a braking system.
That increased demand translates to more heat. Upping the size of your cast-iron rotor helps deal with all that heat better and improves the brake's ability to effectively slow a car. For obvious reasons, though, you can only make rotors so big, both for packaging and weight.
Brake rotors are unsprung, which means their mass has a disproportionately high effect on ride and handling relative to a car's sprung masses. They're also rotating masses, which have a big effect on a vehicle's ability to accelerate, brake, and turn.
"If you can shave weight off your car and more importantly, unsprung weight and evenly more importantly unsprung rotating weight, which is what a rotor is, [there are] huge gains to be had in performance," says James Walker Jr., a racer, engineer, and author on a book about braking systems.
Chasing lightness, Dunlop developed the first carbon-fiber reinforced carbon brakes for the Concorde in the 1960s, and by the 1980s, these became common in Formula 1. However, these carbon-carbon brakes, still in use at the top levels of motorsport, are entirely unsuitable for road use, as they don't work well at cold temperatures. They're also extremely expensive and time-consuming to make, even now.
A carbon-reinforced silicon-carbide matrix brings some of the weight-saving benefits of carbon-carbon brakes, but in a package that actually works at cold temperatures. And while still expensive and time-consuming to make, a carbon-ceramic brake disc is a lot easier and cheaper to manufacture than a carbon-carbon disc. We're talking a production time of around a couple days vs four months here. (That said, Brembo can make a cast-iron disc in about two hours.)
Photo by: Porsche
Photo by: Ferrari
German company SGL Carbon introduced carbon-ceramic brakes in a road car, with the 2001 Porsche 911 GT2. Brembo's first carbon-ceramic brakes arrived a year later, with the Ferrari Enzo. In 2009, SGL and Brembo formed a joint venture for the development and manufacture of carbon-ceramic brakes, and today, it's one of, if not the largest, suppliers of brakes of this type.
Bruletti says the carbon-ceramic matrix it uses has about a third the density of its cast iron. In terms of actual weight savings, you see all sorts of numbers thrown out. A good example is the brake discs in the previous-generation M3 and M4. In a technical document, BMW quotes a 30.6-pound weight for the car's standard front rotors and 17.1 pounds for the carbon-ceramics. So nearly half. The proportional weight savings for the carbon-ceramic rear rotors on the old M3 and M4 are similar, and that's despite the fact that BMW's carbon discs were slightly larger than their cast-iron counterparts.
So great! But, we also need to talk about what carbon-ceramic brakes don't do. As Walker explains, a brake system is, essentially, a series of hydraulic levers that turn the relatively light force the driver applies to the brake pedal into a huge force at the road that slows the car down. In a road car, a 20 to 30 pound pedal input can translate to 1G of deceleration. This is called gain. Here, carbon-ceramic brakes don't have an advantage.
"There's nothing that's done with a carbon-ceramic system compared to a cast-iron system that increases the mechanical output of the brake system," Walker explains. "So there's no real advantage to them in that space. The only reason people say, 'Oh, they feel better, they stop better,' is not because it's carbon ceramic, it's because [the automaker has] tuned that carbon-ceramic system to have a higher gain."
Taking things a step further, Walker also points out that the braking system is only as good as the tire you have attached to it.
Imagine you could have two identical cars, on the same model tires, the only difference being that one has cast-iron brakes, the other has a carbon-ceramic brake package. The brakes don't change the level of grip the tire is capable of. On the flipside, and to Bruletti's earlier point, the tire has a profound effect on the energy that goes into the braking system.
Photo by: Ferrari
As we've established, a carbon-ceramic disc is materially very different from a cast-iron disc. Carbon-ceramic has a much lower level of thermal conductivity than iron, but also far less mass and heat capacity.
Which is a good and bad thing. Good because the brake disc can withstand the higher temperatures that today's faster/heavier/grippier vehicles generate in extreme braking events, courtesy of that ceramic chemistry.
Brembo says carbon-ceramic discs can comfortably operate between 1,000 and 1,400 degrees Fahrenheit and can even withstand temperatures beyond 1,800 degrees. That's why carbon-ceramic brakes are frequently praised for their resistance to fade on track.
But since the lighter and less-dense carbon-ceramic rotors gain and lose temperature quickly, that leads to huge thermal stresses on the rest of the braking components as they heat and cool in rapid succession. A cast-iron rotor better contains its heat, which keeps everything else cool.
"You need to find a way of dissipating that heat away from the pads in some other way, and this is where it becomes very important to provide the necessary cooling at the brake system," Bruletti says.
Photo by: Porsche
Photo by: Porsche
Beyond carefully designed cooling from both external components and internal rotor ducting, the fact that carbon-ceramic rotors aren't made from a homogenous material also has implications . The length, diameter, and orientation of the individual carbon fibers all have an effect on the material's thermal capacity.
Adding additional layers and coatings also improves thermal capacity, which is why Brembo and SGL offer CCB brakes with additional ceramic friction layers on both sides, and CCW brakes, which use five carbon-ceramic layers. These options allow automakers to size down components, further saving weight, but their manufacturing processes are more time-consuming and thus, expensive.
That's helpful because generally, carbon-ceramic rotors are larger than their cast-iron equivalents, in cars where both are optional. This is a direct result of the heat a carbon-ceramic rotor reflects into the pad during large braking events.
'In order to guarantee a stability of the friction material, you need to go larger with the pad,' Bruletti explains. And when you make the pad larger, you make the caliper larger, and the rotor larger. It's all cyclical.
Yet, there's also a virtuous cycle here. Reducing unsprung, rotational mass means there's less weight to control. In theory, an automaker can use the weight reduction from carbon-ceramic brakes to employ smaller tires, lower spring and damper rates, smaller anti-roll bars, and so on.
Photo by: McLaren
All because of the outsize effect that a brake rotor's weight has on the rest of the car. That's a big part of why Ferrari and McLaren only use carbon-ceramic brakes, beyond the simple need for a brake system that can handle the huge stresses these fast cars generate.
And now, we get to the original claim, the thing that started me down this path. Does a carbon-ceramic rotor last the life of the car? Yes. In some cases.
'The wear of the components really depends on usage, how you use them,' Bruletti says. 'If we assume that the usage, the cycles will be the same, yes, it is fair to say that in normal driving and non-track usage, just everyday driving, a carbon ceramic rotor will last in my opinion almost the entire life of your vehicle.'
It's not just the guy from Brembo saying that too. Walker agrees that in normal street use, a carbon-ceramic rotor will last a very long time. Obviously you'll need to replace pads, but the rotors could have incredible longevity. But add track use into the mix, and the calculus becomes very different.
With lots of heavy braking events, the carbon fibers in a carbon-ceramic rotor will eventually burn out. They'll lose thermal capacity. At road speeds, this won't happen much, if at all, but depending on what sort of car you drive on track, what sort of tracks you go to, and how you drive it, the carbon fibers can burn out very quickly.
Photo by: BMW
Let's say you're running your new, 5,300-pound BMW M5 at Road America, where you'll regularly blow past 150 mph on the track's long straights. Let's also say you're one of the last of the late brakers, pushing your brake zones as deep into the corner as you dare, hitting the pedal as hard as you can. If you've got carbon brakes on, you shouldn't expect those rotors to last very long at all.
But say you've got a Porsche 911 GT3, which weighs in around 3,330 pounds, and you're at Lime Rock Park, which has only one heavy braking zone. And let's also say that you're a bit more measured. Rather than braking hard and late, you brake a little lighter, a little earlier. In that case, you can reasonably expect a more life out of your carbon-ceramic rotors.
That difference, though, is why Porsche still offers cast-iron rotors on its GT cars, even the mighty GT3 RS. It knows that some customers will use up their brakes tracking their cars often, and in that instance, it makes sense to go for cast-iron discs, which are much cheaper to replace.
Some other things to consider: With usage, a carbon-ceramic rotor doesn't lose thickness like a cast-iron rotor, but when those carbon fibers burn out, they do decrease in weight. This means a carbon-ceramic rotor won't develop cracks or warp like a cast-iron rotor would on track, so there's another point in favor. It's also why the hats on many carbon-ceramic rotors list a minimum weight. Once the rotor goes below that weight, it's time for a replacement.
So, there isn't a simple answer to whether carbon-ceramic brakes are 'worth it.' But given what we all now know, their high upfront cost can be offset by rotor longevity, and the myriad other benefits the technology brings. It becomes a question of you, the customer. How are you going to use your car, and what do you value at the end of the day?
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