No problem - I had plenty of my own confusion on this too

 

Christ. This thread almost had me opting for PCCB on my car. Let me get the hell out of here...

 

Sicom lists 1185 Euros (about $1326) per rotor. Still a lot more than $400 but if it can be done, perhaps more companies will start doing it soon and competition will bring the price down?

BTW, don't knock blue with yellow calipers. I think it looks hot! Still thinking of steel though for the GT4 even though I love the PCCB.

 

Haha, seriously. Some of these threads can be extremely dangerou$$$.

 

Yes but sicom can refurb ccm rotors it seems, which is different from CCB

 

By the way, why are PCCB generally larger than their steel counterparts. Is it because given the compound more area of friction is necessary? Or is it that they can be because they are lighter? And if the two are the same diameter, are PCCB's any stronger than steel, or even weaker?

 

Actually, they claim to refurb both kinds...

 

Oh boy... I've become the disseminator of bad info, which I always despise. I'm just going to sit and read ab this brake issue until ordering day.. Then I'll play eeny-Meany-miney-mo if I haven't decided yet and pick red or yellow

 

The cynic in me thinks it might be because there is more brake torque applied when you clamp the rotor a longer radius from the hub, so you get more stopping sensation for the same amount of pedal pressure (assuming all other variables equal). Helps make the driver think there is a braking improvement...

 

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.

Link to the technical specs of PCCB material:
http://www.sglgroup.com/cms/internat...ml?__locale=en

 

Thanks, Pete!

 

Also identical to the brakes on the new RS (both iron and PCCB options) which has even more power, bigger tires, and more downforce. If Porsche thinks the brakes are adequate for that car and the type of hard use it will see, they are really generously over-engineered for the GT4.

 

Of these it's really only the power that impacts the peak temperature the brakes. If anything the downforce and extra stick from the tires will let the car carry higher cornering speeds, reducing how much you need to use the brakes. One of the big differences between factory test drivers who tested the early PCCBs and Porsche's amateur customers was that the test drivers were too good. They brake less for the corners and they never went off into gravel traps!

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 would've thought that the extra downforce and bigger tires would raise the threshold of how hard you could apply the brakes without locking a wheel (and putting more heat into the brakes). Good point on higher cornering speeds reducing braking requirements though...

 

On second thought, I guess the downforce and tires just allow you to brake later and absorb the same kinetic energy over a shorter time. Thanks for pointing out the 918 case too, Pete!