Why are people talking of increased heat transfer with ceramics?

Ceramics are excellent insulators, not conductors, right?

So isn't the problem likely that the ceramic pads are not able to transfer any of the heat to the caliper, leaving it all in the rotor?

 

I run Stan's rotors with Hawk Super Duty with braided Stainless lines front and back and brake shields removed all around.

I ran them on the track Saturday and Sunday 30 minute sessions from 9:00am to 5:00pm full out.

The results are extremely satisfactory. The fronts run cooler than the back due to the solid rotors but removing the shields really helped.

TB

 

Yes, Tim, thats exactly the problem. Ceraminc pads on iron rotors are not the best combo. The C-5 Vettes as raced by Andy Pilgrim and Kelly Collins had both ceramic pads and ceramic rotors. That same option is available on the new 911's for a mere $9000 or so. If you watched any of the Daytona 24 hr. both the Vetes and the Audis had ceramic/ceramic packages and they could be seen glowing red hot for most of the race. Some of the Daytona prototypes used them as well and the excess heat contributed to front suspension failure. The bootm line is that the configuration of the vented Lightning rotor cannot absorb the amount of heat dumped into it by the ceramic pads without causing problems with the rotors. The rotors are so thick that the temperature variations between the vented area in the center of the rotor and the surface where the ceramic is actually dumping the heat is so great that the high temperature variations and consequent vaariations in the thermal expanses lead to stress cracking. As I indicated before, a 600 Benz has a rotor that's about 1/2" thick with no vent slots. As such the heating is uniform and all the way through. Fewer thermal imbalances and less thermal stress. Vented rotors the size of those found on the Lightning are happier with a pad that is soft enough to grip properly without boiling the fluid in the caliper. Once you boil the fluid, it is time to bleed and toss it or live with a spongy pedal until you do.

 

Look at it with the shuttle with the ceramic tiles protecting it. It will absorb heat and a tremendous amount at that protecting the shuttle and at the same time it will dissipate heat just as fast keeping the shuttle cool as long as there is some kind of cooling air, liquid etc. This goes with the ceramic pads. It will draw the heat from the rotor as long there is effecient cooling for the pad. If there is not effecient cooling to dissipate the heat eventually the rotor will heat up with the ceramic pad and the pad will keep heating up more than the rotor can handle.

 

Umm, oops. That happened to me a few months ago, did a post about it.

 

AZ fun, I have to take issue with that. The tiles on the shuttle have the atttribute of absorbing a lot of heat at the sufrace without transmitting it into the mounting substrate and hence into the shuttle surface. As a restatement, you could put a shuttle tile in your hand, heat the top surface red hot with a torch and continue to hold it. Yes, the heat willeventually soak through, but the design criteria is such that by that time the shuttle velocity is sufficiently diminished that in conjunction with the very low upper apmospheric temperature,the tiles begin cooling, rather than heating before the heat gets to the shuttle. One of the major challenges in the tile design was getting the thickness and specific heat correct to make the transtion from apsorption to disappation to match the decelration curve of the spacecraft.

So:

If you can design a system that actualls cools the pads quickly enough to allow them to play a significant role in absorbing the heat of decelrating the vehilce, I'd be interested in using that design and putting it into production. As is, the sole heat sinking device for the pads, (other than the rotor) is the caliper. Last time I looked at mine, it was obvious that there was very little, if any, consideration given to the way they dissapate heat. Yes, they do have vestigal ribs, but if you look at a properly designed aluminum caliper (like the Aussie designed 1LE Corvette caliper, you can see that they were designed to radiate off heat. The lightning calipers may be best described as a big lump of cast iron, with a surface configuration designed for ease of manifacture with no thermal engineering thought given toward proper cooling. Yes, I love my Lightning, but the bean counters got to the calipers. The fronts aren't much. The rears belong on a focus. The true problem is not the pads themselves getting hot, but the pads dumping heat into a caliper that is just not properly designed to get rid of it. The rotor is too thick to heat uniformly. Non uniform heating equals distortion and cracking. Several long high speed stops lead to higher brake pedal effort. Why? Because the fluid can't handle the heat and boils. The older the fluid is, the more water it absorbs. The more water, the lower the boiling point and the worse the braking. If you are going to use you brakes hard, change the fluid at least once a year.

 

I agree with you kerno on what you say. The key word though is cooling. Our or any brake system can handle ceramics but without proper cooling eventually the ceramic will heat up to a point where everything around it will fail. You can run ceramics on metal all day long with the proper cooling. Ceramics will draw the heat from any surface but with the constant contact with the rotor eventually the rotor catches up with pad temperature causing even more heat. The ceramic will heat up faster and hotter first before everything else. Thats why you need good cooling and the brake systems on the lightning can't handle it. As for the corvette brake system what happens there is that the pad and rotor will heat up faster and cool faster at the same time before anything else will heat up.