Brake Aero Cooling

[olefud]

Very true. But it’s important to distinguish between heat energy and temperature. During braking the pads have a 100% duty cycle while any particular point on the rotor has a rather low duty cycle. Thus the rotor will tend to run hotter than the rotor which, as your point out, tends to divert heat energy to the lower temperature rotor

Do you mean heat transfer and temperature, rather than heat energy and temperature? I would agree with your comment that the pad is subject to a 100% "duty cycle" during braking, while the rotor is not. But consider this hypothetical: Let's assume we have a pad material that conducts absolutely no heat, and a rotor material that has a very high thermal conductivity. At the instant the brakes are applied and there is friction heat generated at the pad/rotor interface, the relative pad and rotor surfaces would assume a state of thermal equilibrium based on how efficiently the rotor conducts the thermal energy away from the contact interface. Since the pad material does not conduct heat, while the pad outer surface will become hot, there will be no conductive heating of the pad body. And all of the heat load produced at the friction interface will be forced into the rotor body.

My point is a bit different. As braking continues, the rotor tends to run cooler than the pad (pad surface in your no-pad-conductivity example) though both are exposed to the same “friction interface” temperature. Not only is rotor a better heat conductor but only a relatively small portion of the swept area is exposed to the friction heat source at any given time, i.e. a lower duty cycle. Thus the rotor would have not only a greater conductive coefficient h, but would also see a greater ΔT relative to the “friction interface” area that would be neither pad nor rotor but the common area there between.

No conductivity is a tricky concept. I’m thinking it would be like very low conductivity ceramic heat shields that still show surface heating, i.e. space craft reentry heat shields.

[riff_raff]

What you describe is the situation in a nutshell. Without a relative deltaT between the pad/rotor surfaces, there would be no conductive heat transfer.

[olefud]

What you describe is the situation in a nutshell. Without a relative deltaT between the pad/rotor surfaces, there would be no conductive heat transfer.

My thinking is that the pad and rotor see the same very high temperature at the friction interface there between. However, my thinking went astray in expecting the cooler rotor to conduct more heat energy from the interface thereby lowering the pad temperature somewhat. So now I’m thinking that the rotor normally carries cooler air to the pad that cools it. However, with the vanes this flow is interrupted such that he pads run a bit hotter despite the substantial cooling of the rotor.

[riff_raff]

It is true that a metal rotor would conduct more of the thermal energy away from the friction interface, due to its higher thermal conductivity, higher specific heat, greater thermal mass, more efficient heat transfer to the cooling airflow, and greater deltaT at the friction interface. Metal brake rotors work well because of their ability to efficiently dissipate the heat energy from braking friction. But even though C-C brakes have much lower thermal conductivity, they have the advantage of being able to operate at much greater temperatures, without a degradation of friction coefficient or mechanical strength.

[olefud]

It is true that a metal rotor would conduct more of the thermal energy away from the friction interface, due to its higher thermal conductivity, higher specific heat, greater thermal mass, more efficient heat transfer to the cooling airflow, and greater deltaT at the friction interface. Metal brake rotors work well because of their ability to efficiently dissipate the heat energy from braking friction. But even though C-C brakes have much lower thermal conductivity, they have the advantage of being able to operate at much greater temperatures, without a degradation of friction coefficient or mechanical strength.

Metal rotors, while rejecting some heat energy during braking, serve primarily as heat sinks under hard braking. When the heat sink capacity is swamped, temps climb to effective radiant cooling temperatures, but these temperatures also cause heat checking. C-C, having much less thermal mass and being able to withstand much higher temperatures, reject heat contemporaneously by thermal radiation. Since radiant heat rejection is a fourth order of the ΔT of the absolute temperature of the rotor and the surroundings it sees, the higher rotor temperature of C-C drives radiant heat rejection nicely –if it doesn’t overheat the wheel/tire assembly.
Yesterday’s Phoenix NASCAR race appears to illustrate the problem of uncontrolled radiant rotor cooling overheating the tire n view of the number of early, in terms of wear, tire failures. Of course conductive heat could also reach the tire, but conductive heat usually gets the bearing more than the tire. It would be interesting to see how NASCAR teams deal with this.

[riff_raff]

Whether the rotor is carbon or metal, acting as a heat sink is only part of the rotor's function. The complete function of the rotor is to use its mass to absorb the thermal energy produced by the friction of braking, and then transfer that thermal energy to the passing airflow. A brake rotor's capacity to achieve this is based on many factors, such as rotor mass, rotor material specific heat, rotor material thermal conductivity, and rotor surface area available for heat transfer to the passing airflow.

[autogyro]

Does this method of brake cooling increase or decrease braking efficiency?

[olefud]

Does this method of brake cooling increase or decrease braking efficiency?

Efficiency is a slippery term – increasing one operating parameter relative to another. I’m not aware of a formal definition for brakes.

However, if we designate efficiency as more effectively rejecting friction heat from the system to increase the maximum work a brake system can do, it would increase thermal efficiency. More specifically, brakes falter when overheated. By maintaining the capacity to convert kinetic energy to heat energy under greater brake loading and facilitating more effective brake temperatures, it would be fair to say that the method increases efficiency, though it makes little difference if the brakes are not stressed.

A secondary contribution to efficiency would be allowing lighter brake assemblies with performance equal to heavier conventional brakes, but this would be overall efficiency rather than specifically brake efficiency.

[olefud]

Whether the rotor is carbon or metal, acting as a heat sink is only part of the rotor's function. The complete function of the rotor is to use its mass to absorb the thermal energy produced by the friction of braking, and then transfer that thermal energy to the passing airflow. A brake rotor's capacity to achieve this is based on many factors, such as rotor mass, rotor material specific heat, rotor material thermal conductivity, and rotor surface area available for heat transfer to the passing airflow.

Semantics can be troublesome. By heat sink I intended to say that braking heat is transferred and stored for rejection largely out of phase with the braking event. But you are correct if your point is that the rotor will be heated during braking and is thus inherently a heat sink.

Both convective heat rejection by my vane and radiant heat rejection are largely contemporaneous with the braking event and rely on necessary short term heat storage in the rotor to primarily reach working temperatures. But heat rejection occurs mostly during the braking event rather than being stored in the rotor as is internally vented, or even solid, metal rotors. Carbon rotors have low thermal mass and are comfortable with rather elevated temperatures. Thus radiant cooling is effective.

Metal rotors don’t do so well with temperatures that facilitate IR radiant heat rejection, i.e. fairly bright red glow. Therefore conventional metal rotors, when pushed, work better as heat sinks that store energy for rejection in large part after the braking event.

The rotor factors you list are properly characterized. I’m just observing that these factors, as a generality, function as described above.

[riff_raff]

Does this method of brake cooling increase or decrease braking efficiency?

As the old joke goes," Brakes only stop the wheel. It's the tires that stop the car."

The efficiency of a brake system would be based on the ability to transfer kinetic energy to the cooling airflow without exceeding the traction limits of the tires.

[autogyro]

As the old joke goes," Brakes only stop the wheel. It's the tires that stop the car."

The efficiency of a brake system would be based on the ability to transfer kinetic energy to the cooling airflow without exceeding the traction limits of the tires.

Thats a bit over simplified isnt it?

The ability to control the brakes without lock up is essential.
As is achieving a high temperature from hard use without producing brake fade.
Brakes are not just used at a maximum.

[olefud]

As the old joke goes," Brakes only stop the wheel. It's the tires that stop the car."

The efficiency of a brake system would be based on the ability to transfer kinetic energy to the cooling airflow without exceeding the traction limits of the tires.

Thats a bit over simplified isnt it?

The ability to control the brakes without lock up is essential.
As is achieving a high temperature from hard use without producing brake fade.
Brakes are not just used at a maximum.

Big picture, I agree with riff_raff –brakes fail in general because of overheating. Since the vanes provide an additional and additive means of rejecting heat, they enable brake operation under greater thermal loading and particularly under sustained use. The enhanced heat rejection is progressive with temperature such that normal operating temperature is not much affected. Normal use is pretty much normal with the vanes. Except for the pad temperature idiosyncrasy, brakes operate otherwise normally but are much more resistance to fade and metal rotor heat checking and warping with extreme use.
Optimum braking would not be increased for a given combination of brakes and tire. Rather, the optimum would be maintained rather than fading under hard use.

[riff_raff]

Thats a bit over simplified isnt it?..........Brakes are not just used at a maximum.

With the brakes on a road race car, I would mostly disagree. The brake systems on road race cars are typically designed such that they can be applied as hard as needed without fade or overheating. With F1 cars, the brakes are applied to the maximum that the individual tire tractions permit, at every braking event.