Brake Aero Cooling

[riff_raff]

Maybe I'm missing something, but I still believe with a carbon-carbon brake system, the pads will always run far hotter than the disc. This is due to the lower thermal mass of the pads, the less efficient path for conducting heat away from the pad friction surface, and the limited path for heat transfer from the pad to the cooling airflow. The rotor likely has about 3-4 times the thermal mass of the pads, and the rotor also has many times the surface area exposed to the cooling airflow if you consider both the rotor friction surface and the internal vane surfaces.

Of course, since the pads are more thermally isolated than the rotor, there will be far more heat rejected from the rotor body.

[olefud]

Maybe I'm missing something, but I still believe with a carbon-carbon brake system, the pads will always run far hotter than the disc. This is due to the lower thermal mass of the pads, the less efficient path for conducting heat away from the pad friction surface, and the limited path for heat transfer from the pad to the cooling airflow. The rotor likely has about 3-4 times the thermal mass of the pads, and the rotor also has many times the surface area exposed to the cooling airflow if you consider both the rotor friction surface and the internal vane surfaces.

Of course, since the pads are more thermally isolated than the rotor, there will be far more heat rejected from the rotor body.

Let me restate my findings to see if we really have an issue. I found that a control run under rather demanding parameters did, as you state, develop somewhat higher temperatures in the pads than in the iron rotor. A subsequent run with the vanes showed a marked decrease in the rotor temperature and a noticeable increase in the pad temperatures. Thus, while the rotor heat rejection, by far the greater portion of the heat load, was enhanced by the vanes, there was a disruption of an unsuspected (by me) cooling flow to the pad. I now think this cooling was cool air at the outer portion of the rotor boundary layer, but this is just conjecture at this point.
The problem with the greater ΔT between the rotor and the pads is that volatile material from the hot pads deposits on the cooler rotor well beyond the usual transfer layer. This may not be a real problem since the runs were under conditions beyond those of even competition.
This phenomenon may not be a problem with C-C given the lower volatiles and similar pad/rotor materials. Rather, C-C may not like reduced temperatures. I’m at a point where I consider the concept sound but requiring tweaking for specific applications. Full development of the theory details such as pad cooling should shake out during such testing.

[riff_raff]

OK, my bad. Apparently I missed the part about using an iron rotor. The fact that the rotor is iron makes a huge difference. While the iron rotor has a much lower safe thermal structural limit than a carbon rotor, it also has much more efficient conductive heat transfer away from the friction surface.

As such, it would also be logical to assume that more of the heat load produced at the pad/rotor interface would be transferred into the iron rotor mass, since it would have a larger delta-T than that of the pad surface, and would also have a greater thermal conductivity rate through its thickness.

[olefud]

Apparently I missed the part about using an iron rotor. The fact that the rotor is iron makes a huge difference. While the iron rotor has a much lower safe thermal structural limit than a carbon rotor, it also has much more efficient conductive heat transfer away from the friction surface.

As such, it would also be logical to assume that more of the heat load produced at the pad/rotor interface would be transferred into the iron rotor mass, since it would have a larger delta-T than that of the pad surface, and would also have a greater thermal conductivity rate through its thickness.

Carbon would be a reasonable assumption this being a F-1 forum. However, when I can get access to a brake dyno, I gratefully take what I can get –and there’s a lot more iron than carbon in the braking world.
When considering the heat transfer it’s useful to think in a dynamic time domain rather than static analysis. For instance, when tested at triple digit equivalent road speed, the friction heat generated essentially in two dimensions, i.e. very high initial temperatures with “no” volume. During the short period between heat creation at a rotor contact area and the rotor again contacting the leading edge of the pad, the very high surface temperature heats both the rotor and the adjacent boundary layer. While a temperature profile develops in the rotor –very hot at the surface and much cooler internally- the boundary layer is largely parted and removed by the vane to be replaced by much cooler ambient air each revolution. This differing ΔT drives heat transfer more to the boundary layer despite the lower heat conductivity of air.
Though the total heat energy rejected with the boundary layer removal during a single rotation is not great in the big picture, the heat generated in a single cycle is also not that substantial. It’s just that there are a really large number of cycles that drive both sides of the equation.

[riff_raff]

It's not a simple problem to analyse for sure. But with an organic pad compound and an iron rotor, the fact that the pad compound has extremely low thermal conductivity means that under braking the pad friction surface rapidly becomes very hot, and most of the heat transfer is to the cooler and more thermally conductive iron rotor body. As for the heat rejection from the vented iron rotor, I would think that more heat transfer would take place within the internal slots due to the fact that the air mass flow rate and velocity is much greater from the effect of the vanes.

[godlameroso]

Perhaps introducing a turbulent flow directly in the path of the pads would help with cooling as turbulent air has better heat transfer.

[olefud]

It's not a simple problem to analyse for sure. But with an organic pad compound and an iron rotor, the fact that the pad compound has extremely low thermal conductivity means that under braking the pad friction surface rapidly becomes very hot, and most of the heat transfer is to the cooler and more thermally conductive iron rotor body. As for the heat rejection from the vented iron rotor, I would think that more heat transfer would take place within the internal slots due to the fact that the air mass flow rate and velocity is much greater from the effect of the vanes.

My initial analysis missed a couple of anomalies according to the empirical data. I’m in discussions to obtain an understanding that the test data are a product of our joint work product and thus not subject to the Confidentiality Agreement. However, if I can’t reach an understanding, I will defer since I do appreciate the help I received.

Rotor temperatures are a somewhat slippery subject. These are reported by a single thermocouple while there is a substantial temperature gradient. Thus there is serious spatial aliasing of the reported temperature. The rotor surface can be cherry red hot while the internal fins are relatively cool. Results should be comparable, however, since the thermal couple should be consistently located.

Conventionally, the rotor serves as a heat sink since it can’t reject heat at the rate hard braking generates heat. Since braking duty cycle is usually rather low, the accumulated heat in the rotor is largely rejected during nonbraking, which is often not good enough for competition. The exterior aero vanes add an addition means of heat rejection, cumulative to the conventional means, that more aggressively rejects the boundary layer heat with increasing temperature at the rotor surface.

A wild card with the abusive testing conducted –clamping down a fixed hydraulic pressure and rotor rotational speed- is, as you suggest, variation in pad coefficient of friction with temperature. A cooler rotor with vanes but with hotter pads would probably be doing a bit less work if the pads lost friction with temperature.
There’s still much to be learned. However, I can categorically state that the vanes illustrated in the more detailed Brake Heat Rejection Concept thread do in fact provide a very substantial temperature reduction in the rotor under hard use relative to a control sans the aero vanes. What I’m having a problem with is a concurrent increase in the pad temperature when I expected the cooler rotor, which deals with by far the greater amount of heat energy, to modestly also cool the pad.

[olefud]

Perhaps introducing a turbulent flow directly in the path of the pads would help with cooling as turbulent air has better heat transfer.

Yes, in competition, with slipstream or forced convection cooling, this would seem to be a good answer. However, I’ve been surprised once by the hotter pads and would need to test such convection cooling before signing off.

Also, I’m really interested in figuring out why the pads run hotter with the vanes. It may not be a problem in real world use, or perhaps with different friction material. The feedback has convinced me that there’s something going on with pad temperature that hasn’t been recognized.

[godlameroso]

Do you think the pads are absorbing heat that would otherwise be absorbed by the rotor with your device? If so this principle could be used in brake caliper design. For example higher performance pads that need to be heated to perform could be used with conventional iron rotors improving performance without having to use expensive carbon components.

[olefud]

Do you think the pads are absorbing heat that would otherwise be absorbed by the rotor with your device? If so this principle could be used in brake caliper design. For example higher performance pads that need to be heated to perform could be used with conventional iron rotors improving performance without having to use expensive carbon components.

What I think -operative word “think”- is happening is that the vanes while detaching the hot boundary layer are also redirecting an attached outer, cooler boundary layer. This outer layer serves to cool the pads/caliper. Brake experts I have talked with have no idea what’s happening –one suggested it might be a data error. Further testing showed it’s real.\

The rotor takes by far a greater heat load than do the pads. By keeping an iron rotor cool, destructive heat checking and warping, the usual failure mode, can be avoided. I see the pad heating as more of a tuning challenge and frankly have posted the result to see if somebody might have seen such pad heating elsewhere. Apparently it’s new ground.

To be clear, the test failure occurred at relatively low pad and rotor temperatures. The failure mechanism appeared to be a result of the rather large ΔT between the somewhat hotter pads and the substantially cooler rotor which, in my view, caused excess transfer of pad material from the hot pad to the cool rotor resulting in a buildup of pad material on the rotor surface.

[godlameroso]

Interesting, perhaps it is like you say, simply a matter of tuning and finding the right pad material.

[olefud]

Interesting, perhaps it is like you say, simply a matter of tuning and finding the right pad material.

Maybe. We used a common Corvette competition setup with PFC01 pads. These pads are known to deposit a tenacious, tough to remove dust.

[riff_raff]

Do you think the pads are absorbing heat that would otherwise be absorbed by the rotor with your device? If so this principle could be used in brake caliper design. For example higher performance pads that need to be heated to perform could be used with conventional iron rotors improving performance without having to use expensive carbon components.

The thermal energy produced from the braking friction has to go somewhere. And the principles of conductive heat transfer would imply that most of this heat load would be forced into the iron rotor, with its higher thermal conductivity, greater thermal mass, greater specific heat, and more efficient heat rejection rate, rather than the asbestos(?) pad body with far inferior thermal properties.

[olefud]

Do you think the pads are absorbing heat that would otherwise be absorbed by the rotor with your device? If so this principle could be used in brake caliper design. For example higher performance pads that need to be heated to perform could be used with conventional iron rotors improving performance without having to use expensive carbon components.

The thermal energy produced from the braking friction has to go somewhere. And the principles of conductive heat transfer would imply that most of this heat load would be forced into the iron rotor, with its higher thermal conductivity, greater thermal mass, greater specific heat, and more efficient heat rejection rate, rather than the asbestos(?) pad body with far inferior thermal properties.

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

[riff_raff]

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.