Brake Heat Rejection???

[Overrun]

Is there a need for enhanced brake heat rejection? There seems to be an attitude that heat rejection is fully mature with no room for improvement –not even a brake thread in this forum.

What I have is a cross discipline new means –additive to the existing convection/radiation mechanisms -for heat rejection that avoids the conventional heat sinking for delayed cooling. I’d be happy to go into more detail is there’s interest, but my question is essentially to the need and profile of the brake industry. My background is a bit eclectic but includes suspension enhancements and midlevel race engineering (shoestring but the team did win a TV race). But I have few present contacts.

Any thoughts?

 

[chicopee]

I am amazed that no one has mentioned regenerative power as a braking system on gasoline and diesel powered engines. What a waste of energy when using conventional braking systems.

 

[Overrun]

Chicopee, regenerative braking and the subject vanes are actually complementary. The former is useful for low speed braking but becomes less attractive for the rare high speed braking event because of the cost and weight of batteries, cables etc. as well as the need to reserve battery capacity the store the regenerated energy. Since the energy load increases with the square of speed, these requirements quickly escalate. High speed events are conventionally dealt with –or not- by heavy rotors that sink and store the heat energy over time.
The vanes deal with the higher heat by rejecting rather than sinking the heat with increasing effectiveness as the rotor temperature increases thereby reducing the parasitic energy loss resulting from accelerating and decelerating the heavier rotors.

 

[Tmoose]

"High speed events are conventionally dealt with –or not- by heavy rotors that sink and store the heat energy over time."

Figure 9 on Page 97 here shows rotor temperature rise hitting the brakes in 10 second bursts, followed by 35 second cooling cycles. at a mere 65 mph

http://www.gt40s.com/images/howto/LynnMiner/Reference/SAE_Papers.pdf

I did not see any mention of relative rotor masses.
The difference between the inboard and outboard surfaces after the first few stops is kind of interesting.

 

[Overrun]

Tmoose, thanks -and thanks again. A bit saddening in that the Riverside tests mentioned I expect are those that killed Ken Miles.

While the results are qualitatively what I would expect, it was heartening to see that the solid rotor showed the same internal and external temperatures. This would seem to support my theory of the insulating high temp boundary layer. The experts have had a reluctance to buy in on this.

The inboard/outboard difference is likely due to testing with the wheel on, though I haven’t yet read enough to confirm this.
It’s interesting that NASCAR was having the same heat checking problems just a few years ago. But they seem to have solved it or stopped talking about it –probably the former since there doesn’t seem to be as many rotors rolling down the track lately.

Again, thanks, good info even from almost fifty years ago is very helpful.

 

[NormPeterson]

Have you measured any caliper temperatures - or perhaps more importantly, brake fluid temperatures?

Norm

 

[Overrun]

Norm, we only measured midpoint rotor and pad temperatures during two brake dyno tests. The first was a “standard” test that didn’t show much at the low temperatures. However, at the end the operator offered a “burn down” that involved clamping down and running at high speed. It was supposed to last maybe 10 to 15 seconds. He stopped the test after a minute or two since it was stressing the dyno.

The second test I can’t discuss because of a confidentiality agreement that was supposed to relate to anything I might see concerning race team testing. After the test the company turned hostile and refused any release of the results. The agreement probably doesn’t actually cover the data but the test was gratis and I really don’t much want to fight about it. But I know a good bit more than can be discussed.

Only the pad temps were measured and they ran a bit hotter with the vanes than without in the first test. Thus I think the brake fluid would also be a bit hotter. But there’s still a lot to learn about vane placement and pad material when the rotor runs substantially cooler than the pad. You want a transfer layer of pad material on the rotor to promote micro welding and shearing of the micro welds. But too much pad transfer could get out of hand if not managed.

While the pad temperature was a bit hotter the amount of energy involved is relatively small compared to a similar temperature rise in the rotor.

 

[tbuelna]

Going back to the discussion of C-C brakes used on large commercial aircraft, these brake systems use stacks of rotor and stator plates that are clamped by caliper surrounding the entire circumference of the rotor.

These brake systems are designed for a single brake application that requires absorbing huge amounts of energy. The brake system uses multiple rotor/stator plates to provide a large thermal mass capable of absorbing the thermal energy created by braking the aircraft at landing. These C-C brakes don't require any real amount of air cooling.

 

[NormPeterson]

Do you suppose that in addition to the vanes 'tripping' the boundary layer off the rotor they are also rendering the leading surfaces of the caliper less effective at shedding their heat gain?

I can see why you'd place the vanes immediately leading the caliper or close to such a location - it takes a finite amount of time for the heat to flow from rotor to the air, and at 750 or so revs/mile and much road speed at all there isn't going to be a lot of time before the 'new' boundary layer air gets scraped off.

I understand the confidentiality aspects, though the refusal to release data in spite of such an agreement seems a little odd.


Norm

 

[Overrun]

Tbuelna, the C-C brakes are highly developed and do the job, particularly with thrust reversers. They require high temperatures to avoid high wear rates. Still, carbon doesn’t have a high specific heat and rejecting heat is better than storing it as in taxiing or a missed approach with braking. Weight is always a concern and rejected heat doesn’t have to have a heat sink. Just theory on my part. Getting something new tested and certified can’t be easy.

NormPeterson, truthfully, I don’t know from the limited testing. My theory is the close-coupled vane is diverting the hot boundary layer and cooler air being carried with it. My testing was more concerned with extreme failure-certain conditions as a proof of concept. Refinement is still to be done for differing needs such as truck brakes that, with big wheels, low RPM would probably do well with multiple vanes per rotor side.

As to the agreement, I suspect this came from corporate when they got wind that the engineers were doing the testing.

 

[140Airpower]

Overrun, I think what Norm is saying is the reason the pads run hotter. That is, whatever airflow is cooling the pads and calipers is partially blocked by the vanes.
The vanes are an extremely effective way to disturb the boundary layer. Holes and slots are not as effective, but do blow or suck the boundary layer and provide some additional surface area (as well as sites for cracks to form, etc).
Your comment about the pads not benefiting from a cooler rotor is interesting. I think the pads should benefit from a cooler rotor. Is it the caliper-pad assembly or just the pads? You could use air ducted behind the vanes. This should make things better for the caliper and pad.

 

[tbuelna]

With auto disc brakes that use calipers and pads, the pad friction surface normally runs much hotter than the rotor surface. The pad material typically has poor conductive properties thru its thickness, so more of the friction heat generated at the contact interface between the pad and rotor is forced into the cooler rotor surface. It is not too difficult to thermally isolate the caliper pistons and body from heat conducted thru the pad body. So the main concern with cooling the caliper is keeping the brake fluid temperature within acceptable limits.

Going back to the picture of the C-C aircraft brake I posted, you'll note there are no brake "pads" used. The caliper pistons simply bear against two outer stator discs that don't rotate. No provisions for cooling airflow anywhere on the rotor or stator discs. The amount of energy absorbed by these brakes is huge, and they provide extremely precise anti-skid performance.

 

[NormPeterson]

But if the pads are in fact running hotter with the vane(s) in place, so are the calipers and ultimately the fluid.

FWIW and I won't attempt to guess why, episodes of brake fade - either pads fading or fluid boiling - seem to be more frequent occurrences among track day enthusiasts than rotor failures. Rotor cracks that have been "encouraged" to start by drilled thermal discontinuities perhaps excepted.


Norm

 

[Overrun]

Tbuelna, I agree with your characterization of C-C brakes –though couldn’t open your link. C-C brakes are a bit unique and live well with high temperatures –see below. But if substantial heat could be rejected real time the brakes could be even lighter and avoid retracting a hot brake after taxiing or a go around after an aborted landing attempt. But, as you observe, there’s no crying need.

The hotter pads were measured by thermocouples and showed the difference with identical runs other than the vanes. For competition this could be addressed with ducted cooling air. For trucks or street vehicles it probably could be addressed with appropriate pad compounds.

Brakes used hard rely on a transfer layer of pad material on the rotor to develop optimum friction. The transfer layer develops a like-material frictional interface that favors the micro fusion of asperities between the rotor and pad that make and shear. This is workable with the normally modestly hotter pad material. However, when the pad gets overly hot, too much material transfers and presents the risk of welding larger portions of the pad to the rotor. It’s sort of the same but opposite of C-C brakes in which the like C pad and rotor materials bond more aggressively at lower temperatures to accelerate low temperature wear.

The anomalies I’m reporting should –till you’ve done so you haven’t- be readily solved with further development. However, the solutions will probably differ for competition and truck brakes, for instance.