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?

 

[gruntguru]

I am sure you will get plenty of interest here if you want to discuss your concept. With the exception of trucks, brake fade is rarely a problem in road use. Heat rejection will become even less an issue with the emergence of brake energy recovery.

Engineering is the art of creating things you need, from things you can get.

 

[Overrun]

Agreed as to normal street use and energy harvesting. However, there would seem to be an opportunity even on these vehicles to lighten the rotor by decreasing the heat sink load. Class 8 trucks (new distance requirements) and competition vehicles would seem to be the more likely. My take is that the brake suppliers serve various users.

I have no qualms about the tech and theory discussion. But I’m walking a bit of a fine line as a new poster as to spam. I’d rather do so with interest pull rather than push by me.

 

[IRstuff]

This site considers it spam only when you are massively posting in multiple forums. Staying in this one forum should alleviate any concerns with regard to spam. Since you don't appear to be, per se, selling anything, at least, not right now, you are not infringing on other site rules.

TTFN
faq731-376

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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!

 

[GregLocock]

Brake fade may not be a real life problem in some countries, but when towing 1 tonne trailers behind family cars it is an issue. Every manufacturer I have worked for has a brake fade test, and passing it involves non-trivial compromises with the rest of the pad formulation.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[BrianPetersen]

I've overheated brakes when going downhill in mountainous areas with a trailer in tow. This was on a car with 4 wheel disks and no trailer brakes. You can smell brakes constantly on many roads like that, so yes, it can be an issue!

 

[gruntguru]

I will kick off the discussion with an observation. Much of the mass of a conventional rotor exists to maintain dimensional stability/prevent distortion but also to act as a heat sink to store heat energy produced by short duration, high power braking events. Dissipation by convection and radiation is a much lower power process. Videos I have seen of rotors in action clearly show that the high temp "glow" is primarily a surface effect and dims rapidly as heat transfers to the core metal of the rotor. The "glowing skin" effect must present an opportunity for rapid heat rejection during that event - especially radiation which is proportional to T^4.

Engineering is the art of creating things you need, from things you can get.

 

[dicer]

So if a person gives out a patentable idea on here can others take the credit? I do have an idea for this brake heat rejection issue.
Here is some ones engineering progress.

https://www.youtube.com/watch?v=c_ItwLcBw5Q

 

[Overrun]

Some of this is a bit counterintuitive so keep an open mind.

I observed that a solid-rotor car I raced did not respond to convection cooling as it should. After a good bit of investigation the problem was determined to be a thick, tenacious boundary layer that insulated the swept faces. Air has the property of becoming more viscous with increasing temperature that resulted in the isolating boundary layer. Some idea can be gained from the telltale glowing bits in the ring of fire evident in a stressed brake. The high temperature swept area was insulated by this aggressive boundary layer.

Once the mechanism was determined it was exploited by placing a simple, fixed aerodynamic vane adjacent to but space outside of runout each swept area. The “tacky” boundary layer was continuously diverted, rejected and replaced with cool air each rotor revolution without adding to the rotor heat sink load. Brake dyno testing has confirmed the effectiveness of the concept. It does little at lower temperatures but becomes progressively more effective with increasing rotor/pad temperature.

I haven’t worked out just how the conventional pad affects the boundary layer but I suspect that the boundary layer largely flows over the pad/caliper and reattaches to the rotor in the conventional setup.

Two series of brake dynamotor test have clearly shown the concept is valid. There are material, mechanism and anomalies to be worked out but it would make sense to do so in the context of a particular application.

 

[TheBlacksmith]

Isn't this what brake cooling ducts do? By forcing cool air onto the rotor it displaces the hot air. Several guys on the Focus ST forum that autocross have devised wings similar to a Porsche to divert air onto the front rotors. For those unfamiliar with the Focus ST, Ford uses the front brakes (it is FWD) as a pseudo limited slip by braking a spinning wheel, so these cars can really heat the front brakes.

 

[JayMaechtlen]

You're saying that at high temperatures, the boundary layer is tenacious and insulates the rotor surface from an ordinary flow of cooling air?
In that case, several vanes (scrapers) might help by allowing the flow of cooling air to reach the surface of the rotor.
I wonder if that's what the holes and slots in racing rotors actually accomplish?

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[TheBlacksmith]

Heated brake pads used to off gas and that would try to push the pads off the rotor; holes and slots were made to address pad gassing. Newer pads don't off-gas so much, so holes and slots are primarily for looks while wearing out the pads faster.

 

[Overrun]

I’ve tested with multiple vanes with one per swept area working best, at least at high speeds. Understand that the subject vanes are fixed adjacent the moving rotor. Not much in common with internal rotor vanes.

 

[JayMaechtlen]

Only one vane - presumably about 180 degrees from the caliper assembly?
Of course, all this is totally apart from any internal venting, vanes, bumps, etc.

Jay Maechtlen

http://www.laserpubs.com/techcomm

 

[Overrun]

At higher speeds one vane per swept area located almost immediately leading the caliper works well. At lower speeds the boundary layer would have more time to heat and it perhaps would be better to have more –I’m thinking of a truck with load descending a long grade here. The work hasn’t been done to optimize the concept for varying conditions.

 

[gruntguru]

All sounds feasible except "I suspect that the boundary layer largely flows over the pad/caliper and reattaches to the rotor in the conventional setup". Am I missing something here? Are you suggesting a pad-rotor gap (brakes not applied) for the BL to sneak through or a "homing" BL that will do the seemingly impossible to get back to the rotor?

Engineering is the art of creating things you need, from things you can get.

 

[Overrun]

Gruntguru, the operative word is “suspect” in that I’m inferring from faint evidence and need to do more testing to find out one way or the other. However, I have observed that, while the rotor cools nicely with the vane, the pads run a bit hotter. I had expected the cooler rotor to conduct heat energy from the pads. A fuller discussion of this can be found at;

http://www.f1technical.net/forum/viewtopic.php?f=6&t=10630

And a general discussion and more detailed description at;

http://www.f1technical.net/forum/viewtopic.php?f=6&t=14562

 

[tbuelna]

When air cooling a metal brake rotor surface, the heat transfer primarily occurs at the boundary layer, as noted by Overrun. And it is well understood that the heat transfer rate can be increased by tripping up the cooling air boundary layer flow at the rotor surface. But due to factors like temperature limits of metals used for brake rotors, there is still a practical limit to the amount of heat transfer that will occur with this set up. The passing air and rotor surface will always have a certain temperature delta, and there is only a given amount of time available for the heat transfer to take place before the rotor surface passes back under the brake pad.

The friction contact at the pad/rotor interface produces huge amounts of heat energy, and since the metal rotor has far better thermal conduction properties than the brake pad material, most of the friction heat produced is forced into the metal rotor. To improve the capacity of a rotor brake system you can either increase the thermal mass of the metal brake rotor, increase the surface area of the rotor exposed to cooling airflow, or make the rotor from a material with increased temperature capabilities like ceramic or carbon. Ultimately, with commercial vehicle applications, cost is usually the most important factor. And that's why cars still use cast iron brake rotors.

 

[TheBlacksmith]

"Breaking up the boundary layer" finally clicked for me; I think another way to look at this is that the vanes introduce turbulence. Since the heat transfer mechanism from the rotor to the air is largely through convection, the turbulence makes the heat transfer more effective.

 

[Overrun]

Tbuelna, I’m not sure as to what is meant by “tripping up”. What I’m doing is probably not tripping so much as aerodynamically stripping and conducting the coherent boundary in a number of possible directions. The high Reynolds number of the boundary layer makes a great difference. Most experts I’ve consulted with thought the pad would essentially disrupt the heated boundary layer and thus provide a good bit of the advantage of my rig. Not so. For some reason the pad runs a bit hotter with my gizmo when I would think that detaching the boundary layer would shield the pads from the ring-of-fire blast as well as seeing a cooler –by a good bit- rotor. I agree that the initial pad/rotor friction is almost a singularity with regard to the heat energy generated in essentially two dimensions. T-piles buried in the rotor can be misleading in this regard.

Just in case it was missed, the perhaps most significant advantage of what I’m doing is the heat rejection essentially real time and not adding to the rotor heat sink burden. This gets rid of much of the heat without affecting –other than heat load- the classical sinking/convection/radiation means by which brakes deal with heat.

Cost and cast iron are indeed driving factors. Iron is a bit iffy at the temperatures needed to drive radiant cooling since this is a fourth order Δ absolute T mechanism it can be quite effective at high rotor temps. Pony cars are practically begging for better brakes, though they’re much improved from the one stop wonders they were.

Aircraft brakes are a bit of puzzlement to me. As I understand the design carbon rotors and stators are stacked to gain swept area. But his would defeat much of the radiant cooling potential since the components would only “see” other similarly heated components. At such temps I can see giving up some volume to my vanes to reject the heat energy real time with essentially forced convection of the boundary layers. The high C-C temps would really make my vanes super effective.