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How hot is the actual air expelled from center dividers on a brake rotor?

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j-c-c
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How hot is the actual air expelled from center dividers on a brake rotor?

Post by j-c-c »

Not sure this site is a good candidate for an answer for this question, but I'll take a stab at it.

We are talking circuit type braking conditions, so feeding cool air with typical materials (Alum, CF, etc) is no problem.

I'm toying with, in addition to the typical inlet (albeit slightly smaller if this idea works out), fabbing a low profile hood over the center opening of the brake caliper to "suck" air out thru a side mounted NACA duct.

But I am uncertain what exhaust air temps might be. There is very little structural loading involved here.

Weight is of course important so I'm leaning towards CF, which means under the 400-500F range depending on epoxy chosen(CF itself is in 2000F range).

Guesses with reasoning appreciated.
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by Protech Racing »

For starters. Look up your brake pad operating temps. Mine are 800 -1200F
The rotor temps can be taken with an IR gun or temp paint . Near 600* is common for race cars.
US Trans Am cars used water mist in the brake ducts for a while . I used water in my SCCA touring cars until it was outlawed for Sebring (very hard on brakes).
NASCAR uses a stack of fans mounted in 3 -5 hoses. no water .
Wheel fans work .
I now just blow air at the calipers , trying to cool the fluid. The modern pads are so good that I see the fluid as the weak link usually. The pads work to 1200* before gassing off or disintegrating, while the fluid gives up around 500 continuous.
If you wanna make something from CF, make a new diffuser around the new track bar.....
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by FC-Pilot »

CF should be just fine depending on how you have it mounted. If on a steel caliper it could be an issue. Be careful though as you may be surprised as to how best to make air flow. Ferrari on the 458 Special used ducts to push air in through the top of the caliper, but when ford tried to copy it on the GT 350 it was a total fail. So much depends on the airflow around the vehicle and especially in the wheel well. As stated, your fluid is important for track conditions and good use of thermocouples and a reader will tell you a decent amount. I would suggest building a number of different ideas and taking them all to the track with multiple TC’s on certain components. You can deduct much based off of the trends seen. Or go to the next level and install TC’s in your rotors and use wireless transmitters to record the actual rotor temps as well as installing TC’s in the pads as well.

Paul
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by Circlotron »

Protech Racing wrote: Mon Apr 13, 2020 1:03 pm The rotor temps can be taken with an IR gun or temp paint .
Be aware that the infra red emissivity of a surface depends a lot on it's darkness. For example I have an electric kettle with a silvery brushed metal surface and with boiling water in it and the outside surface extremely hot it only measures about 35 deg c using an IR gun. If it was matte black (like the surface of a tyre after a burnout) the measurement would be much closer to true. Brake rotor surfaces are pretty shiny so probably a source of error.

https://en.wikipedia.org/wiki/Emissivity

https://www.flukeprocessinstruments.com ... ity-metals
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by Rick! »

j-c-c wrote: Mon Apr 13, 2020 10:41 am Not sure this site is a good candidate for an answer for this question, but I'll take a stab at it.

We are talking circuit type braking conditions, so feeding cool air with typical materials (Alum, CF, etc) is no problem.

I'm toying with, in addition to the typical inlet (albeit slightly smaller if this idea works out), fabbing a low profile hood over the center opening of the brake caliper to "suck" air out thru a side mounted NACA duct.

But I am uncertain what exhaust air temps might be. There is very little structural loading involved here.

Weight is of course important so I'm leaning towards CF, which means under the 400-500F range depending on epoxy chosen(CF itself is in 2000F range).

Guesses with reasoning appreciated.
Brute forcing air flow will not pay off so you need to learn where the favorable pressure gradients are.
Certainly a duct near the front face of the car will feed airflow to the caliper as long as the static pressure near the caliper is lower than the static pressure in the duct or duct front opening.
If you stand a few back of a vented rotor, it kinda looks like a centrifugal pump doesn't it.
This would lead one to deduce that there is a low pressure in that area as the air would be slung out the outer openings of the vents.
The non-intuitive deduction or discovery is that there may be slightly higher static pressure there that assists feeding the rotor vents. A thermocouple here would help out a bunch, along with a MAP sensor.
The air going out the vents needs to not be impeded by a wheel hub/rim that is really close to the rotor. Drilled rotors may work better in this application as wheels are usually just big enough to clear the really expensive caliper.
Flow inside wheelwells is not intuitive.
I've had to reshape inner fenders to allow air to flow through them from the radiator as that rascally pressure gradient thing needs to be present for flow to happen.
Just like any fluid, air flows best with the least amount of bends and diameter changes. Since air doesn't have much mass, it doesn't carry a lot of momentum to bull its way out the exit. So, avoid dryer ducting unless absolutely unavoidable.
Lastly, the lightest rotors and calipers offer the least amount of heat capacitance and durability. Any heat that can be bled off the rotor is less heat into the pads and subsequently the calipers. Cooling calipers is where you make hay as every 10 degrees cooler the fluid is kept, the longer you can run hard and avoid Flintstone braking events.
Good luck.
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by dannobee »

If it's just the temps that you need, I can tell you first hand that the air coming out is hot enough to burn the paint on any of the stock car style wheels. Burnt as in grey but still sticking to the wheel. I'm guessing somewhere around 400 degrees. But remember, that's not just air, but brake pad material as well. Sure, air itself doesn't have much mass, but the remnants of brake pads do. If you need a more accurate measurement, get some Tempilstik crayons of various ratings and stick it anywhere you think you need to know the temp, then do some hot laps.

https://megadepot.com/product/tempil-02 ... gKZkfD_BwE
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by FC-Pilot »

We struggled with airflow sensors in the wheel well. Any tiny movement between heats and parts changes made the data completely useless. It was easier for us to identify trends with solidly placed thermocouples. We also used two different types of contact TC’s on the rotors. The sacrificial ones worked better than the “ski” type as the amount of pressure varied quite a bit on the ski thermocouples. When we really needed accurate data, placing the TC’s in the rotor with a .020” depth gave very accurate and and consistent data. Then running the wires to a wireless slip-ring (transmitter) can still yield data accumulation at 100hz (which for TC’s is more then you need).

I had not thought of the temp stick, as that could get you by in a pinch. I love outside the box thinking.

We had done some GoPro video in wheel wells before which was useful. We were looking for other issues, but maybe small strands of yarn on suspension components might help you “see” some of the airflow.

Paul
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by j-c-c »

dannobee wrote: Wed Apr 15, 2020 11:30 pm But remember, that's not just air, but brake pad material as well. Sure, air itself doesn't have much mass, but the remnants of brake pads do.
That might be my idea killer right there. Dealing with 1000F? particles would more problematic then the benefits I likely would receive with my original idea. And as mentioned, getting the calipers cooler with forced air, has little downsides and is proven.

So, Nevermind. :D
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by dannobee »

Porsche obviously uses BIG brakes on their factory race cars, with proper ducting and cooling. If you google GT2 and GT3 brake ducts, you'll see the plastic scoops and ducts that snap onto the lower control arm, catching the air under the car and redirecting it into the center of the brake rotor. Cheap and effective. And if those aren't good enough, Porsche Motorsports has even bigger scoops, although those ducts are not cheap, a copy would probably be almost as effective. Another thing that they do is take air from the back side of the radiator, so as to keep the front end cleaner in the slip stream. And I seriously doubt that air only 50-100 degrees hotter makes much of a difference on cooling a 1000 deg F brake rotor.
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by FC-Pilot »

dannobee wrote: Thu Apr 16, 2020 10:16 am Porsche obviously uses BIG brakes on their factory race cars, with proper ducting and cooling. If you google GT2 and GT3 brake ducts, you'll see the plastic scoops and ducts that snap onto the lower control arm, catching the air under the car and redirecting it into the center of the brake rotor. Cheap and effective. And if those aren't good enough, Porsche Motorsports has even bigger scoops, although those ducts are not cheap, a copy would probably be almost as effective. Another thing that they do is take air from the back side of the radiator, so as to keep the front end cleaner in the slip stream. And I seriously doubt that air only 50-100 degrees hotter makes much of a difference on cooling a 1000 deg F brake rotor.
I was thinking of bringing up the Porsche ducting. Great catch as their setup is simple but effective. They really did a nice job of doing a lot with a little. Chevy also did a variation of it with the ZL1 Camaro which I am sure they got the idea from Porsche. Do what you can with what you have. Come up with a few different ideas and try them all. Search for images of each of these and anything else you can to get some more fresh ideas. Sadly all my photos I had went away when I left my job to work for a competitor.

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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by RCJ »

If you found the lowest pressure area on the outer body of the car.This area would give the greatest pressure differential, also this area would have the least down force. You would also be drawing air from the underside of the car.Even if it did nothing to cool the brakes, BY eliminating a low pressure area with a duct and creating lower pressure under the car could be a positive move. I've did things like this and it worked enough to throw the balance of the car off and make it slower also.
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Re: How hot is the actual air expelled from center dividers on a brake rotor?

Post by Monzsta »

Are recirculating brake fluid (for cooling) set ups allowed? Seems that's the easy way to keep brake fluid cool. Of course that does nothing for pad/rotor temps.

http://www.larsenracingproducts.com/pro ... lator.html

As a mechanic I've seen the results from brake failures. I've seen tires melted off rims, bearing grease catch on fire, ball joint and tie rod plastic liners melt due to stuck or seized calipers and ABS wires where the insulation looks like you passed a torch over it. The air coming off the rotors in that scenario is very hot.
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