Brake Cooling for Car

[CFDmech14]

I am currently have a job task to improve brake cooling for a car. What is the most efficiency, cost saving and time consuming way to improve Brake Cooling rate?

Thx,

 

[Komodo86]

Bigger discs, both in mass and surface area.

 

[Tmoose]

I think I'd first spend some time trying to understand the "problem" more specifically.
What components are overheating, and is the actual temperature an issue, //OR// some symptom like fade or fluid boiling?

Are you already using disk brakes with vented rotors ?
Highway speeds? race car?
Do you need to dissipate more than 4,000,000 ft-lbs of kinetic energy in one stop 9 4 wheel brakes)?
By test and analysis Is the airflow under the car and thru the wheels hurting or helping the existing brakes do their job.
Remove the brake dust shields?

http://www.autopartswarehouse.com/brake_dust_shields~pop.html

Too many coats of caliper paint?
Poor choice of pad material?
As Komodo said, undersized components? What size are the wheels now, and is there room for larger brakes without changing the wheel combination currently used?

For building brakes fitting inside 15 inch wheels and capable of nearly stopping a 2900 lb car from 200 mph see pages 91 ff here -

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

 

[CFDmech14]

Thx for the link,
We already used vented disc but the disc cooling rate is still low so it take longer time to cool (not meet our requirement 0.7G). My team currently had to solve this prob. I have a limit of time b'cause the car will be produce in April. To answer your question
1)I can't change material of disc, type etc due to cost effct.
2) city car, air dust is not recommended to be remove.
3) the test and analysis show the cooling rate is high.

Does we need advance research like racing car for city car? Or the cooling rate is not major concern in city car? The rim size is 14" current HTC heat transfer coeff is 32.16 W/m^2-K. Disc rotor dia 256mm. current max tem 823.15 K.

 

[BrianPetersen]

If this is a "city car" - small light car for low speed driving around town - why are you having problems with heat build-up? There should be next to nothing to begin with in that application.

Is this an imaginary problem dreamed up by someone with too much time on their hands (i.e. a problem that will not be apparent in real world conditions)?

VW Golf Mk5 has a simple brake cooling duct moulded into the plastic panels underneath the front of the car, to direct some airflow towards the brakes.

Wheel designs with open spokes are more likely to allow air circulation than the solid wheel covers that the aerodynamics engineer probably wants.

 

[CFDmech14]

Thx,I also think like that. That is not major concern b'cause of low speed propose. VW Golf Mk5 is one of reference to solve this problem. Their major concern is the cooling rate.

For city uses, we have to use a lot of time in braking. So when our cooling time rate is lower, that the problem. Pad, Disc effectiveness, could be reduce due to the low cooling rate at brake disc.

 

[tbuelna]

CFDmech14,
You need to increase the heat transfer rate (or "cooling" rate as you describe it) from the disc to the ambient airflow in order to keep the disc material within its safe operating temperature limits, right? You said you cannot change the disc material, but there are a couple ways you can improve the heat transfer from disc to air by modifying the internal geometry of the disc itself. First, you can increase the disc surface area in contact with the internal cooling airflow by adding more vanes and or increasing the vane surface area. You can increase the air mass flow rate thru the rotor vents by adding ducts/shrouds to smooth the airflow at the inlet & discharge of the vents; it should be possible to add some shrouding and ducting that will not interfere with wheel changes. You can increase heat transfer between the vent surface and the airflow by adding tiny features to the flow surface (such as bumps or ridges) that stir up the boundary airflow and promote mixing with the core airflow. You can also increase the temperature rise in the cooling airflow across the rotor by increasing the distance the air travels between the vent inlet and discharge.

If you are proficient using CFD and FEA tools, you should be able to create a vented rotor design with internal geometry that is optimized for your particular application. Once you have a digital model of the optimized rotor you can have some prototype rotor blanks made quickly and for reasonable cost using DLMS. The rotors would only require a modest amount of finish machining and heat treatment to be ready for use.

Good luck with your project.
Terry

 

[BrianPetersen]

"For city uses, we have to use a lot of time in braking" - The energy to stop from 50 km/h is one-quarter what it is to stop from 100 km/h (suburban speed), one-ninth what it is to stop from 150 km/h (a little above normal motorway speed, and the car had better be able to do that), etc. It might be spending a lot of time with the brakes applied but not necessarily dissipating a lot of energy. The whole situation just doesn't make any sense.

Unless there is something extraordinarily bad about the design or sizing of the brakes, it simply shouldn't be an issue. If the car is stopped, thermal radiation and conduction to adjacent parts likely matters more than the air circulation through the rotor. Make sure the rim is in good thermal contact with the hub and make sure the path of heat transfer allows dissipation via that route before it gets to the wheel bearings. Use open spoked wheels to give a thermal radiation path to the outside. If there is a dust/splash shield on the inside, make sure it is not blocking air movement too much and make sure it isn't a thermal insulator.

I've never had overheated brakes driving in the city ... ever. Steep downgrades in mountains while towing a trailer, different story.

 

[Tmoose]

Can you post pictures of components, and relative positions.
The wheel design ( and wheel covers) and location and obstruction of rotor inlet might be important.

When Volvo upgraded 240s to vented rotors they also added a chin spoiler and opened up the steel wheel cooling holes.
see attached image.
There were warnings not to modify the chin spoiler for fear of compromising the (high speed, mountain)brake cooling.

How many test vehicles are there. Do they all "do that?" Do both (all) brake rotors get hot?
If the rear brakes stay cool ( not common with FWD with drum rear brakes, at least during mild stops ) maybe getting the rears more involved during the test cycle would help.

Especially If there is only one test vehicle, does the car roll roll freely? If a glitch in the brake system design or manufacturing has the brakes dragging it may increase the thermal load unexpectedly. As can pads with friction characteristics that improve at higher temperatures. In the drum brake days GM offered some Heavy Duty metallic brakes that had friction characteristics that varied a lot with temperature (poor when cool). They were to be adjusted MANY clicks looser than a slight rub since at speed each wheel would have different friction and pull badly on the first stop.

How well finished are the rotor vents? At low speed ( and slow wheel/brake rotation ) forced conduction //may// be of relatively small value compared to radiation of a blazing hot rotor. Still, vents with poorly formed inlets or internal blockages may be helping less than expected.

http://milford.nserl.purdue.edu/weppdocs/usersummary/usrimages/figure11.gif

nicely finished rotor

http://img822.imageshack.us/img822/4162/56213842.jpg

How hot are the wheels when the rotors are real hot?
If they are cool Maybe a coat of high emissivity paint (not necessarily any old flat black) on the inside of the wheels could get them to help out a little more with heat dissipation.

 

[yoshimitsuspeed]

I have been wondering how much gain could be had by using a coating like this.

http://www.cerakotehightemp.com/finishes/C-187Q/

Iron oxide has a much lower thermal conductivity than bare metal and this coating is supposed to have a much better thermal conductivity.
Even though the layer of iron oxide is realaitvely thin it is right at the interface to the air which makes me think it may actually help.
This is more in regards to keeping brakes from rusting and maintaining the original or slightly better than uncoated properties over time.

I am also really confused and have the same questions as Brian. Overall heat input into the rotors driving around town is going to be a small fraction of what they see in say an emergency stop on the highway or a spirited descent down a windy mountain. If you are seeing high brake temps in the city then either of the above situations could prove catastrophic.
At the same time I just don't see it being possible to overheat the brakes driving around town even on dangerously undersized brakes.

 

[Tmoose]

OP CFDmech14 joined on Feb 19 and last logged on the next day, Feb 20.

I think he is putting all our advice to good use