Temperature of the Brake Pads

[Andrew07]

Hey guys.
I'll probably be doing most of the brake design this year for our car (or at the least, the heat transfer side of things). Obviously the pads have some temperature range of near max Cf. I did not know whether this temperature range was from measured pad surface temp or rotor temp. The customer service at Wilwood told me to essentially ignore the pad temp and just look at my rotor temperatures to base my pad compound selection off of. This is also what others do.
Even though the rotors have the vast majority of heat transferred into them rather than the pads, the pads still get hot. In fact usually Ive seen from other studies that they get significantly hotter than the rotor due to contact resistance and different material properties. Given the temperature difference is quite sizable, Id like to hear your guy's thoughts on its effects. A graph I happen to have open right now shows a 600 C difference between rotor surface and pad surface.

[Andrew07]

Hey guys.
I'll probably be doing most of the brake design this year for our car (or at the least, the heat transfer side of things). Obviously the pads have some temperature range of near max Cf. I did not know whether this temperature range was from measured pad surface temp or rotor temp. The customer service at Wilwood told me to essentially ignore the pad temp and just look at my rotor temperatures to base my pad compound selection off of. This is also what others do.
Even though the rotors have the vast majority of heat transferred into them rather than the pads, the pads still get hot. In fact usually Ive seen from other studies that they get significantly hotter than the rotor due to contact resistance and different material properties. Given the temperature difference is quite sizable, Id like to hear your guy's thoughts on its effects. A graph I happen to have open right now shows a 600 C difference between rotor surface and pad surface.

[thewoundedsoldier]

What rotor material? It's very hard to believe that a 600C difference can exist on the contact surface.

[Andrew07]

Author does not specify but he does say the disk and pad have the following properties:

Conductivity (W m-1 K-1)____Disk:43.5___Pad:12
Density (kg m-3)____________Disk:7,850__Pad: 2,500
Heat capacity (J kg-1 K-1)__Disk: 445___Pad: 900

The graph mentioned comes from this article.

Analysis of heat conduction in a disk brake system
Faramarz Talati Æ Salman Jalalifar
Received: 3 July 2008 / Accepted: 5 January 2009 / Published online: 27 January 2009
Springer-Verlag 2009

[Drew Price]

It has been a long time since I have seen a reference posted in a new thread, good work Andrew!

I too am surprised that there could be such a large temp. difference at the contact interface, but it may have some to do with the caliper doing a fair amount of insulating of the pads? I have a feeling that the difference, like WoundedSoldier said, is highly dependent on the specific pads and rotors you're talking about.

I am a little surprised to see the quoted heat capacity of the pad material to be double that of the rotor, I had always assumed it would be lower but I never actually looked into it, might be something to ask around about?

I guess the assumption that the contact surfaces will always have pretty close to the same temperature, at the interface at least, but I also did pretty badly in our heat transfer class. That may be because one of the midterm questions was the temp. gradient along the length of a hot dog inserted end-on into boiling water though - couldn't wrap my head around stuff like that....

Best,
Drew

[mp24]

I agree with Drew, 600C diffrence is woah!!

Does the book specify the pads or rotors?

[thewoundedsoldier]

From those numbers the rotor is definitely a low grade steel, probably gray cast iron. The pad material is tough to tell because different manufacturers use such different matrix combinations.

I totally remember the hot dog lab! The best part was the free lunch afterwards http://fsae.com/groupee_common/emoticons/icon_smile.gif . I am not a heat transfer buff myself, but I believe one of the premises of the field is that between two temperature zones there exists a gradient--meaning no abrupt changes. I'm willing to bet that the matrix allows for the increase in heat (greater heat capacity) and may, in fact, have an increased Cf as the temperature gradient rises.

I know a lot of people would give me flack if they heard this, but I don't think brake temperature needs to be the largest concern of your design. Wilwood will do a very good job of getting you on the right track; just make sure you use the right rotor material. I would consider it more important to get the mechanical ratio of the pistons dead on--it is easier to miss than you might think.

[Andrew07]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">I believe one of the premises of the field is that between two temperature zones there exists a gradient--meaning no abrupt changes </div></BLOCKQUOTE>
If you look in you're heat transfer book (and im about to assume most heat transfer books are about the same) and look at the section with composite walls, you'll see that the assumption is perfect contact. In my book, at the end of the section, it shows a small picture and a short explanation of two surfaces with imperfect contact (contact resistance). There is a temperature jump in said section. Given that we probably dont have perfect contact with the pad, Im willing to believe there is a temperature difference. But yes, I agree 600C is alot more than one would think.

[buggaero]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Posted August 11, 2010 01:04 PM Hide Post
From those numbers the rotor is definitely a low grade steel, probably gray cast iron. The pad material is tough to tell because different manufacturers use such different matrix combinations.

I totally remember the hot dog lab! The best part was the free lunch afterwards . I am not a heat transfer buff myself, but I believe one of the premises of the field is that between two temperature zones there exists a gradient--meaning no abrupt changes. I'm willing to bet that the matrix allows for the increase in heat (greater heat capacity) and may, in fact, have an increased Cf as the temperature gradient rises.

I know a lot of people would give me flack if they heard this, but I don't think brake temperature needs to be the largest concern of your design. Wilwood will do a very good job of getting you on the right track; just make sure you use the right rotor material. I would consider it more important to get the mechanical ratio of the pistons dead on--it is easier to miss than you might think.

"Good judgement comes from experience. Experience comes from bad judgement </div></BLOCKQUOTE>


I have few things to point out.

what makes the soldier think of it as gray cast iron??
i was researching the brake pad material property since a min before you put that reply and my little research showed this....

Material with
Thermal Conductivity 12 : HASTELLOY B2 (12.1 ACTUALLY) that too at 100 degC http://fsae.com/groupee_common/emoticons/icon_wink.gif
DENSITY 2500 : ALUMINIUM (2700 ACTUALLY)
SPECIFIC HEAT CAPACITY 900 J Kg-1 K-1 : ALUMINIUM (897 ACTUALLY)



So anyone please clear the air in this thread by enlightening us guys.

[thewoundedsoldier]

That sounds like an excellent question to ask a brakepad manufacturer. My assumption would be to get the contact resistance as small as possible to allow the heat transfer (to avoid buildup like your model is showing you), but the higher the Cf the higher the contact resistance. Interesting catch-22...I would definitely want to hear what a pad maker has to say about the two conflicting goals.

Is the 600C example that we are talking about based on an 18-wheeler going down hill or an F1 car or something? I can tell you with certainty that you don't have to worry about such a large gradient in FSAE cars.

[thewoundedsoldier]

haha buggaero my bad...the pad is an aluminum composite matrix...the problem is figuring out what kind...

the rotor is steel

[buggaero]

Then i guess the user should machine an aluminium rotor as well because the Aluminium-Aluminium coefficient of friction is the highest amongst most general pairings.

Cf Static Sliding
Al-Al 1.05-1.35 1.4
Al-MS .61 .47

This high,even without any abrasive in dry running.

[Andrew07]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by thewoundedsoldier:
That sounds like an excellent question to ask a brakepad manufacturer. My assumption would be to get the contact resistance as small as possible to allow the heat transfer (to avoid buildup like your model is showing you), but the higher the Cf the higher the contact resistance. Interesting catch-22...I would definitely want to hear what a pad maker has to say about the two conflicting goals.

Is the 600C example that we are talking about based on an 18-wheeler going down hill or an F1 car or something? I can tell you with certainty that you don't have to worry about such a large gradient in FSAE cars. </div></BLOCKQUOTE>

You can download the pdf, its free. But in short is was dissipating some energy of a moving passenger car over a 4 second time interval. Total energy absorbed was 165KJ. Rotor outer diameter was 227mm. Not sure about how big the pad was.

I tried to get the wilwood people I emailed to elaborate on the pad temperature and its possible effects but they didn't comment on it at all. They just repeated the idea to look at rotor temp.

[Andrew07]

I'm starting to think that this guys results are all baloney. First, I cannot find where he came up with his heat partition equation. I found one that looks like it though. But it was derived on the basis of equal temperatures on the interface surface of both materials. Also, the contact resistance manifests itself when a heat flux has to pass through a interface material or void. BUT the heat int actually passing threw this void, its being generated at the interface and either going to the left or right of the interface. Finally, the paper I referenced is all an exercise in mathematics and he never backs up his results with a physical experiment from what I can see. Given the fact that he is published and I am not, I am reluctant to say this, but it looks like bs. In other words I think he miss used an equation and carried that mistake all the way threw his model.

[Drew Price]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">..... Finally, the paper I referenced is all an exercise in mathematics and he never backs up his results with a physical experiment from what I can see. ...... </div></BLOCKQUOTE>


This is one of the things that continues to discourage me from being motivated to continue my education. Baloney belongs on sandwiches, not on race cars.


And I think you're on the right track about the contact assumptions:

Imperfect surface contact
Deformable material (pad at least)
Non-uniform through-thickness composition
Non-uniform contact pressure

But most of all....

Temp. gradiant within the contact periphery (mostly from leading to trailing edge, but radially as well I'm sure...) which would also lead to a Coeff. friction gradient through the pad interface.


This all seems like a lot of mental masturbation to me, since you will be using off the shelf pad compounds anyway. I think you're making this much too difficult for yourself, you will probably get yourself just as close to a good setup by making some reasonable assumptions about the other constraints in your system.

Best,
Drew

[Andrew07]

Bringing a little more closure to this thread.
From the book "Brake Design and Safety" the proper heat partition equation is:

(q''rotor/q''pad) = ((p.rotor*c.rotor*k.rotor)/(p.pad*c.pad*k.pad))^(1/2)

Just thought I'd make this thread a little more definitive for people who see it in the future.