Re: [OT] car brakes

jobst.brandt@stanfordalumni.org

Kerry Montgomery wrote:

>>> [...]

>>>> I find the defense of the gas theory, that arose from auto
>>>> mechanics, a form of religious faith. "I read it in Road &
>>>> Track". As I explained, in auto repair the cause of drum brake
>>>> lockup was not recognized as the same effect that caused total
>>>> brake fade. That disk brakes decouple application force and
>>>> brake force, being at right angles to one another, seems to elude
>>>> many folks. When I worked in brake development in F1 design in
>>>> Europe, we were aware of myth and lore that surrounded brakes.

>>> Since you worked in brake development, do you know what causes
>>> pads to glaze and what are the best solutions to the problem?

>> The occurrences I witnessed came from using drum brake friction
>> material hardly modified for disk use. The problem was that higher
>> temperatures developed with the pressure needed to make pads with
>> smaller area than drum shoes caused molten material to agglomerate
>> on the surface.

>> There is a basic rule in friction that when two materials slide
>> over one another, one or both must melt locally at asperity
>> contacts in the absence of a lubricant. Therefore, surface
>> morphology will change if the material is not suited for the
>> operating temperature at the interface, rather than the bulk
>> temperature. As I mentioned previously, brake friction material is
>> an insulator, making its surface extremely hot while the bulk
>> remain far below that. Matthauser didn't know that and put useless
>> cooling fins on the back of his brake pads.

>> Today's racing F1 brakes use carbon disks and pads whose wear
>> debris is CO2, the major advantage of this material although
>> lighter weight is also an asset. Oxides materials produce granular
>> debris whose melting temperature must be high enough for the
>> application to not become glazing paste. This whole problem
>> retarded disk development at the onset. Curbstone mechanics had
>> hole, slot and odd pad surfaces as solutions rather than
>> recognizing that it was a materials problem and that people skilled
>> in the art were working on it... with success.

>>> Slotted disks scrape the glaze off and perhaps also create
>>> channels for fumes and powder (or something-- whatever it is it
>>> smells bad) to depart via thus preventing it from vitrifying.

>>> Cross-drilling seems to be done for similar reasons-- to let
>>> material of some kind escape (although the cheese-grater effect
>>> seems to be an unwanted side-effect).

>> It's a materials problem and the appropriate people are developing
>> friction materials that makes this crude solutions go away, and
>> they have to a large degree except for those who like folklore.

> Some of the F1 wear debris may be CO2, but some is just plain C, as
> shown by the clouds of black stuff coming off when the wheels are
> changed in the pits.

This first came to light years ago when hard disk drives began to use
sputtered Carbon as a wear layer. Using a ceramic slider in a test
chamber with a controlled atmosphere or vacuum, the production of
carbon wear debris in a nitrogen atmosphere was achieved and its
presence detected after which in an oxygen atmosphere all wear debris
became CO2. This was an early experiment I generated to show why much
harder oxides were damaging while carbon was benign. After that the
common wear layer changed to carbon coating. This work led to a PhD
for my summer student who verified my friction and wear theories.

Any carbon dust you see coming off race car brake disks is a "don't
care" because it is not remaining in the interface and the interface
is purging itself of CO2, the only significant "gas" that occurs in
carbon disk brakes. Tribology is a largely misunderstood subject.
However, good publications were published about instantaneous flash
temperatures in sliding interfaces that were formerly considered to be
close to room temperature, the bulk temperature.

Jobst Brandt

jim beam

Zog The Undeniable wrote:
> jobst.brandt@stanfordalumni.org wrote:
>> Robert Lorenzini wrote:
>>
>>>> Cross-drilled seems to be more about looks than anything else.
>>>> "Vented" or "ventilated" rotors are another matter. They appear to
>>>> be reasonably effective, but it's not a *huge* effect (+40%, rather
>>>> than 100%).
>>
>>> Vented (cross-drilled or slotted) rotors came from racing brakes
>>> which use pads that boil gasses under heat. This is most noticeable
>>> when bedding green pads and they act just like a tire hydroplaning
>>> until most of the volatiles have disapated.
>>
>> Be more skeptical with these old storied of "gas bearing", something
>> that occurred with drums and was not gas as understanding of brakes
>> verified. If there were any truth to the gas idea, all passenger cars
>> sold in the last decade or more do not have cross drilling that are
>> found only on vehicles sold as racing sport machines such as MTB's,
>> "Sports cars", and Motorcycles.
>>
>> How does the brake know on what type vehicle it is performing? How do
>> car companies get away with smooth surface disks with no gas vents for
>> family cars, trucks, commercial aircraft and high speed trains?
>
> Cross-drilled discs almost always develop cracks with age, something
> unacceptable on a mass-market car with a long warranty period.

that was indeed the case in the early days. today though, this can be
largely mitigated by re-finishing each hole to induce compressive
residual stress. but it costs more.


> Some
> discs are blind-drilled to reduce the risk of cracking, but modern pads
> don't outgas significantly and the pits are really there to make the
> brakes look pimpin'.
>
> Grooved discs have more effect because the sharp edges of the groove
> de-glaze the pad. They're also quite noisy and the pads wear quicker.

if cut with a square bottom, grooves too can cause cracking. as can
crappy casting.

bottom line: brakes are under a lot of thermal stress and they wear
anyway. just replace them if there is any doubt.

jim beam

Ben C wrote:
> On 2008-05-05, jobst.brandt@stanfordalumni.org <jobst.brandt@stanfordalumni.org> wrote:
> [...]
>> I find the defense of the gas theory, that arose from auto mechanics,
>> a form of religious faith. "I read it in Road & Track". As I
>> explained, in auto repair the cause of drum brake lockup was not
>> recognized as the same effect that caused total brake fade. That disk
>> brakes decouple application force and brake force, being at right
>> angles to one another, seems to elude many folks. When I worked in
>> brake development in F1 design in Europe, we were aware of myth and
>> lore that surrounded brakes.
>
> Since you worked in brake development, do you know what causes pads to
> glaze and what are the best solutions to the problem?
>
> Slotted disks scrape the glaze off and perhaps also create channels for
> fumes and powder (or something-- whatever it is it smells bad) to depart
> via thus preventing it from vitrifying.
>
> Cross-drilling seems to be done for similar reasons-- to let material of
> some kind escape (although the cheese-grater effect seems to be an
> unwanted side-effect).

so when is one of the knuckle-draggers going to mention the effect of
water on braking surfaces??? wet brakes can be spectacular for almost
total ineffectiveness. drilled/slotted brakes otoh are much more
effective in this situation.

jim beam

jobst.brandt@stanfordalumni.org wrote:
> Kerry Montgomery wrote:
>
>>>> [...]
>
>>>>> I find the defense of the gas theory, that arose from auto
>>>>> mechanics, a form of religious faith. "I read it in Road &
>>>>> Track". As I explained, in auto repair the cause of drum brake
>>>>> lockup was not recognized as the same effect that caused total
>>>>> brake fade. That disk brakes decouple application force and
>>>>> brake force, being at right angles to one another, seems to elude
>>>>> many folks. When I worked in brake development in F1 design in
>>>>> Europe, we were aware of myth and lore that surrounded brakes.
>
>>>> Since you worked in brake development, do you know what causes
>>>> pads to glaze and what are the best solutions to the problem?
>
>>> The occurrences I witnessed came from using drum brake friction
>>> material hardly modified for disk use. The problem was that higher
>>> temperatures developed with the pressure needed to make pads with
>>> smaller area than drum shoes caused molten material to agglomerate
>>> on the surface.
>
>>> There is a basic rule in friction that when two materials slide
>>> over one another, one or both must melt locally at asperity
>>> contacts in the absence of a lubricant. Therefore, surface
>>> morphology will change if the material is not suited for the
>>> operating temperature at the interface, rather than the bulk
>>> temperature. As I mentioned previously, brake friction material is
>>> an insulator, making its surface extremely hot while the bulk
>>> remain far below that. Matthauser didn't know that and put useless
>>> cooling fins on the back of his brake pads.
>
>>> Today's racing F1 brakes use carbon disks and pads whose wear
>>> debris is CO2, the major advantage of this material although
>>> lighter weight is also an asset. Oxides materials produce granular
>>> debris whose melting temperature must be high enough for the
>>> application to not become glazing paste. This whole problem
>>> retarded disk development at the onset. Curbstone mechanics had
>>> hole, slot and odd pad surfaces as solutions rather than
>>> recognizing that it was a materials problem and that people skilled
>>> in the art were working on it... with success.
>
>>>> Slotted disks scrape the glaze off and perhaps also create
>>>> channels for fumes and powder (or something-- whatever it is it
>>>> smells bad) to depart via thus preventing it from vitrifying.
>
>>>> Cross-drilling seems to be done for similar reasons-- to let
>>>> material of some kind escape (although the cheese-grater effect
>>>> seems to be an unwanted side-effect).
>
>>> It's a materials problem and the appropriate people are developing
>>> friction materials that makes this crude solutions go away, and
>>> they have to a large degree except for those who like folklore.
>
>> Some of the F1 wear debris may be CO2, but some is just plain C, as
>> shown by the clouds of black stuff coming off when the wheels are
>> changed in the pits.
>
> This first came to light years ago when hard disk drives began to use
> sputtered Carbon as a wear layer. Using a ceramic slider in a test
> chamber with a controlled atmosphere or vacuum, the production of
> carbon wear debris in a nitrogen atmosphere was achieved and its
> presence detected after which in an oxygen atmosphere all wear debris
> became CO2. This was an early experiment I generated to show why much
> harder oxides were damaging while carbon was benign. After that the
> common wear layer changed to carbon coating. This work led to a PhD
> for my summer student who verified my friction and wear theories.

friction and wear are not gas evolution jobst. but don't let that get
in the way of you trying to claim credit for someone else's research.


>
> Any carbon dust you see coming off race car brake disks is a "don't
> care" because it is not remaining in the interface and the interface
> is purging itself of CO2, the only significant "gas" that occurs in
> carbon disk brakes.

you're bullshitting jobst. the reaction rate of carbon and oxygen at
braking temperatures is so low, you'd have to wait months to achieve the
mass loss affected by just one hard brake application.


> Tribology is a largely misunderstood subject.

by some, most evidently.


> However, good publications were published about instantaneous flash
> temperatures in sliding interfaces that were formerly considered to be
> close to room temperature, the bulk temperature.
>

bullshitter.

jim beam

Kerry Montgomery wrote:
> <jobst.brandt@stanfordalumni.org> wrote in message
> news:481f837b$0$34571$742ec2ed@news.sonic.net...
>> Ben C wrote:
>>
>>> [...]
>>>> I find the defense of the gas theory, that arose from auto
>>>> mechanics, a form of religious faith. "I read it in Road & Track".
>>>> As I explained, in auto repair the cause of drum brake lockup was
>>>> not recognized as the same effect that caused total brake fade.
>>>> That disk brakes decouple application force and brake force, being
>>>> at right angles to one another, seems to elude many folks. When I
>>>> worked in brake development in F1 design in Europe, we were aware
>>>> of myth and lore that surrounded brakes.
>>> Since you worked in brake development, do you know what causes pads
>>> to glaze and what are the best solutions to the problem?
>> The occurrences I witnessed came from using drum brake friction
>> material hardly modified for disk use. The problem was that higher
>> temperatures developed with the pressure needed to make pads with
>> smaller area than drum shoes caused molten material to agglomerate on
>> the surface.
>>
>> There is a basic rule in friction that when two materials slide over
>> one another, one or both must melt locally at asperity contacs in the
>> absence of a lubricant. Therefore, surface morphology will change if
>> the material is not suited for the operating temperature at the
>> interface, rather than the bulk temperature. As I mentioned
>> previously, brake friction material is an insulator, making its
>> surface extremely hot while the bulk remain far below that.
>> Matthauser didn't know that and put useless cooling fins on the back
>> of his brake pads.
>>
>> Today's racing F1 brakes use carbon disks and pads whose wear debris
>> is CO2, the major advantage of this material although lighter weight
>> is also an asset. Oxides materials produce granular debris whose
>> melting temperature must be high enough for the application to not
>> become glazing paste. This whole problem retarded disk development at
>> the onset. Curbstone mechanics had hole, slot and odd pad surfaces as
>> solutions rather than recognizing that it was a materials problem and
>> that people skilled in the art were working on it... with success.
>>
>>> Slotted disks scrape the glaze off and perhaps also create channels
>>> for fumes and powder (or something-- whatever it is it smells bad)
>>> to depart via thus preventing it from vitrifying.
>>> Cross-drilling seems to be done for similar reasons-- to let
>>> material of some kind escape (although the cheese-grater effect
>>> seems to be an unwanted side-effect).
>> It's a materials problem and the appropriate people are developing
>> friction materials that makes this crude solutions go away, and they
>> have to a large degree except for those who like folklore.
>>
>> Jobst Brandt
>
> Jobst,
> Some of the F1 wear debris may be CO2, but some is just plain C, as shown by
> the clouds of black stuff coming off when the wheels are changed in the
> pits.
> Kerry
>
>

of course. but jobst doesn't check facts, he just makes presumptions.

Ben C

On 2008-05-06, jim beam <spamvortex@bad.example.net> wrote:
> Ben C wrote:
>> On 2008-05-05, jobst.brandt@stanfordalumni.org <jobst.brandt@stanfordalumni.org> wrote:
>> [...]
>>> I find the defense of the gas theory, that arose from auto mechanics,
>>> a form of religious faith. "I read it in Road & Track". As I
>>> explained, in auto repair the cause of drum brake lockup was not
>>> recognized as the same effect that caused total brake fade. That disk
>>> brakes decouple application force and brake force, being at right
>>> angles to one another, seems to elude many folks. When I worked in
>>> brake development in F1 design in Europe, we were aware of myth and
>>> lore that surrounded brakes.
>>
>> Since you worked in brake development, do you know what causes pads to
>> glaze and what are the best solutions to the problem?
>>
>> Slotted disks scrape the glaze off and perhaps also create channels for
>> fumes and powder (or something-- whatever it is it smells bad) to depart
>> via thus preventing it from vitrifying.
>>
>> Cross-drilling seems to be done for similar reasons-- to let material of
>> some kind escape (although the cheese-grater effect seems to be an
>> unwanted side-effect).
>
> so when is one of the knuckle-draggers going to mention the effect of
> water on braking surfaces??? wet brakes can be spectacular for almost
> total ineffectiveness. drilled/slotted brakes otoh are much more
> effective in this situation.

I think the link Carl posted did mention that.

I don't think you want slotted disks on shopping cars because they wear
the pads too quickly.

I've not known wet brakes on modern cars (with plain vented disks) to be
a problem in practice. I suppose the pad just squeegees the water off,
which is why a gentle reapplication the other side of the ford is
recommended instead of waiting till the deer jumps out of the hedge
around the next corner.

clare at snyder dot ontario dot canada

On Mon, 05 May 2008 21:02:26 -0700, jim beam
<spamvortex@bad.example.net> wrote:

>Ben C wrote:
>> On 2008-05-05, jobst.brandt@stanfordalumni.org <jobst.brandt@stanfordalumni.org> wrote:
>> [...]
>>> I find the defense of the gas theory, that arose from auto mechanics,
>>> a form of religious faith. "I read it in Road & Track". As I
>>> explained, in auto repair the cause of drum brake lockup was not
>>> recognized as the same effect that caused total brake fade. That disk
>>> brakes decouple application force and brake force, being at right
>>> angles to one another, seems to elude many folks. When I worked in
>>> brake development in F1 design in Europe, we were aware of myth and
>>> lore that surrounded brakes.
>>
>> Since you worked in brake development, do you know what causes pads to
>> glaze and what are the best solutions to the problem?
>>
>> Slotted disks scrape the glaze off and perhaps also create channels for
>> fumes and powder (or something-- whatever it is it smells bad) to depart
>> via thus preventing it from vitrifying.
>>
>> Cross-drilling seems to be done for similar reasons-- to let material of
>> some kind escape (although the cheese-grater effect seems to be an
>> unwanted side-effect).
>
>so when is one of the knuckle-draggers going to mention the effect of
>water on braking surfaces??? wet brakes can be spectacular for almost
>total ineffectiveness. drilled/slotted brakes otoh are much more
>effective in this situation.

I've seen rotors SHATTER when they hit ice cold water when red hot
from heavy braking in my rally days.
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