Re: [OT] car brakes

jobst.brandt@stanfordalumni.org

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?

http://en.wikipedia.org/wiki/Brake_..._in_drum_brakes

Jobst Brandt

Tom Sherman

jobst.brandt@stanfordalumni.org aka Jobst Brandt wrote:
> [...]
> 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?
>
The manufactures get away with using smooth surfaced brake rotors since
the buyers are not looking to use the brakes as status symbols.

--
Tom Sherman - Holstein-Friesland Bovinia
The weather is here, wish you were beautiful

carlfogel@comcast.net

On Sun, 04 May 2008 18:41:55 -0500, Ben C <spamspam@spam.eggs> wrote:

>On 2008-05-04, A Muzi <am@yellowjersey.org> wrote:
>>>> Peter Cole <peter_cole@verizon.net> 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%).
>>
>>> Robert Lorenzini <bob@newportharbornet.com> wrote:
>>>> 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.
>>
>> Michael Press wrote:
>>> What is the chemical composition of the boiling gasses?
>>> Do the boiling gasses come from the pads?
>>
>> Counterintuitive, isn't it? This guy thinks it happens:
>> http://www.innerauto.com/Auto_Part/Brake_Disc/
>>
>> I'm not convinced ('gas' goes in the little hole??) or that taking one's
>> foot off a brake can break it.
>
>If gas did come off the pad where it touched the disk then it would have
>nowhere to go except to try and squeeze round the edge of the pad or
>build up into a sort of bubble. Either way the gas pressure would try to
>push pad and disk apart. The holes would prevent that happening.
>
>So I can see how the holes would work if the gas is there.
>
>And just because I don't know what gas would come off brake pads doesn't
>mean no gas does. After all what I don't know about brake pads would
>fill a book.

Dear Ben,

If you google for "disc brakes" "outgas" -excessive, you'll eliminate
much of the echoing of a familiar passage.

Browsing will suggest that there's some oversimplification of an
apparently problem well-known from the 1960s.

People often mention problems with organic pads that tended to outgas
on Formula One cars with early disc brakes.

The trouble was not that the gas pushed the pad away from the rotor.

What happened was that at high temperatures, the gas and the particles
eroding from the pad would mix and form a slick surface--a horrible
glazing effect rather than a gas layer.

Drilling holes through the discs may allow the gas and powdered
residue to escape before forming a film on the pad, or the holes may
simply help to scrape the stuff off, leaving a clean pad surface to
press against the rotor.

Many articles claim that modern pads have eliminated this problem and
that the drilling is now merely cosmetic.

Other articles suggest that competition level braking can still lead
to the problem.

Here's a typical passage leading up to a description of the problem in
the last sentence:

"Manufacturers define the performance -- the grippiness -- of their
brakepads by measuring the Mu, a function of the material coefficient
of friction over temperature. Pads are said to have rising, linear or
digressive rates, based on the Mu. A digressive rate pad means you
have to push harder to maintain the friction of the brakepad on the
rotor. A rising rate means you have to push less as the braking effect
increases. A linear rate means you maintain the same pressure. The
friction increases with temperature in a rising rate pad. Linear Mu
means the friction value is maintained as the temperature increases.
Many performance pads show rising friction values as the temperature
goes up, then drop off as the temperature reaches a point where the
pad outgasses, and powders, reducing the friction with the breakdown
film."
http://www.lemd.com/motorsport/previous.cfm?id=25

So it doesn't seem to be a matter of the gas pressing the pad away
from the rotor. It's the combination of very hot gas and powdered pad
forming a glaze on the surface of the pad.

Cheers,

Carl Fogel

Ben C

On 2008-05-05, jobst.brandt@stanfordalumni.org <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?

Obviously the requirements are different. The brakes on a racing car
operate in a higher temperature range altogether since they need a much
higher dissipation power. They also need to be warmed up, but brakes on
a passenger car need to work well from cold.

Disks on trains is an interesting one. I doubt trains brake down hills
(and in any case they don't go down steep hills). They just have to stop
at the stations. The disks may work as heat sinks rather than as
continuous dissipators, hence you probably want them big and heavy.

clare at snyder dot ontario dot canada

On 05 May 2008 05:11:16 GMT, 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?
>
>http://en.wikipedia.org/wiki/Brake_..._in_drum_brakes
>
>Jobst Brandt

Because they CAN.
They can get "adequate" braking with plain rotors
** Posted from http://www.teranews.com **

clare at snyder dot ontario dot canada

On Mon, 05 May 2008 05:10:44 -0500, Ben C <spamspam@spam.eggs> wrote:

>On 2008-05-05, jobst.brandt@stanfordalumni.org <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?
>
>Obviously the requirements are different. The brakes on a racing car
>operate in a higher temperature range altogether since they need a much
>higher dissipation power. They also need to be warmed up, but brakes on
>a passenger car need to work well from cold.
>
>Disks on trains is an interesting one. I doubt trains brake down hills
>(and in any case they don't go down steep hills). They just have to stop
>at the stations. The disks may work as heat sinks rather than as
>continuous dissipators, hence you probably want them big and heavy.

Dynamic braking handles something like 89+% of train braking
requirements
** Posted from http://www.teranews.com **

Andrew Price

On Mon, 05 May 2008 11:47:58 -0400, clare at snyder dot ontario dot
canada wrote:

>>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?
>>
>>http://en.wikipedia.org/wiki/Brake_..._in_drum_brakes
>>
>>Jobst Brandt
>
>Because they CAN.
>They can get "adequate" braking with plain rotors

That is basically it - TGV trains (which first used ventilated
cast-iron disks) later used plain steel disks, which proved
satisfactory in service. However, Alstom then went on to develop
ventilated steel disks which it builds into some of its current
rolling stock.

jobst.brandt@stanfordalumni.org

Clare who? 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?

>> Obviously the requirements are different. The brakes on a racing
>> car operate in a higher temperature range altogether since they
>> need a much higher dissipation power. They also need to be warmed
>> up, but brakes on a passenger car need to work well from cold.

>> Disks on trains is an interesting one. I doubt trains brake down
>> hills (and in any case they don't go down steep hills). They just
>> have to stop at the stations. The disks may work as heat sinks
>> rather than as continuous dissipators, hence you probably want them
>> big and heavy.

> Dynamic braking handles something like 89+% of train braking
> requirements

Next time a passenger train passes, listen to the wheels and note that
in spite of using disk brakes they have flat wheels that go thump thump
thump, here and in other parts of the world. Trains use their brakes
heavily even for speed control while descending grades under dynamic
braking. They must because doing that with the engine brakes alone
would cause the engine to skid and get flat wheels, something that is
far more expensive to rectify because their axles have attached motors
whose gears are integral with them.

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.

Today I am amazed how those stories survived, although I shouldn't be,
considering how this is the age of appearances rather than substance,
as we see from the cars people drive (SUV's and Trucks) with $1000+
wheels and low cross section tires... many and huge exhaust pipes.

Jobst Brandt

Zog The Undeniable

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. 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.

Frank Krygowski

On May 5, 4:06 pm, jobst.bra...@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".

But... isn't Road & Track at _least_ as reliable as Buycycling???

- Frank Krygowski

Ben C

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).

jobst.brandt@stanfordalumni.org

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

clare at snyder dot ontario dot canada

On Mon, 05 May 2008 21:30:11 +0100, Zog The Undeniable
<hrothgar19@yahoo.com> 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. 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.

The grooved rotors on my 1996 Mystique are much quieter than the
brakes I replaced. We will see what the pad life is like - using
Kevlar pads. The braking is very smooth and effective - and did I
mention quiet??. NEVER squeal.
** Posted from http://www.teranews.com **

Ben C

On 2008-05-05, jobst.brandt@stanfordalumni.org <jobst.brandt@stanfordalumni.org> wrote:
> 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.

Thanks for the explanation.