What does brake fade feel like?
pop4
> Pad fade is what most people think about when they talk about brake
> fade. Pad fade (pads outside their design temperature range due to
> overheating) feels about the same as green fade, except that the car
> stops progressively worse until you get them so hot they stop working
> (for all practical purposes). There is plenty of warning if your brakes
> are in decent condition and anywhere *near* up to the task at hand (this
> is obviously a judgement call). Progressively higher braking effort is
> required to get the same effect as fade progresses. Cooling brings them
> back in a few to several minutes, and with practice you can judge how
> much gentler you have to be to get consistent brake performance.
Why does overheating cause the pad to fail? TIA.
pop
S. McClendon
The pads/brakes get so hot they heat the brake calipers. The calipers then
boil the brake fluid, and the brake pedal goes to the floor. No brakes at
all, sometimes you can pump and get some back. The type mentioned below is
basically a "glazing" of the pads and rotors. In effect, the pads get hot
enough to melt slightly. the glaze on the pads does not have the same
friction abilities and real brake pads, and you start getting longer and
longer stop. All brakes really do is convert motion (speed) into heat. The
hotter the brakes, the less heat before they "melt down". Less heat, less
stop (to slightly over simplify!)
"pop4" <vote...@altavista.com> wrote in message
news:9c3d64f3.02011...@posting.google.com...
JB
> Why does overheating cause the pad to fail? TIA.
Brakes work by turning kinetic energy into thermal energy. An average
family sedan stopping from 100kph could boil a litre of water with the
heat created by its brakes. Brakes continue to work one application
after another because the heat is disappated into the structure of the
car by conduction, radiated to the surrounding environment and is
carried away by the air rushing over the brake body and rotor between
applications. That's one of the main reasons disc brakes are better
than drum brakes (sure there are other reasons too) but the exposed
rotor is much easier to cool. Fade is caused by the brake creating
more heat than it can get rid of. Eventually the brakes get so hot
they can't get rid of the heat fast enough and the conversion of
kinetic to thermal energy stops. This is experienced by the driver as
described above. Once you get up to these serious temperatures you
start to get the odd materials effects when the pads start to break
down, the fluid boils and the rotors turn into 'potato chips'. Ways to
avoid brake fade (other than modifying your behavior) are bigger
ventilated and/or drilled rotors, finned brake bodies, ducts to direct
cold air onto the brakes and spoked wheels with wide gaps. If you
stand near a corner on an F1 track (apart from going deaf) you can
feel the radiated heat from the brakes of the cars. Remember as with
all things automotive e=mv^2, more speed, exponentially more energy.
Hope that helps.
JMB
pop4
> Many call "brake fade" a total loss of brakes for all practical purposes.
> The pads/brakes get so hot they heat the brake calipers. The calipers then
> boil the brake fluid, and the brake pedal goes to the floor. No brakes at
> all, sometimes you can pump and get some back.
Does this mean that the brake fluid cannot put pressure on the
calipers (or is it pistons?) because the fluid turns to air (boils) on
contact; and air is much more compressible than liquid? If not, why
does the brake fluid boiling prevent the brakes from working? If the
brake pedal literally falls to the floor, how is it possible to pump
it since it can't go any further (farther?)?
pop
The Al Bundy
news:4186f4aa.02011...@posting.google.com...
Nice story, but with F1 it once happend to someone from the Compaq supported
group (i thought) that during a pitstop his engine stalled by some mistake
and within 10 seconds or so the brakes were on fire, since they were not
cooled by the airflow....
The_Al
Chris Matthaei
I've always wondered this myself. Why does boiling fluid let the pedal
drop? I assume when a liquid boils, it expands. If you're applying 2000PSI
of pressure to this boiled fluid, it will compress, but never to a point
where the gas takes up less volume than the liquid. If you can compress it
to the volume of a liquid, it would just liquify, right? What exactly
happens when you boil your brake fluid?
thanks,
Chris
Art
turns to mush.
"Chris Matthaei" <ch...@notherealadd.com> wrote in message
news:a29k9n$m8s$1...@bob.news.rcn.net...
Chris Matthaei
>Bubbles. As soon as there's a bubble or two in your brake fluid, the pedal
>turns to mush.
But the gas bubble should have a higher volume than the liquid that boiled
to create it, right? The pedal should go up as the fluid boils and
expands! Obviously this is not what happens. I'm wondering about the
physics involved. What am I missing?
Chris
Charlie Spitzer
"Chris Matthaei" <ch...@notherealadd.com> wrote in message
news:a29o65$55b$1...@bob.news.rcn.net...
Dave Plowman
Chris Matthaei <ch...@notherealadd.com> wrote:
> But the gas bubble should have a higher volume than the liquid that boiled
> to create it, right? The pedal should go up as the fluid boils and
> expands! Obviously this is not what happens. I'm wondering about the
> physics involved. What am I missing?
Gas can be compressed. Liquids can't. (In practice)
--
* A woman drove me to drink and I didn't have the decency to thank her
Dave Plowman dave....@argonet.co.uk London SW 12
RIP Acorn
Elroy the Seedy Impaler
physics/thermodynamics/chemistry behind it)...
Elroy the Seedy Impaler
Chris Matthaei
>In article <a29o65$55b$1...@bob.news.rcn.net>,
> Chris Matthaei <ch...@notherealadd.com> wrote:
>> But the gas bubble should have a higher volume than the liquid that boiled
>> to create it, right? The pedal should go up as the fluid boils and
>> expands! Obviously this is not what happens. I'm wondering about the
>> physics involved. What am I missing?
> Gas can be compressed. Liquids can't. (In practice)
So the gas can be compressed to a higher density than the liquid? This is
counterintuitive, but I suppose it's possible.
Chris
Art
why, once you've compressed the bubbles down to the same density of the
original liquid, wouldn't you hit the hard spot in the brake pedal,
rendering the bubbles moot?
It does seem counterintuitive, and I'm going with that. I don't think you
can compress the bubble to a higher density than the liquid.
I think (or am guessing) that here's what happens: When you're not pressing
the pedal, the system is not closed. The bubbles that increase the volume
don't simply push upward on the brake pedal. Instead, they expand the fluid
up into the container. The system closes when you begin to apply the
brakes; it takes the amount of fluid within the closed part and compresses
it. (This would allow it to take in new fluid from the reservoir each time
you use the brakes, to compensate for leakage and pad wear.) That way, with
the bubbles having forced some fluid up into the reservoir, there is room to
compress and the pedal gets mushy.
You can't get down to the "hard spot" because the brake cylinder has a very
short throw, since it's normally compressing liquid which doesn't "give."
It can't move far enough to compress the bubbles much.
"Chris Matthaei" <ch...@notherealadd.com> wrote in message
news:a2a3ss$h4r$1...@bob.news.rcn.net...
Alex Carreira
> drop? I assume when a liquid boils, it expands. If you're applying 2000PSI
> of pressure to this boiled fluid, it will compress, but never to a point
> where the gas takes up less volume than the liquid. If you can compress it
> to the volume of a liquid, it would just liquify, right? What exactly
> happens when you boil your brake fluid?
Although I am not sure what exactly happens, I have a guess for you. Based
on PV = nRT (1), lets just simplify and pull out n and R. Now we have PV =
T (2). As the temperature, T, of the fluid goes up, let us assume a
constant volume, V, to store the brake fluid. Thus, only P can change to
mathematically satisfy equation 2 above. P, pressure, is equal to force
over area, P=F/A (3). As the temperature of the brake fluid rises from
braking, the pressure will increase and exert greater forces on the master
cylinder (and other components in the system). The area to which this force
is exerted can be assumed constant, thus only force can compensate for
increased pressure to mathematically satisfy equation 3. Eventually, a seal
(in the piston area I'm guessing) will yield enough to allow some of the
fluid to escape the closed volume. Then, the fluid can expand enough to
form a binary system of gas and liquid which is compressible and leads to
increased pedal travel with decreased deflection of the pistons in the brake
caliper.
Like I said, this is only a guess and my knowledge of brake systems is
limited (I suspect there is some solution to the above described scenario, I
just have yet to learn about it.).
A :)
Dave Plowman
Chris Matthaei <ch...@notherealadd.com> wrote:
> So the gas can be compressed to a higher density than the liquid? This
> is counterintuitive, but I suppose it's possible.
I'd guess the fluid when it boils and changes to gas will expand (at the
caliper end) and push the excess back into the reservoir, leaving a
mixture of gas and fluid in the line. This is what compresses. If you keep
the brakes *on* until they fade, I'd guess it can't happen like this.
--
* INDECISION is the key to FLEXIBILITY *
Unknown
>Although I am not sure what exactly happens, I have a guess for you.
Boiling fluid displaces into the resevoir, leaving you with
compressible bubbles the next time you apply the brakes,
ergo, pedal to floor.
Colin
J. Harvey
1) Mount Washington, NH. 1986 VW GTi. Going up was a blast (yee haw...).
Going down the steeper sections (perhaps a bit too quickly...) the brakes
overheated and started to show signs of 'major' fade. Dropped down to the
lowest possible gear and the car still accelerated down the (very!) steep
sections - engine over-reving. Used the parking brake until it got hot.
Back to the normal (mostly front) brakes. Round the loop (brakes, engine,
parking brake) a few times until the plunge to oblivion finally leveled out.
Phew... Great fun on a beautiful Sunday afternoon.
2) Somewhere else. Same car. Car stuffed to the gills with heavy
equipment. Way, way, way (!) overloaded. Driving at a 'brisk' pace (leave
it at that). Going along the long, moderately down-hill stretch that ends
at a stop light. Didn't think about the load. Leave the braking until the
usual point. Brakes fade out to nothing at about the point where it is
passing through the speed limit. Madly double-clutch downshift and apply
the parking brake. Ain't gonna make it.... Light turns green (Thank you
God.) Slide through intersection into a huge flat parking lot. Jump out
and look at front wheels - rotors are just fading back to a dull red glow.
Those examples are what brake fade feels like.
pop4
Thanks alot for your anecdotes. Anybody with a BMW e36 328 have
similar experiences in the "real world" (not on a race track)? MTIA.
pop
self
> "Art" <a...@bikerider.com> writes:
>
> >Bubbles. As soon as there's a bubble or two in your brake fluid, the
pedal
> >turns to mush.
>
> to create it, right? The pedal should go up as the fluid boils and
> expands! Obviously this is not what happens.
Actually, it does.
But, that 'extra' displaced fluid is returned to the reservoir, when the
pedal is returned to TOS. From that time forward, until bleeding the air
out, you get the spongy
pedal.
Take a look at the diagram at
http://www.autoshop-online.com/auto101/brake1.html
Notice that if the piston goes back far enough (brake pedal goes high
enough), then there is a connection between the brake pressure system and
the reservoir. The reservoir is roughly at atmosphere, so if you somehow
have air under pressure somewhere in the pressure side of the system it will
'relieve' excess fluid into the reservoir, leaving that air pocket behind!
Spongy!
Greg Stoner
self
> "Art" <a...@bikerider.com> writes:
>
> >Bubbles. As soon as there's a bubble or two in your brake fluid, the
pedal
> >turns to mush.
>
> But the gas bubble should have a higher volume than the liquid that boiled
> to create it, right? The pedal should go up as the fluid boils and
> expands! Obviously this is not what happens.
Actually, it does.
But, that 'extra' displaced fluid is returned to the reservoir, when the
pedal is returned to TOS.
From that time forward, until bleeding the air out, you get the spongy
pedal.
Greg Stoner
self
> What am I missing?
This:
"When the forward movement of the pistons causes their primary cups to cover
the bypass holes, hydraulic pressure builds up and is transmitted to the
wheel cylinders"
Notice - bypass holes. This diagram gives the idea (Figure 4):
http://www.howstuffworks.com/master-brake2.htm
Greg Stoner
JB
> Nice story, but with F1 it once happend to someone from the Compaq supported
> group (i thought) that during a pitstop his engine stalled by some mistake
> and within 10 seconds or so the brakes were on fire, since they were not
> cooled by the airflow....
Sure, another illustration. I'm not sure why you say "Nice story,
but...", radiation is just one of the heat dissapation mechanisms,
along with convection.
Cheers
JMB
C.R. Krieger
> similar experiences in the "real world" (not on a race track)? MTIA.
Not likely. It doesn't even happen very often *on track* (and the one I
hang around as a BMW CCA instructor at - Road America - can be hell on
brakes) in cars like yours. The most 'underbraked' BMW was the 2002
followed closely by the E21 320i. Since the E30, most are up to almost
any reasonable *street* use and with minor upgrades even stand up well
to track use. If you get brake fade in the E36, it's more likely a
problem with *your* braking techniques and habits, not the car's brakes.
--
C.R. Krieger
"Don't argue with 'em, dear; they're beneath our dignity." - W.C. Fields
Chris Matthaei
>Actually, it does.
>Greg Stoner
Thank you Greg, Art, Colin, and anyone else who answered this for me. I
think my confusion came from the fact that I've never experienced boiled
braked fluid before. I had assumed it was similar to fade, and that the
pedal would go to the floor in the middle of braking as soon as the fluid
boiled. From what you guys are saying, the pedal won't go to the floor
until the *next time* you hit the brakes. This makes perfect sense, and
has restored my faith in physics. :)
Chris
ray
> >pedal is returned to TOS.
> >From that time forward, until bleeding the air out, you get the spongy
> >pedal.
>
> >Greg Stoner
>
> Thank you Greg, Art, Colin, and anyone else who answered this for me. I
> think my confusion came from the fact that I've never experienced boiled
> braked fluid before. I had assumed it was similar to fade, and that the
> pedal would go to the floor in the middle of braking as soon as the fluid
> boiled. From what you guys are saying, the pedal won't go to the floor
> until the *next time* you hit the brakes. This makes perfect sense, and
> has restored my faith in physics. :)
>
> Chris
That would also explain why I've seen NASCAR drivers "check" the brakes
on a road course. The foot-cam shots show them "tap" the brakes a lot.
I assumed they were using it to "set" the car for the corner, but
they are probably just checking to see if there's any brakes at all.
Ray
--
01 Trans Am Ram Air - 13.49@104 mph (stock)
For my REAL return email, change the bitbucket to my first name,
and it's only a dot com. The rest is spammer bait.
piggy
is higroscopic meanig it absorbes moisture change it every 12 months is
ya best bet if your getting brake fade its mostlikly your driveing than
the brakes god gave us gears for a reasion so use them
Greg Reed
asked a colleague with racing experience. Here's his explanation:
When the brake pedal is pressed, the piston in the master cylinder closes
off the opening to the brake fluid reservoir on top, to provide a solid
fluid system for braking. Because of the pressure applied during actual
braking, the boiling of brake fluid doesn't occur until the brake pedal is
released, and the pressure returns to atmospheric. (FYI: The temperature
at which all fluids boil rises and drops with pressure.) This boiling
happens at the caliper, and while the opening to the brake reservoir is
opened. The boiling fluid at the caliper then expands and displaces fluid
backward through the brake lines, through the master cylinder, and up into
the reservoir. The lack of braking happens the *next* time you try to apply
the brakes, and all you get is compression of the bubbles that are down at
the caliper -- bubbles which are now at atmospheric pressure. Since the
actual movement of brake fluid under braking is very tiny, at doesn't take
much of a gas void at the caliper to cause a lack of braking.
Note also that the lack of braking will likely only be at the wheel where
the boiling occurred. You won't lose *all* braking until all your brake
calipers have gas voids at them. Of course, the front calipers are where
most of the work is happening, and therefore boil first. They're also the
ones to cause you to "feel" the biggest loss of braking power, as anybody
who's driven a car with only rear brakes can tell you. (Leave *LOTS* of
following distance if you ever find yourself in this situation.)
Once the bubbles collapse, "pumping" the brake pedal will draw fluid back
out of the reservoir and push it back down to the caliper, refilling the
system and restoring braking power.
My racing friend told me that in NASCAR (and probably other racing series as
well) they have a brake "loop" instead of a single brake line going from the
master cylinder to each of the calipers. The fluid passes *to* the caliper
down the brake line (just as on a regular car), but instead of passing back
*up* the brake line when the pedal is released (as on a regular car), the
fluid returns through a separate return line. This permits a continuous
fresh supply of brake fluid to the calipers, and the long round trip from
the caliper back up to the master cylinder and then back down to the caliper
allows the brake fluid lots of time to cool.
I didn't know any of this before asking, and thought that others on this
list (and the others in the crosspost) might find it as interesting to learn
as I did.
- Greg Reed
'89 200 TQ
'90 V8Q
'00 Olds Intrigue
'76 Cadillac Fleetwood 9-pass sedan (Pimp-Mobile)
'01 Chevy Astro Van (for the wife)
"Chris Matthaei" <ch...@notherealadd.com> wrote in message
news:a2hle0$2ao$1...@bob.news.rcn.net...
D Autry
be hot for them to work properly. If they're too cold you won't have
optimum braking either.
"ray" <bitb...@rollingviolation.comnet.org> wrote in message
news:3C4C5FE0...@rollingviolation.comnet.org...
C.R. Krieger
news:3c4dc...@news3.calweb.com...
> Question: do the BigTime racers have to bleed their brakes
> all the time, or have they engineered around this problem?
> Seems like an antiquated practice, what with computer controlled
> engines, ABS, etc.
All the time. Even the Small Time Racers do it - usually before each
race weekend.
Tom Tweed
wrote:
>Question: do the BigTime racers have to bleed their brakes
>all the time, or have they engineered around this problem?
>Seems like an antiquated practice, what with computer controlled
>engines, ABS, etc.
I believe good brake cooling ducts and a (fairly inexpensive)
recirculating system for the fluid is used by most teams to
prevent fading. Huntley Racing used to sell the recirc systems
for about $500, I think. To a high-bucks race team, this would
be cheap.
TT
---------------------------------
Tom Tweed mailto:twe...@ucsd.edu
La Jolla, CA, USA or ttw...@san.rr.com
http://members.rennlist.com/tweedt/
---------------------------------
C.R. Krieger
news:GYu38.335$DT5.2...@news.uswest.net...
> I quess it depends on when their tapping. Many race pads actually
have to
> be hot for them to work properly. If they're too cold you won't have
> optimum braking either.
About one good lap at a track like the Glen gets these kind of pads hot
enough that they won't significantly cool down during the race. If the
drivers are touching the brakes outside the braking zones in the middle
of the race, especially in the latter portion of a long straight, it's
definitely just to see if they're 'still there'.
Steven Shields
that has collected in your brake fluid. Break fluid should be changed
at least once a year and more frequently, with track events. Brake
Fluid attracts moisture. Steam in your brake lines causes fade/failure.
(2) Brake fluid can boil. There are brake fluids with higher boiling
points, look for them. (3) Pads give off gasses as they heat from
tremendous friction. These gases tend to come between the pads and
rotors inhibiting compression against the rotors,"float" if you will.
Slotted, Hole forged and drilled rotors are attempts to relieve the
buil-up pad gases. The latter tends to cause metal failures around the
drilling sites do to metal stresses. (4) Different break compounds are
better under different driving conditions, such as Track vs Street.
Personally, I am a fan of Pagid Orange, which I use for street and
track. There are much cheaper pads to use for the street, but time and
laziness comes into play for changing the pads after track events.
However, there are trade-offs. Track brakes like to be heated before
their best "bite" and tend to "squeal like a pig" around town. "Softer"
brakes create more immediate "bite" and alot more dust. (5) Cooling of
the rotors and pads improves braking performance. There are many
cooling kits available, depending upon vehicle and application (6) You
should NOT be experiencing "brake fad" while driving around town, unless
of course you are re-filming "Rendevous." If normal driving causes you
to experience brake fade then a step by step evaluation and tesing of
your cars brake SYSTEM is appropriate.
regards
Steve
pop4 wrote:
> JRE <not...@nowhere.com> wrote
>
>>Pad fade is what most people think about when they talk about brake
>>fade. Pad fade (pads outside their design temperature range due to
>>overheating) feels about the same as green fade, except that the car
>>stops progressively worse until you get them so hot they stop working
>>(for all practical purposes). There is plenty of warning if your brakes
>>are in decent condition and anywhere *near* up to the task at hand (this
>>is obviously a judgement call). Progressively higher braking effort is
>>required to get the same effect as fade progresses. Cooling brings them
>>back in a few to several minutes, and with practice you can judge how
>>much gentler you have to be to get consistent brake performance.
>>
>
> Why does overheating cause the pad to fail? TIA.
>
> pop
>
--
Steven Shields, CRNA, MHA
mailto:rbcs...@ptd.net
Locum Tenens CRNA
RBC Savers Peri-Operative Cell Saving
J. Harvey
> Break (brake) fluid should be changed
> at least once a year and more frequently,
> with track events.
Are you suggesting that the average schmoe should change his automobile's
brake fluid "at least once a year" ? Does this mean that "twice a year"
would be about right and "three times a year" would be even better ?
I thought that (track events aside) the average schmoe would be treating his
car pretty good if it got changed once every two years or so.
Dave Plowman
J. Harvey <JM001....@sympatico.ca> wrote:
> I thought that (track events aside) the average schmoe would be treating
> his car pretty good if it got changed once every two years or so.
As indeed will happen with a properly serviced BMW - it's part of the
schedule.
--
* Half the people in the world are below average.
Luke Osborne
news:3c4d44b3$1...@iridium.webone.com.au...
God didn't give you gears to slow down. Read a book called "Speed Secrets".
Gears are in fact used to give the fastest acceleration *out* of a corner
and not to be used to slow the car down. That's what brakes are for...
JB
> Brake Fade is mutifacited.....Pads give off gasses....Different break compounds......
For my $0.02 these are limit effects, the basic cause of fade is the
brakes can't shift the heat, if you can't get rid of the heat, you
can't convert the energy. For most people who don't drive race
modified cars that will be the cause.
JMB