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Ben C June 23rd 09 12:12 PM

Vented Discs
 
On 2009-06-19, Michael Press wrote:
In article ,
Ben C wrote:

On 2009-06-19, wrote:
SJM who? wrote:

[...]
1. To cool the disks
2. To vent gases between pads and disks
3. To allow disk heat expansion/contraction without subtle warp

1. Drilled disks have less cooling surface than solid ones.


If you drill a disk it has a higher surface-area to volume ratio than it
did before. So I'm not sure what you mean. You are reducing the area of
the bit that's directly conducting heat away from the pads. But heat
will conduct from there to the inside surfaces of the holes whence it
will be cooled by the air. So it's difficult to say which is
theoretically better.


Denote by

h: thickness of the disk
r: the radius of the drilled hole

the change in surface area = 2.pi.rr - 2.pi.r.h


You've only counted one end of the cylinder that you drilled out, and
you're considering only the change in surface area, not the change in
surface area to volume ratio.

See
http://groups.google.co.uk/group/rec...d90ad43f00c793

so the surface area increases if r h
and decreases if r h.


Almost (if you count both ends, the turning point would be 0.5h), but
the surface-area to volume ratio always increases, whatever the hole
size.

Having said all this though, I think you're right that surface area is a
more important consideration than surface area to volume ratio.

The volume affects the heat capacity, but disks (on cars) heat up
quickly and then reach a thermal equilibrium where radiation matches
heat generated by friction. So we don't really care a lot about the heat
capacity.

(Disks on trains, for example, may work mainly as heat sinks instead,
like bicycle rim brakes.)

Now you must show that the heat transfer out of the walls of the hole
is as great as that off the surface of the disk.
The air speed through the hole decreases with the radius.
Good luck.


I guess another question to ask is is heat dissipation typically limited
by pad-disk conduction or by disk-air radiation/convection?

If the former, you don't want holes (for cooling). If the latter, maybe
you do.

Ben C June 23rd 09 06:03 PM

Vented Discs
 
On 2009-06-23, Phil W Lee phil wrote:
Ben C considered Tue, 23 Jun 2009 06:12:28 -0500
the perfect time to write:

On 2009-06-19, Michael Press wrote:
In article ,
Ben C wrote:

On 2009-06-19, wrote:
SJM who? wrote:
[...]
1. To cool the disks
2. To vent gases between pads and disks
3. To allow disk heat expansion/contraction without subtle warp

1. Drilled disks have less cooling surface than solid ones.

If you drill a disk it has a higher surface-area to volume ratio than it
did before. So I'm not sure what you mean. You are reducing the area of
the bit that's directly conducting heat away from the pads. But heat
will conduct from there to the inside surfaces of the holes whence it
will be cooled by the air. So it's difficult to say which is
theoretically better.

Denote by

h: thickness of the disk
r: the radius of the drilled hole

the change in surface area = 2.pi.rr - 2.pi.r.h


You've only counted one end of the cylinder that you drilled out, and
you're considering only the change in surface area, not the change in
surface area to volume ratio.

See
http://groups.google.co.uk/group/rec...d90ad43f00c793

so the surface area increases if r h
and decreases if r h.


Almost (if you count both ends, the turning point would be 0.5h), but
the surface-area to volume ratio always increases, whatever the hole
size.

Having said all this though, I think you're right that surface area is a
more important consideration than surface area to volume ratio.

I don't think surface area is relevant at all - for a given caliper
force, reducing surface area just increases pressure on the remaining
area in proportion (so if the caliper exerts 200lb pressure, you can
utilise that as 200psi over a square inch, or 100psi over 2 square
inches) the resulting friction is going to be nearly identical.


I wasn't thinking of the friction force, but of having a larger area to
conduct heat into.

But in many applications you will get a higher friction normal force
with a larger area-- that's why racing cars have wide tyres.

[...]
There must be a limit on how much of the disk you can cover with pad
before the ability of the disk to pass heat to the air is compromised,
although with vented disks the limit would be higher.


Yes, and since most disks _are_ vented, it implies that the disk-air
boundary was the limiting factor.

Radey Shouman June 24th 09 01:49 AM

Vented Discs
 
Ben C writes:

On 2009-06-19, Michael Press wrote:
In article ,
Ben C wrote:

On 2009-06-19, wrote:
SJM who? wrote:
[...]
1. To cool the disks
2. To vent gases between pads and disks
3. To allow disk heat expansion/contraction without subtle warp

1. Drilled disks have less cooling surface than solid ones.

If you drill a disk it has a higher surface-area to volume ratio than it
did before. So I'm not sure what you mean. You are reducing the area of
the bit that's directly conducting heat away from the pads. But heat
will conduct from there to the inside surfaces of the holes whence it
will be cooled by the air. So it's difficult to say which is
theoretically better.


Denote by

h: thickness of the disk
r: the radius of the drilled hole

the change in surface area = 2.pi.rr - 2.pi.r.h


....

Now you must show that the heat transfer out of the walls of the hole
is as great as that off the surface of the disk.
The air speed through the hole decreases with the radius.
Good luck.


I guess another question to ask is is heat dissipation typically limited
by pad-disk conduction or by disk-air radiation/convection?

If the former, you don't want holes (for cooling). If the latter, maybe
you do.


Holes through the disk increase turbulence. Greater turbulence
increases heat, mass, and momentum transfer. Correlations for
turbulent transfer typically include the effect of surface roughness
-- sharp edged holes are the equivalent of a very rough surface.
Even though there may be no net flow through the holes, there will be
enough flow in and out of them to cause the air inside the holes to
quickly approach the conditions just outside.

Improved heat transfer through increased turbulence may well help to cool
the disk.

Also, the contents of the holes provide a bulk flow of air past the
*pad*. Without holes, when not braking, there is only a narrow space
for air to flow past the pad, which makes convection cooling very
slow. When braking there is no space for air to flow and cooling can
occur only by conduction, to the disk or to the caliper. With holes
there is significant flow of air past the pad, whether braking or not.
The air inside the holes will be well-mixed, and quickly approach
equilibrium with the surface of the pad. Once past the pad, air in
the holes will be exchanged with the free stream of air flowing past
the disk, thus cooling the pad.

The above is just intuition on my part, I have neither calculated nor
experimentally verified any of it, nor do I have any experience
designing brakes.


Nate Nagel[_2_] June 24th 09 02:59 AM

Vented Discs
 
wrote:
Radey Shouman wrote:

[...]


1. To cool the disks
2. To vent gases between pads and disks
3. To allow disk heat expansion/contraction without subtle warp



1. Drilled disks have less cooling surface than solid ones.


If you drill a disk it has a higher surface-area to volume ratio
than it did before. So I'm not sure what you mean. You are
reducing the area of the bit that's directly conducting heat away
from the pads. But heat will conduct from there to the inside
surfaces of the holes whence it will be cooled by the air. So
it's difficult to say which is theoretically better.


Denote by


h: thickness of the disk
r: the radius of the drilled hole


the change in surface area = 2.pi.rr - 2.pi.r.h


Now you must show that the heat transfer out of the walls of the hole
is as great as that off the surface of the disk.
The air speed through the hole decreases with the radius.
Good luck.


I guess another question to ask is is heat dissipation typically
limited by pad-disk conduction or by disk-air
radiation/convection?


If the former, you don't want holes (for cooling). If the latter,
maybe you do.


Holes through the disk increase turbulence. Greater turbulence
increases heat, mass, and momentum transfer. Correlations for
turbulent transfer typically include the effect of surface roughness
-- sharp edged holes are the equivalent of a very rough surface.
Even though there may be no net flow through the holes, there will
be enough flow in and out of them to cause the air inside the holes
to quickly approach the conditions just outside.


Improved heat transfer through increased turbulence may well help to
cool the disk.


Also, the contents of the holes provide a bulk flow of air past the
*pad*. Without holes, when not braking, there is only a narrow
space for air to flow past the pad, which makes convection cooling
very slow. When braking there is no space for air to flow and
cooling can occur only by conduction, to the disk or to the caliper.
With holes there is significant flow of air past the pad, whether
braking or not. The air inside the holes will be well-mixed, and
quickly approach equilibrium with the surface of the pad. Once past
the pad, air in the holes will be exchanged with the free stream of
air flowing past the disk, thus cooling the pad.


The above is just intuition on my part, I have neither calculated
nor experimentally verified any of it, nor do I have any experience
designing brakes.


This is beginning to sound like a "stress relieving" thread. If the
effects are so scientifically definable, why are most disks of disk
brakes not cross-drilled? Almost no passenger cars that are not
"sports cars" are cross drilled nor are HSR disks. Clutch plates on
most vehicles using friction clutches, single- or multi-plate, are not
cross drilled... (because the public can't see them so it has no
benefit), the process apparently has as much benefit as spoilers on
family sedans and street legal "sports cars", all of which can be
validated by similar arguments used in favor of cross drilling disks.

Religion is great. To each his own supreme being and beliefs. My car
tires are lower profile and wider than yours, so I can commute to work
faster than you. I need them!

Jobst Brandt


Well, there's no point in drilling a clutch, because except when
starting from a dead stop it should be either in or out; that is,
there's no slipping so no heat buildup.

nate

--
replace "roosters" with "cox" to reply.
http://members.cox.net/njnagel

Tom Sherman °_° June 24th 09 03:01 AM

Vented Discs
 
Nate Nagel wrote:
[...]
Well, there's no point in drilling a clutch, because except when
starting from a dead stop it should be either in or out; that is,
there's no slipping so no heat buildup.

You can slip a clutch? ;)

--
Tom Sherman - 42.435731,-83.985007
I am a vehicular cyclist.

Michael Press June 24th 09 03:12 AM

Vented Discs
 
In article ,
Ben C wrote:

On 2009-06-19, Michael Press wrote:
In article ,
Ben C wrote:

On 2009-06-19, wrote:
SJM who? wrote:
[...]
1. To cool the disks
2. To vent gases between pads and disks
3. To allow disk heat expansion/contraction without subtle warp

1. Drilled disks have less cooling surface than solid ones.

If you drill a disk it has a higher surface-area to volume ratio than it
did before. So I'm not sure what you mean. You are reducing the area of
the bit that's directly conducting heat away from the pads. But heat
will conduct from there to the inside surfaces of the holes whence it
will be cooled by the air. So it's difficult to say which is
theoretically better.


Denote by

h: thickness of the disk
r: the radius of the drilled hole

the change in surface area = 2.pi.rr - 2.pi.r.h


You've only counted one end of the cylinder that you drilled out,


Area of a circle is ?
Answer: pi.rr

Area of the two ends of a right circular cylinder is 2.pi.rr.

and
you're considering only the change in surface area, not the change in
surface area to volume ratio.


That is so. I am not considering the surface to volume ratio.


See
http://groups.google.co.uk/group/rec...d90ad43f00c793

so the surface area increases if r h
and decreases if r h.


Almost (if you count both ends, the turning point would be 0.5h), but
the surface-area to volume ratio always increases, whatever the hole
size.

Having said all this though, I think you're right that surface area is a
more important consideration than surface area to volume ratio.

The volume affects the heat capacity, but disks (on cars) heat up
quickly and then reach a thermal equilibrium where radiation matches
heat generated by friction. So we don't really care a lot about the heat
capacity.

(Disks on trains, for example, may work mainly as heat sinks instead,
like bicycle rim brakes.)

Now you must show that the heat transfer out of the walls of the hole
is as great as that off the surface of the disk.
The air speed through the hole decreases with the radius.
Good luck.


I guess another question to ask is is heat dissipation typically limited
by pad-disk conduction or by disk-air radiation/convection?


I do not know.
Heat is mostly generated in the pad, is transferred to the rotor
by conduction, and is mostly dissipated from the rotor by convection.

If the former, you don't want holes (for cooling). If the latter, maybe
you do.


--
Michael Press

Radey Shouman June 24th 09 04:57 AM

Vented Discs
 
writes:

Radey Shouman wrote:

....

The above is just intuition on my part, I have neither calculated
nor experimentally verified any of it, nor do I have any experience
designing brakes.


This is beginning to sound like a "stress relieving" thread. If the
effects are so scientifically definable, why are most disks of disk
brakes not cross-drilled? Almost no passenger cars that are not
"sports cars" are cross drilled nor are HSR disks. Clutch plates on
most vehicles using friction clutches, single- or multi-plate, are not
cross drilled... (because the public can't see them so it has no
benefit), the process apparently has as much benefit as spoilers on
family sedans and street legal "sports cars", all of which can be
validated by similar arguments used in favor of cross drilling disks.


There is no flow of outside air past a clutch disk, so convection is
much less effective. As I said, I don't know for sure that cross
drilled disks run cooler, but it seems plausible that they might.

Even if drilled rotors have a real advantage for heavy braking, it
might still be the case that they are typically chosen only for
appearance. The fact that they are marketed primarily to "sporty"
drivers is not much of an argument either way.




Ben C June 24th 09 08:50 AM

Vented Discs
 
On 2009-06-24, wrote:
Radey Shouman wrote:

[...]
Holes through the disk increase turbulence. Greater turbulence
increases heat, mass, and momentum transfer. Correlations for
turbulent transfer typically include the effect of surface roughness
-- sharp edged holes are the equivalent of a very rough surface.
Even though there may be no net flow through the holes, there will
be enough flow in and out of them to cause the air inside the holes
to quickly approach the conditions just outside.


Improved heat transfer through increased turbulence may well help to
cool the disk.


Also, the contents of the holes provide a bulk flow of air past the
*pad*. Without holes, when not braking, there is only a narrow
space for air to flow past the pad, which makes convection cooling
very slow. When braking there is no space for air to flow and
cooling can occur only by conduction, to the disk or to the caliper.
With holes there is significant flow of air past the pad, whether
braking or not. The air inside the holes will be well-mixed, and
quickly approach equilibrium with the surface of the pad. Once past
the pad, air in the holes will be exchanged with the free stream of
air flowing past the disk, thus cooling the pad.


The above is just intuition on my part, I have neither calculated
nor experimentally verified any of it, nor do I have any experience
designing brakes.


This is beginning to sound like a "stress relieving" thread. If the
effects are so scientifically definable, why are most disks of disk
brakes not cross-drilled?


Most road cars in normal use don't have such big problems with brake
cooling, and the holes have side-effects the normal user doesn't want
like faster-wearing pads and possibly cracked disks in what a normal
user would consider a low mileage.

A racing car driver on the other hand needs lots of hard braking from
high speed for frequent tight turns on a track without much cooling-down
time in between. He doesn't mind so much if he needs to use fresh disks
and pads for every race.

Different priorities for different applications.

Almost no passenger cars that are not "sports cars" are cross drilled
nor are HSR disks. Clutch plates on most vehicles using friction
clutches, single- or multi-plate, are not cross drilled... (because
the public can't see them so it has no benefit), the process
apparently has as much benefit as spoilers on family sedans and street
legal "sports cars", all of which can be validated by similar
arguments used in favor of cross drilling disks.


Some of those spoilers do do something even for road cars. Don't know if
you get the Audi TT in America, but a few of them spun off the autobahn
while changing lanes at 100mph because the back end of the car
experienced aerodynamic lift. The problem was solved by adding a small
spoiler.

Ben C June 24th 09 08:52 AM

Vented Discs
 
On 2009-06-24, Still Just Me wrote:
On Tue, 23 Jun 2009 20:49:34 -0400, Radey Shouman
wrote:

Also, the contents of the holes provide a bulk flow of air past the
*pad*. Without holes, when not braking, there is only a narrow space
for air to flow past the pad, which makes convection cooling very
slow. When braking there is no space for air to flow and cooling can
occur only by conduction, to the disk or to the caliper. With holes
there is significant flow of air past the pad, whether braking or not.
The air inside the holes will be well-mixed, and quickly approach
equilibrium with the surface of the pad. Once past the pad, air in
the holes will be exchanged with the free stream of air flowing past
the disk, thus cooling the pad.


This might apply in a system with a center vented rotor. The holes
would not cool the pads in a single, non vented disk configuration
except by virtue of the fact that they might lower the rotor
temperature while it's not between the pads. Even with the vented
rotor, the flow to the pad is minimal. I suspect that if the holes
have any cooling effect in either configuration, it's when the rotor
is NOT between the pads.


But the rotor's always between the pads?

Ben C June 24th 09 08:54 AM

Vented Discs
 
On 2009-06-24, Michael Press wrote:
In article ,
Ben C wrote:

On 2009-06-19, Michael Press wrote:
In article ,
Ben C wrote:

On 2009-06-19, wrote:
SJM who? wrote:
[...]
1. To cool the disks
2. To vent gases between pads and disks
3. To allow disk heat expansion/contraction without subtle warp

1. Drilled disks have less cooling surface than solid ones.

If you drill a disk it has a higher surface-area to volume ratio than it
did before. So I'm not sure what you mean. You are reducing the area of
the bit that's directly conducting heat away from the pads. But heat
will conduct from there to the inside surfaces of the holes whence it
will be cooled by the air. So it's difficult to say which is
theoretically better.

Denote by

h: thickness of the disk
r: the radius of the drilled hole

the change in surface area = 2.pi.rr - 2.pi.r.h


You've only counted one end of the cylinder that you drilled out,


Area of a circle is ?
Answer: pi.rr

Area of the two ends of a right circular cylinder is 2.pi.rr.


Sorry, my mistake, you are quite right.


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