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 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.

Hi,

Radey Shouman wrote:
Improved heat transfer through increased turbulence may well help to cool
the disk.

*IF* you wanted to achieve effective cooling by venting, you'd have to
do it like the railway disk brakes do it: use double discs with venting
channels between them.

A lightweight version might be to cut slots into the discs from inside
to outside, so that the disc would look like a "comb". These slots would
have to be "diagonal" so that the centripetal force acting on the air
trapped in the slots forces the air outwards, resulting in a steady
airflow. Looking at the low radial speeds on a bike, this would probably
not work well.

The accepted solution to the cooling problem seems to be "larger discs"
- and since disc brakes are quite effective, a large disc might be
un-manageable by the (inexperienced) rider, so by reducing the effective
surface area, the disc brake gets "softer"... If you're afraid of heat
on disc brakes, you need maximum-size discs which would be the wheel
rims - so get a rim brake for maximum heat capacity...

... 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.

Actually the amount of heat transferred from pad to "air-in-hole" will
be so small, you can simply ignore it. The duration of contact between
air and pad is almost zero, so the effect is almost zero, too.

Stop trying to find a reason for the holes in discs - as I stated in my
other post, the most probable reason is "looking cool" - the only
technical effect of the holes probably is cleaning the pads - and the
disc linked in the original post (with the huge holes) would be
unsuitable for that because of the hole arrangement.

If the holes had any sensible effect, rims would have holes on the
braking surface, because rim brakes are large-diameter disc brakes...
And they have no holes... Actually, rim brakes do seldom heat to
critical temperatures according to (1).

(1) sorry, german only: http://fahrradzukunft.de/fz-0603/0603-11.htm
The author used temperature-measuring strips on the rims and found that
extreme temperatures (long downhill passages with all-time breaking and
lots of luggage) leads to failure of tube patches beyond 80°C and fading
due to brake pad overheating beyond 110°C.

Ciao...

..

On Jun 25, 9:49*am, Bernhard Agthe wrote:

If the holes had any sensible effect, rims would have holes on the
braking surface, because rim brakes are large-diameter disc brakes...
And they have no holes...

And no, you can't bore experimental holes in my rims!

Actually, rim brakes do seldom heat to
critical temperatures according to (1).

(1) sorry, german only: http://fahrradzukunft.de/fz-0603/0603-11.htm
The author used temperature-measuring strips on the rims and found that
extreme temperatures (long downhill passages with all-time breaking and
lots of luggage) leads to failure of tube patches beyond 80°C and fading
due to brake pad overheating beyond 110°C.

Gee. I know the RBT readers are supercyclists who wear their
underpants over the rest of their clothes, but how many
1. Are capable of going to places where it is possible to induce such
extreme conditions?
2. Haver ever cycled or will ever cycle in such places?
3. Having reached there, are stupid enough to cycle in such a
dangerous manner as to induce such conditions?
4. Having induced such conditions, are stupid enough not to notice?
5. Having noticed, are so thick as not immediately to take the
necessary counter-measures?

Hell, consider this sequence of facts. Jobst reported the other day
that he got up to 50mph or 80kph down some notorious mountain in
California. If his brake pads faded, never mind failed to stop him
(or melted!), you can bet we would have heard of it. I needed truck
assistance to get up to 100kph, and had to stop in a hurry afterwards
to avoid crossing a dangerous intersection (or, as bad, hitting the
back of my own truck). Neither disc brakes nor roller brakes got too
warm to touch.. I didn't at the time have a bike with rim brakes but I
do now and often ride the brakes down long descents so as not to speed
ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard
braking from over 50kph at the bottom of a hill makes no impression
either, as I've reported here before.

Sounds to me like this German test proves that existing bicycle brakes
are plenty good enough, and that venting is an affectation, just like
you say.

On the other hand, if holes make the fashion victims think they stop
cooler, let the disc brake manufacturers take their money. And who
knows, maybe some fashion victims will ride and brake hard enough (or
believe they do) to get real value out of narrow diagonal slots
designed to increase airflow, the only kind that can be said even
theoretically to work. I doubt though that objectively venting works
perceptibly at bicycle speeds and weights.

In conditions where bicycle brakes are genuinely stretched to the
point of heating up, say descending an Alp, discs should be made
larger rather than vented. But disc brakes are probably not the right
choice anyway. If such extreme, extended use is intended, discs should
swapped out for hydraulic rim brakes, because the rim is by definition
the longest circumference metal surface you can fit to a bicycle
wheel, passing through a lot more air for every revolution of the
wheel than a disc ever can..

Andre Jute
"The brain of an engineer is a delicate instrument which must be
protected against the unevenness of the ground." -- Wifredo-Pelayo
Ricart Medina
"Lateral thinking" -- Edward de Bono

On Jun 25, 2:38 pm, Andre Jute wrote:
snip
Gee. I know the RBT readers are supercyclists who wear their
underpants over the rest of their clothes, but how many
1. Are capable of going to places where it is possible to induce such
extreme conditions?
2. Haver ever cycled or will ever cycle in such places?
3. Having reached there, are stupid enough to cycle in such a
dangerous manner as to induce such conditions?
4. Having induced such conditions, are stupid enough not to notice?
5. Having noticed, are so thick as not immediately to take the
necessary counter-measures?

I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV. I have
personally experienced tire failure due to brake heat riding down Big
Cottonwood Canyon east of SLC UT.


Hell, consider this sequence of facts. Jobst reported the other day
that he got up to 50mph or 80kph down some notorious mountain in
California. If his brake pads faded, never mind failed to stop him
(or melted!), you can bet we would have heard of it. I needed truck
assistance to get up to 100kph, and had to stop in a hurry afterwards
to avoid crossing a dangerous intersection (or, as bad, hitting the
back of my own truck). Neither disc brakes nor roller brakes got too
warm to touch.. I didn't at the time have a bike with rim brakes but I
do now and often ride the brakes down long descents so as not to speed
ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard
braking from over 50kph at the bottom of a hill makes no impression
either, as I've reported here before.

Sounds to me like this German test proves that existing bicycle brakes
are plenty good enough, and that venting is an affectation, just like
you say.

The best reason for disc brakes has nothing to do with heat. Rim
brakes use the rim as a friction surface, which destroys the rim over
time. Disc brakes remove that source of wear from a soft aluminum or
carbon fiber rim to something designed to be a consumable item.
Mountain bikers and bike tourists can wear out a rim in just a single
season of riding in muddy conditions as the pads and the dirt combine
to form an abrasive slurry applied under high pressure against the
rim. The same issues apply to cyclocross in which the UCI recently
banned the use of discs. I guess the "purity" of the sport out weighed
actually making it affordable to run over extended periods of time
without costly parts replacement.

On Jun 25, 4:10*pm, Opus wrote:
On Jun 25, 2:38 pm, Andre Jute wrote:snip
Gee. I know the RBT readers are supercyclists who wear their
underpants over the rest of their clothes, but how many
1. Are capable of going to places where it is possible to induce such
extreme conditions?
2. Haver ever cycled or will ever cycle in such places?
3. Having reached there, are stupid enough to cycle in such a
dangerous manner as to induce such conditions?
4. Having induced such conditions, are stupid enough not to notice?
5. Having noticed, are so thick as not immediately to take the
necessary counter-measures?

I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV.

I'll leave that one to the local scoffjaws, who're just engaging motor-
drive.

I have
personally experienced tire failure due to brake heat riding down Big
Cottonwood Canyon east of SLC UT.

Why don't you tell us how long and steep this ride is, how
irresponsibly you rode down there, how the tire failed, how you
determined that it was due to brake heat. Note that I never said it is
impossible, merely that the circumstances are rare and avoidable.

Hell, consider this sequence of facts. Jobst reported the other day
that he got up to 50mph or 80kph down some notorious mountain in
California. *If his brake pads faded, never mind failed to stop him
(or melted!), you can bet we would have heard of it. I needed truck
assistance to get up to 100kph, and had to stop in a hurry afterwards
to avoid crossing a dangerous intersection (or, as bad, hitting the
back of my own truck). Neither disc brakes nor roller brakes got too
warm to touch.. I didn't at the time have a bike with rim brakes but I
do now and often ride the brakes down long descents so as not to speed
ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard
braking from over 50kph at the bottom of a hill makes no impression
either, as I've reported here before.

Sounds to me like this German test proves that existing bicycle brakes
are plenty good enough, and that venting is an affectation, just like
you say.

The best reason for disc brakes has nothing to do with heat. Rim
brakes use the rim as a friction surface, which destroys the rim over
time. Disc brakes remove that source of wear from a soft aluminum or
carbon fiber rim to something designed to be a consumable item.

I agree with you. If you read the archives, you will find me saying
so.

Mountain bikers and bike tourists can wear out a rim in just a single
season of riding in muddy conditions as the pads and the dirt combine
to form an abrasive slurry applied under high pressure against the
rim. The same issues apply to cyclocross in which the UCI recently
banned the use of discs. I guess the "purity" of the sport out weighed
actually making it affordable to run over extended periods of time
without costly parts replacement.

Well, here we drift out of agreement again. I think discs and their
maintenance are too expensive, though it is true that wheel rebuilding
because the rim is worn through is also very expensive. But the
solution is roller brakes. I have Shimano's 70/75 series rollerbrakes
on my Cyber Nexus Trek, and they're every bit as good as disc brakes,
but cost a tiny fraction in maintenance. You can see my Trek Smover
with these excellent enclosed brakes at:
http://www.audio-talk.co.uk/fiultra/...%20Smover.html

Andre Jute
I'm not cheap, I'm poor!

Opus aka DC wrote:
[...]
I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV.[...]

Presumably the full streamliners that exceed 120 kph.

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

Hi,

Opus wrote:
I have seen bicycles that set their brakes on fire stopping after a
straight line run on level ground, at Battle Mountain NV. I have
personally experienced tire failure due to brake heat riding down Big
Cottonwood Canyon east of SLC UT.

Sure this is possible, though the article I linked earlier says it's
unlikely in "normal operation" for most riders. And there's ample
warning to prevent brake overheating on long downhill passages - just
look in the manual ;-)

Apart from that, you can do a lot to keep your brakes cool by not
breaking steadily but in intervals. And most brakes don't fail
catastrophically. When you notice brake fading, you should stop and wait
for the brakes to cool ;-)

I needed truck
assistance to get up to 100kph, and had to stop in a hurry afterwards

Be careful with that - rather find a nice downhill section than use a
truck...

Sounds to me like this German test proves that existing bicycle brakes
are plenty good enough, and that venting is an affectation, just like
you say.

He concludes that rim brakes do take a lot of abuse and it's unlikely
they'll fail catastrophically. He is an experienced long-distance
touring cyclist and he doesn't see any special danger in (correctly set
up) rim brakes, even on long descends.

The best reason for disc brakes has nothing to do with heat. Rim
brakes use the rim as a friction surface, which destroys the rim over
time.

Well, usually the rims will take a lot of breaking before they
disintegrate. At least when you use pads that are not too "abrasive"...
You know, I once saw a disintegrated rim, but they used very narrow pads
(which reduces the area of friction quite much)...

Though I do remember an article (by J.Forrester?) on coaster brakes
overheating quickly and failing catastrophically... So I'd guess, any
type of small drum brake needs some cooling capacity... Remember the
cooler on SRAM's iBrake?

Disc brakes remove that source of wear from a soft aluminum or
carbon fiber rim to something designed to be a consumable item.

You could mount "discs" to your rims - at least if you build such a
system... But I guess that nobody would do that ;-)

rim. The same issues apply to cyclocross in which the UCI recently
banned the use of discs. I guess the "purity" of the sport out weighed
actually making it affordable to run over extended periods of time
without costly parts replacement.

Well, I'm not so happy with UCI technical rules - they should at least
allow for a "special bike" category - for example, allow recumbent bikes
for one week out of the Tour de France - or allow 'bents on time-trial
races - I do understand the rule about racing-style handlebars in group
races (they allow very good control), but not any rules on bike fit,
frame form or whatever. What's the problem if somebody uses a smaller
front wheel, disc brakes or curved stays? At least parts of the rules
are not strictly necessary, so I'd rather see them eased...

Ciao...

..

On Thu, 25 Jun 2009 07:38:19 -0700 (PDT), Andre Jute
wrote:

(snip)

Hell, consider this sequence of facts. Jobst reported the other day
that he got up to 50mph or 80kph down some notorious mountain in
California. If his brake pads faded, never mind failed to stop him
(or melted!), you can bet we would have heard of it. I needed truck
assistance to get up to 100kph, and had to stop in a hurry afterwards
to avoid crossing a dangerous intersection (or, as bad, hitting the
back of my own truck). Neither disc brakes nor roller brakes got too
warm to touch.. I didn't at the time have a bike with rim brakes but I
do now and often ride the brakes down long descents so as not to speed
ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard
braking from over 50kph at the bottom of a hill makes no impression
either, as I've reported here before.

(snip)

Data point: 220 lb. rider, reasonable 700c rims (Alex DM-18). Coming
down Gannett Hill
http://www.mountainzone.com/mountain...sp?fid=6276756
the road isn't too straight and there is a deep ditch on either side.
I kept speed down to 20 mph.

Both rims were very hot at the bottom. How hot? Wish I knew. It
wasn't boiling, but uncomfortable to the touch. The temperature
measuring strips are a good idea.

Now scale the hill up to Rocky Mountain size...

On Jun 26, 2:57*am, somebody wrote:
On Thu, 25 Jun 2009 07:38:19 -0700 (PDT), Andre Jute

wrote:

(snip)

Hell, consider this sequence of facts. Jobst reported the other day
that he got up to 50mph or 80kph down some notorious mountain in
California. *If his brake pads faded, never mind failed to stop him
(or melted!), you can bet we would have heard of it. I needed truck
assistance to get up to 100kph, and had to stop in a hurry afterwards
to avoid crossing a dangerous intersection (or, as bad, hitting the
back of my own truck). Neither disc brakes nor roller brakes got too
warm to touch.. I didn't at the time have a bike with rim brakes but I
do now and often ride the brakes down long descents so as not to speed
ahead of my pedalpals -- result: at worst a slightly warmed rim. Hard
braking from over 50kph at the bottom of a hill makes no impression
either, as I've reported here before.

(snip)

Data point: *220 lb. rider, reasonable 700c rims (Alex DM-18). *Coming
down Gannett Hillhttp://www.mountainzone.com/mountains/detail.asp?fid=6276756
the road isn't too straight and there is a deep ditch on either side.
I kept speed down to 20 mph.

IOW you were riding the brakes almost constantly.

Both rims were very hot at the *bottom. *How hot? *Wish I knew. *It
wasn't boiling, but uncomfortable to the touch. *The temperature
measuring strips are a good idea.

Makes me wonder if "uncomfortable to the touch" is anywhere near the
failure of patches due to overheating beyond 80 degrees C found by the
the German researcher cited by Bernhard.

Now scale the hill up to Rocky Mountain size...

Did you read about Opus's ride on steel rims and narrow tyres with
bead failure on stopping? (Everything I've reported is on relatively
wide rims or very wide rims, and with tyres between 37 and 60mm wide.)
Again, it makes you wonder about heat build-up when you stop moving,
when the air no longer flows around the rim, IOW whether the danger
isn't the hard ride but cutting off the airflow. Maybe those recumbent
riders with fairings aren't doing their brakes much good either.

Andre Jute
Curioser and curioser