With all this talk about cfrp....

Tim McNamara wrote:

I think it's likely for example that bicycle disk brakes
dissipate energy to the air much better than rim brakes.

I'm curious as to what the mechanism of this would be.

Because they get hotter which may more than make up for their lower
surface area.

Here are the calculations where I estimated the heat capacity of a
disk to be a bit less than half that of a rim:

http://groups.google.co.uk/group/uk....1d32532f671264

It doesn't really show anything except that a rim brake may well
just work as a heat sink, and that if a disk brake also worked that
way, it would suck a lot more. My conclusion is that disk brakes
must work by dissipation if they work at all and are therefore a
better choice for long drags downhill.

Disk brakes can be heated much higher than a rim brake because there
is no tire attached. You could make a disk brake glow red from heat
if you wanted. Rim brakes need to dissipate heat quickly to avoid
blowing off the tire, disk brakes can dissipate heat slowly and it
doesn't matter.

I matters. At some temperature steady stat4 must be reached or the
disk would melt. Therefore, dissipation rate must be at least as good
as for rim brakes, although at a higher temperature. I have not heard
from anyone who, with a disk brake, descended a mountain pass like the
Stelvio, but I'm sure it has been done. The question is whether the
disk got visibly red hot. I am doubtful of the technology, because I
have not yet found any manufacturer at InterBike that could explain
why there are holes in the disks and why they are made as flimsy as
they ar... other than to save weight.

A rim brake has a higher surface area, but doesn't get hot enough
(or if it does the tyre blows off).

Aluminum rims easily get hot enough to boil water. We've had
reports of observing this posted to the newsgroup- I've never seen
it myself, but the hills round here are only 600-700 ft gain/loss
from base to top, and the roads are straight enough to not require
significant braking.

Indeed, but I think much hotter than that and you may get tyre
problems. A disk on the other hand can get much hotter provided
you make the pad material out of some appropriate material.

Angel Rodriguez reported putting adhesive heat-sensitive stickers
on his tandem rims and finding that the rims got up over 200F very
quickly on steep descents requiring heavy braking.

I make that 93C, a bit less than the boiling point of water. How
hot does a disk get? Dunno, but it could easily be more than twice
that.

I said 200F because I was confident about that and I didn't feel
like searching my bookshelves for Rodriguez's book. My recollection
was that his rims heated above 250F in a surprisingly short time of
hard braking with two riders on the tandem, like 45 seconds of
braking on a steep descent, but I'm not confident in my
recollection. The numbers have been quoted in the newsgroup before,
though; perhaps Carl "the Google" Fogel can ferret them out.

I'll assure you that it is more than 212°F because I generated steam
while braking into each hairpin turn on the Nufenen Pass (CH) that
blew with a loud hissss from the valve stem hole of a front wheel that
had taken on water in stream crossings. That is, until the rim was
dry. Fiamme red label rims and Clement Campionato del Mundo tires!

Jobst Brandt

Tom Kunich wrote:

D'ohBoy wrote:

But I am hoping they will be sufficient to handle the max
2-4 mile downhills we have around here that I don't brake
on anyhoo.

You seem to be missing what's being said. IF you need to stop quickly with
carbon rims you cannot. There's no place for the heat energy to go save to
melt the rims or the pads since the carbon cannot conduct the heat away
effectively.

Carbon-epoxy composite is a wildcard with regard to its ability to
transmit and reject heat. The thermal conductivity of the fibers
themselves varies according to their quality and what kind of original
organic material was used to make them. Basically they straddle the
same two-orders-of-magnitude range of thermal conductivity that
structural metals do, from worse than the least conductive stainless
steel to better than silver (the most conductive metal).

The epoxy resin matrix in which the fibers are encapsulated has poor
thermal conductivity similar to that of other familiar plastics, and
comprises a variable portion of a CFRP mixture. This resin can be
filled with other powdered materials to change its thermal
characteristics.

The bottom line is that between the huge conductivity variation
between fibers themselves and the huge variation between fiber-resin
mixtures, you can't make a valid generalization about how well CFRP
dissipates heat. I think it's fair to say that just about any mixture
would be harder on brake pads than aluminum, but the thermal
conductivity of a CFRP composite can vary by a factor of more than
100.

Chalo

Ben C? wrote:

I think it's likely for example that bicycle disk brakes
dissipate energy to the air much better than rim brakes.

I'm curious as to what the mechanism of this would be.

Because they get hotter which may more than make up for their
lower surface area.

Here are the calculations where I estimated the heat capacity of a
disk to be a bit less than half that of a rim:

http://groups.google.co.uk/group/uk....1d32532f671264

It doesn't really show anything except that a rim brake may well
just work as a heat sink, and that if a disk brake also worked
that way, it would suck a lot more. My conclusion is that disk
brakes must work by dissipation if they work at all and are
therefore a better choice for long drags downhill.

Disk brakes can be heated much higher than a rim brake because
there is no tire attached.

Yes exactly.

You could make a disk brake glow red from heat if you wanted. Rim
brakes need to dissipate heat quickly to avoid blowing off the
tire,

I don't think they do dissipate heat very quickly. In normal use
it's OK for them just to soak it up because they have a high heat
capacity. If put heat into them continuously because you're keeping
them on all the way down a mountain, the tyre _does_ blow off.


As I said, they must or they would melt on heavy braking descents.
Weighing only a few grams, their heat capacity is less than that of an
aluminum 700c rim. I think this is a difference in semantics. They
get hotter but do not store more energy than a rim. As hot as they
must get, radiant cooling also plays a role beside the convective
cooling.

disk brakes can dissipate heat slowly and it doesn't matter.

Well there is still going to be a temperature at which they stop
working properly. Actually for bicycle disks you read stories of
the fluid boiling, a problem that has been pretty much eliminated in
cars with modern brake fluids.

Yes, but no one I have met can tell me anything about the limits. As
often occurs in the bicycle business, no significant testing is
performed and documented. Just talking to manufacturers at interbike,
gives me doubt about many of these brakes.

Take for instance among cars and trains. Only "Sport" cars have disks
with many transverse drilled holes... line most bicycle disks. I am
sure these are vestiges of the "gas bearing brake fade scenario" that
is entirely BS. The holes allow the gas emitted from friction
materials to escape instead of causing brake fade, as the faithful
believe.

Jobst Brandt

Chalo Colina wrote:

But I am hoping they will be sufficient to handle the max 2-4 mile
downhills we have around here that I don't brake on anyhoo.

You seem to be missing what's being said. IF you need to stop
quickly with carbon rims you cannot. There's no place for the heat
energy to go save to melt the rims or the pads since the carbon
cannot conduct the heat away effectively.

Carbon-epoxy composite is a wildcard with regard to its ability to
transmit and reject heat. The thermal conductivity of the fibers
themselves varies according to their quality and what kind of
original organic material was used to make them. Basically they
straddle the same two-orders-of-magnitude range of thermal
conductivity that structural metals do, from worse than the least
conductive stainless steel to better than silver (the most
conductive metal).

The epoxy resin matrix in which the fibers are encapsulated has poor
thermal conductivity similar to that of other familiar plastics, and
comprises a variable portion of a CFRP mixture. This resin can be
filled with other powdered materials to change its thermal
characteristics.

The bottom line is that between the huge conductivity variation
between fibers themselves and the huge variation between fiber-resin
mixtures, you can't make a valid generalization about how well CFRP
dissipates heat. I think it's fair to say that just about any
mixture would be harder on brake pads than aluminum, but the thermal
conductivity of a CFRP composite can vary by a factor of more than
100.

So why, as often, is nothing measured and documented on these
characteristics of rims? Why is the bicycle industry so lacking in
science? If there was good thermal conductivity, no special brake
pads would be needed as a start. Where do we go from here?

Jobst Brandt

On Jan 23, 9:54*am, wrote:

I'll assure you that it is more than 212°F because I generated steam
while braking into each hairpin turn on the Nufenen Pass (CH) that
blew with a loud hissss from the valve stem hole of a front wheel that
had taken on water in stream crossings.

Water boils at 196°F at the top of that pass (2478m
above sea level) and 205°F at the bottom (1159m).

Tom Ace

On Jan 23, 10:54 am, wrote:
Tim McNamara wrote:

It doesn't really show anything except that a rim brake may well
just work as a heat sink, and that if a disk brake also worked that
way, it would suck a lot more. My conclusion is that disk brakes
must work by dissipation if they work at all and are therefore a
better choice for long drags downhill.
Disk brakes can be heated much higher than a rim brake because there
is no tire attached. You could make a disk brake glow red from heat
if you wanted. Rim brakes need to dissipate heat quickly to avoid
blowing off the tire, disk brakes can dissipate heat slowly and it
doesn't matter.

I matters. At some temperature steady stat4 must be reached or the
disk would melt. Therefore, dissipation rate must be at least as good
as for rim brakes, although at a higher temperature. I have not heard
from anyone who, with a disk brake, descended a mountain pass like the
Stelvio, but I'm sure it has been done. The question is whether the
disk got visibly red hot. I am doubtful of the technology, because I
have not yet found any manufacturer at InterBike that could explain
why there are holes in the disks and why they are made as flimsy as
they ar... other than to save weight.


Radiative energy loss goes as area * T^4 where T is absolute
temperature (eg degrees Kelvin) of the disk or rim, assuming
the disk radiates roughly as a blackbody, while conductive
energy loss should go linearly, as area*(T-T_ambient). So if
a disk gets very hot, it is possible (I haven't calculated the actual
numbers) that the radiative loss dominates. This doesn't tell
you whether the rate of energy loss is greater for a disk
or rim at low T. At equal temperatures the rim should still
dissipate energy faster because it has more area.

In order to assert that a disk brake would be better for long
drags downhill one would need to know something about
the temperature at which the disk stops working as
effectively (probably has to do with glazing the pads?)
and actually calculate the energy losses from conduction
and radiation to see how hot you could expect the disk
to get.

Ben

On 2009-01-23, wrote:
Ben C? wrote:

I think it's likely for example that bicycle disk brakes
dissipate energy to the air much better than rim brakes.

I'm curious as to what the mechanism of this would be.

Because they get hotter which may more than make up for their
lower surface area.

Here are the calculations where I estimated the heat capacity of a
disk to be a bit less than half that of a rim:

http://groups.google.co.uk/group/uk....1d32532f671264

It doesn't really show anything except that a rim brake may well
just work as a heat sink, and that if a disk brake also worked
that way, it would suck a lot more. My conclusion is that disk
brakes must work by dissipation if they work at all and are
therefore a better choice for long drags downhill.

Disk brakes can be heated much higher than a rim brake because
there is no tire attached.

Yes exactly.

You could make a disk brake glow red from heat if you wanted. Rim
brakes need to dissipate heat quickly to avoid blowing off the
tire,

I don't think they do dissipate heat very quickly. In normal use
it's OK for them just to soak it up because they have a high heat
capacity. If put heat into them continuously because you're keeping
them on all the way down a mountain, the tyre _does_ blow off.


As I said, they must or they would melt on heavy braking descents.
Weighing only a few grams, their heat capacity is less than that of an
aluminum 700c rim.

I was talking about rims. My theory is that disks must dissipate because
their heat capacity is so low, but that in normal use rims don't need to
dissipate because their heat capacity is high enough just to soak up the
energy.

I think this is a difference in semantics. They get hotter but do not
store more energy than a rim.

They may well store less.

As hot as they must get, radiant cooling also plays a role beside the
convective cooling.

Yes.

disk brakes can dissipate heat slowly and it doesn't matter.

Well there is still going to be a temperature at which they stop
working properly. Actually for bicycle disks you read stories of
the fluid boiling, a problem that has been pretty much eliminated in
cars with modern brake fluids.

Yes, but no one I have met can tell me anything about the limits. As
often occurs in the bicycle business, no significant testing is
performed and documented.

Well I've heard various reasons why they don't just use car fluid or
something like it: it would damage the seals they use (but why not use
different seals?); and it strips the paint if you spill it on your pride
and joy-- but that applies equally to cars.

"Ben C" wrote in message
...

Well there is still going to be a temperature at which they stop
working properly. Actually for bicycle disks you read stories of
the fluid boiling, a problem that has been pretty much eliminated in
cars with modern brake fluids.

I think you may be attributing rather too much to "modern brake fluids".

Well I've heard various reasons why they don't just use car fluid or
something like it: it would damage the seals they use (but why not use
different seals?); and it strips the paint if you spill it on your pride
and joy-- but that applies equally to cars.

Actually bikes use similar brake fluid to cars and motorbikes. Either
conventional DOT fluid (which is the yeuchy stuff), or mineral oil - but the
latter is also used in some cars (hint - they have a lever to make them go
up and down, and are known for having good brakes :-) ).

Mineral oil isn't hygroscopic, which I regard as a positive point - it means
it doesn't go off when exposed to the atmosphere. For that reason alone I'd
prefer it if my bike brakes used it. And as you should know, boiling fluid
in car brakes isn't the fluid, it's water in the fluid, typically absorbed
from the atmosphere.

So it seems there's no advantage to be gained from using DOT-style fluids.

Jobst Brandt wrote:

Chalo Colina wrote:

The bottom line is that between the huge conductivity variation
between fibers themselves and the huge variation between fiber-resin
mixtures, you can't make a valid generalization about how well CFRP
dissipates heat. *I think it's fair to say that just about any
mixture would be harder on brake pads than aluminum, but the thermal
conductivity of a CFRP composite can vary by a factor of more than
100.

So why, as often, is nothing measured and documented on these
characteristics of rims? *Why is the bicycle industry so lacking in
science? *

I think the bicycle industry is traditionally lacking in science,
depending instead on a long history of trial and error resulting in a
kind of genetic fitness for purpose. I'm not sure that is still the
case for the sorts of very expensive exotic parts we're discussing
here. It is true, though, that the manufacturers of such parts are
not generally forthcoming with properties data or test results.

If the manufacturer of expensive CFRP rims (just as an example) had
lots of good and informative data about their rims that they could
share with the public, there are a few reasons they might not want to
do so. They might fear that differences from common rims, where they
are not to the advantage of their product, could expose them to extra
liability claims. They might fear that such data would give a
marketing advantage or even technical assistance to their
competitors. And they might not want their more astute buyers looking
at the prospects for real improvement over what they already have and
doing cost/benefit analysis.

Nonspecific promises of technical benefits, if believed, do the
manufacturer more good than hard data, because the significance of the
benefits can become inflated in prospective buyers' minds. We see
this effect every single time someone makes a part out of CFRP that
was not previously made of that material-- at the outset, magical
properties and unrealistic advantages are ascribed to the new
gadget.

There is also a likelihood that scientific principles have been
applied, and some testing has occurred, but that the testing methods
are not rigorous. Rigorous testing costs time and money. If the
development budget or schedule doesn't support good conclusive and
presentable testing, then a small manufacturer will do just what he
can to make himself confident that the product is good. At that
point, you don't have data, you have "we put the thing between 2x4s in
the hydraulic press and it didn't break until we mashed it really
hard." In other words, you can be confident in the product without
having hard numbers or even without wanting to share the details of
why you are confident in it.

If there was good thermal conductivity, no special brake
pads would be needed as a start. *

That's not necessarily true. Even if a CFRP rim had spectacularly
good thermal conductivity equivalent to aluminum, you might still need
to use a pad specially formulated to be non-abrasive to the particular
materials in the rim's surface.

Chalo

Ben C? wrote:

I think it's likely for example that bicycle disk brakes
dissipate energy to the air much better than rim brakes.

I'm curious as to what the mechanism of this would be.

Because they get hotter which may more than make up for their
lower surface area.

Here are the calculations where I estimated the heat capacity of
a disk to be a bit less than half that of a rim:

http://groups.google.co.uk/group/uk....1d32532f671264

It doesn't really show anything except that a rim brake may well
just work as a heat sink, and that if a disk brake also worked
that way, it would suck a lot more. My conclusion is that disk
brakes must work by dissipation if they work at all and are
therefore a better choice for long drags downhill.

Disk brakes can be heated much higher than a rim brake because
there is no tire attached.

Yes exactly.

You could make a disk brake glow red from heat if you wanted.
Rim brakes need to dissipate heat quickly to avoid blowing off
the tire,

I don't think they do dissipate heat very quickly. In normal use
it's OK for them just to soak it up because they have a high heat
capacity. If put heat into them continuously because you're
keeping them on all the way down a mountain, the tyre _does_ blow
off.

As I said, they must or they would melt on heavy braking descents.
Weighing only a few grams, their heat capacity is less than that of
an aluminum 700c rim.

I was talking about rims. My theory is that disks must dissipate
because their heat capacity is so low, but that in normal use rims
don't need to dissipate because their heat capacity is high enough
just to soak up the energy.

My experience is that they cool rapidly and that switching speed
control braking between front and rear appears to keep maximum
temperatures loser than using both brakes equally. I find such
decisions difficult to evaluate without instrumentation.

I think this is a difference in semantics. They get hotter but do
not store more energy than a rim.

They may well store less.

As hot as they must get, radiant cooling also plays a role beside
the convective cooling.

Yes.

disk brakes can dissipate heat slowly and it doesn't matter.

Well there is still going to be a temperature at which they stop
working properly. Actually for bicycle disks you read stories of
the fluid boiling, a problem that has been pretty much eliminated
in cars with modern brake fluids.

Yes, but no one I have met can tell me anything about the limits.
As often occurs in the bicycle business, no significant testing is
performed and documented.

Well I've heard various reasons why they don't just use car fluid or
something like it: it would damage the seals they use (but why not
use different seals?); and it strips the paint if you spill it on
your pride and joy-- but that applies equally to cars.

Prima donnas.

Jobst Brandt