With all this talk about cfrp....

Ben C? wrote:

In friction pairs, heat is generated in the softer medium and
usually dissipated in a harder thermally conductive part. No
reasonably flexible brake pad material can dissipate brake heat,
most brake material for unassisted manual brakes being relatively
soft insulators. Therefore, their surface quickly melt and rub
off.

That is (one) of the reason you have special brake pads for carbon
rims. Don't ever use pads meant for alu rims on carbon rims.
They melt. When you switch between alu rim wheels and carbon rim
wheels often you have to switch the brake pads too. That's really
annoying. Like I said braking with carbon rims suck big time.
After a couple of real descents in the Alps I didn't trust the
braking anymore and always switched to a alu rimmed front wheel
after that until I sold those wheels. Except for time trial
wheels carbon rims are a bad idea IMHO.

Well, special brake pads seems not to resolve the thermodynamic
issue. The heat must go somewhere to be dissipated to the
atmosphere or there is no braking.

If the brake pads got much hotter they would dissipate faster to the
surrounding air because you'd have a greater temperature difference.

I Think you are ignoring their tiny cooling surface surface and that
the pad is an insulator so that it gets hot only on the friction
surface that is covered by an equally good insulator. I suggest you
touch your brake pads on the back side after a hard braking to see if
you can detect any warmth, and I mean nay bicycle brake.

I don't know how much hotter and whether that would make up for
their small area. One would have to do some math...

Math won't help if you can't describe the thermal model well enough to
recognize it's failings. So why do you throw out a spurious defense
of such a system. What's in it for you to cast doubt about this
technically obvious problem?

I think it's likely for example that bicycle disk brakes dissipate
energy to the air much better than rim brakes. A rim brake has a
higher surface area, but doesn't get hot enough (or if it does the
tyre blows off).

That is not a parallel example. The disk has about 50 times more
surface than a brake block that doesn't even get warm except for its
friction surface that remains shielded from the atmosphere.

Jobst Brandt

In article ,
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.

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.

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.

My assumption about carbon fiber braking surfaces has been that- like
wood, which is another composite material- the pads got hot and melted,
vaporized, etc. I hadn't thought about whether than would waste enough
heat, since braking on wooden rims does work.

Ryan Cousineau wrote:

Chalo wrote:

Coaster brakes, disc brakes, and
band brakes are all able to circumvent the shortcomings of rim braking
on plastic wheels.

Hm. I wasn't otherwise occupied during the Mag-wheel BMX era. I assume
they used the conventional caliper brakes on the mag wheels, did they
work with any special pad compounds?

There were special brake pads for nylon rims. I can't attest to
whether they worked better for braking than normal brake pads. I
think they were mostly designed not to leave dark smears on colored
nylon.

These, like all things consumable and old-skool BMX, are now usually
priced out of the experimentation regime. Plus there's no telling
what effect the years have had on them.

Chalo

On Jan 22, 4:43*pm, wrote:

Just the same, the carbon faithful in this forum insist they work well
in spite of no evidence that it is possible.

Wow, that looks like a whole lot of fun! Nice pics.

wrote:
I haven't seen anyone do heavy braking with carbon rims so I must ask,
where does the kinetic energy go, the energy that heats metal rims
enough to melt tubular tire glue or blow clinchers off the rim?

I don't ride carbon rims myself, but based on the testimony of others
it sounds like the pads themselves change phase if there is heavy
braking on a long descent... which is why they don't last long.

Tom Kunich wrote:
"Ben C" wrote in message
...
On 2009-01-22,
wrote:

If the brake pads got much hotter they would dissipate faster to the
surrounding air because you'd have a greater temperature difference.

I don't know how much hotter and whether that would make up for their
small area. One would have to do some math...

Perhaps you could think about this? Aluminum rims can get so hot on a
downhill that they'll burn your fingers, melt sewup glue, explode
innertubes and I've even seen them melt the tire around the rim wire and
blow the tire off.

While indeed heat dissipates faster from a hotter surface than a cooler
one, there is a gigantic difference in surface area that the heat
difference simply can't address.

I think it's likely for example that bicycle disk brakes dissipate
energy to the air much better than rim brakes. A rim brake has a higher
surface area, but doesn't get hot enough (or if it does the tyre blows
off).

The problem is more likely that the rim is pretty well protected from
the wind over a large portion of its diameter due to the tire.

I'd say the problem is aerodynamics. If you want the wheel to be
slippery through the air, it won't be able to vent off much heat. If you
extruded an aluminum rim with cooling fins, it wouldn't be aero.

Carl Sundquist wrote:

If the brake pads got much hotter they would dissipate faster to
the surrounding air because you'd have a greater temperature
difference.

I don't know how much hotter and whether that would make up for
their small area. One would have to do some math...

Perhaps you could think about this? Aluminum rims can get so hot on
a downhill that they'll burn your fingers, melt sew-up glue,
explode inner tubes and I've even seen them melt the tire around
the rim wire and blow the tire off.

While indeed heat dissipates faster from a hotter surface than a
cooler one, there is a gigantic difference in surface area that the
heat difference simply can't address.

I think it's likely for example that bicycle disk brakes dissipate
energy to the air much better than rim brakes. A rim brake has a
higher surface area, but doesn't get hot enough (or if it does the
tyre blows off).

The problem is more likely that the rim is pretty well protected
from the wind over a large portion of its diameter due to the tire.

I'd say the problem is aerodynamics. If you want the wheel to be
slippery through the air, it won't be able to vent off much heat.
If you extruded an aluminum rim with cooling fins, it wouldn't be
aero.

Aero dynamic losses are caused by the eddies from non streamlined
shapes. That means there is little laminar flow over the surface,
something that diminishes cooling rather than increasing it. Don't
confuse this with supersonic turbulence that heats airfoils.

I don't see any merit to that concept. Surface heat exchangers on
aircraft rely on laminar flow for best results. The drag on a non
streamlined rim is insignificant anyway, unless riding at TT speeds on
flat courses.

Jobst Brandt

On 2009-01-23, Tim McNamara wrote:
In article ,
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.

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.

In article ,
Ben C wrote:

On 2009-01-23, Tim McNamara wrote:
In article ,
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. 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.

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.

On 2009-01-23, Tim McNamara wrote:
In article ,
Ben C wrote:

On 2009-01-23, Tim McNamara wrote:
In article ,
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.

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.