With all this talk about cfrp....

Ron Ruff wrote:

I was surprised how little wear my carbon rims showed in the two
years I used them. Alu rims wear faster, but the carbon brake pads
wore much faster and they are expensive.

Yep... it is the pads that wear out. They recommend Kool Stops for
these rims, which aren't too expensive.

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? My
experience with such things was with wooden rims that dissipated no
heat while burning off brake pads fast, similar to the problem with
ceramic coated rims.

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.

Friction (tribology) remains a mystery to most people, even those who
work with it often. Methods to deal with it are found but how it
works remains obscure.

http://www.sheldonbrown.com/brandt/brakes.html

Jobst Brandt

schreef:
Ron Ruff wrote:

I was surprised how little wear my carbon rims showed in the two
years I used them. Alu rims wear faster, but the carbon brake pads
wore much faster and they are expensive.

Yep... it is the pads that wear out. They recommend Kool Stops for
these rims, which aren't too expensive.

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? My
experience with such things was with wooden rims that dissipated no
heat while burning off brake pads fast, similar to the problem with
ceramic coated rims.

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 brakepads too. That's really annoying. Like I said braking
with carbon rims suck big time. After a couple of real descents in the
Alpes I didn't trust the braking annymore 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.

Lou

Lou Holtman wrote:

I was surprised how little wear my carbon rims showed in the two
years I used them. Alu rims wear faster, but the carbon brake
pads wore much faster and they are expensive.

Yep... it is the pads that wear out. They recommend Kool Stops for
these rims, which aren't too expensive.

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? My experience with such things was with wooden rims that
dissipated no heat while burning off brake pads fast, similar to
the problem with ceramic coated rims.

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. Nearly all the work that goes into climbing a hill
must go into brake heat on the descent unless it is straight enough to
blow it away in wind drag if not too steep. I think of Sonora pass or
best of all, the stelvio in the Alps. Two possibilities are that the
brakes don't work much and therefore don't generate much heat or the
brake pads burn off as they ablate.

http://tinyurl.com/agnvh
http://tinyurl.com/9ah2r

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

Jobst Brandt

On Jan 22, 3:43*pm, wrote:
Lou Holtman wrote:
I was surprised how little wear my carbon rims showed in the two
years I used them. Alu rims wear faster, but the carbon brake
pads wore much faster and they are expensive.
Yep... it is the pads that wear out. They recommend Kool Stops for
these rims, which aren't too expensive.
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? *My experience with such things was with wooden rims that
dissipated no heat while burning off brake pads fast, similar to
the problem with ceramic coated rims.
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. *Nearly all the work that goes into climbing a hill
must go into brake heat on the descent unless it is straight enough to
blow it away in wind drag if not too steep. *I think of Sonora pass or
best of all, the stelvio in the Alps. *Two possibilities are that the
brakes don't work much and therefore don't generate much heat or the
brake pads burn off as they ablate.

*http://tinyurl.com/agnvh
*http://tinyurl.com/9ah2r

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

Jobst Brandt

Well, Jobst, I must say, empiric evidence will ALWAYS top theorizing
based on principles. Just because in your experience and as you have
heard from others that they don't work well doesn't mean that mine
necessarily won't. You remind me of the engineers who continually
tell me that it worked on their system. Hah! If it fails in the
production build it fails. And vice versa.

Plus I already paid for the rims If you carbon-rim-braking-
doubters are right, and my combination of brake pad/caliper/lever and
rim "stops" me like Huffy brakes, I will certainly report it. 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.

Empirically yours,

D'ohBoy (aka Appkiller)

"D'ohBoy" wrote in message
...
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.

On Jan 22, 2:10*pm, "Tom Kunich" cyclintom@yahoo. com wrote:
"D'ohBoy" wrote in message

...

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.

I thought the point was not about stopping once quickly, which doesn't
require a lot of heat capacity, but about controlling speed on long
descents.

-pm

schreef:
Lou Holtman wrote:


http://tinyurl.com/agnvh
http://tinyurl.com/9ah2r

;-) sweet memories. Climbed both sides on one day last August. Filmed my
descent to Bormio with a helmet camera. Wow....

Lou

"pm" wrote in message
...
On Jan 22, 2:10 pm, "Tom Kunich" cyclintom@yahoo. com wrote:
"D'ohBoy" wrote in message

...

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.

I thought the point was not about stopping once quickly, which doesn't
require a lot of heat capacity, but about controlling speed on long
descents.

Both of those scenarios are dangerous with carbon rims. In a race you can
use them where they're practical but if you ride one bike around and have
carbon rims on it simply because it looks cool and "racy" then you can get
yourself in a lot of trouble quite rapidly.

Mind you, I'm sure there are people who don't have any problems because they
don't ride them in conditions that require a great deal of braking power.

On 2009-01-22, wrote:
Lou Holtman 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 don't know how much hotter and whether that would make up for their
small area. One would have to do some math...

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).

"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.