Is Lennard Zinn wrogn carbon rims?

On 2004-07-22 16:55:22 -0700, Tim McNamara said:

From http://www.velonews.com/tour2004/tec...es/6642.0.html

Carbon wheel issues

Carbon wheels are a case in point. Almost all of the riders are using
carbon wheels on mountain stages now because of the weight savings,
but braking continues to be a problem with them. Simply getting
carbon-specific pads is insufficient, because carbon is an insulator
and seeks to retain heat, while aluminum is a conductor and seeks to
dissipate it. This can cause all sorts of problems.

As I understand it, this is wrong. An insulator does not conduct
energy, which is different from "seeking to retain it." A designer
might use an insulator to retain heat by surrounding the hot stuff
with the insulator, but that works because the insulator simply
doesn't conduct heat well.

Fair enough. Carbon rims don't "conduct heat well." Heat is
generated, and as such if it's not carried away it gets concentrated
"somewhere."

Most Tour riders are using the red Corima cork pads (see photo) on
their carbon rims, although U.S. Postal is using a special pad that
Keith Bontrager came up with. It is of the utmost importance to have
a good pad. I have descended our steep, five-mile Flagstaff Mountain
west of Boulder many times with various carbon wheels special carbon
pads, as well as standard pads. All of them, save for the ADA cork
pads (I have not used the Corima cork pads, but I assume they are
similar), wore down a huge amount with each descent. Besides the
visible reduction of the pad thickness, the pads built up a lot of
melted pad material ahead on the leading end

It seems to me that melting brake pads are not the result of the rim
getting hot from absorbing heat, but of the pad getting hot because
the rims is *not* absorbing heat. Is cork used because it has no
melting point, not being a rubber based material, and perhaps a high
vaporization point?

Cork doesn't melt the same way that rubber does. I can tell you from
personal experience that "standard" rubber brake pads don't work well
with carbon pads. In fact I was riding carbon rims on a descent, and
melted the rear pad. I was going about 40 through a corner, and was
drag braking (just barely -- almost no pressure on the rear brake).
Rear wheel locked up, and I lowsided.

I had actually locked up the rear wheel 3 or 4 times previously to
this... I wasn't sure what was going on, I though perhaps someone had
put bleach on my Corsa CX's -- they were that prone to lockup.

http://www.altovelo.org/rr01/results/




Braking on carbon rims tends to be grabby anyway, and the rim heating
accentuates it.

Are carbon rims grabby because they are getting hot or because of
coefficient of friction between rim and pad being too high? Or
because the brake pads are getting hot, as they retain the heat and
have no way to shed it rapidly enough, and the surface of the pad is
melting and sticking to the rim? Everything I've read about this thus
far in r.b.t suggested to me that carbon rims do not heat up, do not
conduct heat away from the brake pads, and thus the pads get hot and
melt or vaporize material. Hence the pads wear out in short order.

Because the pads are melting. Intially they are not that "grabby" but
after 3 or 4 seconds of braking, the friction coefficient goes up
significantly. It's decidedly non-linear, and that's "a bad thing."

When there is a lot of heat with tubular wheels, there is always the
possibility of rolling tires. Even with aluminum rims, the glue gets
soft on a hot descent, and the tires visibly slip around the rim,
causing the valve stem to be angled sharply. We used to flip our
front wheel around on subsequent mountain stages so that the valve
stem would straighten back out. But the heat with a carbon can be far
more extreme. I think Joseba Beloki could have avoided his crash last
year had he not been using carbon rims. The hot rims grabbed when he
braked hard, locking his rear wheel. When the wheel came back down,
the hot glue allowed the tire to roll right off of the rim.

Again, Zinn seems to be stating that this insulator, the carbon fiber
rim, is absorbing and storing heat from braking. If the rim is hot
enough to melt brake pads, how could any glue hold on any carbon rim
when descending? The melting point of glue must be far below the
melting point of brake pads! If this is the case, then carbon rims
must be totally unsuited to this application. Or is it that the pads
get too hot and stick to the rim, since the latter does not conduct
the heat out of the pads, causing grabbing?

Agreed here -- the pads are going to melt and get grabby FAR before the
tubular glue will melt. However, if he was using cork pads, they
probably wouldn't have melted. It would be a pretty colossal gaffe for
a mechanic to *not* use carbon-compatible pads these days... it
wouldn't take long for it to become evident it was dangerous.

I've never actually used cork pads on CF rims (I sold the rims soon
after my crash), so I don't know how hot things would get on a long
descent with cork pads. Tons and tons of heat would be generated and
it wouldn't be conducted away very well, so it seems the braking
surface and pads would have to get awfully hot.


This year, Thomas Voeckler switched bikes a number of times on
mountain stages while wearing the yellow jersey. His director
sportif, Jean Rene Bernadeau (who won epic mountain stages a rider
himself - remember his win in Sondrio in the Giro stage over the
Passo Stelvio with Bernard Hinault in the early 1980s?), said it was
due to fear of rolling a tire, since Voeckler's carbon rims were
getting so hot.

From everything I've read about this in rec.bikes.tech, I think that
either Bernardeau and Zinn really don't understand what's going on
here- or I have totally misunderstood everything I've ever read on the
subject!

Bill Lloyd writes:

I've never actually used cork pads on CF rims (I sold the rims soon
after my crash), so I don't know how hot things would get on a long
descent with cork pads. Tons and tons of heat would be generated
and it wouldn't be conducted away very well, so it seems the braking
surface and pads would have to get awfully hot.

Since CF is an insulator, wouldn't it just be the pads that are
getting hot, and not the braking surface of the rim? This would mean
that all the heat from the friction of braking is retained in the pad-
which is pretty small- rather than being conducted away from the pad
as by an aluminum rim. Hence the pad melts or vaporizes. ISTR that
brake pad material is also a poor conductor of heat, so it may be that
it is just the part of the pad in contact with the rim that gets hot
and melts or vaporizes taking the heat with it.

Cork is also not much of a conductor of heat, but as it's made of
cellulose could it perhaps burn? (Now *there's* a mental image).

Bill Lloyd writes:

I've never actually used cork pads on CF rims (I sold the rims soon
after my crash), so I don't know how hot things would get on a long
descent with cork pads. Tons and tons of heat would be generated
and it wouldn't be conducted away very well, so it seems the braking
surface and pads would have to get awfully hot.

Since CF is an insulator, wouldn't it just be the pads that are
getting hot, and not the braking surface of the rim? This would mean
that all the heat from the friction of braking is retained in the pad-
which is pretty small- rather than being conducted away from the pad
as by an aluminum rim. Hence the pad melts or vaporizes. ISTR that
brake pad material is also a poor conductor of heat, so it may be that
it is just the part of the pad in contact with the rim that gets hot
and melts or vaporizes taking the heat with it.

Cork is also not much of a conductor of heat, but as it's made of
cellulose could it perhaps burn? (Now *there's* a mental image).

On 2004-07-23 20:30:05 -0700, Tim McNamara said:

Bill Lloyd writes:

I've never actually used cork pads on CF rims (I sold the rims soon
after my crash), so I don't know how hot things would get on a long
descent with cork pads. Tons and tons of heat would be generated
and it wouldn't be conducted away very well, so it seems the braking
surface and pads would have to get awfully hot.

Since CF is an insulator, wouldn't it just be the pads that are
getting hot, and not the braking surface of the rim? This would mean
that all the heat from the friction of braking is retained in the pad-
which is pretty small- rather than being conducted away from the pad
as by an aluminum rim. Hence the pad melts or vaporizes. ISTR that
brake pad material is also a poor conductor of heat, so it may be that
it is just the part of the pad in contact with the rim that gets hot
and melts or vaporizes taking the heat with it.

I'd think the braking surface itself could get hot -- I mean it is in
direct contact with the pad. So the fact that it's an insulator would
mean the heat wasn't being conducted to other places on the rim, other
than directly on the braking surface.

Can't state empirically, though -- haven't ridden CF rims in 3 years
and I didn't touch them after descents.

On 2004-07-23 20:30:05 -0700, Tim McNamara said:

Bill Lloyd writes:

I've never actually used cork pads on CF rims (I sold the rims soon
after my crash), so I don't know how hot things would get on a long
descent with cork pads. Tons and tons of heat would be generated
and it wouldn't be conducted away very well, so it seems the braking
surface and pads would have to get awfully hot.

Since CF is an insulator, wouldn't it just be the pads that are
getting hot, and not the braking surface of the rim? This would mean
that all the heat from the friction of braking is retained in the pad-
which is pretty small- rather than being conducted away from the pad
as by an aluminum rim. Hence the pad melts or vaporizes. ISTR that
brake pad material is also a poor conductor of heat, so it may be that
it is just the part of the pad in contact with the rim that gets hot
and melts or vaporizes taking the heat with it.

I'd think the braking surface itself could get hot -- I mean it is in
direct contact with the pad. So the fact that it's an insulator would
mean the heat wasn't being conducted to other places on the rim, other
than directly on the braking surface.

Can't state empirically, though -- haven't ridden CF rims in 3 years
and I didn't touch them after descents.