T
Tim McNamara
Guest
From http://www.velonews.com/tour2004/tech/articles/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.
>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?
>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.
>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?
>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!
>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.
>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?
>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.
>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?
>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!
