Re: Exploding tires II

Quoted message said:

Tom Sherman said:

Quoted message said:

... That rim heating is significant has been experienced by tandem
riders who ride in mountains as well as singles on steep roads such
as those in Austria, typically Zirlerberg with winding 18% grade
and where bicycling are absolutely prohibited downhill, and for
good reason. Even for cares and trucks, several steep run-away
tracks exit from curves in the event of brake failure.

http://tinyurl.com/jhiu

Quoted message said:

Is this actually a bad Polish joke?

What do you find odd about this. There are runaway tracks on US
railroads and highways. The Grapevine in California was equipped
with these before it became Interstate-5 and in this web site, take a
look at runaway lanes in America:

http://modena.intergate.ca/personal/pl8s/BC5/Hwy_5N_C.htm

Yes, they're quite common. I presume Tom was referring to the
story on the page you first referenced. It refers to a Polish family
that made the unfortunate choice to have their picnic at the site of the
emergency runaway lane. While they were there a truck with a 40 ton
load lost its brakes and needed to use the lane.

Quoted message said:

Quoted message said:

Quoted message said:

What would colored water do?

Make coloured steam.

Too bad it isnt that easy.

But you get the idea. Add something to the water so the steam is colored.
Then you can pretend you're the Great Waldo Pepper on a bike.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

Quoted message said:

Frank Krygowski said:

I wonder about the change in the interface between the tire and the
rim when the dimensions change with temperature. The steel bead
wire (assuming that's what you've got) would change the least, by
virtue of lowest coefficient and being most insulated. The rim
would grow a bit (tightening the fit, I suppose), but the rubber
would be trying to grow significantly. Is there a chance this
(combined with, say, softening at higher temps) would cause
distortion of the bead shape, and cause the bead to lose its grip on
the rim?

It's the clinch, not the wire. Wire bead tires blow off the rim
equally well with Kevlar ones. Believe me, I've tried it and observed
it. Stop raising new specters behind every turn. Brake heating
causes blow-offs. Accept it!

What is this all about anyway. What are you trying to generate?

Jobst, it's a discussion. I'm just curious about the details of the
phenomenon! Saying "Accept it!" about tires blowing off the rim this is
like saying "Accept it, people die of cholera!"

If the discussion sheds light to enable someone to design a
blow-off-proof tire, that would be wonderful. If the discussion merely
teaches us something about the physics of the situation, that would at
least be beneficial.

As I see it, this is at least as valuable as harping about whether the
word "strech" should be used to describe elongation of chain pitch.

Sheesh!

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]

Quoted message said:

Quoted message said:

Are you suspecting that the air temp in the tube actually exceeds
100C? In the 0-100C example, the pressure differential didn't
appear to be enough to blow a (properly mounted) tire off of a rim
(from 100psi to 136psi).

That depends on how long the tube is exposed to how high a
temperature. What is proven is that you can blow a tire off the rim
with brake heating.

That it happens isn't questionable. Why it happens is. A tire ought to be
able to handle a pretty significant amount of pressure over what it's rated
for. A typical tire will easily handle 150psi before blowing off a rim
(easily proven in the shop). But I'm not seeing the physics (yet) that show
a high-enough temperature differential to accomplish this. That's why I'm
thinking there's more to it than just an increase in pressure.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

<[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Mike Jacoubowsky said:

Quoted message said:

Actually things are worse than you think because rims get much
hotter than suspected. My first introduction to how high rims get
was with steam generated from water in the rim on a mild descent
with hairpin turns at the end of several straight runs. The
experience also showed that on exiting the turn, steam stopped
escaping which revealed how fast air cooling reduces rim
temperatures.

Quoted message said:

So perhaps for a cheap thrill or two, a cyclist could put a small
amount of colored water (to make the steam easier to observe) into
their rim prior to a descent?

What would colored water do?

Quoted message said:

Are you suspecting that the air temp in the tube actually exceeds
100C? In the 0-100C example, the pressure differential didn't
appear to be enough to blow a (properly mounted) tire off of a rim
(from 100psi to 136psi).

That depends on how long the tube is exposed to how high a
temperature. What is proven is that you can blow a tire off the rim
with brake heating.

Quoted message said:

I have seen a far greater number of tubes exhibiting snake-bite-type
damage (as you'd see on a compression cut) on "blowouts" on steep
descents, making me wonder if the material properties of the rubber
itself change (for the worse) as temperatures rise.

It makes no difference, the tire blows off the rim. That is why you
hear a loud bang that produces a long slash in the tube.

http://draco.acs.uci.edu/rbfaq/FAQ/8b.4.html

Jobst Brandt
[email hidden]

Quoted message said:

On the other hand, I wasn't aware that there was any doubt that rim
heating cause blow-offs, especially among bicycle shop operators.

There is no question that rim heating causes tube/tire failure. There is a
question as to the mechanism that causes this. Previously, before the
physics was made clear, I had assumed it was simply high pressure... but
since that same amount of pressure (according to the physics lessons
presented in this thread, which nobody has seriously questioned yet) doesn't
accomplish this in the shop, that causes me to question the assumption.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

<[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

dianne_1234567890? said:

Quoted message said:

I have seen a far greater number of tubes exhibiting
snake-bite-type damage (as you'd see on a compression cut) on
"blowouts" on steep descents, making me wonder if the material
properties of the rubber itself change (for the worse) as
temperatures rise.

Quoted message said:

I wonder if the tube gets the snakebite cut after the blowout,
before the rider comes to a stop?

That's a red herring! If the tire got a flat first, then what made it
blow off the rim and go BANG! This makes no sense.

On the other hand, I wasn't aware that there was any doubt that rim
heating cause blow-offs, especially among bicycle shop operators.

Jobst Brandt
[email hidden]

Quoted message said:

What do you find odd about this. There are runaway tracks on US
railroads and highways. The Grapevine in California was equipped
with these before it became Interstate-5 and in this web site, take a
look at runaway lanes in America:

http://modena.intergate.ca/personal/pl8s/BC5/Hwy_5N_C.htm

There are presently at least two runaway truck ramps on I5 northbound (at
least there were Sunday night when I rode past), as well as a number of them
on westbound I80 in the Sierras. Just out of curiosity, I always look for
evidence of recent use when I pass them, but I suspect they "recondition"
them after each incident (smooth out the gravel).

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

Mike Jacoubowsky said:

There are presently at least two runaway truck ramps on I5 northbound (at
least there were Sunday night when I rode past), as well as a number of
them on westbound I80 in the Sierras. Just out of curiosity, I always
look for evidence of recent use when I pass them, but I suspect they
"recondition" them after each incident (smooth out the gravel).

While every situation is different, every runaway truck ramp does get used.
For some, it may be a couple times a year, for others it may be a couple
years between uses, but they justify their existence.

Austin

Quoted message said:

OK, nothing lost if you put a few heat indicators on your rear rim,
find a steep road (steeper than 12%) in your area and with a tire
inflated over 100psi, roll down the hill at between 5 and 8 mph with
only the rear brake applied. When the tire blows off, use the front
brake to stop and check the highest temperature reached. This will
cost you no more than one tube, preferably one that had a few patches.
There is no hazard here if you don't have any tight turns.

You want a real hazard? Try riding an medium-gear fixed wheel bicycle
down a long 16% hill with just a front brake. You're on the verge of
spinning out at 150rpm (or 200rpm if you're a track ace), the brake
smells like burning Bakelite, and then you remember you pumped up the
front tyre until it was rock hard. Do you:

[ ] release the brake and let the bike throw you off
[ ] pray the front tyre doesn't explode
[ ] aim for the hedge?

I was there a couple of weeks ago. I chose option 2 and the tyre held,
but on a real mountain I'd have been toast.

Mike Jacoubowsky said:

Quoted message said:

Quoted message said:

Are you suspecting that the air temp in the tube actually exceeds
100C? In the 0-100C example, the pressure differential didn't
appear to be enough to blow a (properly mounted) tire off of a rim
(from 100psi to 136psi).

That depends on how long the tube is exposed to how high a
temperature. What is proven is that you can blow a tire off the rim
with brake heating.

That it happens isn't questionable. Why it happens is. A tire ought to be
able to handle a pretty significant amount of pressure over what it's rated
for. A typical tire will easily handle 150psi before blowing off a rim
(easily proven in the shop). But I'm not seeing the physics (yet) that show
a high-enough temperature differential to accomplish this. That's why I'm
thinking there's more to it than just an increase in pressure.

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

Mike Jacoubowsky said:

Quoted message said:

Quoted message said:

> What would colored water do?

Make coloured steam.

Too bad it isnt that easy.

But you get the idea. Add something to the water so the steam is colored.
Then you can pretend you're the Great Waldo Pepper on a bike.

--Mike-- Chain Reaction Bicycles
www.ChainReactionBicycles.com

Dear Mike,

No matter what color the water is, the steam is just the
water vapor. There's nothing else in it, so it's always the
same color.

Boiling doesn't steam off dirt, salt, sugar, or food
coloring. What hisses out of the tea kettle is just the
stuff with the lowest boiling point. Left behind is a more
and more concentrated solution.

Skywriting airplanes trail smoke.

Carl Fogel

Jim Smith said:

Mike Jacoubowsky said:

Quoted message said:

> Are you suspecting that the air temp in the tube actually exceeds
> 100C? In the 0-100C example, the pressure differential didn't
> appear to be enough to blow a (properly mounted) tire off of a rim
> (from 100psi to 136psi).

That depends on how long the tube is exposed to how high a
temperature. What is proven is that you can blow a tire off the rim
with brake heating.

That it happens isn't questionable. Why it happens is. A tire ought to be
able to handle a pretty significant amount of pressure over what it's rated
for. A typical tire will easily handle 150psi before blowing off a rim
(easily proven in the shop). But I'm not seeing the physics (yet) that show
a high-enough temperature differential to accomplish this. That's why I'm
thinking there's more to it than just an increase in pressure.

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

Dear Jim,

Maybe a little more than a few hundred square cm? More like
a thousand?

My touring bike's front rim is about 25 inches in diameter.
It's about 2 inches around the curve from tire edge to tire
edge.

pi * 25 * 2 = 157 square inches = 1013 square cm

That's the outside of the rim. The inside surface area is
about the same, but it's insulated by the rubber tube and
the trapped air, which heats up.

I also wonder whether the spokes act as radiators. They have
lots of surface area for their mass, but they also have such
a tiny connection to the hot rim that I'm guessing that they
don't dissipate much heat. (My guesses often entertain
engineers.)

Why would you use only one brake on a steep descent?

Still, I agree with your conclusion that it could get pretty
hot. I think that what's interesting the other posters is
whether all that heat expands a rim enough to contribute to
its tire coming off.

Carl Fogel

Jim Smith said:

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

So, two possible solutions:

1) Hollow spokes delivering cooling water on demand to the inner
surface of the rim. Reservoir under the saddle, connected to a suitable
fitting at the hub. The optimum rim design would be like the old
Weinmann Concave rims mentioned in another thread, so the water would be
kept at the inner surface and away from the braking surfaces. (A Mr.
Goldberg may already have a patent on this ides, of course.)

2) The water-filled inner tubes from yet another thread.

--
Frank Krygowski [To reply, remove rodent and vegetable dot com.
Substitute cc dot ysu dot
edu]

Quoted message said:

Jim Smith said:

Mike Jacoubowsky said:

>> Are you suspecting that the air temp in the tube actually exceeds
>> 100C? In the 0-100C example, the pressure differential didn't
>> appear to be enough to blow a (properly mounted) tire off of a rim
>> (from 100psi to 136psi).
>
> That depends on how long the tube is exposed to how high a
> temperature. What is proven is that you can blow a tire off the rim
> with brake heating.

That it happens isn't questionable. Why it happens is. A tire ought to be
able to handle a pretty significant amount of pressure over what it's rated
for. A typical tire will easily handle 150psi before blowing off a rim
(easily proven in the shop). But I'm not seeing the physics (yet) that show
a high-enough temperature differential to accomplish this. That's why I'm
thinking there's more to it than just an increase in pressure.

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

Dear Jim,

Maybe a little more than a few hundred square cm? More like
a thousand?

My touring bike's front rim is about 25 inches in diameter.
It's about 2 inches around the curve from tire edge to tire
edge.

pi * 25 * 2 = 157 square inches = 1013 square cm

sounds right

Quoted message said:

That's the outside of the rim. The inside surface area is
about the same, but it's insulated by the rubber tube and
the trapped air, which heats up.

I also wonder whether the spokes act as radiators. They have
lots of surface area for their mass, but they also have such
a tiny connection to the hot rim that I'm guessing that they
don't dissipate much heat. (My guesses often entertain
engineers.)

The area of the spokes in contact with the rim is pretty small, for 36
spokes the total is going to be a lot less than 1.0 cm^2, or much less
than one tenth of one percent of the area of the rim. Also, stainless
steel is a fairly poor conductor of heat, about one fifteenth as good
as aluminum.

Quoted message said:

Why would you use only one brake on a steep descent?

Just thinking of a worst case scenario.

Quoted message said:

Still, I agree with your conclusion that it could get pretty
hot. I think that what's interesting the other posters is
whether all that heat expands a rim enough to contribute to
its tire coming off.

Right. To get a tire from 100psi to 150psi by heating, we would have
to go from 75 degrees F to 308 degrees F. Seems we could get that
hot. Assume the area of our rim is 2000 cm^2 and we are putting 900
watts in on a 75 degree F (297K) day. The wind rushing by our rim
ensures that it is surrounded by 297K air and most of the heat is lost
by radiation. Q = e*s*A(T2^4 - T1^4) The emissivity of anodized Al
is about 0.8. We plug our numbers in and we get a temperature of 570
degrees F. So, by "pretty hot" I meant hot enough to get the pressure
high enough to blow the tire off the rim.

Frank Krygowski said:

So, two possible solutions:

1) Hollow spokes delivering cooling water on demand to the inner
surface of the rim. Reservoir under the saddle, connected to a suitable
fitting at the hub. The optimum rim design would be like the old
Weinmann Concave rims mentioned in another thread, so the water would be
kept at the inner surface and away from the braking surfaces. (A Mr.
Goldberg may already have a patent on this ides, of course.)

2) The water-filled inner tubes from yet another thread.

3) Disc brakes.

Austin

Jim Smith said:

"Mike Jacoubowsky" <[email hidden]> writes:

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

For those less adept at the computation:

For a cyclist continually braking while descending a constant slope (s),
the speed that generates the maximum power dissipation in the brakes is

Vwc = Vt/sqrt(3).

The max power dissipated in the brakes is

Pmax = 2/(3*sqrt(3)) * Fg*Vt

where
Vt = terminal velocity of rider (descending w/out braking)
Fg = s*m*g
s = slope of road
m = mass of bike and rider
g = gravitational acceleration.

Plugging in some typical values

m = 225lb = 68kg
s = 10% = 0.10
Vt = 54mph = 87kph [a guess]
Fg = 10m/s^2

we get

Vwc = 50kph = 31*mph
Pmax = 930watt,

which is about what Jim came up.

Joe Riel

[email hidden] wrote in message ...

Quoted message said:

Carl Fogel said:

Quoted message said:

My first introduction to how high rims get was with steam generated
from water in the rim on a mild descent with hairpin turns at the
end of several straight runs. The experience also showed that on
exiting the turn, steam stopped escaping which revealed how fast
air cooling reduces rim temperatures.

Quoted message said:

Is this rapid heat loss after you exit the turn and stop braking
mostly air cooling, radiating, or convection into the tires, tube,
and compressed air?

In this case it probably went into making steam but air cooling is a
major sink judging from the absence of blow-offs when I descent the
same road fast.

Quoted message said:

I think that you've mentioned a tire bursting when you were just
dawdling down a steep hill, barely riding the brake at around 10 mph
while waiting for a friend who'd stopped to take a picture, so this
would seem to support air cooling.

This was on a 12%+ grade on which I was waiting for my friend who was
taking pictures. For convenience I used only the rear brake so I
could easily look over my shoulder. I have ridden this route often
with no problem, before that and since with the same normal inflation
pressure. It is also not the first descent encountered on my European
bicycle tour.

Quoted message said:

But I'm wondering how much heat just radiates off the huge exposed
surface area of the rim in still air and how much heat is absorbed
by the tire-tube-compressed-air covering the unexposed surface.

Enough to blow the tire off.

Quoted message said:

Are the other two heat losses negligible?

I don't know or care.

Tubulars do not have this problem, they creep with certain glues.
They definitely cannot blow off the rim like a wired on tyre.

Trevor

[email hidden] wrote in message ...

Quoted message said:

Frank Krygowski said:

Quoted message said:

> There are commercially available stick-on temperature indicators that
> record maximum temperatures of the surface to which they're stuck.
> http://www.tempil.com/Tempilabel.htm

Quoted message said:

Quoted message said:

Forget about that, it's both temperature and duration that cause
dangerous heating and overpressure blow-offs. I've done it as have
others with whom I have ridden on steep roads. Unobservant riders
might attribute the blow-off to a faulty tire or poor tire mounting
but it is heat.

Quoted message said:

As a guy who likes data, I'd still be interested in a "maximum
temperature" reading. If someone wanted to log temperature versus
time instead, that would be even more interesting, but much more
difficult.

Quoted message said:

Quoted message said:

> ISTR someone checking tandem rims on mountain descents with these
> things, but I don't recall what the maximum temperature was.

Quoted message said:

Quoted message said:

After you have ridden a few hundred miles in mountainous terrain
and then have a blow-off should be definitive.

Quoted message said:

I've done the first, but not the second. I'd hope to find a less
scary way of learning about this!

OK, nothing lost if you put a few heat indicators on your rear rim,
find a steep road (steeper than 12%) in your area and with a tire
inflated over 100psi, roll down the hill at between 5 and 8 mph with
only the rear brake applied. When the tire blows off, use the front
brake to stop and check the highest temperature reached. This will
cost you no more than one tube, preferably one that had a few patches.
There is no hazard here if you don't have any tight turns.

This is an easy test that I have done inadvertently without
instrumentation. I'm satisfied that it occurs easily. I have had an
opportunity to testify on a tandem case where the rider concocted a
story that was immediately apparent to me, because the scenario was
based on the belief that brake heating did not cause the tire to bow
off. I could prove by the evidence on the bicycle that his story was
false and also how the failure actually occurred.

It may predominantly be the braking in itself which causes the tyre to be
pulled around the rim and so release the tyres grip on the rim, it is only
typical tube pressure which then lifts the tyre from the rim. Degreasing of
the rim and tyre before fitting will ensure satisfactory adhesion to prevent
this lifting. Alternatively use tubulars.

Trevor

[email hidden] wrote in message ...

Quoted message said:

This is service temperature, not peak survival temperature, which is
far higher. There is also the problem of tube adhesion to the tire
casing, which is also enhanced by higher temperature. I just got a
snake bite flat that went down over about 30 seconds, mainly because
the tube was securely adhering to the tire. After the tube was out of
the tire it leaked like a sieve.

Should have been using tub's. Not liable to snakebites. No snake can catch
yer. Tubs will flat over 30min's due to the close fit between tyre and
tube, not generally available with wired on tyres.

Trevor

Frank Krygowski wrote in message <[email hidden]>...

Quoted message said:

Jobst, it's a discussion. I'm just curious about the details of the
phenomenon! Saying "Accept it!" about tires blowing off the rim this is
like saying "Accept it, people die of cholera!"

If the discussion sheds light to enable someone to design a
blow-off-proof tire, that would be wonderful. If the discussion merely
teaches us something about the physics of the situation, that would at
least be beneficial.

As I see it, this is at least as valuable as harping about whether the
word "strech" should be used to describe elongation of chain pitch.

Blow off proof tyre - tubular.

Trevor

Jim Smith said:

Quoted message said:

Jim Smith said:

"Mike Jacoubowsky" <[email hidden]> writes:

> >> Are you suspecting that the air temp in the tube actually exceeds
> >> 100C? In the 0-100C example, the pressure differential didn't
> >> appear to be enough to blow a (properly mounted) tire off of a rim
> >> (from 100psi to 136psi).
> >
> > That depends on how long the tube is exposed to how high a
> > temperature. What is proven is that you can blow a tire off the rim
> > with brake heating.
>
> That it happens isn't questionable. Why it happens is. A tire ought to be
> able to handle a pretty significant amount of pressure over what it's rated
> for. A typical tire will easily handle 150psi before blowing off a rim
> (easily proven in the shop). But I'm not seeing the physics (yet) that show
> a high-enough temperature differential to accomplish this. That's why I'm
> thinking there's more to it than just an increase in pressure.
>

Well, a quick back of the email calculation shows that for a bike and
rider weighing 1000N (225lbs) and a 10% grade, maximum energy
dissipation by the brakes will occur at a speed of around 50 km/h. At
that speed the bike is losing gravitational potential energy at a rate
of 1388 watts and it is taking 465 watts for wind and rolling
resistance, so that leaves 923 watts for the brakes to get rid of. If
we are using only one brake that is 923 watts into a 450 gm piece of
Al with a surface area of a few hundred cm^2. I suspect it could get
pretty hot.

Dear Jim,

Maybe a little more than a few hundred square cm? More like
a thousand?

My touring bike's front rim is about 25 inches in diameter.
It's about 2 inches around the curve from tire edge to tire
edge.

pi * 25 * 2 = 157 square inches = 1013 square cm

sounds right

Quoted message said:

That's the outside of the rim. The inside surface area is
about the same, but it's insulated by the rubber tube and
the trapped air, which heats up.

I also wonder whether the spokes act as radiators. They have
lots of surface area for their mass, but they also have such
a tiny connection to the hot rim that I'm guessing that they
don't dissipate much heat. (My guesses often entertain
engineers.)

The area of the spokes in contact with the rim is pretty small, for 36
spokes the total is going to be a lot less than 1.0 cm^2, or much less
than one tenth of one percent of the area of the rim. Also, stainless
steel is a fairly poor conductor of heat, about one fifteenth as good
as aluminum.

Quoted message said:

Why would you use only one brake on a steep descent?

Just thinking of a worst case scenario.

Quoted message said:

Still, I agree with your conclusion that it could get pretty
hot. I think that what's interesting the other posters is
whether all that heat expands a rim enough to contribute to
its tire coming off.

Right. To get a tire from 100psi to 150psi by heating, we would have
to go from 75 degrees F to 308 degrees F. Seems we could get that
hot. Assume the area of our rim is 2000 cm^2 and we are putting 900
watts in on a 75 degree F (297K) day. The wind rushing by our rim
ensures that it is surrounded by 297K air and most of the heat is lost
by radiation. Q = e*s*A(T2^4 - T1^4) The emissivity of anodized Al
is about 0.8. We plug our numbers in and we get a temperature of 570
degrees F. So, by "pretty hot" I meant hot enough to get the pressure
high enough to blow the tire off the rim.

Dear Jim,

Yikes!

Since half that 2000 cm^2 surface is insulated by tube and
trapped air, it sounds as if your calculation is on the low
side, if anything.

Curiously, 570 F is almost the same as 570 K, so I doubt
that this high figure is due to the kind of mistake that I
often make.

But do bicycle rims really reach 570 degrees F or K?

Or is this just a pardonably rough approximation that tends
toward the high side, sort of like taking Chalo Colina as
having a roughly normal weight for a human being?

What happens to rubber tires and butyl inner tubes and rim
tape pressing firmly against 570 degree F aluminum rims?

Carl Fogel