Re: Exploding tires II

Frank Krygowski said:

Checking the Matweb site,
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=P0RUB1
for properties of vulcanized natural rubber, there are two things that
strike me as interesting.

One is "Maximum service temperature, Air = 176 deg. F." They don't
specify the consequences of exceeding this, and I don't know if they're
related to long term degredation or short term failure, but it's
interesting.

The other is the coefficient of thermal expansion, 125 microinches per
inch per degree F. That's pretty high, of course, about ten times as
much as aluminum and 20 times steel.

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 distorion of the bead shape, and cause the
bead to lose its grip on the rim?

"Microinches"? Barf! What kind of units are those, join the 20th
century already! Anyway, "microinches per inch" are just parts per
million.

The tire is given shape by the cords, not the rubber - I think the
rubber's expansion is not relevant. I'm not sure which Matweb
listing applies to cotton or nylon fibers that might make it into
a tire casing, but

http://www.matweb.com/search/SpecificMaterial.asp?bassnum=O2406
http://www.matweb.com/search/SpecificMaterial.asp?bassnum=PDUNOM1

say extruded nylon has a CTE of 45 microinches/inch/deg F, while
aramid (Kevlar) yarn has a CTE of only 10 microinches/inch/deg F.

Peter said:

Quoted message said:

Tom Sherman 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.

Your presumption is correct.

The name of the author of the site [1], Morscher is suspicious in light
of its English translation, "more rotten".

[1] The "tinyurl" link seems broken:
<http://www.sagen.at/texte/gegenwart/oesterreich/tirol/allgemein/picknickamzirlerberg.htm>.

--
Tom Sherman

Trevor Jeffrey said:

[email hidden] wrote in message ...

Quoted message said:

Frank Krygowski 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:

> 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:

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

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

Dear Trevor,

Since the tire's braking is confined to a small contact
patch, it seems unlikely that there is enough traction to
spin a normally inflated tire on its rim by braking.

There's about 160 linear inches of tire striping the insides
of the rim, perhaps a quarter of an inch wide, all under the
same pressure per square inch as the contact patch. That's
roughly 40 square inches versus one to two square inches of
contact patch.

The tire would skid under braking long before it began to
creep around the rim, barring impressive greasing. And on
dry pavement, you'd flip a normal bicycle before the front
tire began to skid.

Carl Fogel

AustinMN said:

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 idea, of
course.)

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

3) Disc brakes.

<sigh> There's always someone who wants to take the easy way out.

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

Frank Krygowski said:

Quoted message said:

Quoted message 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?

Quoted message said:

Quoted message said:

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!

Quoted message said:

Quoted message said:

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

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!"

You use an odd way to ask. Telling us what else could cause a tire to
come off the rim, raises specters that attain a life of their own. We
have enough myth and lore to quash without you introducing new ones.
You've been around this newsgroup long enough to have read all this
about tire blow-off several times. That's why I ask "what are you
trying to generate?"

Quoted message said:

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.

If your idea of discussion is the classic: "Tires blow off because..."
followed by a series of remotely plausible reasons, followed by a
belated,"that's right isn't it?" somewhere in a disconnected
sentence, you are not asking. That is a typical "end run" offered by
folks who have pet ideas but have no way to prove them or substantiate
that they ever occur.

Quoted message said:

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

Only if you say stretch without defining what you mean by the word.
From misuse of stretch, we have seen many posts in which riders
believe they stretched their chain from riding on too steep a hill.

Quoted message said:

Sheesh!

Quoted message said:

Yes? That is probably appropriate for your stretch of the term

"discussion".

Jobst Brandt
[email hidden]

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).

Quoted message said:

Quoted message said:

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:

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.

Did you try riding that wheel with the tire at 150psi? I suspect that
riding it on flatland, with no braking, that it would come off. I am
not convinced by a static test. Would you test ride that?

Jobst Brandt
[email hidden]

On Wed, 25 Aug 2004 01:10:37 GMT, [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.

Jobst Brandt
[email hidden]

Sorry, let me try to describe it differently:
Maybe the tube gets the snakebite cut *after* the blowout, while the
rider rides on the flat tire, the rim bumping along on the tire and
tube, while slowing to a stop?

Mike Jacoubowsky said:

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.

Quoted message said:

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.

The "physics lesson" used an inappropriately low temperature as I see
it.

Jobst Brandt
[email hidden]

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.

Hold the phone! I'll propose that the rim will get hotter at 1/10
that speed because there is practically no convection cooling when
riding slowly. Convection is the primary coolant of rims in the
absence of rain water. Besides, the rate of conversion of kinetic
energy to heat increases with speed until the brake pad fails. Where
do you get 50km/h?

Jobst Brandt
[email hidden]

Trevor Jeffrey said:

[email hidden] wrote in message ...

[snip Jobst]

Quoted message said:

Carl Fogel writes:

[snip Carl and Jobst]

Quoted message said:

Quoted message said:

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.

[snip Trevor]

Quoted message said:

Trevor

Dear Jobst,

"I don't know or care."

There's the motto for your coat of arms!

Carl Fogel

Quoted message said:

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.

I knew that Skywriting planes trail smoke, but figured there must be
something that could be added to water that would evaporate and leave a
colored trail of some kind. But of course, if there was, you would just use
that and not water, right? So the question becomes what boils at a relevant
temperature and trails darkly-colored smoke.

On the other hand, this obscures what I think actually *is* a relevant
discussion... why tires blow off on descents. I'm still looking for reasons
why it doesn't happen statically (in the shop with an over-pressured tire)
but frequently does out on the road. If it's only air pressure that's
causing this to happen, just how hot is the air in the tube actually
getting???

I'm just now seeing some math that indicates the amount of energy being used
(about 900 watts), and now await somebody telling me what that does to the
temperature/air pressure inside the tire. If we can see a psi of, say, 160
or so, *then* I'm willing to say that air pressure, raised by increasing
temperatures, is the culprit. But if we cannot demonstrate such high
pressures, then I fail to see why we assume that high pressure is the only
thing causing the problem.

--Mike Jacoubowsky
Chain Reaction Bicycles
www.ChainReaction.com
IMBA, BikesBelong, NBDA member

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

Quoted message said:

Mike Jacoubowsky 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

Carl Fogel 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.

Than test has been done. Descending the same hard braking hill with
about 90psi inflation pressure does not blow the tire off while 110psi
does. Anyway, expansion is trivially small in the rim, especially
because it doesn't change it's shape and it is the shape of the rim
bead that holds the tire.

Jobst Brandt
[email hidden]

Quoted message said:

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

Carl Fogel

If the rim is actually heating to 570 degrees, all bets are off. I'm sure
that someone running the numbers would discover that's high enough to exceed
my threshold of 160psi, but what would really concern me is how well the
tube, rimstrip & tire hold up under such high temperatures. But I suspect
that it doesn't approach anything close to 570 degrees, since I think you'd
literally smell rubber starting to burn at such temps. For an experiment,
someone (who's single) might want to toss a tube into an oven and see how it
reacts at various temperatures. Uninflated tube, mind you; we're just
looking for what happens physically to a tube when it gets that hot.

--Mike Jacoubowsky
Chain Reaction Bicycles
www.ChainReaction.com
IMBA, BikesBelong, NBDA member

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

Quoted message said:

Jim Smith said:

Quoted message said:

On 25 Aug 2004 12:33:50 -0500, Jim Smith
<[email hidden]> wrote:

>"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

Quoted message said:

Mike Jacoubowsky said:

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.

Quoted message said:

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.

The "physics lesson" used an inappropriately low temperature as I see
it.

Jobst Brandt
[email hidden]

Dear Jobst,

So what's an appropriate rim temperature for your physics
lesson?

Elsewhere in this thread, people are calculating rim
temperatures of 570 degrees K (or F, take your pick).

Carl Fogel

Mike Jacoubowsky wrote

--snip--

Quoted message said:

On the other hand, this obscures what I think actually *is* a relevant
discussion... why tires blow off on descents. I'm still looking for

reasons

Quoted message said:

why it doesn't happen statically (in the shop with an over-pressured tire)
but frequently does out on the road. If it's only air pressure that's
causing this to happen, just how hot is the air in the tube actually
getting???

I'm just now seeing some math that indicates the amount of energy being

used

Quoted message said:

(about 900 watts), and now await somebody telling me what that does to the
temperature/air pressure inside the tire. If we can see a psi of, say,

160

Quoted message said:

or so, *then* I'm willing to say that air pressure, raised by increasing
temperatures, is the culprit. But if we cannot demonstrate such high
pressures, then I fail to see why we assume that high pressure is the only
thing causing the problem.

I have had one blown tire and, as a result, want to suggest an idea.

Years ago, I had a Michelin tire that was too fat. It must have been
miss-labeled. I put it on the rear even though I knew it was dangerously
fat. It gave no problem even when it got hot as long as I was moving at a
reasonable speed. But then, while it was hot, I stopped and sat at a
stoplight. And after a few seconds, it blew. I thought the world had come to
an end there for a half second or so.

Is it possible that the bead got slippery with the heat and slid out?
Perhaps the weight I had on it while stopped pushed the bead at the bottom
of the wheel up so that it disengaged somewhere else. Or perhaps my weight
pushed the sidewall out where the tire was in contact with the road enough
so that the slippery bead slid out from the hook of the rim. I don't
remember where on the wheel the tube was blown.

If the blowout somehow results from the bead getting soft and slippery,
blowouts from hot tires would occur when the rider was either stopped or
moving slowly enough for the bead to slide out of the rim hook at the road
contact area.

Just a thought.

Quoted message said:

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.

Hold the phone! I'll propose that the rim will get hotter at 1/10
that speed because there is practically no convection cooling when
riding slowly. Convection is the primary coolant of rims in the
absence of rain water. Besides, the rate of conversion of kinetic
energy to heat increases with speed until the brake pad fails. Where
do you get 50km/h?

I should let Jim respond, but from my post it is clear that this
is the speed that maximizes the power dissipation in the brakes.
As you suggest, this may not be the worst-case for rim temperature.

What's a reasonable model for convection losses vs speed?

Joe Riel

dianne_1234567890? said:

Quoted message 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:

Quoted message said:

Quoted message said:

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

Quoted message said:

Quoted message said:

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.

Quoted message said:

Sorry, let me try to describe it differently: Maybe the tube gets
the snakebite cut *after* the blowout, while the rider rides on the
flat tire, the rim bumping along on the tire and tube, while slowing
to a stop?

Maybe, maybe the moon is made of green cheese... If the tire blows
off, you know about it. There is no doubt and the tube will have a
long slash. Why do you care if there are also snake bite holes? The
tube is dead. You've got to come up with a better scenario if you
want your original quarry to fly.

Jobst Brandt
[email hidden]

Carl Fogel 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.

That test has been done. Descending the same hard braking hill with
about 90psi inflation pressure does not blow the tire off while 110psi
does. Anyway, expansion is trivially small in the rim, especially
because it doesn't change it's shape and it is the shape of the rim
bead that holds the tire.

Jobst Brandt
[email hidden]

On Thu, 26 Aug 2004 02:42:22 GMT, "Mike Jacoubowsky/Chain
Reaction Bicycles" <[email hidden]> wrote:

[snip]

Quoted message said:

On the other hand, this obscures what I think actually *is* a relevant
discussion... why tires blow off on descents. I'm still looking for reasons
why it doesn't happen statically (in the shop with an over-pressured tire)
but frequently does out on the road. If it's only air pressure that's
causing this to happen, just how hot is the air in the tube actually
getting???

I'm just now seeing some math that indicates the amount of energy being used
(about 900 watts), and now await somebody telling me what that does to the
temperature/air pressure inside the tire. If we can see a psi of, say, 160
or so, *then* I'm willing to say that air pressure, raised by increasing
temperatures, is the culprit. But if we cannot demonstrate such high
pressures, then I fail to see why we assume that high pressure is the only
thing causing the problem.

--Mike Jacoubowsky

Dear Mike,

I suspect that the rims heat up impressively, that the
trapped air in the tube does the same thing, and that the
tire pressure rises dramatically.

The tires could also behave differently if the rims are
getting as hot as some people think.

So could the rims.

But the heat seems to be available.

To rise from 100 psi to your target 160 psi is a 60%
increase.

English-major physics holds (somewhat shakily) that this
would require a 60% increase in absolute temperature.

A 60% absolute temperate increase from 75 degrees F means a
60 % increase from 297 degrees K, so we need to go from 297
to 475 K, which is 395 degrees F.

Here's a nice calculator that does F, K, and C all at once:

http://members.aol.com/javawizard/tture.html

Elsewhere in this thread, people are estimating up to 570
degrees K, which is coincidentally about 570 degrees F, so
it seems plausible that things are getting that hot.

And Jobst has made the point that the 160 psi figure may be
unrealistically high--ordinary tires probably come off
loaded rims rolling over bumps at lower pressures.

I'm just impressed with the idea of bike rims getting up to
400 degrees F and higher.

Carl Fogel

Carl Fogel said:

Quoted message said:

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.

Quoted message said:

Quoted message said:

Quoted message said:

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.

Quoted message said:

Quoted message said:

The "physics lesson" used an inappropriately low temperature as I
see it.

Quoted message said:

So what's an appropriate rim temperature for your physics
lesson?

Quoted message said:

Elsewhere in this thread, people are calculating rim
temperatures of 570 degrees K (or F, take your pick).

How about starting a 110psi and, following Mike's test of at what
pressur a tire comes off wehn ridden at room temperature, re-evaluate
what temperature that takes. As I said, I could make plenty of steam
on a not-so-steep Alpine pass. I know that you can burn hands
(blisters) at the tempreature I developed on an 18% grade in 500m
using both brakes at 40km/h. I had no way of measuring it but it was
hot enough that we stopped at the runaway lane for the rims to cool to
warm-to-the-touch. There was a breeze on the W?rzen Pass.

Jobst Brandt
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