Exploding tires II

On Tue, 24 Aug 2004 12:26:31 -0400, Frank Krygowski
wrote:

Mike Jacoubowsky wrote:

For those of us who have forgotten our high-school physics, could
you give a
few examples of absolute vs Celsius (or Fahrenheit) temperatures? In
the real world, we might conceivably see a tire/tube start the day at
40 degrees (F) and reach a peak of well over 100 (F), possibly 150? To
tell you the truth, I don't honestly know just how hot the air in a
tube might get on a very long, very steep descent with ambient air temp
at 90 degrees. However, that's a real-world example that some of us
experience.


OK, the relationship is (P1*V1)/T1 = (P2*V2)/T2 where P and T refer to
absolute pressures and temperatures, V is volume. If volume is
(reasonably) assumed constant, it cancels out, and you can rearrange to:

P2 = P1 *(T2/T1)

But again, P & T have to be measured on absolute scales. They're
usually not, so you have to convert.

For metric measurements like the originally quoted Celsius, the absolute
temperature scale is Kelvin. To convert Celsius to Kelvin, add 273.15

If you're starting with degrees Fahrenheit, the corresponding absolute
scale is Rankine. To convert Fahrenheit to Rankine, add 459.67 (or
alternately, convert Fahrenheit to Celsius and use the Kelvin scale.)

For pressures, your gage measures "gage pressure," psig. To convert to
absolute pressure (psia) add the pressure of the atmosphere, 14.7 psia.

So, 40 deg. F = 499.67 deg R 150 deg F = 609.67 deg R
90 psig = 104.7 psia

and P2 = 104.7 psia*(609.67/499.67) = 127.7 psia

But now we have to get that absolute pressure converted back to gage
pressure. Subtract 14.7 and you get P2 = 113 psig.

So in general, it's not as bad as you might think.



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

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


Long response, wow.
I think the main thing is that the tire/tube combination is much
more sensitive to temperature than the change in air pressure.
Rubber like materials don't fare well over about 150 F.
Hot pavement with braking on a long descent can cause material
troubles.
Bill Baka


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Frank Krygowski writes:

For metric measurements like the originally quoted Celsius, the
absolute temperature scale is Kelvin. To convert Celsius to Kelvin,
add 273.15

If you're starting with degrees Fahrenheit, the corresponding
absolute scale is Rankine. To convert Fahrenheit to Rankine, add
459.67 (or alternately, convert Fahrenheit to Celsius and use the
Kelvin scale.)

For pressures, your gage measures "gage pressure," psig. To convert
to absolute pressure (psia) add the pressure of the atmosphere, 14.7
psia.

So, 40 deg. F = 499.67 deg R 150 deg F = 609.67 deg R
90 psig = 104.7 psia

and P2 = 104.7 psia*(609.67/499.67) = 127.7 psia

But now we have to get that absolute pressure converted back to gage
pressure. Subtract 14.7 and you get P2 = 113 psig.

So in general, it's not as bad as you might think.

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.

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

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

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.

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

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

Jobst Brandt

wrote:

Frank Krygowski writes:


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


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.

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.

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


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

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


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

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.

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?

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).

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.

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

Frank Krygowski writes:

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

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.

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.

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

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

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.

Jobst Brandt

Mike Jacoubowsky writes:

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.

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?

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.

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

wrote:

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.


No hazard of crashing maybe, but when it happened to me I had a big welt
on the side of my calf from the explosion. I suggest wearing protective
clothing if anyone tries this.

wrote in message

Mike Jacoubowsky writes:

[...]

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?

Make coloured steam.

--

A: Top-posters.
Q: What is the most annoying thing on Usenet?

Mike Jacoubowsky/Chain Reaction Bicycles wrote:


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).

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.

Checking the Matweb site,
http://www.matweb.com/search/Specifi...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?

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

"DRS" writes:

wrote in message

Mike Jacoubowsky writes:

[...]

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?

Make coloured steam.


Too bad it isnt that easy.