Re: Rim brake heat and clincher blowoff, was Re: Potential good news for Mt. Washington access.

[Tim McNamara]

[email protected] writes:

> Tom Sherman writes:
>
>> There still needs to be a control for temperature in the static
>> test, since of course compressing air into the tire raises the
>> temperature of the air and casing. However, if the tire is inflated
>> slowly, actual temperature rise will be insignificant.
>
>> In addition to the motion between the tire and rim that Jobst
>> Brandt mentions as possibly affecting blow-off pressure, there is
>> the effect of heating the tire, since the rubber will change
>> properties with temperature.
>
> I don't believe inflating a tire makes enough temperature difference
> to worry about. I have not noticed hot tires from inflation nor
> even a hot hose or brass hose-to-valve coupler (hose chuck). I am
> aware of Silca frame fit pump heads getting hot in the hand while
> pumping but that is a big difference from a floor pump and hose.

Hmm, well, if pressure increase from heating is the *only* cause of
blow-off, then one would expect to be able to blow off the tire at the
same pressure whether in the garage or rolling down the hill. Or at
least so it seems to me as a non-engineer. As I recall from the prior
discussion, heating the rim to the temperatures normally seen doesn't
cause a huge increase in pressure.

> I think we are scraping at nits. Besides, who gives a damn how much
> pressure a stationary wheel can hold. We are interested in safely
> riding down steep grades.

Or is there some other factor involved- the heat of the rim affecting
the coefficient of friction between the tirm and tire? There are
different designs at the bead- some tires have a fabric chafing strip
over the bead, some are just a rubber coating over the casing of the
tire- could this make a difference and could it be tested?

Or perhaps the effect of braking causing some kind of pulling on the
bead on a line between the contact patch and the rim- drawing the bead
tight ahead of the contact patch and loosening it behind the contact
patch? Does rim have to be hot or can this happen cold (if the latter
is possible, I'd expect to see it happen in criteriums or during panic
stops).

And out of this, if the mechanism can be determined, is the question
of how to prevent it. Closer tolerances for tire fit on the rim? A
change in the materials at the rim-tire interface? A rim strip that
insulates the tube?

In a thread last August, Jobst stated:

> The idea that the tire bead gets soft occurred to me but I later
> rejected it because I have been in many situations where high rim
> temperatures occurred only for a short duration, not long enough to
> heat the air in the tube. There was no residual effect over many
> miles in which the tires wore out while repeating the rim heating.
> I get to review this every summer in riding over many mountain roads
> and have reduced the tire blow-off to air temperature in the tire
> alone.

This seems to me to suggest that temperature of the rim and not
pressure in the tube may be the culprit. What effect does heating the
rim have? The only thing I can think of is that there is some effect
on the interface between the tire and the rim- reduction of friction,
change in bead position at some point on the rim, etc. And it also
seems that the condition must be pretty specific and difficult to
achieve, or we'd have this happening on many rides, not just a few..

 

[Tim McNamara]

Tom Sherman <[email protected]> writes:

> Jobst Brandt wrote:
>
>> ... Ride bike. In fact, make a point to ride the Alps. You'll
>> love it.
>
> Unfortunately, most cyclists in North America will never have both
> the vacation time and money for an Alpine cycling vacation.

Well, many won't have either the time or the money but perhaps not
"most." Mostcyclists I know make more than I do. I saved up for it
and went, spending two weeks in France and 10 days in the Alps. I had
a wonderful time. The cost total was about $2000 inclusive of air
fare, meals, lodging, miscellaneous purchases and one tire. Another
week would have only raised the costs about $250. Of course this was
2002 when $1 = E1 +/- $0.05. Times have changed, the same vacation
today would cost ~$2600.

But I'll warn you- if you go and ride in the Alps once, you keep
thinking about how much you want to go back!

 

[Tom Sherman]

Tim McNamara wrote:

> Tom Sherman <[email protected]> writes:
>
>>Jobst Brandt wrote:
>>
>>>... Ride bike. In fact, make a point to ride the Alps. You'll
>>>love it.
>>
>>Unfortunately, most cyclists in North America will never have both
>>the vacation time and money for an Alpine cycling vacation.
>
> Well, many won't have either the time or the money but perhaps not
> "most." Mostcyclists I know make more than I do. I saved up for it
> and went, spending two weeks in France and 10 days in the Alps...

How many people in the US can not only accumulate 4 weeks vacation, but
also be allowed to use it all at once?

> ... The cost total was about $2000 inclusive of air
> fare, meals, lodging, miscellaneous purchases and one tire. Another
> week would have only raised the costs about $250. Of course this was
> 2002 when $1 = E1 +/- $0.05. Times have changed, the same vacation
> today would cost ~$2600....

The lower working classes in the US and Canada could not reasonably
afford that, nor could the majority of those in Mexico (since the
original contention involved North Americans).

> But I'll warn you- if you go and ride in the Alps once, you keep
> thinking about how much you want to go back!

I will have to ask my bicycle about this, since it toured the Alps with
a previous owner.

--
Tom Sherman - Near Rock Island

 

[Paul Kopit]

On Mon, 10 Jan 2005 12:32:09 -0600, Tom Sherman
<[email protected]> wrote:

>It would be a simple matter to use a tandem (e.g. Arai) drag brake on a
>single bicycle and/or use adequately sized disc brakes.

It's not that easy to get the components to fit onto a 135 mm spaced,
threaded hub. Also, the chain stay needs to hold the brake arm
somehow. Tandems frequently have the fitting already there and the
chainstay is sturdy enough to mount a bracket if it's not integral.

 

[Tom Sherman]

Paul Kopit wrote:

> On Mon, 10 Jan 2005 12:32:09 -0600, Tom Sherman
> <[email protected]> wrote:
>
>
>>It would be a simple matter to use a tandem (e.g. Arai) drag brake on a
>>single bicycle and/or use adequately sized disc brakes.
>
>
> It's not that easy to get the components to fit onto a 135 mm spaced,
> threaded hub. Also, the chain stay needs to hold the brake arm
> somehow. Tandems frequently have the fitting already there and the
> chainstay is sturdy enough to mount a bracket if it's not integral.

I was addressing the issue from a design point of view, not necessarily
modifying existing bicycles. Using proper dropout spacing for a tandem
hub and including the brake arm fitting are a trivial matter at the time
of frame construction.

--
Tom Sherman - Near Rock Island

 

[[email protected]]

Matt O'Toole writes:

>>> Ride bike. In fact, make a point to ride the Alps. You'll love it.

>> Unfortunately, most cyclists in North America will never have both
>> the vacation time and money for an Alpine cycling vacation.

> Not if they keep dumping their money into the tanks of their SUVs.

Or don't buy that carbon, titanium, bicycle with Pkzsyferium wheels,
or the SUV and you'd have more than enough for the trip and all the
hotels for a month on the way.

> It's all about priorities.

That what I say!

http://www.paloaltobicycles.com/alps_photos.html

Jobst Brandt
[email protected]

 

[Mark and Christine]

On Mon, 10 Jan 2005 00:05:19 -0500, Frank Krygowski
<[email protected]> wrote:

>[email protected] wrote:
>

>This is a continuation of a somewhat spirited discussion that took place
>about a month ago on r.b.tech. IIRC, at that time, several people
>pointed out that room temperature, stationary tires can withstand very
>high (over 160 psi) pressures without blowoff, and that therefore
>blowoffs must be caused by something other than _just_ the pressure
>increase from the heat of braking. I was certainly one who felt that way.
>
>Others (I'll let Jobst say who) seemed to be claiming that the only
>relevant factor was the pressure increase.
>
>Thus, after Jobst determines pressure at blowout during hard braking,
>I'd like to see the same rim subjected to a room temperature static test
>at the same pressure. We'll then know if _only_ pressure causes those
>blowouts. It's an opportunity to learn.
>

I am coming into this discussion a little late, so if I am
repeating something that has already been pointed out then I
appologize in advance. There have been a couple of "factual
errors" in recent posts. If no one with an engineering
background has corrected them then...

Expanding air cools, compressing air heats. When you fill a
tire you are expanding the air (cooling) and then as the
tire inflates the amount of expansion decreases. It does
not compress and heat. For all practical purposes the tire
"might" cool slightly. (Disclaimer: If you are using a hand
pump then the pump is performing work on the air and the air
is in fact be compressed into the tire so it will heat up -
how much? No idea, I have a 250 PSI compressor that fills
my tires - doesn't everyone?)

When you are braking there are 2 major reactive friction
forces. The calipar against the rim, and the tire against
the road. How much of each occurs depends on a lot of
things, but I would think that on a road bike they are both
significant to the overall heating and must be considered.

I can't imagine the tire moving relative to the rim enough
to cause heat. In order to move, the relative force between
the rim and the tire would exceed the force that other
components could withstand. Blow out would be one of your
last worries.

When the rim heats it will expand in all directions. So the
rim will become larger in diameter (taller) and it will get
wider. This will cause the tire to stretch to fill the rim
and decrease its aspect ratio.

As the tire heats its pressure will increase, but since it
is a small volume of air I suspect that the increase will be
insignificant. More importantly the tire will soften.

So...

From just a thought experiment view, it would stand to
reason that if the tire changes its shape enough combined
with the softening of the bead/sidewall material to yeild
under the internal pressure, then it is going to blow.

 

[Frank Krygowski]

Joe Riel wrote:

>
>
> Jobst's test should reveal that; he's logging the rim temperature and
> air pressure.
>
> Something we [meaning those of us in the peanut gallery] might want
> to do is ascertain whether the road test is sufficient to settle the matter.
>
> I believe that three factors have been proposed as directly
> contributing to tire blow-off:
>
> 1. tire (air) pressure (i.e. increased pressure due to heating),
> 2. tire movement (i.e. sidewall flexing while rolling under load).
> 3. tire (cord/rubber) temperature
>
> Jobst is measuring the first while applying the second. No attempt is
> being made to measure or independently vary the third factor; however,
> we may be able to indirectly determine whether it could be
> significant. For example, if during steady state braking, the tire
> pressure quickly rises to its final value but the tire doesn't blow
> off until significantly later, that would be consistent with (3) being
> a factor [because the thermal mass of the tire will delay its
> temperature rise].
>
> One reason for measuring the static blowoff pressure is that it serves
> as a baseline against which to compare the dynamic blowoff pressure.
> As it is, we [the peanut gallery] should be able to determine whether
> they are roughly the same by first measuring the static blowoff pressure,
> then pumping a tire to slightly below that and just riding around to see
> if doing so induces a blowoff. Any takers? Ear-protection is recommended.

I have a friend who routinely runs insanely high pressures (ISTM) and
who's never mentioned blowoffs. This is not a mountainous area, though.

This leads me to believe that item #2 may need either revised, or added
to. IOW, it may not be the almost pure lateral flexing that matters.
Rather, it may be that lateral flex, when combined with a
circumferential (or longitudinal) tug on the tire from the road surface.

Based on my friend's lack of blowoff experience, I'd bet that "tug" is
more important than the sideways sidewall flex.


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

 

[Tom Sherman]

Mark and Christine wrote:

> ...
> Expanding air cools, compressing air heats. When you fill a
> tire you are expanding the air (cooling) and then as the
> tire inflates the amount of expansion decreases. It does
> not compress and heat. For all practical purposes the tire
> "might" cool slightly. (Disclaimer: If you are using a hand
> pump then the pump is performing work on the air and the air
> is in fact be compressed into the tire so it will heat up -
> how much? No idea, I have a 250 PSI compressor that fills
> my tires - doesn't everyone?)....

I would expect more bicycle tires to be inflated with hand pumps (even
in industrialized countries, and certainly worldwide).

For most people, owing a compressor is as relatively wasteful as owning
a luxury SUV.

--
Tom Sherman - Near Rock Island

 

[Frank Krygowski]

Mark and Christine wrote:

>
>
> I am coming into this discussion a little late, so if I am
> repeating something that has already been pointed out then I
> appologize in advance. There have been a couple of "factual
> errors" in recent posts. If no one with an engineering
> background has corrected them then...

Several people with strong engineering backgrounds have been following
this thread. Your "corrections" have grevious factual errors, which is
why nobody else has brought them up. Comments below:

>
> Expanding air cools, compressing air heats. When you fill a
> tire you are expanding the air (cooling) and then as the
> tire inflates the amount of expansion decreases. It does
> not compress and heat. For all practical purposes the tire
> "might" cool slightly.

When you take atmospheric air at zero gage pressure and pump a tire to
100 psi gage pressure, you are _certainly_ compressing the air, not
expanding it. During the pump compression stroke, the air within the
pump is at a higher pressure still; but this is largely immaterial.

> When you are braking there are 2 major reactive friction
> forces. The calipar against the rim, and the tire against
> the road. How much of each occurs depends on a lot of
> things, but I would think that on a road bike they are both
> significant to the overall heating and must be considered.

It's not necessary to go into such detail. The source (and maximum
rate) of energy addition to the system is the rider's drop in potential
energy. By far, the greatest work of friction is at the braking
surface. The heat input from the tire tread is negligible.

You can verify this by taking your car out, performing a panic stop from
55 mph, then touching first the tire tread, then the brake disk. All
usual disclaimers (such as "don't try this at home") apply, of course!

>
> I can't imagine the tire moving relative to the rim enough
> to cause heat.

I don't think anyone is proposing that. We know the tire moves relative
to the rim as the blowout happens. We're trying to find out why it
moves. The heat comes from elsewhere.

>
> When the rim heats it will expand in all directions. So the
> rim will become larger in diameter (taller) and it will get
> wider. This will cause the tire to stretch to fill the rim
> and decrease its aspect ratio.

Spoke tension will restrain the diameter increase. The width increase
will be only a few thousandths of an inch.

> As the tire heats its pressure will increase, but since it
> is a small volume of air I suspect that the increase will be
> insignificant.

We'll see. Gas laws tell us with great exactness how pressure relates
to temperature. They work for small volume or large volume.
(P1*V1/T1)=(P2*V2/T2)

More importantly the tire will soften.

Many of us think that may be happening at the bead. But again, we'll see.


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

 

[Joe Riel]

Mark and Christine <[email protected]> writes:


> Expanding air cools, compressing air heats. When you fill a
> tire you are expanding the air (cooling) and then as the
> tire inflates the amount of expansion decreases. It does
> not compress and heat. For all practical purposes the tire
> "might" cool slightly. (Disclaimer: If you are using a hand
> pump then the pump is performing work on the air and the air
> is in fact be compressed into the tire so it will heat up -
> how much? No idea, I have a 250 PSI compressor that fills
> my tires - doesn't everyone?)

I suspect most people, at least those with presta valves,
pump their tires with a hand (floor) pump. Regardless,
no one is (or should be) suggesting that the pumping air
into a tire significantly raises its temperature (significant
being high enough to change the mechanical properties of the
rubber).

> When you are braking there are 2 major reactive friction
> forces. The calipar against the rim, and the tire against
> the road. How much of each occurs depends on a lot of
> things, but I would think that on a road bike they are both
> significant to the overall heating and must be considered.

Only the first matters and is significant. Friction with the road
dissipates energy only if the tire is skidding, which is not of
interest.


> When the rim heats it will expand in all directions. So the
> rim will become larger in diameter (taller) and it will get
> wider. This will cause the tire to stretch to fill the rim
> and decrease its aspect ratio.

Rim expansion is close to insignificant. The thermal coefficient
of aluminum alloys is around 2.5e-5/degC. For a 100degC rise
the rim diameter will increase by about 0.07inch. Note that
a steel wire bead has a smaller thermal coefficient of expansion
(about 1.5e-5/degC).

> As the tire heats its pressure will increase, but since it
> is a small volume of air I suspect that the increase will be
> insignificant.

Wrong. The volume has nothing to do with it. The increase
in absolute pressure is proportional to the increase in
absolute temperature. This is almost certainly the major
factor in tire blow-off. Note that Jobst is not attempting
(I believe) to demonstrate that. Rather, he is assuming that
that is the cause, measuring it (how quickly the pressure rises)
and attempting to determine whether rim strips can effectively
decrease the rate of heat transfer from the rim to the tube.

> More importantly the tire will soften.

This may contribute to blow-off, however, I suspect that it
is a minor effect.

Joe

 

[[email protected]]

Tim McNamara writes:

>>> There still needs to be a control for temperature in the static
>>> test, since of course compressing air into the tire raises the
>>> temperature of the air and casing. However, if the tire is
>>> inflated slowly, actual temperature rise will be insignificant.

>>> In addition to the motion between the tire and rim that Jobst
>>> Brandt mentions as possibly affecting blow-off pressure, there is
>>> the effect of heating the tire, since the rubber will change
>>> properties with temperature.

>> I don't believe inflating a tire makes enough temperature
>> difference to worry about. I have not noticed hot tires from
>> inflation nor even a hot hose or brass hose-to-valve coupler (hose
>> chuck). I am aware of Silca frame fit pump heads getting hot in
>> the hand while pumping but that is a big difference from a floor
>> pump and hose.

> Hmm, well, if pressure increase from heating is the *only* cause of
> blow-off, then one would expect to be able to blow off the tire at
> the same pressure whether in the garage or rolling down the hill.
> Or at least so it seems to me as a non-engineer. As I recall from
> the prior discussion, heating the rim to the temperatures normally
> seen doesn't cause a huge increase in pressure.

As I said, the chafing strip on tire beads is not there for nothing,
and from observation of aluminum on the strip and cloth marks in the
aluminum, there is motion. Friction is not a consideration in tire
retention when moving.

>> I think we are scraping at nits. Besides, who gives a damn how
>> much pressure a stationary wheel can hold. We are interested in
>> safely riding down steep grades.

> Or is there some other factor involved- the heat of the rim
> affecting the coefficient of friction between the rim and tire?
> There are different designs at the bead- some tires have a fabric
> chafing strip over the bead, some are just a rubber coating over the
> casing of the tire- could this make a difference and could it be
> tested?

Friction is not a consideration in tire retention when moving. I
don't think rubber coatings last long at high pressure in this
interface. What HP tires have no chafing strip? I'm considering road
tires because fat tires have entirely different problems and rims.

> Or perhaps the effect of braking causing some kind of pulling on the
> bead on a line between the contact patch and the rim- drawing the
> bead tight ahead of the contact patch and loosening it behind the
> contact patch? Does rim have to be hot or can this happen cold (if
> the latter is possible, I'd expect to see it happen in criteriums or
> during panic stops).

This would only be possible with an elastic bead because the entire
circumference of the bead would have to slip for any of it to move. I
have not seen any evidence of circumferential tire creep as I formerly
saw on tubulars.

> And out of this, if the mechanism can be determined, is the question
> of how to prevent it. Closer tolerances for tire fit on the rim? A
> change in the materials at the rim-tire interface? A rim strip that
> insulates the tube?

I think I mentioned that as part of the experiment.

> In a thread last August, Jobst stated:

>> The idea that the tire bead gets soft occurred to me but I later
>> rejected it because I have been in many situations where high rim
>> temperatures occurred only for a short duration, not long enough to
>> heat the air in the tube. There was no residual effect over many
>> miles in which the tires wore out while repeating the rim heating.
>> I get to review this every summer in riding over many mountain
>> roads and have reduced the tire blow-off to air temperature in the
>> tire alone.

> This seems to me to suggest that temperature of the rim and not
> pressure in the tube may be the culprit. What effect does heating
> the rim have? The only thing I can think of is that there is some
> effect on the interface between the tire and the rim- reduction of
> friction, change in bead position at some point on the rim, etc.
> And it also seems that the condition must be pretty specific and
> difficult to achieve, or we'd have this happening on many rides, not
> just a few..

I don't understand. How do you draw that conclusion, one that is
exactly the opposite from the one I derive from the cited paragraph
that I wrote?

Jobst Brandt
[email protected]

 

[Tom Sherman]

Frank Krygowski wrote:

> Mark and Christine wrote:
>

>> I am coming into this discussion a little late, so if I am
>> repeating something that has already been pointed out then I
>> appologize in advance. There have been a couple of "factual
>> errors" in recent posts. If no one with an engineering
>> background has corrected them then...
>
> Several people with strong engineering backgrounds have been following
> this thread. Your "corrections" have grevious factual errors, which is
> why nobody else has brought them up. Comments below:
>
>> Expanding air cools, compressing air heats. When you fill a
>> tire you are expanding the air (cooling) and then as the
>> tire inflates the amount of expansion decreases. It does
>> not compress and heat. For all practical purposes the tire
>> "might" cool slightly.
>
> When you take atmospheric air at zero gage pressure and pump a tire to
> 100 psi gage pressure, you are _certainly_ compressing the air, not
> expanding it. During the pump compression stroke, the air within the
> pump is at a higher pressure still; but this is largely immaterial...

I believe that Mark and Christine are using a compressor to pressurize a
reservoir to ~250 psi, letting the compressed air cool down to ambient
temperature, then inflating the bicycle tire to normal inflation
pressure. In this case, the air is being expanded into the tire, so some
cooling would take place.

--
Tom Sherman - Near Rock Island

 

[Jim Smith]

Frank Krygowski <[email protected]> writes:

> Spoke tension will restrain the diameter increase. The width increase
> will be only a few thousandths of an inch.

I agree with all of your comments except for this one. The force
required to prevent the wheel from expanding are much larger than the
spokes can apply. In any case, with a coefficient of thermal expansion
somewhere in the neighborhood of 2x10-5 per kelvin, a 622mm rim is
only going to expand a fraction of a millimeter.

 

[Joe Riel]

Frank Krygowski <[email protected]> writes:

>> When the rim heats it will expand in all directions. So the
>> rim will become larger in diameter (taller) and it will get
>> wider. This will cause the tire to stretch to fill the rim
>> and decrease its aspect ratio.
>
> Spoke tension will restrain the diameter increase. The width increase
> will be only a few thousandths of an inch.

Dang, I forgot about the (isothermal) spokes. So the thermal
expansion of the wire bead will tend to make the fit looser. That is,
for a 100degC rise the bead diameter increases about

(100degC)(1.5e-5/degC)(27inch) = 0.04inch.

Not insignificant, but not huge. Note that Kevlar has a negative CTE
(at least at room temperature, I don't know what it does at
100degC)---do Kevlar beaded tires blow-off less easily than wire
beaded tires during braking descents?

Joe

 

[Frank Krygowski]

Tom Sherman wrote:

> I believe that Mark and Christine are using a compressor to pressurize a
> reservoir to ~250 psi, letting the compressed air cool down to ambient
> temperature, then inflating the bicycle tire to normal inflation
> pressure. In this case, the air is being expanded into the tire, so some
> cooling would take place.

Ah. OK, that would be correct.


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

 

[Benjamin Lewis]

Frank Krygowski wrote:

> Mark and Christine wrote:
>
>> I am coming into this discussion a little late, so if I am
>> repeating something that has already been pointed out then I
>> appologize in advance. There have been a couple of "factual
>> errors" in recent posts. If no one with an engineering
>> background has corrected them then...
>
> Several people with strong engineering backgrounds have been following
> this thread. Your "corrections" have grevious factual errors, which is
> why nobody else has brought them up. Comments below:
>
>> Expanding air cools, compressing air heats. When you fill a tire you
>> are expanding the air (cooling) and then as the tire inflates the amount
>> of expansion decreases. It does not compress and heat. For all
>> practical purposes the tire "might" cool slightly.
>
> When you take atmospheric air at zero gage pressure and pump a tire to
> 100 psi gage pressure, you are _certainly_ compressing the air, not
> expanding it. During the pump compression stroke, the air within the
> pump is at a higher pressure still; but this is largely immaterial.

PV = NkT. Assuming volume inside the tire remains relatively constant
(once the tire is mostly inflated), and that the pressure is directly
proportional to the number of molecules, shouldn't the temperature remain
constant? (Once it has entered the tube, that is). I'm not sure what
"Mark and Christine" had in mind; perhaps that the volume inside the tube
is increasing as it inflates.

--
Benjamin Lewis

Luke, I'm yer father, eh. Come over to the dark side, you hoser.
-- Dave Thomas, "Strange Brew"

 

[[email protected]]

Mark and Christine writes:

> I am coming into this discussion a little late, so if I am repeating
> something that has already been pointed out then I appologize in
> advance. There have been a couple of "factual errors" in recent
> posts. If no one with an engineering background has corrected them
> then...

> Expanding air cools, compressing air heats. When you fill a tire
> you are expanding the air (cooling) and then as the...

This has got to be a troll, it is so bizarre. Let's not get carried
away by it. This is not the place to review high school physics for
those who missed it.

Jobst Brandt
[email protected]

 

[[email protected]]

On Tue, 11 Jan 2005 01:27:29 GMT,
[email protected] wrote:

>Mark and Christine writes:
>
>> I am coming into this discussion a little late, so if I am repeating
>> something that has already been pointed out then I appologize in
>> advance. There have been a couple of "factual errors" in recent
>> posts. If no one with an engineering background has corrected them
>> then...
>
>> Expanding air cools, compressing air heats. When you fill a tire
>> you are expanding the air (cooling) and then as the...
>
>This has got to be a troll, it is so bizarre. Let's not get carried
>away by it. This is not the place to review high school physics for
>those who missed it.
>
>Jobst Brandt
>[email protected]

Dear Jobst,

It isn't?

If I'm following the idea, M&C are talking about what
happens when air previously compressed to 250 psi and
allowed to cool to room temperature is then released into an
empty inner tube to a pressure between say 25 and 125
psi--you know, what happens at most gas stations with a
compressor.

True, no hairy-chested he-men bent on telling everyone what
not to talk about here on rec.bicycles.tech ever use a
gas-station air-chuck, but . . .

Isn't Nashbar having a sale on CO2 inflators? You know, the
kind that get real cold and frosty when you use them to fill
empty inner tubes?

It's all kind of irrelevant to the interesting test that
you're working on, but it's still kind of fascinating to see
a different point of view on how something as simple as an
inner tube gets pumped up.

Carl Fogel

 

[[email protected]]

Carl Fogel writes:

>>> I am coming into this discussion a little late, so if I am
>>> repeating something that has already been pointed out then I
>>> appologize in advance. There have been a couple of "factual
>>> errors" in recent posts. If no one with an engineering background
>>> has corrected them then...

>>> Expanding air cools, compressing air heats. When you fill a tire
>>> you are expanding the air (cooling) and then as the...

>> This has got to be a troll, it is so bizarre. Let's not get
>> carried away by it. This is not the place to review high school
>> physics for those who missed it.

> It isn't?

> If I'm following the idea, M&C are talking about what happens when
> air previously compressed to 250 psi and allowed to cool to room
> temperature is then released into an empty inner tube to a pressure
> between say 25 and 125 psi--you know, what happens at most gas
> stations with a compressor.

Hold the phone. The tire is being inflated. That means compressed
air enters the tire. If that air for one reason or another is colder
than the tire it is entering then it will warm up and give a little
more pressure than the gauge on the inflater registers, but not much
because the surface to volume ratio of a bicycle tire is far smaller
than a car tire where tires are routinely inflated from zero (ambient
atmospheric pressure). It's even more so for CO2 cartridges and even
they don't present a measurable change.

> True, no hairy-chested he-men bent on telling everyone what not to
> talk about here on rec.bicycles.tech ever use a gas-station
> air-chuck, but...

> Isn't Nashbar having a sale on CO2 inflators? You know, the kind
> that get real cold and frosty when you use them to fill empty inner
> tubes?

I think we get into theoretical effects of insignificant proportions
here and overlook the practical aspect of what counts. This posting
remains a troll no matter how you look at it.

> It's all kind of irrelevant to the interesting test that you're
> working on, but it's still kind of fascinating to see a different
> point of view on how something as simple as an inner tube gets
> pumped up.

I am not fascinated.

Jobst Brandt
[email protected]