Potential good news for Mt. Washington access.

[Benjamin Lewis]

jobst brandt wrote:

> Frank Krygowski writes:
>
>>> I am working on a test for blow-off. I made a valve stem pressure
>>> sensor adapter that, when installed, opens the Presta valve to an
>>> aneroid pressure sensor that sends its readings to a data-logger
>>> (about the size of a deck of cards) that also reads a thermocouple
>>> attached to the rim. The data logger is tied to the inside of the
>>> wheel near the hub and has a USB connector to download test data to
>>> a PC where it can be charted.
>
>>> Not to worry, the results will be announced.
>
>> Excellent!
>
>> When you do this, please compare with a static pressure test of the
>> same tire and rim at room temperature.
>
> What do you mean by this. Inflation pressure is inflation pressure
> whether the wheel is rotating or not.

Are you saying that tire blow off depends only on pressure and not on
temperature? The test Frank proposes could confirm this.

--
Benjamin Lewis

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

 

[[email protected]]

Benjamin Lewis writes:

>>>> I am working on a test for blow-off. I made a valve stem
>>>> pressure sensor adapter that, when installed, opens the Presta
>>>> valve to an aneroid pressure sensor that sends its readings to a
>>>> data-logger (about the size of a deck of cards) that also reads a
>>>> thermocouple attached to the rim. The data logger is tied to the
>>>> inside of the wheel near the hub and has a USB connector to
>>>> download test data to a PC where it can be charted.

>>>> Not to worry, the results will be announced.

>>> When you do this, please compare with a static pressure test of the
>>> same tire and rim at room temperature.

>> What do you mean by this. Inflation pressure is inflation pressure
>> whether the wheel is rotating or not.

> Are you saying that tire blow off depends only on pressure and not
> on temperature? The test Frank proposes could confirm this.

As I mentioned, rims reach their peak temperatures quickly on steep
descents and we don't have problems because these are transients. Rim
temperatures drop rapidly as soon as the brake is released if the road
is fairly straight and allows cooling speeds. Treacherous roads are
steep ones that require nearly continuous braking.

The problem with a static test is that I believe wheel rotation may
have an influence on how predictably the tire releases, there being
constant motion between rim and tire. This is evident from the wear
marks in the rim and the presence of a chafing strip on the tire bead
at that interface.

Jobst Brandt
[email protected]

 

[Dennis Ferguson]

Matt O'Toole <[email protected]> wrote:
>[email protected] wrote:
>> This isn't about MTB's. It is primarily a road bicycle problem. It
>> takes the same pressure to blow a skinny tire off a rim as a fat one,
>> the interface being the inside width of the rim, on which inflation
>> pressure acts (between tire and rim). If your 2.5" MTB tire on a
>> narrow rim can withstand 140psi then you may be able to experience
>> blow-off. On the other hand, starting at 45psi, I doubt that you
>> could get enough pressure even if you have a wider rim.
>
>So perhaps a 25mm or 28mm tire at 90psi is safer than a 23mm one at 120psi?
>Care to make an educated guess at what the threshold of safety is? Wouldn't it
>be interesting to test this?

Finding tires/pressures/insulation methods to make a failure at
high rim temperatures less likely is one way at the problem. Reducing
the rim temperatures might be another.

Since the increase in rim temperature above ambient should be inversely
proportional to the external surface area of the rim, and since unseparated
air flow over the rim's entire surface should enhance cooling even further,
one might expect an all-alloy, aerodymanic, deep-V rim to run significantly
cooler than a normal U-shaped rim.

Dennis Ferguson

 

[Frank Krygowski]

[email protected] wrote:

> Benjamin Lewis writes:
>
>
[FK wrote:]
>>>>When you do this, please compare with a static pressure test of the
>>>>same tire and rim at room temperature.
>
>
>>>What do you mean by this. Inflation pressure is inflation pressure
>>>whether the wheel is rotating or not.
>
>
>>Are you saying that tire blow off depends only on pressure and not
>>on temperature? The test Frank proposes could confirm this.
>
>
> As I mentioned, rims reach their peak temperatures quickly on steep
> descents and we don't have problems because these are transients. Rim
> temperatures drop rapidly as soon as the brake is released if the road
> is fairly straight and allows cooling speeds. Treacherous roads are
> steep ones that require nearly continuous braking.
>
> The problem with a static test is that I believe wheel rotation may
> have an influence on how predictably the tire releases, there being
> constant motion between rim and tire. This is evident from the wear
> marks in the rim and the presence of a chafing strip on the tire bead
> at that interface.

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.


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

 

[Tom Sherman]

Frank Krygowski wrote:

> [email protected] wrote:
>
>> Benjamin Lewis writes:
>>
>>
> [FK wrote:]
>
>>>>> When you do this, please compare with a static pressure test of the
>>>>> same tire and rim at room temperature.
>>
>>
>>
>>>> What do you mean by this. Inflation pressure is inflation pressure
>>>> whether the wheel is rotating or not.
>>
>>
>>
>>> Are you saying that tire blow off depends only on pressure and not
>>> on temperature? The test Frank proposes could confirm this.
>>
>>
>>
>> As I mentioned, rims reach their peak temperatures quickly on steep
>> descents and we don't have problems because these are transients. Rim
>> temperatures drop rapidly as soon as the brake is released if the road
>> is fairly straight and allows cooling speeds. Treacherous roads are
>> steep ones that require nearly continuous braking.
>>
>> The problem with a static test is that I believe wheel rotation may
>> have an influence on how predictably the tire releases, there being
>> constant motion between rim and tire. This is evident from the wear
>> marks in the rim and the presence of a chafing strip on the tire bead
>> at that interface.
>
>
> 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.

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.

--
Tom Sherman - Near Rock Island

 

[[email protected]]

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.

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.

Jobst Brandt
[email protected]

 

[Alfred Ryder]

Frank Krygowski 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.
>

While we are all offering gratuitous suggestions to the one doing the work,
let me put in mine.

What about expanding Frank's suggested lab test as follows. Attach a heating
element to the rim, either bonded to the braking surfaces or inside the rim
somewhere. The amount of thermal energy put into the wheel would be easy to
know. Also monitoring the temperature and pressure of the air in the tube,
temperature of the rim, etc. would be much easier in the lab than on the
road. Then plot the blow-off point as a function of rim temperature and air
pressure, both being controllable variables.

Such a lab experiment would not replace the road test. But I agree with
Frank that it would add information, probably necessary information. (My
suspicion is that the rim can get very hot for a long time before the tube
air temperature changes very much.)

I would be interested in knowing whether the blow-off point is affected by
whether someone is sitting on the bike or not. The only blow-off I have had
was a few seconds after coming to a complete stop. And the only ones I have
heard about where when the rider was going very slowly. Is it possible that
the weight of the rider adds some asymmetry that facilitates blow-off?

Forgive the above. If I were doing the experiment, I would be offended by
everyone standing around offering suggestions..

 

[[email protected]]

Alfred Ryder writes:

>> 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.

> While we are all offering gratuitous suggestions to the one doing
> the work, let me put in mine.

> What about expanding Frank's suggested lab test as follows. Attach a
> heating element to the rim, either bonded to the braking surfaces or
> inside the rim somewhere. The amount of thermal energy put into the
> wheel would be easy to know. Also monitoring the temperature and
> pressure of the air in the tube, temperature of the rim, etc. would
> be much easier in the lab than on the road. Then plot the blow-off
> point as a function of rim temperature and air pressure, both being
> controllable variables.

> Such a lab experiment would not replace the road test. But I agree
> with Frank that it would add information, probably necessary
> information. (My suspicion is that the rim can get very hot for a
> long time before the tube air temperature changes very much.)

> I would be interested in knowing whether the blow-off point is
> affected by whether someone is sitting on the bike or not. The only
> blow-off I have had was a few seconds after coming to a complete
> stop. And the only ones I have heard about where when the rider was
> going very slowly. Is it possible that the weight of the rider adds
> some asymmetry that facilitates blow-off?

> Forgive the above. If I were doing the experiment, I would be
> offended by everyone standing around offering suggestions.

I have seen tires blow off after 1/4 mile of steep descending,
starting from "room temperature" rims after reaching the summit of the
climb. As I said, heating is fast and hot on grades over 10%.

The Stelvio:

http://www.paloaltobicycles.com/alps_photos/i49.html
http://www.paloaltobicycles.com/alps_photos/i02.html
http://www.paloaltobicycles.com/alps_photos/i43.html
http://www.paloaltobicycles.com/alps_photos/i68.html
http://tinyurl.com/len5

The last picture gives a realistic view of the Stelvio and that even
though it is steep, the straight runs allow reaching terminal velocity
and therefore, adequate cooling to prevent overheating the rims. I
have taken a few tours of the Alps, pumping my tires to 100psi before
departure from SFO and not pumping them again until a couple of weeks
after my return home. On these trips, there were times when I waited
for the rims to cool before continuing down a steep grade.

Jobst Brandt
[email protected]

 

[Benjamin Lewis]

jobst brandt wrote:

> 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.
>
> 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.

Then shouldn't it help to know as much as possible about what factors
affect blowoff? In addition to the static case, you could ride the bike
with increasingly higher pressures, without braking, to test your
hypothesis that motion between the tire and rim is contributory. If
temperature as well as pressure were a factor, perhaps using a different
material for the tire bead would help.

--
Benjamin Lewis

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

 

[Joe Riel]

"Alfred Ryder" <[email protected]> writes:

> While we are all offering gratuitous suggestions to the one doing the work,
> let me put in mine.
>
> What about expanding Frank's suggested lab test as follows. Attach a heating
> element to the rim, either bonded to the braking surfaces or inside the rim
> somewhere. The amount of thermal energy put into the wheel would be easy to
> know. Also monitoring the temperature and pressure of the air in the tube,
> temperature of the rim, etc. would be much easier in the lab than on the
> road. Then plot the blow-off point as a function of rim temperature and air
> pressure, both being controllable variables.
>
> Such a lab experiment would not replace the road test. But I agree with
> Frank that it would add information, probably necessary information. (My
> suspicion is that the rim can get very hot for a long time before the tube
> air temperature changes very much.)

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.

Joe

 

[Mark Hickey]

Benjamin Lewis <[email protected]> wrote:

>jobst brandt wrote:

>> 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.
>
>Then shouldn't it help to know as much as possible about what factors
>affect blowoff? In addition to the static case, you could ride the bike
>with increasingly higher pressures, without braking, to test your
>hypothesis that motion between the tire and rim is contributory. If
>temperature as well as pressure were a factor, perhaps using a different
>material for the tire bead would help.

I think Jobst nailed it on the head. While it might be interesting to
explore all the nuances of this phenomenon, all that really matters is
what the tire does when actually rolling down a steep hill with a
rider on board.

But to add my voice to the chorus of armchair testers, I'd suggest
that it might be interesting to see the effects of using a deep rim on
the rear wheel to see how effective the extra mass is at getting rid
of the heat.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame

 

[[email protected]]

Benjamin Lewis 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.

>> 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.

> Then shouldn't it help to know as much as possible about what
> factors affect blowoff? In addition to the static case, you could
> ride the bike with increasingly higher pressures, without braking,
> to test your hypothesis that motion between the tire and rim is
> contributory. If temperature as well as pressure were a factor,
> perhaps using a different material for the tire bead would help.

OK. Do it. I'm interested in brake heating and the resulting hazard
of crashing... fatally. You can perform the tests you propose without
instrumentation so what's keeping you from doing so? You could "know
as much as possible about what factors affect blowoff."

Jobst Brandt
[email protected]

 

[Frank Krygowski]

Alfred Ryder wrote:

>
> I would be interested in knowing whether the blow-off point is affected by
> whether someone is sitting on the bike or not. The only blow-off I have had
> was a few seconds after coming to a complete stop.

When our tandem was brand new (long, long ago) it suffered two blowouts
as it sat alone in a bedroom, at least an hour after mounting and
inflating the tires. I never did figure that out.


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

 

[Tom Sherman]

Jobst Brandt wrote:

> ...We are interested in safely riding down steep grades.

And one should add, "without increasing weight, expense, and mechanical
complexity."

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.

--
Tom Sherman - Near Rock Island

 

[Benjamin Lewis]

jobst brandt wrote:

> Benjamin Lewis writes:
>
>> Then shouldn't it help to know as much as possible about what
>> factors affect blowoff? In addition to the static case, you could
>> ride the bike with increasingly higher pressures, without braking,
>> to test your hypothesis that motion between the tire and rim is
>> contributory. If temperature as well as pressure were a factor,
>> perhaps using a different material for the tire bead would help.
>
> OK. Do it. I'm interested in brake heating and the resulting hazard
> of crashing... fatally. You can perform the tests you propose without
> instrumentation so what's keeping you from doing so? You could "know
> as much as possible about what factors affect blowoff."

How could I know that without also doing your brake heating experiment
(which I'm not about to do)? Neither experiment by itself will determine
if there is a temperature dependence. If you don't think knowing whether
this is the case would have any practical benefit then fine, but I think
many people would be interested in this.

--
Benjamin Lewis

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

 

[Frank Krygowski]

[email protected] wrote:

> Benjamin Lewis writes:
>
>
>>Then shouldn't it help to know as much as possible about what
>>factors affect blowoff? In addition to the static case, you could
>>ride the bike with increasingly higher pressures, without braking,
>>to test your hypothesis that motion between the tire and rim is
>>contributory. If temperature as well as pressure were a factor,
>>perhaps using a different material for the tire bead would help.
>
>
> OK. Do it. I'm interested in brake heating and the resulting hazard
> of crashing... fatally. You can perform the tests you propose without
> instrumentation so what's keeping you from doing so? You could "know
> as much as possible about what factors affect blowoff."

???

Benjamin could make the test he proposes using Sun rims and Kenda tires.
Jobst could make his test using Rigida rims and Avocet tires. We'd
get different results for pressure at blowout, and then we'd be left to
wonder: How much of the difference in behavior is due to differences in
design and material, as opposed to the effects of temperature and
braking force? We could never tell.

The most difficult part of this, by FAR, is what Jobst is doing. Most
of us would not be able to pull it off. But the knowledge gained from
his work will be FAR greater if he also does a couple more very easy
tests, for example static pressure to blow out.

I can't imagine why he'd do all the hard work, but refuse to do the easy
work that would allow for direct comparisons.


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

 

[Matt O'Toole]

Mark Hickey wrote:

> But to add my voice to the chorus of armchair testers, I'd suggest
> that it might be interesting to see the effects of using a deep rim on
> the rear wheel to see how effective the extra mass is at getting rid
> of the heat.

I thought about this too. It's not the extra mass that gets rid of the heat
though. It's the greater area exposed to airflow. The extra mass makes for a
bigger heatsink, which rises in temperature more slowly. A heavier rim will get
just as hot if the hill is long enough and steep enough, if that heat isn't
dissipated to the air. Ultimately the heat transfer is what makes the
difference.

I agree though, that it would be interesting to compare a normal rim with a
heavy aero one.

Matt O.

 

[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..

 

[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]