Re: Exploding tires II

Quoted message said:

... I don't care what analytical results we get if
it is not shown to cause a blow-off. That they occur and easily, has
been demonstrated to my satisfaction. To calculate that it does not
occur doesn't help.

Perhaps you misunderstand our conclusions. We know the tire pressure
generated in such a descent is not enough, _on its own_, to blow the
tire. Therefore we know there must (also?) be some _other_ cause.

Quoted message said:

Quoted message said:

Sure sounds like myth and lore!

Well it isn't reality.

Jobst, you've got it exactly backwards, which is very unusual. The
reality is that many, if not most, tires routinely withstand 160 psi
when cold and stationary. And some pretty competent calculations have
shown that the pressure inside your tubes is not reaching that level.
So again, there must (also?) be some _other_ cause.

Quoted message said:

Quoted message said:

I'm looking for a model that involves more than the simplistic
notion that air pressure _alone_ is the cause, because I think we've
proven the necessary air pressure isn't there. I haven't seen a
rebuttal for that last idea.

I explained why I believe pressure is the main and probably only cause
and how this can be ascertained. I don't know what you find
"simplistic" about that.

You say air pressure is probably the only cause of these blowouts. Yet
we have reports of people applying far more air pressure and failing to
see blowouts. This makes it clear that _something_ else should be in
the explanation.

At this point, we don't know which tires and rims were used, and that's
potentially important. Maybe the 160 psi tests were on tires that fit
tightly, and the failures you observed were on tires that fit loosely.
If so, the lesson becomes "use tight fitting tires in the mountains."

Quoted message said:

Quoted message said:

Hmmm. If it _were_ just over-pressure caused by heat, then the
solution could be to deflate tires before a descent. Alternately,
we could design pressure-limiting relief valves for the valve core,
that would bleed off excess air - and inconveniently, stop to pump
tires once things had cooled.

Yes. how often would you like to do that?

At least once more often than I'd like to change a blown out tube! ;-)

As you can see, I do much

Quoted message said:

touring in mountains and would therefore do a lot of pumping and
letting out of air.

I understand, and agree. Unless this started happening nearly every
descent, of course.

A pressure relief valve is a difficult device to

Quoted message said:

tune properly and not one that I would like to sell to the average
rider considering liability.

And that makes some sense, too. OTOH, if such a valve existed for bike
tires, the liability sharks would likely come after the first bike shop
that didn't install it for mountain use!

But I digress. Again, neither the physics (thanks to Joe) nor the
observations so far presented indicate that it's about tire pressure
alone. If we want to understand this, we need to do more.

BTW, as I mentioned, we don't have mountains where I live. I can find
some very steep grades, but they're not long, and blowouts have been
almost nonexistent for me. (I can recall only one on a steep downhill
(tandem) and that hill wasn't more than 1/4 mile long.)

So my interest in understanding the details of the cause is mere
curiosity. I'd think yours would be greater.

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

In article <[email hidden]>,

Frank Krygowski said:

But if you believe that, you have to somehow reconcile that the
resulting air pressure during a descent is much less than what tires
have routinely been shown to hold, when cold. How do you explain this
discrepancy?

One phenomenon I've noticed in rubber is that its friction goes down
with heat. In the case of my work boots (pretty similar in material to
tires) if I step on steel plate that's been out in the sun all day it's
surprisingly slick.
Perhaps we should measure the friction coefficient between tire
rubber and aluminum at various temperatures.

--
B.B. --I am not a goat! thegoat4 at airmail.net

B.B. the Goat said:

Quoted message said:

But if you believe that, you have to somehow reconcile that the
resulting air pressure during a descent is much less than what
tires have routinely been shown to hold, when cold. How do you
explain this discrepancy?

Quoted message said:

One phenomenon I've noticed in rubber is that its friction goes down
with heat. In the case of my work boots (pretty similar in material
to tires) if I step on steel plate that's been out in the sun all
day it's surprisingly slick.

Quoted message said:

Perhaps we should measure the friction coefficient between tire
rubber and aluminum at various temperatures.

No tire rubber makes contact with the rim nor is the bead made of
rubber. Tire beads in my blow-off experience have a steel wire core
and a double layer of casing cords bonded with an elastomer that is
not rubber. Various tire manufacturers show schematic cross sections
of this construction. The bead has a fabric wear layer that
withstands chafing and flexing.

Jobst Brandt
[email hidden]

Instrumentation:

This device has two problems. It is too expensive and it attempts to
measure air temperature, something that it cannot do at the end of a
valve stem..., but otherwise it has great specifications:

http://www.microdaq.com/mt/pressure/prtemp1000.php

Jobst Brandt
[email hidden]

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

Quoted message said:

Frank Krygowski said:

There are many extrudable alloys - 2024, 5086, 6061, 6063, 7075 and
many others - but I don't know which go into rims.

I'd bet MATWEB would have much of the info on those alloys. You could
get an idea how variable those properties are between alloys. Density
would be almost uniform, and maybe Cp. Conductivity, I'll bet, varies.

Thanks for the reference, I wasn't aware of that site <www.matweb.com>.
Checking those gives

Type k (W/m/K)
------- ----------
2024-XX 121-193
5086-XX 125
6061-XX 154-180
6063-XX 193-218
7075-XX 130-155

The range depends on the XX (O, T1, T2, etc) which I assume indicates
heat treatment. What would be appropriate for a rim?

Quoted message said:

Also, what is a reasonable number for the

Quoted message said:

cross-sectional area of the material? From a web search I'm currently
using 82mm^2; however it wasn't clear what rim that was for. I'd
prefer a number for an MA-40 type rim. An electronic drawing of the
cross section, so that I can include it in the paper, would be ideal.

I once figured cross sectional areas of a few rims, using two
methods. First is just to take the rim mass and assume the center of
mass is at the bead seat diameter.

mass = area * (circumference at center of mass) * density

and again, density's pretty uniform, about 2.77 g/cm^3

My second method was to use a hacksawed section of rim as a rubber
stamp and break the resulting cross section down into simple shapes.
Tedious.

I have a planimeter, so measuring the area is straightforward,
however, I don't have a rim lying around to sacrifice---I tossed
my old rims when I moved, alas.

Quoted message said:

The two methods agreed acceptably. A Super Champion clincher rim (Mod
58? Not sure) gave 0.152 square inches by one method, 0.153 by
another. That's 9.84 *10^-5 m^2, or 98.4 mm^2. That's a 530 gram rim.

Joe

hey joe i have a3d model of a wheel right down to a pretty accurate mavic rim
drop me a line if you want it

Trevor said:

DRS wrote in message ...

Quoted message said:

"Trevor" <[email hidden]> wrote in message
news:[email hidden]

Quoted message said:

[email hidden] wrote in message ...
> Mike Jacoubowsky writes:
>
>> 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?
>
So what pressure does a track cyclist use with 20mm high pressures?
I think it would be near 150psi in most cases.

FWIW, when Anna Meares was interviewed the other night at one point she

said

Quoted message said:

she was running at 220psi, but obviously she was using tubulars.

That pressure amazes me. Anna Meares, Australian, sprint gold medal?
What makes it obvious tubulars were used?
I do remember something about Specialized blowing a rim apart before their
tyre failed, and it was recommended their 25mm(really 20mm) tyres may be
taken up to 150psi although the tread marking indicated use at 115psi.

Trevor

Dear Trevor,

Amazing, but 145-220 psi is what some racing tubulars are
supposed to be able to handle:

http://www.tufo.com/index.php?lg=en&mn=3&id=26

Carl Fogel

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

Quoted message said:

Jim Smith said:

By definition the flow of air around the rim is not laminar because the
rim is constantly accelerating. Consider a point directly in front
of the axle: it is moving at a speed equal to the bicycle in a direction
pointed 45 degrees towards the ground, one half revolution later, when
this point is directly behind the axle, it is again moving at the
speed of the bicycle but pointing 45 degrees up, the velocity vector
for that point having rotated through 270 degrees.

I'm not sure that is sufficient to say that the flow is not locally
laminar (my term). Surely the flow across the blades of a slow speed
fan is modeled as laminar, even though the velocity vector is changing?

Quoted message said:

Q_convection may be proportional to \sqrt{v}, but the flow is most
definitely not laminar.

So what is it called? I assume it isn't turbulent. The significant
feature, of course, is whether convection is proportional to \sqrt{v},
at least for determining the worst-case velocity.

Quoted message said:

Also, just a minior nitpick, but you state that the "s" in Fg = mgs
is the slope, when of course it is really sin(atan(slope)).

Which is s/sqrt(1+s^2) and within 0.5% at a slope of 10%. A far
better approximation than the others, undoubtedly. But I'll make a
note of it. I did screw up the math there, I drew a vector diagram
and interpreted it incorrectly.

Thanks,

Joe

Didn't know where to put this comment in this thread so I'll just
stick it at the end.

What about thermal expansion of the metals. The rim and bead are both
expanding and at different rates. A heated bead would be looser and
not only be easier to roll off but also metals lose strength (almost
always) at increased temperature. So at a higher temperature it would
require less force to get the tire off. Maybe that's the other
mechanism you're looking for.

By the way, I work for a temperature sensor manufacturer so if someone
wants to run the test I'll supply the thermocouples or RTDs or
thermistors.

Bill Schuh
Watlow

Bill Schuh said:

What about thermal expansion of the metals. The rim and bead are
both expanding and at different rates. A heated bead would be
looser and not only be easier to roll off but also metals lose
strength (almost always) at increased temperature. So at a higher
temperature it would require less force to get the tire off. Maybe
that's the other mechanism you're looking for.

As I said, my experience is exclusively with steel bead tires on
aluminum rims. This would not fit that model but rather one that
makes a tighter fit.

Quoted message said:

By the way, I work for a temperature sensor manufacturer so if
someone wants to run the test I'll supply the thermocouples or RTDs
or thermistors.

What I believe we need is a data-logger that can accept input from a
pressure sensor and thermocouple. The idea is to have a portable unit
that can be secured to a wheel along with a thermocouple on the rim
and a pressure sensor attached to a valve stem extension. This makes
it transportable and the tube replaceable. ZAP glue can attach the
thermocouple to the inner circumference of the rim. No external
connections are needed and the data can be dumped afterwards to a
display device.

The goal is to have rim temperature and air pressure plotted against
time. The test runs can be made at different speeds to give that
effect, but that doesn't require logging other than the tester writing
it down.

Do we have any volunteers?

Jobst Brandt
[email hidden]