Potential good news for Mt. Washington access.

[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]

 

[Mark and Christine]

On Mon, 10 Jan 2005 19:53:20 -0500, Frank Krygowski
<[email protected]> wrote:

>
>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:
>

Since I was trying to be nice and add something, and you
were obviously doing the opposite...

Boyles law only works in absolutes. If you use C for the
temp then you must add 273 to your temp on each side, but
only 14.7 for pressure and nothing for volume. You do the
math and see if temp will do anything to the pressure of a
tire in a temperature range that would not first weaken the
tire.

As for friction, you simply do not understand real world
friction applications if you do not think that friction
occurs unless the tire is skiding. Max heating of a tire
occurs at the speed just before the tire stops rotating.
The moment a tire skids friction drops significantly.
Anti-lock brakes?

Mark

PS, Why am I, and bunch of others, double posting
everything to the group tonight?

 

[Joe Riel]

Mark and Christine <[email protected]> writes:

> As for friction, you simply do not understand real world
> friction applications if you do not think that friction
> occurs unless the tire is skiding.

We understand the real world quite well, however, that
triple-negative is a bit hard to parse.

Your original (incorrect) point was that the friction of the road
against the tire somehow heated up the air in the tube. It doesn't,
essentially all the heat is generated in the brake/rim interface. The
tire friction is significant only in that without we couldn't generate
the braking friction, however, no significant amount of heat is being
generated at the tire/road interface unless the tire is skidding. And
a skidding tire is not of interest to this discussion (for several
reasons).

> Max heating of a tire
> occurs at the speed just before the tire stops rotating.

That makes no sense. There is no given speed at which a tire stops
rotating. Anyhow, the velocity at which maximum power generation
occurs is not the worst-case for temperature rise under steady-state
braking, one has to consider convection cooling at the rim. A
calculation I did some time ago suggests that the worst-case velocity
is Vterm/sqrt(5), where Vterm is the terminal velocity of the rider.

Joe

 

[Tim McNamara]

[email protected] writes:

> Tim McNamara writes:
>
>> 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.
>
>> 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.

Well, to answer this I had to look at the pile of tires in my
basement. All appeared to have some kind of chafing strip, although
most (Avocet Duro, Ritchey Tom Slick, Michelin Synergic) appeared to
be coated with the same rubber covering the sidewalls. The
Continental chafing strip seemed to have little or no rubber
coating. That was, I guess, the visual I had in mind when I wrote my
comment- the chafing strip is more visible on the Continentals.

>> 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.

The insulated rim strip, anyway. But I'm puzzled by the utility of
this, given your comments last August:

>> 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?

I misread, apparently. However, in the context of the thread back in
August it was suggested from the laws of physics that the temperature
increase in the tube would result in a small rise in pressure- from
100 psi to 126 psi with a temperature increase from about 60 F to 250
F, for example, well within the normal capacity of rims and tires to
withstand. This would seem to suggest that there is some other
mechanism other than increased pressure caused by heating the rims
from braking.

However, I'm obviously no physicist nor an engineer, and I'm looking
forward to your findings- one measurement being worth a thousand
opinions.

 

[Joe Riel]

Jim Smith points out that spoke tension is insufficient to
significantly restrain rim diameter from increasing due to thermal
expansion. While contemplating this on my evening walk, I wondered
whether the resulting spoke tension increase could be a significant,
and overlooked, factor affecting spoke life.

A few calculations are in order.

Let
dCr = (delta) increase in rim compressive force
dTs = (delta) increase in spoke tension
n = number of spokes = 36
R = wheel radius
As = spoke cross-sectional area ~ 2mm^2
ks = spoke elasticity ~ 30Mpsi
Fs = ks*As ~ 42,000kgf
a = coefficient of thermal expansion of rim ~ 25e-6/degC
Ar = rim cross-sectional area ~ 80mm^2
kr = rim elasticity ~ 10Mpsi
Fr = kr*Ar ~ 560,000kgf
dT = rim temperature increase
P = rim perimeter
dPt = change in P due to temperature increase in rim
dPc = change in P due to compressive force in rim

(1) dCr = n*dTs/2/pi
(2) dR/R = dTs/Fs
(3) dR/R = (dPt+dPc)/P
(4) dPt/P = a*dT
(5) dPc/P = -dCr/Fr

expanding (3) and plugging in (4) and 5 we get

(6) a*dT - dCr/Fr = dTs/Fs

Using (1) to eliminate dCr gives

(7) a*dT = dTs*(1/Fs + (n/2/pi)/Fr)
(7a) = dTs/Feff

where
1/Feff = 1/Fs + (n/2/pi)/Fr
~ 30,000kgF

From (7a) we find that

(8) dTs/dT = a*Feff ~ 0.75kgf/degC

So a 100degC rise in the rim temperature increases the spoke
tension by 75kgf (165lbf). This is not a trival amount, it
represents about 25% of the ultimate strength of a spoke.

Comments? I have ignored bending in the rim because
I couldn't easily compute its effect. Presumably it
significantly reduces the tension increase in the spokes.


Joe Riel

 

[Joe Riel]

Joe Riel <[email protected]> writes:

> where
> 1/Feff = 1/Fs + (n/2/pi)/Fr
> ~ 30,000kgF

That should be

Feff ~ 30,000kgF

Nothing else changes.

Joe

 

[A Muzi]

>> 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.

Frank Krygowski wrote:
> 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.

Most likely an installation issue.

--
Andrew Muzi
www.yellowjersey.org
Open every day since 1 April, 1971

 

[Chris Neary]

>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.

Rim heating gets a lot of discussion on the tandem list, for obvious
reasons.

Several posters anecdotally believe that deep dish rims are helpful because
of the extra mass and the area which can be cooled.

I've put forward the theory that rims with ceramic braking surfaces *may*
offer additional resistance to heating as the ceramic *may* serve as an
insulating layer reducing heat induction into the aluminum rim. I found at
least one technical source which appeared to support this idea, but no "real
world" testing has been performed to develop real data on bikes.


Chris Neary
[email protected]

Chris & Tracey
1999 Co-Motion Speedster

 

[nobody]

On Tue, 11 Jan 2005 06:37:07 GMT, Joe Riel <[email protected]> wrote:


[snip maths re: spoke tension vs. rim temp]

Following on from your (Joel) and Carl's brief dialogue,
ISTR that at least two people who frequent this group
have constructed FE models of the bicycle wheel
hub-spokes-rim, primarily to investigate the validity
of the "stands on its spokes" statement.

Maybe, if asked nicely, they could be persuaded to
superpose a thermal or equivalent initial strain loadcase
on the rim elements and report their findings thereafter?

 

[Joe Riel]

nobody <[email protected]> writes:

> On Tue, 11 Jan 2005 06:37:07 GMT, Joe Riel <[email protected]> wrote:
>
>
> [snip maths re: spoke tension vs. rim temp]
>
> Following on from your (Joel) and Carl's brief dialogue,
> ISTR that at least two people who frequent this group
> have constructed FE models of the bicycle wheel
> hub-spokes-rim, primarily to investigate the validity
> of the "stands on its spokes" statement.
>
> Maybe, if asked nicely, they could be persuaded to
> superpose a thermal or equivalent initial strain loadcase
> on the rim elements and report their findings thereafter?

Do you recall who they were? I don't. I also don't recall
seeing any followup to that, that is, whether they actually
built the models.

Joe

 

[[email protected]]

Joe Riel writes:

> You [and I] are ignoring the bending stress in the rim due to the
> discrete spoke locations. Isn't that where the yield problem for
> the rim lies?

The rim fails in column buckling at the spoke holes from the
compressive load. the valve hole in some rims being the weakest
location. That's why the rim goes into a buckling "S" bend, being
constrained by the spokes from any single large lateral excursion as a
free standing column (pole vault pole) would.

Jobst Brandt
[email protected]