Potential good news for Mt. Washington access. - Page 18

01-10-2005, 04:11 PM

Frank Krygowski <[Only registered and activated users can see links. ]> 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.

01-10-2005, 04:11 PM

Frank Krygowski <[Only registered and activated users can see links. ]> 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.

01-10-2005, 04:15 PM

Frank Krygowski <[Only registered and activated users can see links. ]> 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

01-10-2005, 04:15 PM

Frank Krygowski <[Only registered and activated users can see links. ]> 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

01-10-2005, 04:15 PM

Frank Krygowski <[Only registered and activated users can see links. ]> 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

01-10-2005, 04:16 PM

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]

01-10-2005, 04:16 PM

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]

01-10-2005, 04:16 PM

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]

01-10-2005, 04:22 PM

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"

01-10-2005, 04:22 PM

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"