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PostPosted: Tue Sep 11, 2012 5:03 pm 
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Does anyone, on this forum, use Co2 for anything else than emergency puncture repair fill up? And does anyone, on this forum, not fill up the tires before every ride, to get the exact pressure? I don't know about you guys, but I only use Co2 when I puncture and it only needs to last the rest of that ride, next ride I'm back on air from the floor pump anyway.

So what's the point, you really need nitrogen for half a ride, every 20:th or 30:th ride (and that's at worst case scenario for most)?

IRL, the whole idea just adds up to being completely useless and the solution to a non-existant problem.

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Posted: Tue Sep 11, 2012 5:03 pm 


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PostPosted: Tue Sep 11, 2012 6:12 pm 
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dkwperth wrote:
one reason nitrogen is used in car race tyres and in aircraft tyres is because it is a cheap way to eliminate oxygen from the tube and replace it with an inert gas.
Yes, but it also used for vehicles/aircraft prevent moisture accumulation which tend to shorten the service life of electronic sensors used for tire pressure monitoring systems.

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PostPosted: Tue Sep 11, 2012 6:59 pm 
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A few quick tidbits, which may help to clear up a few things:

1) Air is a mixture of nitrogen, oxygen, carbon dioxide, water vapor, and a few other gases in trace amounts. The primary reason why (pure) nitrogen is used in motor racing tires is that it is a 'dry' gas (contains no water vapor). Therefore, it expands/contracts with temperature in a very predictable way. Racing tires have very large changes in temperature and because inflation pressure is highly important controlling tire traction, they want to limited and predictable changes in pressure. Water vapor condensation/evaporation with temperature can create larger and less predictable changes in pressure, and therefore unpredictable changes in traction.

The temperature stability of nitrogen is also important in aircraft tires, which are often subject to temperatures well below freezing.

Another advantage to dry gases in motor racing and aircraft tires is water and/or other contaminants can lead to corresion of (highly stressed) metal wheels and, and also damage their electronic pressure monitoring systems.

Bicycle tires do not normally operate under the as large a temperature range as motor racing tires, and the higher pressure of bicycle tires results in less evaporation of any liquid water in the tires, and do not have pressure monitoring systems, so the use of nitrogen has not particular advantage in bicycle tires.


2) Carbon dioxide can be easily stored as a liquid at room temperature. Because of its low boiling point, nitrogen can not be held as a liquid in a small, light, easily portable cartridge. This gives a carbon dioxide much higher expansion efficiency for an inflation device, making it one of the most efficient compressed gases for portable inflation.


3) Carbon dioxide molecules are very large, and do not easily leak from gas containers. However, carbon dioxide shares a greater affinity to rubber than do nitrogen and oxygen, so carbon dioxide diffuses through rubber at a greater rate. This makes carbon dioxide a poor gas for tire inflation for stability over extended time periods.


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PostPosted: Tue Sep 11, 2012 7:03 pm 
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Illuminate wrote:
timzcat wrote:
Reason for using Nitrogen in car tires which is becoming more common now a days is the stability. It contains no oxygen and no moisture so it is not susceptible to temperature change.
Effectively your tires do not get low on pressure when the temperature outside drops. Pressure does not go up when the tire heats up.


Sorry, the scientist in me has to call bullsh!t on this one.

Ideal gas law applies to nitrogen molecules (N2) too.

PV=NRT.

Cheers

Ideal gas law. Real gases are not ideal. The question is the significance of the deviation. Is it possible that nitrogen is less susceptible to temperature change than regular air, and that this difference is due to the non-ideal nature of the gases (difference in molecular sizes and intermolecular attractions)? What sayeth the Van der Waals equation in this matter? Also, MarkMcM makes a point about the predictability of pressure change with temperature change.

The equation of state given here applies only to an ideal gas, or as an approximation to a real gas that behaves sufficiently like an ideal gas. There are in fact many different forms of the equation of state for different gases. Since it neglects both molecular size and intermolecular attractions, the ideal gas law is most accurate for monatomic gases at high temperatures and low pressures. The neglect of molecular size becomes less important for lower densities, i.e. for larger volumes at lower pressures, because the average distance between adjacent molecules becomes much larger than the molecular size. The relative importance of intermolecular attractions diminishes with increasing thermal kinetic energy, i.e., with increasing temperatures. More detailed equations of state, such as the van der Waals equation, allow deviations from ideality caused by molecular size and intermolecular forces to be taken into account.


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PostPosted: Tue Sep 11, 2012 8:32 pm 
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What about hydrogen? Very light, and if you have a blowout you get a fireball as well!

Or Helium? Also much lighter than oxygen......you'd probably float right up climbs!

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PostPosted: Tue Sep 11, 2012 8:33 pm 
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HammerTime2 wrote:
Also, MarkMcM makes a point about the predictability of pressure change with temperature change.


So... does this in any way begin to answer the question I asked on the last page which no one so far has answered?

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PostPosted: Tue Sep 11, 2012 10:10 pm 
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Hi,

If only to make you happy; the answer is yes.

Now that we've said that there's bound to be someone going to state that inner tubes can not blow up anyhow.....Right? :P

Ciao, ;)

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PostPosted: Wed Sep 12, 2012 8:06 pm 
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prendrefeu wrote:
HammerTime2 wrote:
Also, MarkMcM makes a point about the predictability of pressure change with temperature change.


So... does this in any way begin to answer the question I asked on the last page which no one so far has answered?


Theoretically, filling the tires with nitrogen would ever so slightly decrease the chance for a tire blow-off. But in practice, the difference is too small to worry about.

Tests have shown that the increase in pressure with temperature is a secondary factor in tire blow-off. An increase in temperature from 70 deg. F to 250 deg. F would only increase the pressure by about 30%, which for typical inflation pressures would still be below the blow-off pressure of most tire/rim combinations. For example, you typically inflate tires to 100 psi, under the heat of extreme braking energies the pressure might only increase to about 130 psi.

The primary cause of tire blow-off under braking is changes in the tire bead properties under heat - specifically, an increase in bead elasticity (allowing it to be squeezed and flattened) and a decrease in the friction coefficient between tire and rim (allowing the bead to slide more easily over the edge of the rim). Combined with the circumferntial shearing action between tire and rim under braking forces, a hot tire bead can more easily creep out of the rim "hook". The extra pressure with temperature provides additional force to push the tire off the rim, but it is not the primary cause of tire blow-off under heat.


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PostPosted: Wed Sep 12, 2012 8:22 pm 
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Great answer, thank you! :thumbup:

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PostPosted: Wed Sep 12, 2012 8:41 pm 
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While the basic conclusion of MarkMcM may be correct, details of his example are not.
MarkMcM wrote:
An increase in temperature from 70 deg. F to 250 deg. F would only increase the pressure by about 30%, which for typical inflation pressures would still be below the blow-off pressure of most tire/rim combinations. For example, you typically inflate tires to 100 psi, under the heat of extreme braking energies the pressure might only increase to about 130 psi.
130 psi is NOT 30% greater than 100 psi. When using PV=nRT, not only must absolute temperature be used (which MarkMcM did), but absolute pressure must also be used, and MarkMcM did not use absolute pressure. Absolute pressure = 1 bar + gauge pressure. In the example quoted, 100 psi and 130 psi refer to gauge pressure. Going with the more exact figure that 250 F is 34% greater than 70 F, then using MarkMcM's incorrect methodology with regard to pressure would result in 134 psi at 250 F from 100 psi at 70 F. However, accounting correctly for absolute pressure, means that the correct value at 250 F is 139 psi, not 134 psi. At lower pressures, the percentage error resulting from using gauge pressure rather than absolute pressure increases.


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PostPosted: Wed Sep 12, 2012 11:30 pm 
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Hi,

prendrefeu wrote:
Great answer, thank you! :thumbup:


How so? It does not really answer your question. :? Well, not the particulars anyway...

Ciao, ;)

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PostPosted: Wed Sep 12, 2012 11:45 pm 
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Hi,

It answered it better than you did, earlier! :lol:

Anyway, from what I gathered from Mark's answer and HammerTime2's follow up, having a tube full of nitrogen air will help only slightly with relieving the pressure change as a result of temperature increase that comes with excessive heat built upon heavy braking on a descent. However the increase in tire pressure is not the primary cause of tube blowout - as in the tube itself is not the thing to 'pop' or give on its own. What happens instead is that the bead of the tyre loosens off of the hooks of the rim due to the heat increase, and at some point this will give way to the tube's air pressure pushing the tire off the lip of the rim and, immediately, causing a flat which will be rather violent (as things are rotating quickly at that point).

So, really, there are two better areas of technology to address when covering the issue of tire safety and excessive heat from the rims on a tough descent with heavy braking (in no particular order that I know of yet) -
1. Get better heat dissipation from the brake track so that the heat does not affect the tire's bead/hook interface.
2. Design better tire beads to that they are not affected by the heat increase.

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PostPosted: Thu Sep 13, 2012 12:59 am 
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Hi,

:lol: yeah, well at least I gave you the correct answer and it was a short one. :mrgreen:

3. Use tubulars glued up with a high melting point such as Conti's carbon specific cement. (No fan of it other than that specific property)

Regarding point #2. IMHO quite a few of the experienced blow outs are not so much temperature related but due to weak spots in the bead caused by overstretching it locally: use of levers, bad technique, etc. or a combination of all the above.
Most higher end clinchers and so called open tubulars use kevlar beads. Not sure how much those can take temperaturewise before they'd start to stretch. Once they do then yes, that would be a sure recipe for a blow out/ blow of especially when a latex inner tube would be used.
Not to mention carbon clinchers.... I've seen a few selfdestruct at room temperature which led me to the theory of the overstretched beads.

As a bead material Zylon (aka PBO) comes to mind but even though it won't stretch I have no clue about how heat resistant it is. Either way both kevlar and PBO should be far more heat resistant than the material surrounding it. :P

Ciao, ;)

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PostPosted: Thu Sep 13, 2012 7:43 am 
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DMF wrote:
Does anyone, on this forum, use Co2 for anything else than emergency puncture repair fill up? And does anyone, on this forum, not fill up the tires before every ride, to get the exact pressure? [...]

Since you asked, a friend of mine is using it to seat his tubeless tyres, because it apparently is less of a hassle than using a pump, and he doesn't have a compressor.

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PostPosted: Thu Sep 13, 2012 5:14 pm 
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HammerTime2 wrote:
While the basic conclusion of MarkMcM may be correct, details of his example are not.
MarkMcM wrote:
An increase in temperature from 70 deg. F to 250 deg. F would only increase the pressure by about 30%, which for typical inflation pressures would still be below the blow-off pressure of most tire/rim combinations. For example, you typically inflate tires to 100 psi, under the heat of extreme braking energies the pressure might only increase to about 130 psi.
130 psi is NOT 30% greater than 100 psi. When using PV=nRT, not only must absolute temperature be used (which MarkMcM did), but absolute pressure must also be used, and MarkMcM did not use absolute pressure.


Oops, you're right! In my quick, back-of-the-envelope calculations I forgot to convert the tire pressure into an absolute pressure, and so my calculated pressure increase was bit low. But, the basic principle applies, as to increased pressure with temperature not being the primary cause of tire blow-off. According to Frank Berto (http://www.bccclub.org/documents/Tireinflation.pdf), tire manufacturers actually test their tires to twice the rated pressure, and tires don't get hot enough to double their pressure under the heat of braking.


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Posted: Thu Sep 13, 2012 5:14 pm 


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