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Old 01-04-05, 08:07 PM   #1
jeff williams
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Sure felt like it. Added more air today.

O maybe I've slow leaks.

I've recently oversized my front tire, I run it soft and it froze up since Sunday.

??? Doctor Science???

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Old 01-04-05, 08:47 PM   #2
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Yes, but not just because the temperature dropped below 32 F (if by when "it" freezes you mean H2O). Any temperature change will cause a change in pressure in your tires: at higher temperatures the air molecules in the tires are moving faster so they more frequently collide with the tube, and at lower temperatures they move more slowly and have fewer collisions resulting in higher or lower pressure, respectively. This is a measurable effect, but unless you have tens of degrees worth of temperature change you are not likely to notice. For example, if you inflate your tires on a hot day, say 85 F, and then cool them down to 25 F you will notice, but you probably won't notice if you just go from say 40 F to 30 F.

The air in your tires is mostly nitrogen, a decent fraction of oxygen, but not too much water vapor. If,however, you lowered the temperature so far that nitrogen or oxygen went from a vapor to a liquid you would see a sudden and dramatic drop in pressure due to the phase change (discontinuity in volume). That said, you are very unlikely to be riding your bike at such low temperatures-- even with studded tires
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Old 01-04-05, 08:54 PM   #3
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That said, you are very unlikely to be riding your bike at such low temperatures-- even with studded tires
I would ride my bike in those temps if I could. Will I never make it to the ice caps of mars?
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Old 01-04-05, 08:56 PM   #4
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Thanks..yes i'm remembering a bit more from science class now.

And tires highly inflated can bust in high temps.

First time I believe I've noticed it from freeze.

I live in Canada, but on the south tip of Van Island and it rarely goes below 0.
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Old 01-05-05, 09:29 PM   #5
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Remember high school?
PV=nRT?
V=(nRT)/P
So (nRT1)/P1=(nRT2)/P2.
I doubt the pressure will go up THAT much.
If you stick a wheel into a vacuum, the max overpressure you'll get is +15psi.
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Old 01-06-05, 12:31 AM   #6
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I'm pretty sure the uniform gas law dictates an increase in pressure with a decrease in temperature.
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Old 01-06-05, 12:54 AM   #7
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I think that's density, not pressure... The molocules are closer together, so they don't exert as much force on the inside of the tire.
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Old 01-06-05, 01:33 AM   #8
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Originally Posted by Bekologist
I'm pretty sure the uniform gas law dictates an increase in pressure with a decrease in temperature.
hm, no...

PV=nRT, if P goes up, T must go up. I also suspect you may be thinking density.
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Old 01-06-05, 06:22 AM   #9
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I'm pretty sure the uniform gas law dictates an increase in pressure with a decrease in temperature.
That's the can of water in the freezer thing you're thinking of. That's got more to do with materials science and crystal structure than gas laws.
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Old 01-06-05, 06:43 AM   #10
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Instead of trying to memorize and apply the ideal gas law, think about from where it comes:

One of the primary assumptions for an "ideal" gas is that the molecules are far enough from each other that they don't interact: there are no intermolecular forces to worry about (the may collide, but the assumption is that they just bounce off of each other in a way that averages out). Therefore, the phenomenon of pressure comes entirely from collisions of these gas molecules with the walls in which it is contained. You can relate the momentum imparted from each molecule over time to determine the force applied to the walls. That force per area is the pressure.

From that you can deduce that the more collisions, or the greater velocity imparted by each collision, the higher pressure will be.

Now think about what temperature really measures: it is the mean of the kinetic energy of the molecules. Higher temperature means higher kinetic energy (KE is proportional to the product of the mass and the velocity squared). So for gas molecules it is straightforward that higher temperature must imply that the molecules are going faster. If they're going faster they will impart more momentum and they will strike the sides more frequently; this clearly implies a higher pressure at constant volume.

You may be bothered by the fact that in the case of a tire, the volume is not perfectly constant; furthermore, the tube likely has a temperature-dependent elasticity, and the pressure on the outside of the tire can also change. These do tend to lessen the effect (otherwise it would be VERY noticable), but they don't cancel it out, and they certainly do not reverse the situation.
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Old 01-06-05, 06:55 AM   #11
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Hmmm...I used to talk like that. I quit several years ago and now I feel healthier. My lungs aren't as black and my memory is better.
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Old 01-11-05, 10:29 PM   #12
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Yes, the air pressure does decrease, but not by as much as you would expect.

Whenever you insert the "T" into an equation based on the ideal gas law, you must use the Kelvin or Rankine temperature scale. The difference between, say, 528R (68F) and 492R (32F) and isn't that big a difference. Only 6.8%. The pressure isn't affected that much. If your tires were inflated to 60 psi, they would decrease to 56 psi.

On my MTB, this isn't even noticable.

Considering a fixed volume, as you increase the pressure, you will increase the density. They are related.

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Old 01-11-05, 10:50 PM   #13
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Quote:
Originally Posted by eubi
Yes, the air pressure does decrease, but not by as much as you would expect.

Whenever you insert the "T" into an equation based on the ideal gas law, you must use the Kelvin or Rankine temperature scale. The difference between, say, 528R (68F) and 492R (32F) and isn't that big a difference. Only 6.8%. The pressure isn't affected that much. If your tires were inflated to 60 psi, they would decrease to 56 psi.

On my MTB, this isn't even noticable.

Considering a fixed volume, as you increase the pressure, you will increase the density. They are related.

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I run an oversized, soft rubber compound, under-inflated tire. I did notice.
Thanks all.
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Old 01-13-05, 08:51 AM   #14
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Originally Posted by jeff williams
I run an oversized, soft rubber compound, under-inflated tire. I did notice.
Thanks all.
Good point Jeff. The more I thought about this the more variables there are. Better left to engineering class than this forum.

Another point to consider is if you are filling the tire with a pump or compressor. Using a pump will heat the air because you are compressing it from ambient pressure (14.7 psi) to whatever your tire pressure is. P1/T1 = P2/T2 from the ideal gas law (I know this over-simplifies the system, but we have to start somewhere!). Using the tank of a compressor (presuming you let the stored air cool to RT), the air will be expanding as it enters your tire. My compressor stores air at 180 psi. No problem for my 60 psi MTB tires. Expansion = cooling.

I guess for winter riding, you would be better served using a compressor.
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