Homeless in Vancouver: I'd like to figure this out—but no pressure
The last two spells of cold weather weren’t very demanding or difficult to cope with, but they did have their curious moments.
For instance, take the micro-leak I had in in one of my bicycle tires—please (Ba-dum!).
At 9° C—this morning’s temperature—I could expect the tire to leak out in about seven hours. But at -6° C it didn’t appreciably leak at all. Even at 0° C it only lost about 10 pounds of pressure in about three days.
I realize there is an interrelationship between air pressure, temperature and density; there’s a higher density of air at the bottom of the gravity well, and higher temperatures mean more energetic molecular movement, which means higher air pressure.
So I understand that as the temperature drops so does tire pressure. But what I clearly don’t understand is the mechanics of the differential between the air temperature and pressure in a tire versus the ambient outside air.
If I have a slow-leaking tire—fully inflated—in subzero weather, I would think the pressure and temperature is higher inside the tire than outside. I would expect the tire to leak faster to equalize with the outside lower pressure until an equilibrium is achieved.
I must have everything backward; if the ambient outside air pressure was higher than inside the tire then I would not expect the tire to leak, but I don’t see how that could be in minus-degree weather.
Unless the point is that all air pressure is lower in subzero temperatures and less energy means less leaking, period.
Ah well, another thing I don’t understand. So deflating, ego-wise.
Comments
3 Comments
Hacks
Dec 24, 2013 at 3:31am
Take some physics courses on coursera.org or MIT opencourseware, or Khanacademy (all free) and you'll understand.
tao of slow leaks
Dec 25, 2013 at 3:44pm
You might be conflating pressure with temperature, writing first that " [...] pressure *and* temperature is higher inside the tire than outside", then suggesting that you might have it backwards regarding the difference in pressure (presumably the difference between tire pressure and air pressure) on a cold day, which is odd, considering that atmospheric air pressure is not a function of temperature.
In my experience, the temperature of an inflated tire does not behave differently than the temperature of anything else. A cold tire in a warm room will cool the room a bit while the room warms the tire a bit. Over time they will become the same temperature. This is true regardless of the air pressure, or tire pressure. Your tire on a cold day will only be warmer than the air temperature if it came from a warmer place, and if it did, over time it will lose that heat to the air.
When air pressure goes down, the pressure inside the tire increases. That's why fully inflated tires will burst in aircraft cargo bays if not deflated somewhat. Atmospheric air pressure is not a function of temperature. It can be high on cold days and low on hot ones.
If the pressure inside the tire has increased, (maybe the tire has warmed up, or maybe atmospheric air pressure is low), you would usually expect there to be more force pushing air out through the small leak. All other things being equal, the tire would deflate faster, at least until it got to the point where the tire pressure was the same as it was when the temperature or air pressure was lower, when it would be the same again.
In this case all other things are not equal though. Slow leaks are typically caused by something that has penetrated the inner tube but remained lodged in it. How quickly the tire deflates depends on how effective the seal around the foreign object is. Lower tire pressure (due either to lower temperature or higher atmospheric air pressure) will lower the force of the tire pressure pushing outwards against the foreign object. You would think that the less power available to air trying to get out of the tire, the less the tire will leak. However, it's also possible that the lower tire pressure causes a degradation of the seal around the foreign object, resulting in a larger hole for the air to get out of. Ultimately, how things work out depends on the mechanics of the seal around the foreign object
Stanley Q Woodvine
Dec 25, 2013 at 11:44pm
@ Tao
Thanks for commenting
You write "atmospheric air pressure is not a function of temperature."
Yet everything I read reinforces my understanding that pressure, temperature and density are interrelated.
I don't pretend to understand the Equation of State: p = R r T, where p is pressure, r is density, T is temperature, and R is the specific gas constant.
But a transcribed St. Andrews lecture on pressure as it relates to climate refers to that equation and the relation between pressure and temperature thusly:
"Therefore, we can see that any change in any one variable is likely to cause changes in the others. For example, if we heat a mass of air, we increase its pressure, if the air is allowed to expand to equalise the pressure difference with the surrounding air, the density will decrease. When this happens, it will be lighter than the same volume of surrounding air, and will rise."
You are correct that cold can "cause" high pressure ridge. This is complex stuff, but everything agrees that temperature and pressure are connected. Perhaps a lot of people are over-simplifying for us dummies but the fact remains that the connection is made, and specifically in regard to tires.
Many tire-rated web sites offer the same rule of thumb as tirerack.com:
"The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire's inflation pressure will change by about 1 psi (up with higher temperatures and down with lower)."
Bridgestone tires uses the example of a trip from Las Vagas (100°) on tires set to 100 psi to Reno (0°). After parking 3 hours inflation pressure will be down to about 85 psi.
As for your suggestion that I just wasn't on the ball enough to check for a foreign object in the tube. I did, to the best of my ability. I found nothing and a new tube has been trouble-free so there was no apparent problem with the rim. When I have the chance I'll find the leak using a sink full of water.