| **How much does cold temperature affect tire pressure?** | timfire
*Feb 4, 2003 8:38 PM* | | I've just been curious to find some figures. When ever I fill my tires in the warm house and then go out into the winter cold, I can tell that the tire pressure lowers a bit. Last winter I also had the experience were I filled a tire at a gas station in the cold, then after I brought the bike inside it warmed up and the tube exploded from being over pressurized!
So anyway, does anyone know figures? If I fill my tires to 100 psi with 75 degree air and then take them out in 25 degree weather (what it was today in Chicago), how much should I expect the pressure to lower?
-Tim Kleinert |
| **re: How much does cold temperature affect tire pressure?** | ukiahbill
*Feb 4, 2003 8:54 PM* | | (P1/P2)=(T1/T2) if volume is constant, and temperatove is in Kelvins.....so the pressure would drop about 5 psi. |
| **re: How much does cold temperature affect tire pressure?** | Merlin Vince
*Feb 5, 2003 12:15 AM* | | Hi!
The formula presented is correct but the answer is slightly off.
t1 = 75 degrees F = 297 Kelvin
t2 = 25 degrees F = 269 Kelvin
p1= 100 psi (or could use bars or kPa)
p1/p2=t1/t2
Solving for p2 we get:
p2=p1*t2/t1
Substituting the numbers we get:
p2= 100 psi * 269K / 297K = 90.6 psi
Which is a drop of almost 10 psi.
Here is one more example for your viewing pleasure:
Suppose that you inflate your tires to 100 psi with a room temperature of 70 degrees F. And then you take your bike outside and it is 100 degrees F. What will be your new tire pressure (given a few minutes to adjust)? Answer 105.6 psi.
Now, of course, if you use your brakes while you ride that could also increase tire pressure if you have 'traditional' rim brakes!
Hope this helps. |
| **PV=mRT** | Rusty McNasty
*Feb 5, 2003 5:30 AM* | | Since V1=V2, m, R are constant, then P1/T1=P2/T2.
(T2/T1)*P1=P2 T is in absolute temp, with 0k=-273c, and 0r=-460f
((460+25)/(460+75))*100=(485/535)*100=90.65 lb/in^2 |
| **for geeks only** | Continental
*Feb 5, 2003 7:52 AM* | | In case anyone is wondering about the contraction of the rubber under these conditons, the rubber will contract by about 0.6%, so V2/V1 is approximately .994. Also, the ideal gas assumption over estimates the pressure change by about .002%. Both of these are practically insignicant, so the ideal gas calculation gives a good estimate of the tire pressure change. |
| **Fun Experiment (may not work with a bike tube/tire)** | Kristin
*Feb 5, 2003 8:20 AM* | | But it will work with a wheelbarrel tire. Go outside on a cold night and fill a tube with air. (How much air can you put in an unmounted tube? I'm guessing a lot. You'll probably get bored filling it.) Now, bring the tube inside and set it next to your fireplace (while fireplace is on). You may want to wear earplugs. Wait 6-9 hours. Eventually the tube should explode. |
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