How long should Road tyres stay properly inflated?

[Tim Hall]

Don't forget outside air pressure and temperature has dramatic effect on your tyre pressure. That why daily checks of tyre pressure are advised in all manuals before your first ride. I usually just feel firmness of the tyre, and look down at the bulging during a ride in between checks to get a ball park estimate of pressure. I don't need to ride bang on my ideal pressure, but if I were riding at the upper or lower limit of the tyre's capability I might be more inclined to check often too.

For some values of dramatic. Idle Friday afternoon sums follow:

The highest road in the UK is 670m above sea level. Atmospheric pressure is 93% of the sea level value, so down from 14.7psi to 13.7psi - ie you've got about 1 pound per square inch less pressing against the tyre. The highest atmospheric pressure recorded in the UK is 1053.6 hPa, which is about 4% up on the standard value of 1013.25 hPa. Or about just over half a pound per square inch. Looking at temperature, assuming your tyres a filled with an ideal gas, the pressure is proportional to the absolute temperature. Let's take a cold day, at 0 degrees C. That's 273K. The tyre pressure is a nominal 100psi. Double the temperature and we double the pressure, so at 546K we double the pressure. We also melt the tyre and die. A more reasonable example is to take a hot summer day, say 35 deg C. That's 273+35 = 308K. The pressure will increase by 308/273 = 1.12 or 12%. So a 100psi tyre will go up to 112psi. But who pumps their tyre up at 0 deg C and goes out in 35 deg sunshine? Is it more reasonable to go from, say 15 deg C to 30 deg C? That 288K to 303K, giving a pressure rise of 303/278= 1.08 or 8%. So a 100psi tyre will go up to 108 psi.

(Can you tell I'm bored?)

 

[confusedcyclist]

For some values of dramatic. Idle Friday afternoon sums follow:

The highest road in the UK is 670m above sea level. Atmospheric pressure is 93% of the sea level value, so down from 14.7psi to 13.7psi - ie you've got about 1 pound per square inch less pressing against the tyre. The highest atmospheric pressure recorded in the UK is 1053.6 hPa, which is about 4% up on the standard value of 1013.25 hPa. Or about just over half a pound per square inch. Looking at temperature, assuming your tyres a filled with an ideal gas, the pressure is proportional to the absolute temperature. Let's take a cold day, at 0 degrees C. That's 273K. The tyre pressure is a nominal 100psi. Double the temperature and we double the pressure, so at 546K we double the pressure. We also melt the tyre and die. A more reasonable example is to take a hot summer day, say 35 deg C. That's 273+35 = 308K. The pressure will increase by 308/273 = 1.12 or 12%. So a 100psi tyre will go up to 112psi. But who pumps their tyre up at 0 deg C and goes out in 35 deg sunshine? Is it more reasonable to go from, say 15 deg C to 30 deg C? That 288K to 303K, giving a pressure rise of 303/278= 1.08 or 8%. So a 100psi tyre will go up to 108 psi.

(Can you tell I'm bored?)

Glad to be debunked by some solid math! My own anecdotal experience contradicts the theory, but maybe I had a slow puncture or forget some other important variable like what I had for breakfast or the colour of my socks

 

[smutchin]

Double the temperature and we double the pressure, so at 546K we double the pressure. We also melt the tyre and die.

That certainly would be dramatic.

 

[Ming the Merciless]

There is a range of tyre pressures that will work for you once you are above the bottoming out, tyre squirming level. How does 75psi feel on the road for you? You may find it far more comfortable with less road buzz transmitted through to the bars.

 

[ColinJ]

I just checked my tyres in advance of tomorrow's Dales forum ride and my front tyre has lost a noticeable amount of pressure in the last 4 or 5 days. The rear is still very firm but the front isn't quite firm enough. I'll go and top it up and get an idea of how much pressure it had lost. I'll be back ...

PS It was about 20 psi down on what I last pumped it to. I checked the tyre and there does not appear to be anything stuck in it so I am pretty sure that the tube does not have a puncture***.




*** I have had unpenetrated/unsnakebitten tubes, and valves fail in the past but that has usually lead to a more rapid and complete deflation.