• 11-09-2014
    snailspace
    Cold temps and tire pressure
    Now that its actually winter where I live I have a question about tire pressure and cold weather. I store my bikes indoors. Going from room temp to -30C or colder is a big change in temp. I am curious if there is a way to figure out how much psi I will lose based on temp. For example if at room temp I am at 35psi how much pressure will I lose at -30C. Or could someone point me toward a formula?
  • 11-09-2014
    newfangled
    I'd say not to overthink it.

    Using fatbikes as an example, I think a lot of people will say to ignore the gauge and just go by what feels right. I don't have a fatbike, but I take the same approach for my winter 29er and 26er.

    If my tires feel too hard or too bouncy then I'll adjust them when I get a chance.

    And some days a morning ride might be at -20C, with the afternoon at -5C, but you won't feel the difference in tire pressure - the difference in road/snow conditions will be much more noticeable.
  • 11-09-2014
    AndrwSwitch
    Re: Cold temps and tire pressure
    You could use the ideal gas law.

    p*v = n*r*t

    It's pressure times volume equals molecules times a constant times temperature.

    You don't really need to know a bunch of the values because they don't change. Basically what it's telling you is that tire pressure is proportional to temperature. In fact, if you do a little algebra to put pressure and temperature on one side and the constants on the other, you can derive exactly the formula you're asking for.

    The 't' term is in Kelvin. That's important because for someone in a fairly temperate place, it means that the changes in temperature a bike is likely to experience don't matter all that much. For example, I'm unlikely to ride in temperatures below 40 F, which is a few degrees above 0 C, which means the 't' term in that formula changes less than 10% when I go outside.

    If you're really riding in -30 C weather, I guess you could anticipate a pressure change on the order of 20%.
  • 11-11-2014
    jeffscott
    Quote:

    Originally Posted by snailspace View Post
    Now that its actually winter where I live I have a question about tire pressure and cold weather. I store my bikes indoors. Going from room temp to -30C or colder is a big change in temp. I am curious if there is a way to figure out how much psi I will lose based on temp. For example if at room temp I am at 35psi how much pressure will I lose at -30C. Or could someone point me toward a formula?

    Cold pressure = Hot pressure * (Cold Temp + 273)/(Hot Temp + 273) in deg C

    Cold pressure = Hot pressure * (Cold Temp + 460)/(Hot Temp + 460) in deg F