3 replies on “Woofcast #351”

  1. RE: Tire rotation

    For what it’s worth fellas, on my R53, if I don’t rotate tires every 4-5k miles, the rears will cup something awful and get all chirpy and noisy. I’ve found that if I haven’t rotated often enough, it’s not wear on the front I end up cycling, but I end up rotating the rears up front to wear the cup back out of them. So rotate ’em often, folks!

  2. This is something that I sent to Todd a while back, apparently he didn’t read it.

    Hey Todd,

    This has very little to do with any of the content of your shows, I was just doing a little Rain Man work…

    I was wondering if there was a something wrong with your rule of thumb with tire pressure change with temperature, it didn’t sound right, so I figured it out.

    Gasses behave according to the “Ideal Gas Law”, pV = nRT, which relates pressure, volume, number of molecules, and pressure.

    More pressure, same volume, same number of molecules, more temperature.
    Less temperature, same volume, same number of molecules, … anybody? … Bueller? … yes less pressure.

    According to the Ideal Gas Law and using Fahrenheit and psi (just for you), a tyre that is at 36 PSI at 60F will drop to 29PSI at -40F. So 7 PSI over 100 degrees, or about 0.7 PSI for every 10F.

    So, sure, if we round up, and since degrees F are so small. So, you’re within a million miles. I was surprised that the change was so big.

    On the other hand, if you find that your tyres are at 13 PSI (about where you notice radials look squishy), you could not reasonably say that it was caused by temperature changes. You’ve got a leak.

    As for nitrogen vs. air. Some say that nitrogen doesn’t follow the ideal gas law. Some say that nitrogen doesn’t burn, unlike air. All I know is … what a load of crap. The reason that racing teams use it is that it is cheaper to rent a bottle of nitrogen from the gas companies than air, because they rent more of them. Air probably doesn’t sell that well since you can make it yourself with a compressor, but race teams can’t bring a compressor into the pits. It’s also very hard for Tire Warehouse to charge you for air, whereas they can extract a couple of bucks for selling you nitrogen.

    Some say that nitrogen is a bigger molecule, yes it is 300 picometers as opposed to the 292 picometers of that shrinky dink oxygen. Not a big enough difference, I get offers in my email that oxygen should look in to.

    Yes, nitrogen follows the same ideal gas law too.

    Some say nitrogen is dry whereas air is moist. Well, that is why air compressors have dryers on the lines, it keeps the lines from freezing and keeps the air wrenches from failing. Bottled air is dry as well.

    Air is 70% nitrogen, and only 21% oxygen anyways. Same sort of proportions as pressure change with temperature, we could round up air to 100% nitrogen, it’s just polluted nitrogen.

    Don’t pay for nitrogen.


    From the Great White North where it was -46C on December 13th, that’s -51F!

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