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Tire Pressure Questions

Discussion in 'Wheels & Tires' started by Tlamb, Dec 3, 2010.

    Tlamb guest

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    Hi All

    Living in frigid Southern California, I don't have to worry so much about winter tires, but last week it was cold enough to cause my tire pressure sensors to go off on my 2009 e92.

    My question has to do with the fact that it took me a while to find a gas station with a working air pump, and by that time, my tires had warmed up, inflating the tire pressure from what they would have been that morning. It was difficult to gauge how much to add, since all the tire pressure guides indicate "cold" tire pressures.

    How much does tire pressure change going from cold to "warm"? I know that will depend on how cold vs. how warm, but is there a general 5psi or 2psi guideline meaning that a cold 32psi tire will generally warm up to 35psi or something like that?

    Also, there is a sticker on my door frame identifying that I can increase the tire pressure for travel under 100 mph. What is that? I've ignored it, but can someone give some insight into what that is? Thanks in advance!
    • Member

    Brian A

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    To see how much the pressure changes, just fill them to the recommended pressure while warm, drive home and the next morning (after they cool) measure them to see what the difference is.

    I am not an expert or tire professional, but my understanding is that car companies basically specify the lowest pressure they feel is safe (recall Ford's problems). A low pressure improves the ride of the car and handling is barely affected. There is a maximum tire pressure indicated on the sidewall of the tire; its a lot higher than the recommended pressure. Never exceed the maximum pressure. Personally, I keep my tires a few psi above the recommended pressures.

    See page 68 of the April 2010 Roundel for an article on optimizing tire traction for autocross by adjusting tire pressures. The tire pressure that maximizes traction is often higher than the "recommended" pressure. Perhaps the 100 mph thing relates to that.
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    Zeichen311

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    Your memory betrayed you: The increased pressures are for speeds over 100 mph.

    It's mainly about keeping the tire from exploding. Higher inflation pressures reduce sidewall flex, which apart from increasing control also reduces heating of the tire. An underinflated tire builds up a ton of heat at high speed, leading to premature failure. ("Premature failure" at 100+ mph being a nice euphemism for a spectacular blowout and even more spectacular crash. ;))

    Tlamb guest

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    Thanks for the info, both. And you're right, I meant over 100MPH.

    I'll check it again this weekend and might go a bit higher on the pressure side.
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    CRKrieger

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    It's about 1 psi for every ten degrees Fahrenheit. Temperature drop from 60º to 30º will put your pressures down about 3 psi. This isn't REAL critical for normal street driving until you drop down under about 20 psi, so don't go into a panic if your pressure sensors are that close to triggering. If they think the pressure's fine when you drive it a couple of miles, then you weren't all that low to start with.
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    shelbyvnt Baby Bee...

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    Texas, Your Tire Pressure is down too...

    Dallas may not have the same weather as So-Cal, but my 2010 335i seems to be afflicted with the same "soft-shoes". The temperature was 70+ this weekend & today it went down to 25. Went out to lunch to see the Low Tire Pressure signal glowing. All four tires were between 5 to 10 lbs. low. Curious to see if the pressure will hold tomorrow, once I pull this Garage Queen back out into the elements. Did your pressure hold, or have you had to add more air?
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    Zeichen311

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    Welcome to the Combined Gas Law, which as a law of physics (chemistry, really) is in force worldwide, not just in Dallas and SoCal. Pressure varies with temperature not because air escapes from the tire but because the air molecules themselves are less energetic at lower temperatures, exerting less pressure on the walls of the container (i.e., the tire).

    If the temperature returns to the 70s the tire pressures will return to normal. If you added air to correct the pressure at 25F, it will only fall further if the temperature falls further (or if you have a leak, of course). When the temperature climbs back to the 70's your tires will then be about 5 psi over-inflated, because there is now more air confined in the same volume and that air has more energy due to the higher temperature.

    (Have a look at the second half of this post for a simple worked-out example. Read the rest of the thread for more on the gas laws and an entertaining summary of why it's pointless to fill car tires with nitrogen.)

    Tlamb guest

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    I filled up my tires last week to a bit above the recommended psi. 33.5 in front 37 in back (compared to 32 and 35). Over the weekend temps hit the mid 80's, but this week it should be cooler. My tire pressure monitor hasn't gone off again, but I wouldn't expect it to unless there was an actual leak.
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    CRKrieger

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    That link takes you to the end of the epic thread. If you right click on the post number (upper right corner of each post) and copy the link location, you get the exact post you wanted ... which I think was this one. ;)
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    Zeichen311

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    Weird--it seemed to work here. I wanted to link to the post in context, not alone. No matter...it's a fun read either way and about the only use I make of my college chemistry lessons, these days ;).
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    Wretched

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    Ditto I normally keep mine at 34/38. Haven't had a issue yet even though we are getting under 20 temps!
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    ldamario

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    I've always used 3-4 psi as an estimate for the difference between a "cold" tire and a "warm" tire.

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