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09-12-2021, 10:26 AM | #16 | |
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Re: Is This Tire Pressure Chart Legit?
That chart is, at best, very confusing, and at worst, incorrect. It also apparently ignores the fact that 70°F is not the highest temperature ever achieved on the surface of Earth. I guess we just run them overinflated and ignore that, as per the table.
Charles' Law and Boyles Law are physical LAWS. They are not theroies, conjectures, musings, nor axioms. The Standard Temperature and Pressure (STP) for atmospheric air measurement is to have everything equalized to 68°F (20°C), 36% RH, and 14.696 PSIA (1.013 BAR). The linked table seems to make an attempt at that, but fails miserably. As a general guide, it is better than nothing, but also close to nothing of value. The formula to correct for STP is simple. I can accurately predict the pressure of a given mass of air contained within a fixed volume (tire) at any practical temperaures from near absolute zero to the point of auto-ignition of the tire rubber compounds. A substantial wildcard is the relative humidity of the air in the tire. That changes with temperature - Thus the term "relative" humidity. This is one reason for the popularity of dry nitrogen being used as tire fill, in addition to some other minor benefits. Overall, the table is an attempt of someone to partially educate the populus about pressure variations, but doesn't quite meet the need.
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09-12-2021, 11:59 AM | #17 | ||
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Re: Is This Tire Pressure Chart Legit?
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Below that temperature, say, if the ambient dawn temperature is 60°, then inflate the tires to door frame sticker plus one psi. 50° outside? add two psi to door frame sticker cold pressure, until you get down to F20° So to simplify, if you live where the lowest summer temperatures in your part of the world are between 65-75°F, it's ok to inflate to the door sticker value exactly. IE: Me - 32psi cold. If your winter lows in your part of the world average 30°F early in the day, then add 4psi to the door placard values, to compensate for differences in air density at the colder temperatures. IE: Me - 36psi(32 + 4) cold. That's my understanding, on the face of it. My aforementioned retired engineer customer at work feels that 36psi cold, in summer or winter, is a bad idea for a car which Honda specifies 32psi cold, "seasonal air density differences be damned!" lol Last edited by RidingOnRailz; 09-14-2021 at 10:29 AM. |
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12-24-2021, 02:39 PM | #18 | |
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Re: Is This Tire Pressure Chart Legit?
REDUX:
Here is the same chart, revised for clarity of meaning. The original, the subject of this thread, was the property of the RMA(Rubber Manufacturers Assn). |
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12-25-2021, 06:20 AM | #19 | ||
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Re: Is This Tire Pressure Chart Legit?
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Can you provide a link? (Just a side note: I tried to google for it and couldn't find it.) |
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12-25-2021, 07:18 AM | #20 | ||
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Re: Is This Tire Pressure Chart Legit?
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Here is the link to the Tire Inflation sub-page of AA1 Car repair. https://www.aa1car.com/library/tirepres.htm Note that the blue tire inflation table in the above link is now the revised edition(which I shared in post #18). The webmaster of that site himself stated, in our e-mail exchange, that the original chart(in my first post) was from RMA/TMA. He might have had to recreate the table himself owing possibly to legal reasons for simply not being able to include the original there. For me, the revised table makes much more sense, and the original is suggesting something entirely different. As for the temperature, it is common practice in the scientific and engineering communities to use a temperature of C°20(F°68), and a barometric pressure of 29.94"(or something in the ballpark of mean sea level) as baseline atmospheric conditions for experiments and testing. So the producers of the original table probably just rounded up - to F°70 - for underscoring the effect on inflation pressure every positive or negative ten degrees F. |
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12-26-2021, 07:26 AM | #21 | |||
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Re: Is This Tire Pressure Chart Legit?
Quote:
Quote:
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12-26-2021, 10:14 AM | #22 | ||
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Re: Is This Tire Pressure Chart Legit?
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1) Larry's(from AA1Car) words, verbatim: "Thanks for the feedback. I agree that chart is confusing. I am going to redo it to make it less confusing. The chart was originally provided by the Rubber Mfrs Assn. and lists pressures up to 90 PSI which is for heavy-duty over the road trucks. What the chart is trying to indicate is how much additional tire pressure it takes to provide the same weight carrying capacity as the temperature drops." 2) How tire pressure works: You are saying, if I read you correctly, that all I have to do, monthly or otherwise, is adjust my tires to the correct/recommended cold pressures. IE: If 33psi cold is recommended, just maintain that 33psi, from August high temperatures to February lows, and that's enough. Do I have that correct? |
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12-27-2021, 07:26 AM | #23 | |||
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Re: Is This Tire Pressure Chart Legit?
Quote:
Quote:
Tire pressures are supposed to be set at ambient conditions when the tire is at those conditions. We refer to that as "Cold". So if it is 50°F outside and your car is going to be driven in those conditions, the tire pressure should be set such that they are 33 psi (or whatever is specified) when the tire is at 50°F. Now you could let the car sit outside for an hour and a half so that it reaches ambient conditions or calculate what is needed at the temperature the car is at (like a 70°F garage). The problem in winter is that usually tire pressures are set in a warm environment (like a garage or an autumn-like day), but the tire later is going to operate in much colder conditions. The procedure I recommend is to set the pressures such that they are what you want them to be when the temperature drops - which means that you have to estimate what the worst condition is going to be. As an example, let's assume you want 33 psi when the temperature is going as low as -10°F. You are setting the pressures in your garage and it is 50°F inside the garage. So you are targeting for a temperature drop of 60°F = 6 psi, so you set the pressures at 39 psi. |
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12-27-2021, 07:56 AM | #24 | ||
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Re: Is This Tire Pressure Chart Legit?
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Understood completely. Now, please understand this: Where I live, our cars are in "ambient conditions" year-round, as in, parked out on a driveway. Our garage is detached and unheated, used for storage. So it's a lot more straightforward for me to adjust cold tire pressures. Tire gauges are kept inside, in a kitchen "miscellaneous" drawer, if that means anything. During the half hour before sunrise, coolest time of day, before our cars have been driven anywhere, I just go out, check, and adjust. During the cooling months around here(October, November, December, etc), I overinflate the tires the evening before when it's relatively warm outside, maybe 5psi over door pillar placard. Next morning, just before sunrise, bleed them down to specification. No lingering outside forever waiting for the 12V pump to fill them up. During the warming up months(April, May, June, etc), it's just a simple matter of bleeding off pressure that has built up from the month before. Am I doing it right? And in any case, the producer of the table that is the topic of this thread probably tried to explain what we both understand, in a most round-about and confusing way. |
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12-28-2021, 06:32 AM | #25 | |
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Re: Is This Tire Pressure Chart Legit?
Yes
If you want a better understanding of why all inflation pressures are set at ambient conditions, consider that the stiffness of a material does not change with temperature (within the range of ambient temperatures humans inhabit) and that a tire's load carrying capacity is therefore not dependent on temperature, but it is dependent on inflation pressure. |
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12-28-2021, 07:45 AM | #26 | ||
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Re: Is This Tire Pressure Chart Legit?
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In summer, it was soft and supple by comparison. So rubber, like the composition in tires at least, is less susceptible to such variations? Additionally Barry, I have forward the original table to the USTMA: “Greetings: I am writing to inquire if the attached tire pressure table originated from the USTMA or its predecessor, the RMA (Rubber Manufs). I found it on a car repair site/blog, and it seems to be generating lots of confusion, and outright hostility, among those who have viewed it, regarding what the chart might be suggesting. I interpret it as suggesting that operators of cars and light trucks inflate their tires, during colder temperature months, to pressures up to 7psi higher than they would during hotter months. I would appreciate your assessment of this table, its origins, the intent of information displayed, and succinctly, its overall validity. Thanks and kind regards,” I should hopefully have a response from them by next week, as much of their office are on traditional Christmas-New Years holiday break this week Last edited by RidingOnRailz; 12-28-2021 at 08:56 AM. |
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12-29-2021, 07:06 AM | #27 | ||
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Re: Is This Tire Pressure Chart Legit?
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First, they are very wary of potential lawsuits and this query kind of sounds like a lawyer trolling. Second, the source of this table said he got it FROM a similar table from the RMA. He didn't even credit the RMA on the chart in any way. So it's not the exact table RMA is reported to have published. If I were in their shoes, the only response I would give is to point out that the table didn't come from the USTMA and make no more comment than that! Third, Here's a copy of USTMA's manual for truck and bus tires: https://www.ustires.org/sites/defaul...BusTires_0.pdf If that chart (or some form of it) isn't in there, then either the RMA never published it, or they feel it is obsolete. I've looked through this and don't find anything that resembles the chart. That and the fact that I didn't find the chart (or something similar) doing a google image search convinces me that the webmaster is wrong about where he got the chart - and I'll bet he is misremembering. I certainly don't want to accuse him of falsifying the source to give it more credibility - which he has only done in an email to you. |
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12-30-2021, 05:39 AM | #28 | ||
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Re: Is This Tire Pressure Chart Legit?
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I don't know why he would quote such an authority as the source of the table. Otherwise, the automotive information on AA1, not just in the tire section, seems to be accurate and informative. Re: "lawyer trolling" Well I'm certainly not a lawyer, but in my twenty plus years of participating in Usenet news groups, forums like this one, and social media until last year, I have often been described, either as, a "troll" or "trolling". Seekers of verification and of the truth often get branded as such, lol! |
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12-30-2021, 08:27 AM | #29 | |
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Re: Is This Tire Pressure Chart Legit?
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12-30-2021, 08:53 AM | #30 | ||
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Re: Is This Tire Pressure Chart Legit?
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I'm questioning things because I don't accept, all the time, what is printed and am skeptical of some things. 2) "Wording things differently" I was just trying to sound professional in my e-mail to the USTMA |
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