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Is This Tire Pressure Chart Legit?


RidingOnRailz
09-17-2019, 07:29 AM
Since this isn't getting any attention over in the tire sub-forum, I'll ask here.


This chart I found online suggests that in winter we should increase the indicated tire pressure anywhere from 2-6psi during the cold months:




IE my vehicle's door sticker specifies 33psi cold. Does that mean I should keep them at 37-38psi indicated, in January and February?

I also do not subscribe to that ancient myth about lowering indicated pressures 'for better traction' during winter weather. I just make sure that they are at 33psi from July to January here in CT.

Which is correct: moi, or the attached chart?

jerryg2112
09-17-2019, 11:25 PM
I can't believe that they would want you to run with the higher pressures listed in the chart during colder weather. Yes as it gets colder your pressure drops in your tires. That is why you add more air to compensate and bring the pressure back up to the correct amount but not the higher pressure listed in the chart. Wouldn't the manufacture list this information on the placard as well if it was correct? I think maybe the chart has a baseline of 70 degrees and if you were to fill a tire to say 32 lbs at 70 degrees it would lose roughly a pound for every 10 degree drop in temperature. Now if at 70 degrees you over filled the tire to 39 lbs and then lowered the temperature to 0 degrees it would be at 32 lbs. That is how I interpret it anyway.

RidingOnRailz
09-18-2019, 12:29 AM
I can't believe that they would want you to run with the higher pressures listed in the chart during colder weather. Yes as it gets colder your pressure drops in your tires. That is why you add more air to compensate and bring the pressure back up to the correct amount but not the higher pressure listed in the chart. Wouldn't the manufacture list this information on the placard as well if it was correct? I think maybe the chart has a baseline of 70 degrees and if you were to fill a tire to say 32 lbs at 70 degrees it would lose roughly a pound for every 10 degree drop in temperature. Now if at 70 degrees you over filled the tire to 39 lbs and then lowered the temperature to 0 degrees it would be at 32 lbs. That is how I interpret it anyway.


Thanks for replying!

The site in which this chart resides claims that air is "less dense" at lower temperatures. Thus, they contend, that 32psi at, IE 20F is not the same as 32psi at higher(Summer) temperatures.

The way I interpret it is that if your tires indicate 32psi at 70F, that one should add 1psi for ever ten degrees F drop in ambient temperature, then maintain the same pressure for ambient temperatures 0F and below. So at 60F inflate your tires to indicate 33psi, 40F: 35psi, etc.

Since my car rides rough even at factory 33psi, I could not imagine running any more than 1-2psi over that - at any time of the year!

RidingOnRailz
09-18-2019, 06:01 AM
Here is the link for their explanation for that chart:

https://www.aa1car.com/library/tirepres.htm

CapriRacer
09-18-2019, 08:21 AM
Tire engineer here.

The link didn't actually say HOW to use the chart. I suspect what you are supposed to do is if you fill your tires at a certain ambient temperature, use the chart to tell you how much to fill it to. That way at 70F, you will have the proper pressure.

Alternatively, if you are filling your tires at 70F, then use the chart to make sure you have enough tire pressure when it gets down to the temperature indicated.

Either way, I think this creates more confusion than is needed.

So there is a bit of wrong information on the page. For every 10F change in ambient temperature, the pressure inside a passenger car tire changes about 1 psi. For other tires, use 3% for every 10F. (Note: 3% of 30 psi is about 1 psi)

Please note that the tire manufacturers want you to inflate your tires to the vehicle manufacturer's specified pressure for whatever the operating conditions are. That means that if you are operating at 10F, they want you to use the specified pressure, not one specified for another temperature. That also means if you fill your tires at 70F, then they will be too low at 10F (about 6 psi low).

Yes. yes, it is confusing.

Now allow me to express my opinion: Winter tires would benefit from a few extra psi. (3 to 5 psi) That extra pressure helps push the footprint through the snow to reach the pavement. The pavement will always have better traction than snow.

Some people will argue that over inflating your tires will cause the center of the tread to wear faster that the shoulders. That is true, BUT that effect is small compared to most other things that cause rapid wear, such as alignment and the drive vs steering effect.

Drive vs steering effect? Steering tires tend to wear the shoulders and drive tires tend to wear the center. You can see this clearly on a RWD car - where the wear rates are about the same front to rear but in different portions of the tread. That's why rotating tires gets you longer tread life.

On a FWD car, you can't see the steering vs drive difference as the front tires are doing both - BUT - that also means that the front tires are going to wear more rapidly that the rears. I've measured about 2 1/2 times faster. Again rotating tires helps even out the wear.

RidingOnRailz
08-13-2021, 11:26 PM
CAPRI RACER!

Scroll down to Table (3) in this link....

https://patents.google.com/patent/WO2009036547A1/en

It is similar to the blue one linked to here, and discussed above:

https://s19526.pcdn.co/wp-content/uploads/2018/02/tire_inflation_chart.jpg

https://s19526.pcdn.co/wp-content/uploads/2018/02/tire_inflation_chart.jpg

The explanation following my first link explains how such a chart might be used - for adjusting tire pressure in an environment unlike actual operating conditions! - IE: a WARM GARAGE DURING COOLER AUTUMN OR WINTER OUTDOOR TEMPERATURES.

Think about it - it makes sense after reading the explanation that follows my first link-to.

So if I did regularly garage my Honda Accord(recommended tire pressure 32psi) in an attached space that was heated to, IE, F60, and the January avg. dawn temperature was F20, it would behoove me to set my Acccord 'cold' tire pressures to 36psi while in that heated garage.

Once I drove out of the garage, the tires would 'settle down' to, approximately, the Honda-recommended 32psi.

Makes more sense now! Too bad the providers of the blue chart did not make clear that intended purpose. Duhhh.. It implies an enclosed service space heated to F70, and calculates pressure recommended to compensate for setting it inside of said space, before driving out into lower ambient temps.

Actually, it works out for the better that, in reality, our cars reside in our driveway, so no worries about cold-vs-compensated pressures. Just go outside at dawn, regardless of outdoor ambient temperature, and set to carmaker recommended cold pressure. If that pressure is too high, remove air, if too low, add.

CapriRacer
08-14-2021, 07:56 AM
I didn't go into this in my first reply of this thread - BUT - there are some folks who incorrectly believe that the tire load tables are based on STP (Standard Temperature and Pressure) - they are NOT!!

STP is used in the scientific world for testing purposes where the testing environment is different from the specified test environment. Folks doing testing would need to adjust the results to STP so they can do comparisons.

Tires work on a different principle. Tire pressures should be set for ambient conditions.

That presents a problem for situations where the environment the tire will be operating in is different than the environment when the pressures are adjusted - like moving from the seashore to the mountains, or from a cold morning to the hottest part of the day on a desert floor.

There's another aspect that isn't fully understood - tires are fairly forgiving - that is, if you are within a few psi of the target pressure, that will be good enough. Too many people think that if you are off a few psi, that is significant. And by a few, I mean 2 or 3 psi for passenger car tires.

In theory, the tire pressure specified by the vehicle tire placard includes enough extra capacity that 2 or 3 psi will not matter. This is the opposite of what is argued for the Ford Explorer back in the 2000/2001 time frame. Some argue that the pressure specification CAUSED the tire failure - and that is not the case. The specified pressure was adequate - although, it would be better engineering to have specified a higher pressure. What is misunderstood is that the Firestone tires involved had a design problem that wasn't revealed until the tire started failing. Then Firestone stonewalled in issue until forced to recall the tires.

RidingOnRailz
08-14-2021, 08:51 AM
I didn't go into this in my first reply of this thread - BUT - there are
some folks who incorrectly believe that the tire load tables are based on STP
(Standard Temperature and Pressure) - they are NOT!!

STP is used in the scientific world for testing purposes where the testing environment
is different from the specified test environment. Folks doing testing would need to
adjust the results to STP so they can do comparisons.

Tires work on a different principle. Tire pressures should be set for ambient conditions.

That presents a problem for situations where the environment the tire will be operating
in is different than the environment when the pressures are adjusted - like moving
from the seashore to the mountains, or from a cold morning to the hottest part of the
day on a desert floor.

There's another aspect that isn't fully understood - tires are fairly forgiving - that is, if
you are within a few psi of the target pressure, that will be good enough. Too many
people think that if you are off a few psi, that is significant. And by a few, I mean 2
or 3 psi for passenger car tires.

In theory, the tire pressure specified by the vehicle tire placard includes enough extra
capacity that 2 or 3 psi will not matter. This is the opposite of what is argued for the
Ford Explorer back in the 2000/2001 time frame. Some argue that the pressure
specification CAUSED the tire failure - and that is not the case. The specified pressure
was adequate - although, it would be better engineering to have specified a higher
pressure. What is misunderstood is that the Firestone tires involved had a design
problem that wasn't revealed until the tire started failing. Then Firestone stonewalled
in issue until forced to recall the tires.

So how does all that relate to the tables and explanations I shared?

Like I recently stated, at our place we keep our cars in our "standard operating environment" - our driveway! Plus or minus 100feet(33m) variation in local altitudes. So bi-weekly predawn pressure checks - and adjustments if necessary - are the norm here, and our shops have exclaimed accordingly: "Very even wear for 3-5 year old tires!"

RidingOnRailz
08-14-2021, 11:11 AM
CapriRacer wrote:

On a FWD car, you can't see the steering vs drive difference as the
front tires are doing both - BUT - that also means that the front tires
are going to wear more rapidly that the rears. I've measured about
2 1/2 times faster. Again rotating tires helps even out the wear.

Are you kidding?

I can see different wear pattern(more shoulder wear) on the front tires of my Honda Accord, and on the fronts of other front-drivers I've owned.

Same as with my rear-wheel drivers from ages ago - my Chevelle and Buick Century

CapriRacer
08-15-2021, 07:55 AM
So how does all that relate to the tables and explanations I shared?

Like I recently stated, at our place we keep our cars in our "standard operating environment" - our driveway! Plus or minus 100feet(33m) variation in local altitudes. So bi-weekly predawn pressure checks - and adjustments if necessary - are the norm here, and our shops have exclaimed accordingly: "Very even wear for 3-5 year old tires!"

As I explained, it is very common in scientific circles to reference STP (Standard Temperature and Pressure - 0C, 100 kPa = 32F, 14.7 psi) for testing purposes. Every experimental result is adjusted back to those conditions so they can be compared to other test results.

For tires, STP isn't used or referenced - however many people are used to the idea of a reference temperature and pressure, so they try to apply it EVERYWHERE - even when it isn't appropriate - and that is the case with the tables you referenced.

Are you kidding?

I can see different wear pattern(more shoulder wear) on the front tires of my Honda Accord, and on the fronts of other front-drivers I've owned.

Same as with my rear-wheel drivers from ages ago - my Chevelle and Buick Century

The problem here is that some alignment settings will result in uneven wear patterns rather than even wear. For a reason I don't quite understand, many vehicle manufacturers specify lots of camber - and when coupled with a bit of toe, the result is one-sided wear and/or diagonal wear.

Then there is the issue of how a vehicle is used. Vehicles that do a lot of city driving will experience more shoulder wear. So even though the front tires on a FWD SHOULD wear evenly, both the actual alignment and how the vehicle is used will impact evenness of wear. One way to put it is: Your mileage may vary.

RidingOnRailz
08-15-2021, 08:28 AM
Tire engineer here.

The link didn't actually say HOW to use the chart. I suspect what you are supposed to do is if you fill your tires at a certain ambient temperature, use the chart to tell you how much to fill it to. That way at 70F, you will have the proper pressure.

Alternatively, if you are filling your tires at 70F, then use the chart to make sure you have enough tire pressure when it gets down to the temperature indicated.

Either way, I think this creates more confusion than is needed.

So there is a bit of wrong information on the page. For every 10F change in ambient temperature, the pressure inside a passenger car tire changes about 1 psi. For other tires, use 3% for every 10F. (Note: 3% of 30 psi is about 1 psi)

Please note that the tire manufacturers want you to inflate your tires to the vehicle manufacturer's specified pressure for whatever the operating conditions are. That means that if you are operating at 10F, they want you to use the specified pressure, not one specified for another temperature. That also means if you fill your tires at 70F, then they will be too low at 10F (about 6 psi low).

Yes. yes, it is confusing.

Now allow me to express my opinion: Winter tires would benefit from a few extra psi. (3 to 5 psi) That extra pressure helps push the footprint through the snow to reach the pavement. The pavement will always have better traction than snow.

Some people will argue that over inflating your tires will cause the center of the tread to wear faster that the shoulders. That is true, BUT that effect is small compared to most other things that cause rapid wear, such as alignment and the drive vs steering effect.

Drive vs steering effect? Steering tires tend to wear the shoulders and drive tires tend to wear the center. You can see this clearly on a RWD car - where the wear rates are about the same front to rear but in different portions of the tread. That's why rotating tires gets you longer tread life.

On a FWD car, you can't see the steering vs drive difference as the front tires are doing both - BUT - that also means that the front tires are going to wear more rapidly that the rears. I've measured about 2 1/2 times faster. Again rotating tires helps even out the wear.

As I explained, it is very common in scientific circles to reference STP (Standard Temperature and Pressure - 0C, 100 kPa = 32F, 14.7 psi) for testing purposes. Every experimental result is adjusted back to those conditions so they can be compared to other test results.

For tires, STP isn't used or referenced - however many people are used to the idea of a reference temperature and pressure, so they try to apply it EVERYWHERE - even when it isn't appropriate - and that is the case with the tables you referenced.



The problem here is that some alignment settings will result in uneven
wear patterns rather than even wear. For a reason I don't quite
understand, many vehicle manufacturers specify lots of camber - and
when coupled with a bit of toe, the result is one-sided wear and/or
diagonal wear.

Then there is the issue of how a vehicle is used. Vehicles that do a
lot of city driving will experience more shoulder wear. So even
though the front tires on a FWD SHOULD wear evenly, both the actual
alignment and how the vehicle is used will impact evenness of
wear. One way to put it is: Your mileage may vary.


I just naturally assumed - and expected - varying degrees of additional shoulder wear(or at least, more outer tread wear) - on the steer tires, owing to the complex stew of angles occurring during the act of steering, as compared to tires on non-steering axles. Regardless of front- vs rear- vs all-wheel drive.

Variables could include specific tire compound, inflation pressure, steering geometry specific to a given vehicle, condition of those components, and of course, driving style(Earnhardt vs Sundee driver, lol!)

CapriRacer
08-16-2021, 07:24 AM
I just naturally assumed - and expected - varying degrees of additional shoulder wear(or at least, more outer tread wear) - on the steer tires, owing to the complex stew of angles occurring during the act of steering, as compared to tires on non-steering axles. Regardless of front- vs rear- vs all-wheel drive.

Variables could include specific tire compound, inflation pressure, steering geometry specific to a given vehicle, condition of those components, and of course, driving style(Earnhardt vs Sundee driver, lol!)

As you point out, there are lots of variables. That is why it is difficult to convinced people of the basic principles involved - their own experiences and what they read on the internet don't include such subtleties as what has the biggest effect and what can be ignored (not having a significant effect).

RidingOnRailz
08-16-2021, 09:09 AM
As you point out, there are lots of variables.
That is why it is difficult to convinced people of the basic
principles involved - their own experiences and what they
read on the internet don't include such subtleties as what
has the biggest effect and what can be ignored (not having
a significant effect).


So based, probably, on the topmost three statements in this link, regarding pressure, density, and temperature of air:

http://msrc.sunysb.edu/~chang/atm205/Notes/Chapter_1_txtb.pdf

The providers of that blue chart are suggesting that, for every F10 below F70, one should increase their cold tire pressures according to the grid.

IE: The vehicle placard on my Honda Accord recommends 32PSI cold, as long as same size and speed rating of tire are kept on it when replacing.

For temperatures from 70F up, that cold pressure can be maintained.

However, when first thing in the morning temperatures start to average 60F, I should add a pound - to 33psi cold. And so on...

Until the average morning temperature, say in February around here, averages only 20F, then I should be maintaining 37psi cold. Come April, with average dawn Temps around 40F, then it's OK to deflate them back to 35psi cold.

Again, based on the principles, concerning air pressure, density, and temperature, in the above link.

So if I had a big old Econoline or Express van, proper light truck tires, and recommended rear axle cold pressure(80psi) = Max cold pressure on the tires, is it ok, per those principles and that chart, to keep them at 85-87psi cold, in winter?

But, if colder air is more dense than warm air - the opposite of what I assumed for most of my 51 years, why are they suggesting increasing cold tire pressures at lower ambient temperatures?

Barry you can probably tell I spent many years in special ed schools - they never taught us any of the above stuff! lol

CapriRacer
08-17-2021, 10:18 AM
Allow me to start in a different place:

What to ignore:

Elevation: Even at 5,000 feet (Denver), the amount of inflation pressure change from sea level is only 2 1/2 psi - not enough to worry about.

Humidity: Just not a factor

So we are only talking about temperature vs pressure.

In summer, we can expect at 20F change from early morning to the hottest point of the day. That would be a 4% change - 80 psi to 83 psi - again, not enough to worry about.

The problem in winter is 2 fold:

1) Water vapor can freeze about of humid air at as little as 37F. That means that one should avoid check inflation pressures when it gets below - say - 40F. That also means that, in winter, one should anticipate colder temperatures and set the tire inflation pressures to cover the worst temperatures in the coming months. When I lived in Michigan, the worst temperature I anticipated was -20F, but I would set the pressures for 0F, reasoning that if it got to -20F, I was not going to travel fast (which causes additional stresses) nor for very long.

Which brings me to:

2) Setting the pressure in a warn environment, with the expectation of operating in a cold environment.

So lets say the pressures are set at 50F, and the expect operating temperature is 0F, then the expect loss of pressure would be 10%. So an 80 psi tire should be set at 88 psi.

Doesn't that exceed the max pressure on the sidewall? Yes, but not only are tires designed for a much higher pressure (long story why that is!), but the tire isn't expected to operate at that pressure - the tire will cool down to a lower pressure.

You can use that chart to help set pressure for anticipated conditions - except that the chart doesn't show what I just outlined above. Ergo, I would recommend NOT using this chart.

RidingOnRailz
08-17-2021, 12:03 PM
Allow me to start in a different place:

What to ignore:

Elevation: Even at 5,000 feet (Denver), the amount of inflation pressure change
from sea level is only 2 1/2 psi - not enough to worry about.

Humidity: Just not a factor

So we are only talking about temperature vs pressure.

In summer, we can expect at 20F change from early morning to the hottest
point of the day. That would be a 4% change - 80 psi to 83 psi - again, not
enough to worry about.

The problem in winter is 2 fold:

1) Water vapor can freeze about of humid air at as little as 37F. That means
that one should avoid check inflation pressures when it gets below - say - 40F.
That also means that, in winter, one should anticipate colder temperatures and
set the tire inflation pressures to cover the worst temperatures in the coming
months. When I lived in Michigan, the worst temperature I anticipated was
-20F, but I would set the pressures for 0F, reasoning that if it got to -20F, I
was not going to travel fast (which causes additional stresses) nor for very long.


Which brings me to:

2) Setting the pressure in a warn environment, with the expectation of
operating in a cold environment.

So lets say the pressures are set at 50F, and the expect operating temperature
is 0F, then the expect loss of pressure would be 10%. So an 80 psi tire should
be set at 88 psi.

Doesn't that exceed the max pressure on the sidewall? Yes, but not only are
tires designed for a much higher pressure (long story why that is!), but the tire
isn't expected to operate at that pressure - the tire will cool down to a lower
pressure.

You can use that chart to help set pressure for anticipated conditions -
except that the chart doesn't show what I just outlined above. Ergo,
I would recommend NOT using this chart.

Neither does a customer I showed it to where I work.

He said if my car specified 32psi cold tire pressures, setting them to the 37psi the grid suggests, even during below freezing dawn temperatures, would, in his own words, "... f - k up the tires! ..." :D I told him you got most idiots these days running anywhere from 40psi up to what's stamped on their tire sidewalls, and you might be risking your life to suggest they do otherwise!

I do maintain 34psi though, from December through March, and 33psi the rest of the year - primarily to account for possible tire gauge discrepancy. This Accord, even with the 50-series low profile rubber OEM at its trim level, seems to do everything and handle most predictably and securely, at those pressures, even on the rare occasion I push things, IE: take a corner a little faster than I usually do, etc.

So I think Honda does its homework when it comes to what's on their door frame decals.

Blue Bowtie
09-12-2021, 11:26 AM
That chart is, at best, very confusing, and at worst, incorrect. It also apparently ignores the fact that 70F 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 68F (20C), 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.

RidingOnRailz
09-12-2021, 12:59 PM
That chart is, at best, very confusing, and at worst, incorrect. It also apparently ignores the fact that 70F 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 68F (20C), 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.

Could the chart - and the website it comes from - be suggesting that at an ambient temp. of F70 or warmer, it's acceptable to match the recommended cold tire pressure on the door frame decal? That is, assuming the temperature first thing at dawn is F70.

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 65-75F, 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 30F 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

RidingOnRailz
12-24-2021, 03:39 PM
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).

CapriRacer
12-25-2021, 07:20 AM
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).

I'm going to challenge you on the origin of this document. I don't think the RMA (Now the US Tire Manufacturers Association - USTMA) would publish such a document. It implies that the inflation pressure listed on the vehicle tire placard is only valid at 70F - which is wrong.

Can you provide a link?

(Just a side note: I tried to google for it and couldn't find it.)

RidingOnRailz
12-25-2021, 08:18 AM
I'm going to challenge you on the origin of this document.
I don't think the RMA (Now the US Tire Manufacturers Association -
USTMA) would publish such a document. It implies that the inflation
pressure listed on the vehicle tire placard is only valid at 70F -
which is wrong.

Can you provide a link?

(Just a side note: I tried to google for it and couldn't find it.)

Good morning and Merry Christmas, et al.

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 C20(F68), 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 F70 - for underscoring the effect on inflation pressure every positive or negative ten degrees F.

CapriRacer
12-26-2021, 08:26 AM
....... 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. .......

When I said link, I meant to an RMA or USTMA webpage. What you just said is that the webmaster SAID it came from the RMA - and I find NO evidence to support that - which why I made the challenge.

.......As for the temperature, it is common practice in the scientific and engineering communities to use a temperature of C20(F68), and a barometric pressure of 29.94"(or something in the ballpark of mean sea level) as baseline atmospheric conditions for experiments and testing.....

Yes, yes, but that is NOT how tire pressure works. Tires are supposed to be inflated at ambient conditions. STP just doesn't apply.

RidingOnRailz
12-26-2021, 11:14 AM
When I said link, I meant to an RMA or USTMA webpage.
What you just said is that the webmaster SAID it came from the
RMA - and I find NO evidence to support that - which why I made
the challenge.


Yes, yes, but that is NOT how tire pressure works. Tires are supposed
to be inflated at ambient conditions. STP just doesn't apply.


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?

CapriRacer
12-27-2021, 08:26 AM
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." ......

Again he SAID it came from the RMA, but I find no supporting evidence - you know, like the chart he referred to at the RMA website! I can not find ANYTHING that even resembles that chart - let alone one from the RMA. So I don't believe he got it from the RMA. He may have gotten it from somewhere else and misremembers.

.......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?

Allow me to state this in a different way:

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 50F 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 50F. 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 70F 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 -10F. You are setting the pressures in your garage and it is 50F inside the garage. So you are targeting for a temperature drop of 60F = 6 psi, so you set the pressures at 39 psi.

RidingOnRailz
12-27-2021, 08:56 AM
Again he SAID it came from the RMA, but I find no supporting evidence - you know, like the chart he referred to at the RMA website! I can not find ANYTHING that even resembles that chart - let alone one from the RMA. So I don't believe he got it from the RMA. He may have gotten it from somewhere else and misremembers.



Allow me to state this in a different way:

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 50F 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 50F. 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 70F 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 -10F. You are setting the pressures in your garage
and it is 50F inside the garage. So you are targeting for a
temperature drop of 60F = 6 psi, so you set the pressures
at 39 psi.


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.

CapriRacer
12-28-2021, 07:32 AM
...... Am I doing it right? .......
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.

RidingOnRailz
12-28-2021, 08:45 AM
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.

Well, I would beg to differ when it came to the stiffness of the vinyl upholstered bench seat in my old 1981 Buick in winter, lol! Like rocks.

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

CapriRacer
12-29-2021, 08:06 AM
Well, I would beg to differ when it came to the stiffness of the vinyl upholstered bench seat in my old 1981 Buick in winter, lol! Like rocks.

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

Do not be surprised if USTMA doesn't respond.

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/default/files/CareAndService_Commerical_TruckBusTires_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.

RidingOnRailz
12-30-2021, 06:39 AM
Do not be surprised if USTMA doesn't respond.

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/default/files/CareAndService_Commerical_TruckBusTires_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.

Yes, I looked through all the free TMA PDF documents and found nothing resembling either table - the original, or his revision of it.

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!

CapriRacer
12-30-2021, 09:27 AM
...... Seekers of verification and of the truth often get branded as such, lol!

I think it's your approach. It seems like you accept what is printed and aren't skeptical of what people write. Perhaps if you worded things differently.

RidingOnRailz
12-30-2021, 09:53 AM
I think it's your approach. It seems like you accept what is printed and aren't skeptical of what people write. Perhaps if you worded things differently.

1) Isn't that a bit backwards?

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

sunnyreign
01-12-2022, 12:00 AM
First time seeing this chart but survived the winter without changing pressure last year!

RidingOnRailz
01-12-2022, 06:21 AM
First time seeing this chart but survived the
winter without changing pressure last year!

How so? Whats your technique?

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