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Old 10-26-2021, 12:03 PM   #99
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Originally Posted by wolfe10 View Post
Agree with the first half or your statement.


The second half not at all. The PSI on the sidewall is for cold (before driving), and has no relationship to PSI after driving. I promise that the tire engineers are familiar with the Ideal Gas Law and absolutely know what PSI change occurs with increased temperature.
Yes we do know that. It could be argued that we depend on that increase.

I would be surprised if most tires do not exceed the side wall PSI if driven on hot days at highway speeds.

Yes tire pressure can many times exceed tire sidewall number in real life operation. I doubt that the placard pressure for any RV is set based on engineering testing but are set simply based on Load & Inflation tables and regulatory minimums from NHTSA and RVIA.


Now we engineers also know that most passenger car tires would be an exception to the above general statement because their tire pressure is based on many dozens if not hundreds of in-car, and in laboratory engineering evaluations and the final number that goes on the placard is a balance of Ride, Handling, Fuel Economy, Noise and Traction and numerous other tests while it is possible that there is a motorhome model that had a Ride or handling evaluation, I have never heard of any motorhome company selecting any tire based on multiple in-vehicle evaluation or comparison test.

Examples: Most Passenger car tires have a recommended inflation pressure on the tire certification label that is significantly lower than the tire sidewall number, even though car companies have to meet similar minimum inflation numbers based on Federal Regulations. Don't forget to consider the percentage change, and not the absolute number of psi.
My personal cars have sticker / tire inflation of 36/51 (sticker is 29% lower) and 29/36 (sticker is 19% lower). My Class-C RV shows 65/80 on the certification sticker. These inflations are only sufficient to provide a Reserve Load of 1.9% Front and 2.9% rear.
If you stop and think about it, passenger cars having double digit reserve load capacity is probably a major reason for tire longevity in passenger car applications.
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Old 10-26-2021, 12:36 PM   #100
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Originally Posted by Max Headroom View Post
That's a serious case of overthinking, on steroids. OTR trucks go from places like Denver to Phoenix in one trip, in one day. Trust me, they are not doing this.
Are you adjusting your tires for fuel burn every couple of hours too?


A tire that is running and up to temp will be just fine, even if you go from Death Valley, to Vail. Checking the cold pressure where ever you are in the morning will be close enough, and what is recommended on the data sheet for the cold load pressures. There is no hot pressure table for any commercial tire that I've ever seen.
If the tire mfgrs thought this was necessary and prudent, they would recommend it, and they don't. Same goes for whats in the coach owners manual. As others have said, all this has already been thought of by the tire makers, when they published the cold inflation specs.
And if you are 'interpolating' what your hot TP 'should be', and adjusting based on that, then you're basically just guessing, and certainly not following the mfgrs recommended safe practice.


For a while, I've been scratching my head as to why so many RV's have blow outs, and I think I have my answer...

Yes Underinflation, which for the tire is about the same as overloading, IS a primary reason for belt separations and shorter tire life. The data from tens of thousand RV scale readings at FMCA and Escapees and other events tells us that a MAJORITY of RV have one or more tire in overload.
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Old 10-26-2021, 12:41 PM   #101
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Originally Posted by PBK Images View Post
My understanding from Michelin is that the standard temp they use is 68F. Assuming that my OAT in the morning is 45F, and my front tires (with the weight that I am carrying) is recommended to be a minimum of 117psi. But I could go up to the max limit of the tire and/or wheel, say 130PSI, my inflation goal at 45F will be the minimum (117 PSI). Why? Because I know that the OAT will increase during the day and the tire temp will increase as I drive. Just using common sense. If it is much colder, say, 35F, and I know that after a mile or two the tires will heat up to 45F and then continue heating, I might "cheat" a pound or two and inflate to 115psi to avoid higher psi at operating temps. Everyone has their own thoughts.

While I know that Michelin has some ideas that are not supported by all other tire companies, I sincerely believe that either you are mis-remembereing or initially misunderstood that statement about 68F being some "standard". Sorry to hear that you lost the document as I would consider that "ground-breaking" information if accurate. Is it possible that the document was from a tire dealer and not Michelin corporate?
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Old 10-26-2021, 12:43 PM   #102
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Originally Posted by Kid Gloves View Post
There is no standard temperature. Where did you find a load and inflation chart that recommends 117psi? You need to round up in 5psi increments to 120psi. Why would you ever inflate to less than the recommended minimum cold pressure?

Higher psi at operating temperatures is normal. Itís physics, itís inescapable. You are not using common sense.



Correct.
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Old 10-26-2021, 12:45 PM   #103
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Originally Posted by NXR View Post
The minimum pressure I use is 90 PSI. "Minimum" as in not underinflated at all and coincidentally is also the inside DOT sticker. So 90 PSI can fully support the GAWR of each axle.

I usually set the tires to 98 or so to give me some temperature margin.

When I leave Florida at the end of March I bring all of the tires up to 100 PSI from the 95 PSI or so they usually are at that time.

When I arrive in Cleveland, OH and the temps are in the low 40's to high 30's, the next morning the tires are sitting at 90 PSI or a tad more.

It works for me.

We'll be leaving here in late December for Florida so I'll have the tires here at about 95 PSI. Whatever they are when we get to Florida will be where they stay unless somehow they are over the maximum sidewall cold pressure of 110 PSI. That has not happened yet.

I put air in the tires twice a year, when we leave Florida in early Spring and in the late Fall before we leave northern Ohio.

FWIW,

Ray

I live near Akron so understand. Your plan and approach is reasonable.
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Old 10-26-2021, 01:00 PM   #104
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Originally Posted by BigBillSD View Post
Why wouldn't the tire pressure recommendations be to a corrected temperature like maybe 70 degrees? Using a TPMS allows you to see your tire pressures rise within a minute or so of starting to drive. My tires usually run at the same (130lbs) pressure when its 60 degrees outside and at 110 degrees outside, and they start at 115lbs cold. I have watched that many times driving from San Diego over toward Yuma on the way out of town. Changing the pressures every morning due to the outside temperature sounds more like an old wives tale.. (or more appropriately a senile engineer's tale) Or, we have always done it that way mentality. Within a few minutes your tires are hot. Maybe we are saying that its those first couple miles that kill the tire, as after that the pressure & tire temp rise pretty fast. -Bill

PS. This should "heat" up the discussion..

I have never advocated changing tire pressure each morning. I do strongly support the adjustment of inflation that is lower than 110% of what is needed to support your measured load on the tires on the heaver end of an axle.
Having "cold" morning pressure that is greater than 120% of your "set pressure would be when I could possibly consider a change but only if the predicted temperature for the day was + 30F or more than it is where you are located AND you are already at 120% of your set pressure.
In my experience which includes 3 cross country trips and temperature ranges from 24F to 98F I do not ever recall bleeding pressure off.


So I am now expecting someone that travels to Alaska in February and Death valley in August to come up with a situation that is outside of my general guidelines. But lets get real. Most people travel in the lower 49 when temperatures range from 40s to maybe 100F. That should mean your morning pressure might change by 12%.



Lets try this... Set your morning pressure to between 105% and 120% of the minimum needed to support the measured load. I bet if you do that you would only be adjusting cold pressure a couple times a year.
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Old 10-26-2021, 01:36 PM   #105
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After a life time of driving trucks of all sizes (CDL-A) both local and long distance the one thing that has saved countless tires for me has been to check the temperature of each tire when hot (after driving, when stopped for fuel, etc) by touching all of them. If one is hotter than the norm, it's probably low or has some other problem. It's just a habit for me (and most good truck drivers) now to check all the tires when I stop for fuel (a quick walk around). Just caught a low tire the other day in MT, picked up a nail in a trailer tire, and had it repaired before heading over the mountain passes. I always set tires to the max psi indicated on the sidewall cold when hauling heavy and have usually had great tire life out of them. You can over think some stuff.
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Old 10-26-2021, 02:16 PM   #106
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If you really want to worry less about fluctuations in pressure you could find a place that could drain all the air from your tires and replace it with nitrogen air. It will have far less fluctuations between cold pressure and driving pressure. And then only add nitrogen air when it needs any more at some point.



How would you "drain" all the air out? Sucking a vacuum in a tire will simply de-seat it.
There is almost no meaningful difference if the Pressure/Temperature change between dry air (78% N2) and a purged tire with 93% N2.
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Old 10-26-2021, 09:17 PM   #107
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Originally Posted by Tireman9 View Post
How would you "drain" all the air out? Sucking a vacuum in a tire will simply de-seat it.
There is almost no meaningful difference if the Pressure/Temperature change between dry air (78% N2) and a purged tire with 93% N2.

I'm sure happy you joined in this thread Roger! It's incredible what some folks come up with.
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Old 10-26-2021, 09:22 PM   #108
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Just imagine the great achievements that might be possible if the creative minds were to focus on something other than ways to reinvent the tire inflation guidelines provided by the tire manufacturers.
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Old 10-27-2021, 06:46 AM   #109
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Oh my. This will be a lively discussion. The "safe" answer is never run them at all, even for a few minutes, if they are below their minimum. This is exactly why i add 10 psi above the charts ... so on cold mornings i dont have to go out and inflate them.

I have no doubt though that other members here will say otherwise.
I for one disagree with you. Never over inflate your tires. Very dangerous. Set them every travel day to the recommended cold pressure. It only takes a few minutes each time.
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Old 10-27-2021, 06:51 AM   #110
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Ok .... for a clearly serious, and actually very insightful question, I will try to answer .... because it might help cut through some of the misinformation on this thread.

Ray - this is actually a really good question - and the answer is that this is really mixing two very different aspects of physics.

A plane wing flies because of two things: the curved top of the wing which produces lower pressure on the top of the wing than on the bottom (the Bernoulli effect - which is is a pressure thing and has nothing to do with density), and the angle of attack of the wing (where, when the front of the wing is tilted upward and the wing pushed forward, air hitting the bottom of the wing is deflected downward, which exerts an upward force on the wing (newtons third law - when an object exerts a force on another object, the second object exerts an equal and opposit force on the first - strictly a density/mass thing, and has nothing to do with pressure).

Your density altitude thing is mostly a result of the angle of attack of a wing allowing it to "push" downward on the air - the denser (colder) the air, the stronger the upward force (because the air molecules are closer together, and the wing pushes more molecules downward for a given amount of forward motion .... kind of like throwing downward two baseballs versus one ... it takes more force to throw down two of them.

Inside a tire, there is no such "pushing" of air, and so its density becomes a non-issue. The ONLY thing acting inside a tire is the pressure the air exerts on the tread, walls, and rim of the tire. This works because a given pressure pushing on the tire "stiffens" the tire, and limits how much the sidewalls of the tire will deflect for a given load. If the pressure is lower, the tire sidewalls are not held stiffly in place, and can deflect more (very much like a very underinflated balloon is easy to squeeze and deform, but a highly inflated balloon is very stiff, and difficult to deform - it can support more weight without deforming.

To understand pressure - you really need to understand statistical thermodynamics .... but the simple explanation is that pressure is the result of lots and lots of gas molecules hitting the inside of the tire .... it is nothing more than that. It is the summation over time of many, many small "balls" (molecules) each with very very small mass and momentum hitting a wall. So .... the fewer the number of molecules inside the tire (like letting air out of the tire), the fewer will be hitting the wall in a given time, and the pressure is lower (the opposite is true when you add air to the tire.

As for temperature - it turns out that the speed a gas molecule flies through space is directly dependent on the temperature (the maxwell-boltzmann distribution). So for a tire with a certain amount of air in it, if the temperature goes down, the speed that the gas molecules are moving at goes down, and they each hit the inside of the tire with less momentum - and the pressure (and thus stiffness of the tire) goes down - for a given amount of weight on the tire, the tire deforms more. The tire may technically be supporting the weight, but upon each revolution it deforms more than if it were supported by a higher pressure - and it is this ongoing increased amount of deformation that causes increased stress and damage to the tire.
BRAVO !!!!!!!!
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Old 10-27-2021, 07:04 AM   #111
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Adjust tire pressure for cold weather??

Adding 10psi over the recommended inflation pressure is not over inflating the tire unless it exceeds the tires maximum pressure on the sidewall. Remember, the chart lists the minimum pressure required to support a given load. Going above that is basically adding some cushion. The only thing that may be sacrificed when increasing above the recommended pressure is the ride.
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Old 10-27-2021, 10:59 AM   #112
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I for one disagree with you. Never over inflate your tires. Very dangerous. Set them every travel day to the recommended cold pressure. It only takes a few minutes each time.

Please link to one instance where overinflation from load/inflation charts caused tire failure, Then links to tire failures from underinflation/overloading.
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