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-   -   Adjust tire pressure for cold weather?? (https://www.irv2.com/forums/f258/adjust-tire-pressure-for-cold-weather-558461.html)

Ray,IN 10-27-2021 10:05 AM

Quote:

Originally Posted by Kid Gloves (Post 5964526)
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.

Add to that, the information from The USDOT and NHTSA, and Tire mfgrs association.

Kid Gloves 10-27-2021 10:34 AM

Quote:

Originally Posted by Chargerman (Post 5964750)
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.

I would suggest that owners consider a +10% recommendation rather than a +10psi recommendation.

On the heavier coaches that require 90psi or more,+10ps is certainly an appropriate consideration. An additional 10psi may be a bit more than necessary on a rig that has a cold pressure recommendation of 65psi. And a passenger car may only require the addition of 3psi.

I am aware that this is a Class A discussion.

MN_Traveler 10-27-2021 10:43 AM

Quote:

Originally Posted by Kid Gloves (Post 5965051)
I would suggest that owners consider a +10% recommendation rather than a +10psi recommendation.



On the heavier coaches that require 90psi or more,+10ps is certainly an appropriate consideration. An additional 10psi may be a bit more than necessary on a rig that has a cold pressure recommendation of 65psi. And a passenger car may only require the addition of 3psi.



I am aware that this is a Class A discussion.



All my tires are close enough to 100 psi that 10 psi IS 10% (or close to it) . Just me being lazy and not doing the math.. you could also call it an "engineering approximation". [emoji4]

Kid Gloves 10-27-2021 10:51 AM

Quote:

Originally Posted by MN_Traveler (Post 5965071)
All my tires are close enough to 100 psi that 10 psi IS 10% (or close to it) . Just me being lazy and not doing the math.. you could also call it an "engineering approximation". [emoji4]

In your case, that works out. It may not for others.

That was the message I was attempting to get across.

MN_Traveler 10-27-2021 11:14 AM

Quote:

Originally Posted by Kid Gloves (Post 5965085)
In your case, that works out. It may not for others.

That was the message I was attempting to get across.

LOL - I was really mostly joking and trying to insert some humor. agreed that if one's pressures were much lower it would probably pay to be more exact...

Tireman9 10-27-2021 04:30 PM

Quote:

Originally Posted by MN_Traveler (Post 5963299)
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.




From a tire design engineer... Good explanation of "why" it is the air that "supports" the load. Thanks.

Tireman9 10-27-2021 04:33 PM

Quote:

Originally Posted by Ramrod (Post 5964736)
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.




In theory yes. BUT in reality too much work wor the average person and the normal day to day pressure changes of 1 to 3 psi "cold" isn't that significant.


If you make it too much work, and some would consider changing inflation daily way too much work given we can't get most to check inflation once every few months, too much work would discourage some from ever checking psi.

Tireman9 10-27-2021 04:35 PM

Quote:

Originally Posted by Chargerman (Post 5964750)
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.






Yup:dance::thumb:

TommyHarris 10-28-2021 04:29 PM

So, I see some who are saying to set pressure to max cold pressure recommended, and others talk about “minimum +10%”…I’m confused.

2020 Fleetwood Pace Arrow 33D

Ray,IN 10-28-2021 09:36 PM

Quote:

Originally Posted by 1Motorhome (Post 5956521)
I recently noticed my cold pressure has decreased in all tires (by TPMS) by almost 7 pounds in the mornings with temps in the 40's. But within 10 driving miles they are back to normal pressure.
My question is, should I add more air at cold ambient temps to bring pressure to cold normal settings?

I really hope the pages of replies have helped, especially replies from Tireman9, who is a retired tire engineer.

Ray,IN 10-28-2021 09:38 PM

Quote:

Originally Posted by TommyHarris (Post 5966681)
So, I see some who are saying to set pressure to max cold pressure recommended, and others talk about ďminimum +10%ĒÖIím confused.

2020 Fleetwood Pace Arrow 33D

I would follow Tireman9 's advice, being a retired tire engineer he knows more than any of us irv2.com members.

Tireman9 10-29-2021 10:19 AM

Quote:

Originally Posted by TommyHarris (Post 5966681)
So, I see some who are saying to set pressure to max cold pressure recommended, and others talk about ďminimum +10%ĒÖIím confused.

2020 Fleetwood Pace Arrow 33D


Starting at the end.


Assuming you know the actual load on each tire from your measurement on a scale (ya I know about assuming but every RV owner has been told at least once to learn their actual loading).
You take the load on the heavy end of an axle as there are almost zero percent RVs with the load exactly at 50/50% side to side.


The load number is then found in the Load/Inflation charts for your size tire and you go up (to the right) till you find a block with at least or more load than what you measured on the scale. NEVER go lower than your scale reading. DO NOT average the reading from each end of the axle weight measurement. DO NOT try and calculate a pressure between the 5 psi increments. Then look up in the chart to find the PSI. That is the MINIMUM inflation you should ever run in the tires on that axle.


I suggest you add 10% to that inflation number to offer some "protection" in case the temperature drops. If you have added my recommended 10%, you will probably see that you do not have to add air every day the temperature drops 10 degrees.


RVs have Certification labels AKA Tire Placards that have tire size, type, Load range and inflation numbers. They also have GAWR which is the MAXIMUM load you should ever have on that axle. The RV company is required, by DOT to post on the sticker, an inflation number that is sufficient to support 100% of the GAWR. RVIA (a standards organization sticker on the side of your RV now requires inflation level good enough to support 110% of GAWR, which is better than the DOT requirement) Because of these load capabilities most RV companies select the smallest (lowest cost for them) tire that can just barely meed these requirements. The result of this purchasing decision is that you will need to inflate your tires to the level needed to support the tire's MAXIMUM load capacity which is the number on the sidewall of the tire.


Side issue. The wording on the tire sidewall is confusing. The reality of what it means is that any given tire has a MAXIMUM load capacity and an inflation (minimum) required to support that load. What is not printed on the tire sidewall is the fact that there is no increase in inflation that will result in that tire ever being capable of supporting more load. Therefore the "max inflation":facepalm: wording that was decided upon by some committee 50 years ago.

jadatis 11-01-2021 07:46 AM

Made my ( to) long story about why I think pressure as to be higher cold at 100degrF then at 70 degrF.

Here it is.

Why pressure should be calculated back to 70 degrF.

That pressure of tires is related to the temperature of gascompound in tire, is fact.

But that for instance you need for 100 degrF ambiŽnt temperature higher cold filled pressure, then for 70 degrF, and for 40 degrF can do with lower pressure then for 70 degrF, is a conclusion I took, and is the discussion here.

I am going to explain how I came to this conclusion.

I once had a long telephone call with a man from Vredestein , and he explained that goal of pressure determination and maxload of tire, is to not overheat them.

He explained that the molecule-structure of natural rubber can be compared to spagetti, and if you push your nail in it the print remains . Then the ď spagetty-strings" slide over each other.

Tires are vulcanised at the end, and this makes sulfurbridges between the ď spagetti ď wich makes the rubber flexible, so when you push your nail in it , the print directly dissapears, the rubber goes back to its original shape.

The rubber needs a certain amount of sulfurbridges, but to much makes the rubber hard, and if then bending of the rubbber, by the deflection and flexing back every cycle, gives small cracks, wich dont disapear ( logical)

When rubbers temperature goes above a critical temperature, to many sulfur bridges are created,and rubber hardens. Wich temperature this is I dont know, but the vulcanising proces is at 170 degrC is 340 degrF.

Rest is my Einstein way of thinking, and because my average IQ is probly half of that of Mr Einstein, change is 2ce as much that my theory is proven wrong once.

The temperature of the rubber is created by the balance between heating up a second and cooling down a second.

When you begin driving at cold pressure in ambiŽnt temperature of 70degrF, in-and out-side-tire air and all the rubber of tire is 70 degrF. The heat-transport then is zero.
In basics Maxload is calculated for reference-pressure ( E-load 80 psi) and reference-speed ( P- and LT tires mostly 99mph/160kmph) so when you drive that speed constant with maxload on tire and reference-pressure cold filled in tire, no overheating of any part of tire. I practice a bit more complicated.

Yust for the example say you then drive 99mph constant speed, with referencepressure of 80psi ( E-load) and maxload on tire. In the beginning cooling down is marginal, because temperaturedifferences between rubber of tire and in and outside tire gascompound is still zero, but driving long enaugh rubber yust below its critical temperature in the middle of the thickest parts of tire, and at the edges , where rubber meats the gascompound, lets say 300 degrF.
Inside tire gascompound I state as 140degrF. Outside the tire always air 70 degrF.

Then temperature difference between edges of rubber and inside tire gascompound 300-140=160 degrF., and between outside tire air 300-70=230 degrF. Also the inside tire gascompound is cooled down mostly trough the rimm to the air outside the tire with only 140-70= 70 degr temperature difference so 160/70 = 2.28 times as much cooling down by the better transport trough the metal rimm.
So coolingfactor then inside tire 160/230=70% of outside tire. In and outside tire Together 230+160=390 degrF
The rubber temperature then stays in balance ,so heating up factor also 390 degrF.

But the deflection , so heatproduction a second at that speed, then is lesser then at cold pressure, because temp in tire 140degrF, wich gives higher pressure. 80 psi filled at 70 degrF becomes 92.9 psi at 140 degrF.

Now situation ambiŽnt temp 100 degr F.
Temp of rubber still max 300degrF.
Temperaturedifference rubber and outside tire air 300-100= 200 degrF
Temperaturedifference rubber and inside tire gascompound 70% of outside 200= 140 degrF.
Together200+140=340 degrF worth of cooling capacity
So temperature inside the tire 300-140= 160degrF.

So at 70degrF ambiŽnt temperature 390degrF worth of cooling capacity.
At 100degrF ambiŽnt temperature 340degrF worth of cooling capacity.
This has to be compensated by lesser heatproduction a second by lesser deflection of tire is 340/390=87.2 % of the heatproduction at 70degrF ambiŽnt temperature.
Now if 80psi filled at 70degrF
At 100F/85.4psi
At 140F/ 92.5psi
At 160F/ 96.1psi
So 92.5/96.1=96.25% of the surface on the ground then at 70degr F driving 99mph with maxload on tire. Rule of tumb I determined gives 0.9625^2= 92.64 times the deflection . If heatproduction goes lineair with the deflection ( I hope and not more), this gives 92.64% of the heatproduction.

Cooling down is 87.2% , so heating up is even still more then cooling down, if 300degrF is the critical temperature of rubber .
If you then lower the 85.4psi cold pressure at 100degrF ambiŽnt temperature to 80psi, as is stated to be allowed because cold filled is at ambiŽnt temperature, the pressure becomes at 160degrF 90.2psi instead of 96.1psi ,wich gives more deflection so heatproduction a second, so rubber of tire temperature goes above 300degrF, and it hardens and cracks.
In fact , because at 100degr heating up factor is 92.64% and cooling down factor is 87.2% at 100degrF ambiŽnt temperature of the 70degrF situation, you should even pump the tires up a little to give lesser heatproduction a second., so heating up factor goes also to 87.2

Now ambient temp 40degrF.
Outside tire 300- 40= 260 degrF worth of cooling down factor.
Inside tire 300- ( 70% of 260= 182 degrF)= 118 degrF inside tire gascompound
So cooling down factor 260+182= 442 degrF
This is 442/390= 113.3% of coolingdown factor then at 70 degrF.
Then at 40 degrF 80psi filled at 70 degrF becomes
74.6 psi cold and at 118 degrF inside tire temp 88.6 psi
92.5psi/88.6 psi = 1.044 times more surface on ground gives 1.044^2= 1.09x more heatproduction.

So when colder tires rubber stays cooler then 300degrF when driving 99mph with maxload and reference-pressure on tire. So for safety not needed to fill up the cold pressure of 74.6 psi to 80 psi , with difference to hot temps, that you may do so for riding quality and fuell saving.

So I think the lists for filling higher at 70 degrF in a heated garage , to get the 80 psi in this example at for instance 20 degrF is because at those extreme cold ambiŽnt temperatures the deflection gets to much so for instance snake-bite.
Then its not anymore to prefent overheating, but for riding -quality.

jacwjames 11-01-2021 08:17 AM

I like the KISS method.


I use a TPMS to monitor pressure and temp.

I set the tire pressure ~10 psi higher then the placard says. I know this may give a "rougher" ride but my coach seems to handle it, drives straight and true, tire wear is minimal if any.

I visually inspect my tires often.


I do not move the coach until the TPMS shows all good, most of the time I just leave it on, but will shut if off if parked longer periods. I've trained my wife to do the same thing.


I have set low pressure and high temp alarms.


While traveling this summer I went from ~90F temps >>> +100F >>> to 45F temps at which point my low pressure alarm sounded on one tire. So I went around and adjusted all the tires. All good rest of trip. I carry a small pancake compressor to use, use just had to start generator.


Now at homes temps start dipping again. Getting ready to go on a trip so I plugged in the TPMS. Got a low pressure alarm again so I went around and adjusted all the pressures.





I know it can be tedious to get the pressures equalized but what I found is that if I over inflate slightly ~2 psi and let the tires site for a while and monitor the TPMS and then one by one I will unscrew the pressure sensor to bleed air, doing this several times to get the same pressure.

Easier then trying to unscrew sensor add air, screw back in sensor and check repeat!!!!


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