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Old 07-18-2023, 11:19 PM   #1
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Tire inflation - #1 inflate while on jacks, #2 weight shift on slopes

2008 Tiffin Phaeton DP MH 40’
I have two questions on large tire inflation
#1 can you adjust cold tire inflation while up on jacks or will it change when off jacks?
#2 when traveling, say that your on a very long steep grade, I would think that the weight would increase on the rear tires. In thinking about forces in physics … when level the forces are straight down but if I remember vectors, on a hill, for each wheel there would be a force down but also a force to the rear. I just wonder if on your trip if you’re doing a lot of grades does weight increase significantly to the rear and cause the rear tires to heat. I’ve also driven for hours on tight curves on mountainous 2 lanes and I imagine the weight changes on curved as well.
I’m not smart enough to do the physics math on this?
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Old 07-19-2023, 06:59 AM   #2
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Originally Posted by swduns View Post
I’m not smart enough to do the physics math on this?
But you ARE smart enough to know that some very smart people have figured all of this out already.

Inflate tires on the ground, like they are designed to be used. Inflate them to your desired pressure while they are cold, before driving, and before the sun has warmed them up.

Now go and drive. Any temperature/pressure changes due to driving loads and road friction are part of the design parameters. Higher pressure on the ‘sunny side’ is normal.
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Old 07-19-2023, 07:10 AM   #3
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Originally Posted by swduns View Post
2008 Tiffin Phaeton DP MH 40’
I have two questions on large tire inflation
#1 can you adjust cold tire inflation while up on jacks or will it change when off jacks?
#2 when traveling, say that your on a very long steep grade, I would think that the weight would increase on the rear tires. In thinking about forces in physics … when level the forces are straight down but if I remember vectors, on a hill, for each wheel there would be a force down but also a force to the rear. I just wonder if on your trip if you’re doing a lot of grades does weight increase significantly to the rear and cause the rear tires to heat. I’ve also driven for hours on tight curves on mountainous 2 lanes and I imagine the weight changes on curved as well.
I’m not smart enough to do the physics math on this?
You are overthinking this unnecessarily. #1/no. #2/If one goes up a grade, one usually descends again. And a twist in the road usually isn't the only one. Someone will chime in with a worry about your refrigerator.
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Old 07-19-2023, 08:30 AM   #4
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Quote:
Originally Posted by swduns View Post
2008 Tiffin Phaeton DP MH 40’
I have two questions on large tire inflation
#1 can you adjust cold tire inflation while up on jacks or will it change when off jacks?
#2 when traveling, say that your on a very long steep grade, I would think that the weight would increase on the rear tires. In thinking about forces in physics … when level the forces are straight down but if I remember vectors, on a hill, for each wheel there would be a force down but also a force to the rear. I just wonder if on your trip if you’re doing a lot of grades does weight increase significantly to the rear and cause the rear tires to heat. I’ve also driven for hours on tight curves on mountainous 2 lanes and I imagine the weight changes on curved as well.
I’m not smart enough to do the physics math on this?
swduns,

Question #1 can you adjust cold tire inflation while up on jacks or will it change when off jacks?

Answer: the interior volume of the tire does not change whether the tire has a load or not. The cords and belts of the tire keep the volume constant regardless of load so the answer is it doesn't matter if you are on jacks or not.

Question #2 when traveling, say that your on a very long steep grade, I would think that the weight would increase on the rear tires.

Answer: Tire designers have taken all this into account. Changes in load because of slope and changes in pressure because of temperature and altitude change are factored into the load tables. Set your pressures according to the Load/Inflation Chart for your tires and load when the tire temperature is ≈70° F. Pressure changes ≈1 PSI per 10° F. If the ambient temperature of the tire is ±30° from 70° that means a difference of 3 PSI. 3 PSI is about the tolerance of most air gauges. Don't worry about small differences. Note, never adjust your pressure when your tires are hot. The change is too small to be significant. Note: Pressure due to internal tire temperature caused by road friction can be much higher and is also normal.

As someone else said, it is easy to overthink the tire pressure issue. That is SO true. Literally millions of over-the-road trucks have tires just like yours. Conscientious drivers check their pressures daily, average drivers check them when they think about it and the numb-skulls never check them! Be somewhere in between conscientious and average and you will be fine!
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Old 07-19-2023, 09:10 AM   #5
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ignore the circumstance of where and how its parked, check the air pressure cold. up or on the ground, tire pressure is set the same.
uphill or downhill. you cant adjust for both, because you wont know the degree of the incline or decline, speed, or temp of road surface, all of which, in laboratory conditions may make a difference, but on the road they wont. so just take an average, and set it like you were on a level surface.
short version, just set the tire pressure at what is recommend, or to suit your fancy.
just dont be the numbskull that never checks the pressure at all.
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Old 07-19-2023, 09:13 AM   #6
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You are WAY over-thinking this.


Inflate to correct PSI in the morning, before driving, before sun is on them.



Drive and enjoy.


Also recommend a Tire Pressure Sensor System.
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Old 07-19-2023, 09:39 AM   #7
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Originally Posted by yeloduster View Post
swduns,

Question #1 can you adjust cold tire inflation while up on jacks or will it change when off jacks?

Answer: the interior volume of the tire does not change whether the tire has a load or not. The cords and belts of the tire keep the volume constant regardless of load so the answer is it doesn't matter if you are on jacks or not.

Question #2 when traveling, say that your on a very long steep grade, I would think that the weight would increase on the rear tires.

Answer: Tire designers have taken all this into account. Changes in load because of slope and changes in pressure because of temperature and altitude change are factored into the load tables. Set your pressures according to the Load/Inflation Chart for your tires and load when the tire temperature is ≈70° F. Pressure changes ≈1 PSI per 10° F. If the ambient temperature of the tire is ±30° from 70° that means a difference of 3 PSI. 3 PSI is about the tolerance of most air gauges. Don't worry about small differences. Note, never adjust your pressure when your tires are hot. The change is too small to be significant. Note: Pressure due to internal tire temperature caused by road friction can be much higher and is also normal.

As someone else said, it is easy to overthink the tire pressure issue. That is SO true. Literally millions of over-the-road trucks have tires just like yours. Conscientious drivers check their pressures daily, average drivers check them when they think about it and the numb-skulls never check them! Be somewhere in between conscientious and average and you will be fine!
Regarding your answer for question 1. Yes, the tire pressure will increase to some degree if the tire is inflated when the vehicle is up on jacks or off the ground. When the vehicle is then lowered and the full weight is on the tires, the tire internal volume will be slightly less, which will cause a pressure increase? Is that difference significant....NO...but it does happen, and maybe that is what you really meant.
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Old 07-19-2023, 12:10 PM   #8
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yeloduster gave an excellent answer. And I believe xrated is wrong about the the tire volume changing with the jacks down (reduced load). The shape of the tire changes, but not the volume. The tire body would have to actually stretch or contract for the volume to change and that doesn't happen, at least not to any degree you could measure outside of a laboratory.
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Old 07-19-2023, 12:22 PM   #9
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Go to tire shop to have new tires
Tech adjusts tire pressure before even mounting them onto the vehicle

Pressure doesn't change

Adjust COLD prior to travel ----on jack, on ground same
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Old 07-19-2023, 01:24 PM   #10
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Just to be clear, the physical space of volume in the tire will not change because deformity in one direction (should) cause deformity in another direction. In addition, and more importantly for tire pressure, the internal surface area of the tire does not change either.

PSI measures force (pounds) relative to area (square inches). If you fill the tire with no external force applied to it (e.g., on the jacks) and then add external force (e.g., lower the vehicle off jacks), I think the PSI has to increase. Albeit, the increase may not be material. Illustration below (stolen from another site).

The pressure in the tire is 100psi. Pressure equals force divided by area. The surface area inside the inner tube (for a 700x23c tyre) is (very) roughly 7.2cm x 210cm = 1512cm square [or in square inches = 234 sq. in.]. The total forces involved on an unladen wheel are therefore 100 ‘pounds per square inch’ x 234 square inches = 23,400 ‘pounds’. By sitting on the bike, one adds another 100 pounds to the wheel and thus to the forces, i.e. 23,500 ‘pounds’. Assuming the inner tube does not change surface area, i.e. the wall in contact with the ground is deformed in shape but overall not stretched or shortened, then the final pressure will be 23,500 ’pounds’ divided by 234 sq. in. = 100.4 psi. Overall an increase in pressure by 0.4%.

The dimensions of the inner tube are actually external dimensions rather than internal.
The tire (and therefore the inner tube) is assumed not to measurably stretch when at an appropriate pressure. Similarly the deformation of the tire wall from curved to flat is assumed not to change the surface area much.

Overall designed to be relatively quick math (and for the benefit of reading I simplified to fewer significant figures along the way) so still prone to a largish margin of error.
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Old 07-19-2023, 04:23 PM   #11
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yeloduster gave an excellent answer. And I believe xrated is wrong about the the tire volume changing with the jacks down (reduced load). The shape of the tire changes, but not the volume. The tire body would have to actually stretch or contract for the volume to change and that doesn't happen, at least not to any degree you could measure outside of a laboratory.
How could it NOT change? Try filling up a plastic bottle all the way to the top, then gently squeeze the side of it just a bit and watch the water overflow out the top. Liquids, for all practical purposes cannot be compressed, so the liquid flows out the top. Air, on the other hand CAN be compressed, and if the air is in a sealed system, such as a tire, since the air cannot escape out the top, like on the water bottle, it has to compress if it's volume is reduced, so the compression of squeezing the tire by placing weight on it, will simple raise the pressure inside the tire. Granted, and I'm not arguing this point, the increase, for all PRACTICAL purposes will be negligible at best, but it will increase, even if it is not detectable...because the tire's volumetric capacity has been slightly reduced. The same effect occurs when a tire hits a pot hole in the road....the pressure will spike to some degree, but of course it would be measured in milli-seconds.

Volume. Pressure is also affected by the volume of the container. If the volume of a container is decreased, the gas molecules have less space in which to move around. As a result, they will strike the walls of the container more often, and the pressure increases
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Old 07-19-2023, 04:47 PM   #12
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How could it NOT change? Try filling up a plastic bottle all the way to the top, then gently squeeze the side of it just a bit and watch the water overflow out the top. Liquids, for all practical purposes cannot be compressed, so the liquid flows out the top. Air, on the other hand CAN be compressed, and if the air is in a sealed system, such as a tire, since the air cannot escape out the top, like on the water bottle, it has to compress if it's volume is reduced, so the compression of squeezing the tire by placing weight on it, will simple raise the pressure inside the tire. Granted, and I'm not arguing this point, the increase, for all PRACTICAL purposes will be negligible at best, but it will increase, even if it is not detectable...because the tire's volumetric capacity has been slightly reduced. The same effect occurs when a tire hits a pot hole in the road....the pressure will spike to some degree, but of course it would be measured in milli-seconds.



Volume. Pressure is also affected by the volume of the container. If the volume of a container is decreased, the gas molecules have less space in which to move around. As a result, they will strike the walls of the container more often, and the pressure increases
An uncapped water bottle is a bad example, there is no pressure there.

When you add weight to a tire the tread flattens but the sides bulge, so same cubic feet of air. No psi change.

Hit a huge pothole, does your TPMS send an alarm, does the tire explode, no. It may damage the tire and wheel but the psi stays the same.
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Old 07-19-2023, 05:25 PM   #13
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How could it NOT change? Try filling up a plastic bottle all the way to the top, then gently squeeze the side of it just a bit and watch the water overflow out the top. Liquids, for all practical purposes cannot be compressed, so the liquid flows out the top. Air, on the other hand CAN be compressed, and if the air is in a sealed system, such as a tire, since the air cannot escape out the top, like on the water bottle, it has to compress if it's volume is reduced, so the compression of squeezing the tire by placing weight on it, will simple raise the pressure inside the tire. Granted, and I'm not arguing this point, the increase, for all PRACTICAL purposes will be negligible at best, but it will increase, even if it is not detectable...because the tire's volumetric capacity has been slightly reduced. The same effect occurs when a tire hits a pot hole in the road....the pressure will spike to some degree, but of course it would be measured in milli-seconds.

Volume. Pressure is also affected by the volume of the container. If the volume of a container is decreased, the gas molecules have less space in which to move around. As a result, they will strike the walls of the container more often, and the pressure increases
PV=nRT all day for an ideal gas, but V is only a constant in a perfectly rigid, inelastic container that has a constant volume irrespective of pressure.

So I think the assumption here might be that a tire is a constant-volume container. It's not. Rubber tires are somewhat elastic despite the embedded cords designed to rein its elasticity into reasonable bounds. I rather suspect this mitigates the pressure difference one might otherwise expect to see between loaded and unloaded tires to a considerable degree. Enough to render the difference moot in the real world (whatever that is).

I'd love to see what Roger would have to say about this because I have no clue how elastic a modern RV tire might be, but we can actually measure this. If I have time in the next few days, I'll measure the pressures with a tire off the ground and on the ground with its usual load with a decent digital gauge and see if it can even be measured. I think the gauge reads in tenths of a PSI, but I'm on the couch and the gauge isn't. ;-)
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Old 07-19-2023, 05:27 PM   #14
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Pretty easy to VERIFY:


Raise a tire off the ground. Check tire pressure.


Lower tire with full weight on the tire. Recheck tire pressure.


What is the delta??
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