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Old 04-17-2016, 07:51 AM   #1
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Do you measure tire pressure while coach leveled on jacks or off of jacks and resting

I haven't tested this yet (but will do so), but odds are that someone on this forum has. Does the tire pressure change a significant amount when on leveling jacks than when resting on the ground with the jacks stowed for travel?

I ask because we have a FWS, so when preparing to de-camp, we bring in the non-FWS's while on Jacks. We then stow the jacks and I go around checking tire pressures (and adding air if necessary). We then start the coach and air up (ours is a DP). Once this is done, we bring in the FWS and pull out of the campground.

These are heavy coaches with large volume tires. Has anyone tested their air pressure both on and off their leveling jacks? If so, was there a significant change in tire pressure? (This sound like one of those first semester physics questions that I had in college many many many decades ago&#129300.

Maybe the "tire guy" on the forum has the answer.
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Old 04-17-2016, 07:54 AM   #2
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Do you measure tire pressure while coach leveled on jacks or off of jacks and...

In any normal scenario, tire pressure does not change with the changing load. The tire pressure remains the same whether the tires are in contact with the ground or suspended in the air.


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Old 04-17-2016, 07:55 AM   #3
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I haven't tested this yet (but will do so), but odds are that someone on this forum has. Does the tire pressure change a significant amount when on leveling jacks than when resting on the ground with the jacks stowed for travel?

I ask because we have a FWS, so when preparing to de-camp, we bring in the non-FWS's while on Jacks. We then stow the jacks and I go around checking tire pressures (and adding air if necessary). We then start the coach and air up (ours is a DP). Once this is done, we bring in the FWS and pull out of the campground.

These are heavy coaches with large volume tires. Has anyone tested their air pressure both on and off their leveling jacks? If so, was there a significant change in tire pressure? (This sound like one of those first semester physics questions that I had in college many many many decades ago&#129300.

Maybe the "tire guy" on the forum has the answer.
No significant change.
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Old 04-17-2016, 08:09 AM   #4
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That is a very interesting question I am making this post just so I will get a notice on my email.

FWIW. When I was in tire shop changing out A TIRE. the tire guy puts in 60PSI per my instruction (couch unloaded). when I was on the road, from time to time, I check tire pressure. it also read 60# (couch loaded). No discernible difference.
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Old 04-17-2016, 08:31 AM   #5
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Your bending the tire, not decreasing the volume. No change.
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Old 04-17-2016, 09:28 AM   #6
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Thanks to all respondents!! 👍
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Old 04-17-2016, 09:37 AM   #7
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Yup--no change....only temp or a "hole" ...and maybe altitude?????
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Old 04-17-2016, 11:18 AM   #8
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Yup--no change....only temp or a "hole" ...and maybe altitude?????
Not sure what your reference to a hole is about, but yes, Temperature and Altitude are the two factors that will affect tire pressure.
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Old 04-17-2016, 02:48 PM   #9
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There will be a change. Has to be. Sit on a balloon and the pressure increases until it bursts. Apply 'reductio ad absurdum' to the problem. May not be significant with tyres given the normal errors in measuring tyre pressure, but why add extra uncertainty to the equation by shifting a massive weight from 3' overhanging the wheel base to within the wheelbase..
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Old 04-17-2016, 03:18 PM   #10
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There will be a change. Has to be. Sit on a balloon and the pressure increases until it bursts. Apply 'reductio ad absurdum' to the problem. May not be significant with tyres given the normal errors in measuring tyre pressure, but why add extra uncertainty to the equation by shifting a massive weight from 3' overhanging the wheel base to within the wheelbase..

The ballon is a horrible analogy, because it's elasticity causes the material to stretch until there is a mechanical failure. If you did the same thing to a truck tire by subjecting it to extreme overload, it would certainly fail, but without being stretched like the balloon.

Assume that you have a truck tire inflated to 100 PSI with no load on it. Each square inch of the inside surface of the tire, plus the surface area of the wheel that is under air pressure, plus even the minuscule surface area of the inside of the valve stem, has 100 pounds of force applied to it. Now apply a load to the tire. The area where the tire contacts the ground deflects a little bit. The sidewall flexes by bulging outward. How much do you think the volume of air in the tire changes? How much do you think the surface area of the inside of the tire/wheel/valve stem changes? Does it stretch like a balloon? No! In the normal range of temperatures on earth, a properly-rated tire inflated correctly to carry the load of the RV, changes so little in volume and surface area that you cannot measure the difference in pressure with any commercially-available tire pressure gauge. Tire pressure varies by dozens of PSI as it transitions from its cold pre-operational state to the point where it reaches equilibrium, dissipating heat as quickly as it gains heat from driving. This is easily measurable with a standard tire pressure gauge or TPMS. Anything else is a red herring, and portraying it as significant enough to warrant altering tire pressure based on weigh shift between extended and retracted slide-outs is absurd. There is no uncertainty.


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Old 04-17-2016, 03:24 PM   #11
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There will be a change. Has to be. Sit on a balloon and the pressure increases until it bursts. Apply 'reductio ad absurdum' to the problem. May not be significant with tyres given the normal errors in measuring tyre pressure, but why add extra uncertainty to the equation by shifting a massive weight from 3' overhanging the wheel base to within the wheelbase..
How about if you go try it and get back to us? Are you a betting man? Do you have the courage of your convictions?

I've got $5.00 that says you are wrong.
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Old 04-17-2016, 03:32 PM   #12
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Do you jack up you car/truck when checking/adjusting air pressure?

DO you think they lift airplanes up off tarmac to adjust air pressure?


Check/adjust your TIRES when COLD (before driven on or warmed by sun)
This is best done the morning of travel day

Then repeat NEXT travel day etc etc etc
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Old 04-23-2016, 06:50 PM   #13
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How about if you go try it and get back to us? Are you a betting man? Do you have the courage of your convictions?

I've got $5.00 that says you are wrong.
Only $5 eh? so guess you aren't all that certain after all.

You need to read what I said not what you wanted to read. I said
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May not be significant with tyres given the normal errors in measuring tyre pressure, but why add extra uncertainty to the equation
I don't have to try it because I know what the result will be. Just normal Science. Any container is elastic to some degree and if the pressure (inside OR outside) changes, the volume changes. If the volume is forced to change eg by an external load, then the pressure inside will change.
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Old 04-24-2016, 11:27 AM   #14
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Quote:
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Only $5 eh? so guess you aren't all that certain after all.

You need to read what I said not what you wanted to read. I said

I don't have to try it because I know what the result will be. Just normal Science. Any container is elastic to some degree and if the pressure (inside OR outside) changes, the volume changes. If the volume is forced to change eg by an external load, then the pressure inside will change.
In theory, I agree with Tony, there should be some change. As the load increased, let say from 10 ton to 50 ton (until the tire burst), the air inside the tire is compressed more and more, the pressure should increase correspondingly.
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