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 > Another ‘dadgum’ post re: tire PSI…please bear with me…

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rgatijnet1

Florida

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Posted: 11/30/19 04:47am Link  |  Quote  |  Print  |  Notify Moderator

Wandering can be caused by having too little weight on your front axle. Most RV's will handle better if the front axle is loaded close to the maximum weight allowed per your placard.
Try shifting some of your load around and see if you can get a little more weight on your front axle. Weight BEHIND your rear axle tends to lighten your front axle. Move those items forward if you can. Check to see what happens if your fresh water tank is full or empty, as far as front axle weight is concerned.
Obviously the best thing to do would be to at least get both axle weights in lieu of a 4 corner weight.

2bzy2c

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Posted: 11/30/19 06:34am Link  |  Quote  |  Print  |  Notify Moderator

Octaneforce wrote:

2bzy2c wrote:

Captain_Happy wrote:

Something else to consider is inflating your tire with Nitrogen instead of pain air. Nitrogen doesn't heat up like plain air does. You stated that you F53 handles pretty good with PSI at 80 PSI when you start out, but as your tires heat up the handling changes. I've owned 2 F53 and they both handled badly.



Bzzzzzt WRONG!

There is virtually no difference between N2, O2, Air and Co2 thermal expansion rates.

"I'll take Thermal Expansion for $100 Alex."


Geek speak on the subject -->

Ideal Gas Law: PV = nRT

This formula is the "Ideal Gas Law Formula." Although there is no such thing as an ideal gas the formula is pretty accurate for N2, CO2, and oxygen as we assume that the gas molecules are point masses and the collisions of the molecules are totally elastic. (A completely elastic collision means that the energy of the molecules before a collision equals the energy of the molecules after a collision, or, to put it another way, there is no attraction among the molecules.) The formula becomes less accurate as the gas becomes very compressed and as the temperature decreases but here "very compressed" pressures are well above even the highest tire pressures and "decreased temperatures" are extremely cold, too cold for tires. There are some correction factors for both of these factors for each gas to convert it to a Real Gas Law Formula, but the Ideal Gas Law is a good estimation of the way N2, CO2 and "air" should react through temperature changes. What does all this mean? It simply means that "air", nitrogen vapor, and CO2 vapor should all react pretty much the same within normal tire pressures (0-120 PSI) and temperatures.


In the hvac business we use nitrogen for leak down tests under the assumption that it doesn't fluctuate in pressure like compressed air will. However we use pressure way beyond that of a tire (500-600psi). I guess this is within the theory you provided.

Why do race car drivers bother to use nitrogen in their tires? Is it a myth?


Mainly because N2 is dry and does not contain moisture like compressed air. No moisture allows for a more predictable expansion rate, less corrosion, and nitrogen is inert and will not react with the tire rubber or compounds.

See- Article


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CapriRacer

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Posted: 11/30/19 07:17am Link  |  Quote  |  Print  |  Notify Moderator

Octaneforce wrote:

2bzy2c wrote:

Captain_Happy wrote:

Something else to consider is inflating your tire with Nitrogen instead of pain air. Nitrogen doesn't heat up like plain air does. You stated that you F53 handles pretty good with PSI at 80 PSI when you start out, but as your tires heat up the handling changes. I've owned 2 F53 and they both handled badly.



Bzzzzzt WRONG!

There is virtually no difference between N2, O2, Air and Co2 thermal expansion rates.

"I'll take Thermal Expansion for $100 Alex."


Geek speak on the subject -->

Ideal Gas Law: PV = nRT

This formula is the "Ideal Gas Law Formula." Although there is no such thing as an ideal gas the formula is pretty accurate for N2, CO2, and oxygen as we assume that the gas molecules are point masses and the collisions of the molecules are totally elastic. (A completely elastic collision means that the energy of the molecules before a collision equals the energy of the molecules after a collision, or, to put it another way, there is no attraction among the molecules.) The formula becomes less accurate as the gas becomes very compressed and as the temperature decreases but here "very compressed" pressures are well above even the highest tire pressures and "decreased temperatures" are extremely cold, too cold for tires. There are some correction factors for both of these factors for each gas to convert it to a Real Gas Law Formula, but the Ideal Gas Law is a good estimation of the way N2, CO2 and "air" should react through temperature changes. What does all this mean? It simply means that "air", nitrogen vapor, and CO2 vapor should all react pretty much the same within normal tire pressures (0-120 PSI) and temperatures.


In the hvac business we use nitrogen for leak down tests under the assumption that it doesn't fluctuate in pressure like compressed air will. However we use pressure way beyond that of a tire (500-600psi). I guess this is within the theory you provided.

Why do race car drivers bother to use nitrogen in their tires? Is it a myth?


If your HVAC assumes that nitrogen doesn't fluctuate like compressed air, then that assumption is wrong. Nitrogen behaves almost exactly like air. After all, air is 78% N2.

Why are you using nitrogen in HVAC? Because it is easy to get in high pressure bottles. The same reason racers use N2. They use those bottles to power their impact guns, so it's readily available.

Plus nitrogen isn't Freon, which is expensive.

Oh, people will say that they use N2 because it is dry (no water vapor) or that N2 doesn't vary due to temperature, but they have that wrong. It's such a common mistake that it's hard to convince people otherwise.


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wolfe10

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Posted: 11/30/19 07:57am Link  |  Quote  |  Print  |  Notify Moderator

As already posted, there are a lot of untrue myths about using Nitrogen in tires.

But easiest to explain if you understand that there are really THREE common sources you can use to inflate your tires, not just two.

Nitrogen is one.

DRY air is another (like from an air compressor with an air dryer).

WET air (like from an air compressor with no dryer).


Nitrogen and dry air both follow the Ideal Gas Law. In layman's terms, they both increase PSI as temperature rises. VERY little difference.

Wet air-- well, water is not a gas, so it will increase PSI more rapidly than a gas for a given temperature rise. Ya, wet air, like that convenience store air compressor you put quarters in and water comes out along with air.


Is Nitrogen better than dry air-- maybe a couple of bucks worth.

Is either Nitrogen or dry air better than wet air-- absolutely!

* This post was edited 11/30/19 09:49am by wolfe10 *


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2bzy2c

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Posted: 11/30/19 09:10am Link  |  Quote  |  Print  |  Notify Moderator

Well said ^

jspringator

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Posted: 12/04/19 06:24am Link  |  Quote  |  Print  |  Notify Moderator

I noticed on my last trip the MH handeled very well when the TPMS showed the hot pressure at 66-69 across the range of 6 tires. My pump was showing the pressure at 60psi when cold inflated, but the TPMS showed 55. Winnebago recomends 65, but the loading charts for the tires show a huge margin of safety at actual weights at the scale. I thnk the issue with mine is that it is a shorty at 27', and I surmise the pressure of 65 is for the entire Sightseer line. My gage, the tire pump and my TPMS all give different readings.

Actually the improvement in handling at these pressures was nothing short of remarkble. I felt noticably less fatigued. I don't remember the tire temperature, but the alarm didn't go off. I'll check that next trip.


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4x4van

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Posted: 12/05/19 03:27pm Link  |  Quote  |  Print  |  Notify Moderator

jspringator wrote:

I noticed on my last trip the MH handeled very well when the TPMS showed the hot pressure at 66-69 across the range of 6 tires. My pump was showing the pressure at 60psi when cold inflated, but the TPMS showed 55. Winnebago recomends 65, but the loading charts for the tires show a huge margin of safety at actual weights at the scale. I thnk the issue with mine is that it is a shorty at 27', and I surmise the pressure of 65 is for the entire Sightseer line. My gage, the tire pump and my TPMS all give different readings.

Actually the improvement in handling at these pressures was nothing short of remarkble. I felt noticably less fatigued. I don't remember the tire temperature, but the alarm didn't go off. I'll check that next trip.
Winnebago's recommendation (I'm assuming on the Federal tag on/near the driver's door) is NOT for the entire Sightseer line, but rather for your specific model, based on it's maximum GVWR, which is 14,800. If your rig actually weighs less than that max, then it would make sense that it handles better at slightly lower pressures.

However, Michelin's tire pressure tables for the 225/70-19.5 tires that came stock on your chassis don't go below 65psi. So while I have always believed that tire pressures should be based on your actual weight, I also feel that you should never go below the lowest pressure listed in that tire manufacturer's table. If I was significantly under the GAWRs, I might consider 60psi cold, but even that would make me a bit nervous.

The troubling part is that your gauge, pump, and TPMS are all giving you different information, so you really don't know what pressure you are running. Perhaps it would be worth going to a major tire shop and seeing what THEIR gauge says vs what yours says.

* This post was edited 12/05/19 03:58pm by 4x4van *


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jspringator

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Posted: 12/05/19 07:20pm Link  |  Quote  |  Print  |  Notify Moderator

The tires are rated for 2,755 lbs per tire at 65 psi. Actual loaded weight is 2,320, or 870 pounds under max weight for both tires on the front. The rear is rated for 2755 X 2= 5,510 per dual or 11,020 for both rear sides. The actual loaded weight for the rear is 8,240. That is why it performs better with lower pressures.

The chart didn't go below 65 psi, so I extrapolated the weight difference per tire in 10 psi increments in the other direction. The weight increase between 65 to 75 is 285 pounds. The difference between 75 and 85 is an increase of 275 pounds. The difference between 85 and 95 is 325 pounds. Based on the foregoing, I would believe the reduction in weight carrying per tire would be less than 300 pounds per tire for a 10 psi decrease in pressure, or 55 psi. Even that would give me a 200 pound margin of safety on the front, and 1,580 in the rear. So I settled on what I believe to be 60 psi, that will leave an even greater margin of safety.

* This post was edited 12/06/19 09:32am by jspringator *

4x4van

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Posted: 12/09/19 10:23am Link  |  Quote  |  Print  |  Notify Moderator

jspringator wrote:

The tires are rated for 2,755 lbs per tire at 65 psi. Actual loaded weight is 2,320, or 870 pounds under max weight for both tires on the front. The rear is rated for 2755 X 2= 5,510 per dual or 11,020 for both rear sides. The actual loaded weight for the rear is 8,240. That is why it performs better with lower pressures.

The chart didn't go below 65 psi, so I extrapolated the weight difference per tire in 10 psi increments in the other direction. The weight increase between 65 to 75 is 285 pounds. The difference between 75 and 85 is an increase of 275 pounds. The difference between 85 and 95 is 325 pounds. Based on the foregoing, I would believe the reduction in weight carrying per tire would be less than 300 pounds per tire for a 10 psi decrease in pressure, or 55 psi. Even that would give me a 200 pound margin of safety on the front, and 1,580 in the rear. So I settled on what I believe to be 60 psi, that will leave an even greater margin of safety.
Your math makes sense, and I understand your reasoning. I just wanted to point out that I have heard that you shouldn't go lower than the lowest pressure on the chart, regardless of weight carried. True? I have no idea, just relating what I've heard.[emoticon]

jspringator

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Posted: 12/09/19 10:29am Link  |  Quote  |  Print  |  Notify Moderator

I never even considered that. I need to look at the chart carefully. I would get a smaller tire but I don’t want to deviate from stock. I’m going to fill it to 60 psi cold on my electric pump and
Monitor my tire temps.

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