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

[goufgators]

I know and agree with posts relevant to ‘have your coach weighed’ and I have done so although not four-corner weights. Axle weights indicate 80 psi for all tires. In the title, I said this post was about tire psi and that is, at least, partially correct. But, it’s also about that age-old problem of steering wander. Let me explain, if I can. Wandering has been a problem since the coach was new (2017). I’ve added rear stabilizer, front stabilizer, steer safe and had alignment adjusted to 5 degrees camber (not sure that’s the correct terminology for the alignment). After all the add-ons and alignment, the problem still exists. Here’s the interesting part. As you know, as tires turn they build up heat and psi increases. When I start out on a trip, the tires are at the cold 80 psi and steering at that psi is beautiful (Cadillacish good)…well maybe not quiet that good, but no wandering. No wandering continues until psi reaches about 90 psi or slightly above. That’s when the steering wheel gets a work our along with the driver (that’s me). After all of the above, here’s the question: In order to keep the tires’ psi from increasing quickly to a point of ‘wander’, would it be safe to reduce the cold psi to say 75 (remember Goodyear says, based on weight, cold should be 80). In our Florida very warm weather, the tires, starting at 75, would rather quickly heat up along with the psi and exceed the cold 80. But, in so doing, the psi might not reach the wandering problem. At least not as soon. Comments welcomed!

[jspringator]

Maybe. But I examined the Michelin chart closely and it gives no such information. Most likely the weight of my vehicle is Lighter than the standard use of this tire.

[larry_cad]

jspringator wrote:
..........What we don’t know is whether the weight chart extends to pressures less than 65psi.

Maybe because pressures less than 65psi are dangerous and the manufacturer doesn't want "test pilots" driving on their tires.

:@:S:h

[jspringator]

I would say all that is weight dependent. If I was running 30psi it might be different. 5 psi is within the customary 10 percent margin of safety. An overinflated tire only has contact in the middle of the tire. A properly inflated tire has contact the entire width of the tire. What we don’t know is whether the weight chart extends to pressures less than 65psi.

[CapriRacer]

4x4van wrote:
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.;)

My understanding is that a tire can generate a lot more cornering force than the pressure to keep the tire seated on the rim at low pressures.

In other words, if you run low pressures, there is a risk that emergency maneuvers may cause the tire to come off the rim - and the lower the pressure, the greater the risk.

[jspringator]

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.

[4x4van]

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.;)

[jspringator]

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.

[4x4van]

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.

[jspringator]

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.

[2bzy2c]

Well said ^

[wolfe10]

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!

[CapriRacer]

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.

[2bzy2c]

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

[rgatijnet1]

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.