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Old 04-23-2015, 05:07 PM   #15
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I' m not a tire expert but I believe that every 10 degree drop in ambient temperature, there will be at least a one (1) pound drop in tire pressures.
That is common misinformation, found on passenger car tire web sites, e.g. Tire Rack. It's valid for tire using pressures in the 40-60 psi range, but rather far off for tires that run in the vicinity of 100 psi. The ideal gas laws state that the change is 1.8% of the pressure for each 10 degree change, but usually we just say 2% for a simpler rule of thumb. Mixtures of gases, moisture, etc. in the air used to fill the tire can make small differences, but it's not worth worrying your head over those.

You may also see advice that the change is 0.5 psi. That's about right for a 25-30 psi tire, fine for the smaller tires used on many cars.
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Old 04-23-2015, 05:29 PM   #16
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I think Roger Marble has a good idea........this enables you to set your air pressure and be ok for temp variations while traveling...... from his website on RV Tire Safety

Bottom Line
Never bleed down hot pressure. You should consider the inflation on the load tables to be your minimum cold inflation. I usually recommend people run plus 10 % over the table long as they are not exceeding the inflation molded on the tire or the max inflation rating for the wheel. If you can’t run the +10% then you probably have tires and wheels that are lower capacity than what you need based on your actual loading.

RV Tire Safety: Pressure
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Old 04-23-2015, 05:31 PM   #17
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That is normal, and is also why you should not run the minimum air pressure for the corresponding load, as seen in the load/inflation charts. Run the mfgrs. tire pressure listed on the placard in the coach.
Ray,IN
IMO anyone who uses the tire pressures on the placard in the coach, (and ignores the "tire manufacturers recommendations" based on actual wheel weights), is asking for tire problems.
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Old 04-24-2015, 06:19 AM   #18
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I hope you do not mean to let air out as the day warms up?
No where in my post is that recommended.

How to check your tire pressure:
• Tire pressure should be checked when your tires are cold and haven’t been driven more than one mile. The load capacity for a given cold inflation pressure is based on ambient outside temperatures.
The pressure in a hot tire may be as much as 10 to 15 psi higher than the cold tire pressure. That means you’ll only get an accurate reading when you check your tires when they’re cold.

• To ensure your tire pressure readings are accurate, Goodyear recommends that you use a quality truck tire gauge with a dual-angled head. This way, you can check inner and outer dual wheels at the same time.

Do NOT bleed air from hot tires.
• Inflation pressure should be adjusted to the tire carrying the heaviest load, and all tires on the axle should have the same inflation pressure.
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I check the tires, and set the pressure in ND, with the tires at -20. Then forget them, other than making sure they are not losing pressure. Ignore the temp rise as you go south, as in a few days I will be going to cold again.

If however, you leave ND in Jan for FL, and will be staying there for several months, and DRIVING the vehicle, then I would readjust in FL. If you are just going to basically park it there, and drive a toad, I would leave them alone.
How many semi drivers weigh all 6 corners and change PSI for empty or loaded.

I am leaving FL. next Thursday for IN. My tires for weight need 90 PSI in all 6. I will drop them down from the present *120#(long storage) to 100#(cold 85) then they will be fine when I get to the cold weather. No change will be needed.

*Storing your vehicle properly helps protect your tires.
Inflate your tires to recommended operation pressure plus 25%, but don’t exceed the rim manufacturer’s inflation capacity.
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Old 04-24-2015, 08:19 PM   #19
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Ray,IN
IMO anyone who uses the tire pressures on the placard in the coach, (and ignores the "tire manufacturers recommendations" based on actual wheel weights), is asking for tire problems.
Mel
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What problems?
Depends on who you trust the most. The mfgrs who built the RV or the mfgr who built the tire. Even the RMA states to never run less than the mfgrs placard tire pressure. Virtually all tire mfgrs are members of the RMA. What the tire mfrgs load/inflation chart shows is the minimum pressure to support the corresponding load, anything less-even 1#, is under-inflated. This RMA pdf, pg 55 pertains to RV tires.
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Old 04-25-2015, 01:05 AM   #20
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Whatever happened to the good old advice to not sweat about the small stuff.

Weigh each axle with the coach in travelling trim (each wheel is even better), look up the manufacturer's load pressure tables, pump the tyres up to those pressures and hit the road.

Check them every two or three weeks and relax.

"Run the mfgrs. tire pressure listed on the placard in the coach." Nope! Tire manufacturer trumps RV manufacturer every time.

"run nitrogen, it doesn't ....." Baloney

" even one pound under is underinflated" Sheeeesh.

"oxygen molecules are small so leak through the tyres" Sigh!
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Old 04-26-2015, 02:42 AM   #21
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I'm trying to figure out tolerances for tire pressure deviation. We followed TST deviation ranges for set up.

But here is my story.

We installed Borg's on the rear duals and added TST TPMS to all 6 tires. Fast forward a week or so afterwards and we're loosing air pounds. So I tighten up the valves (Schneider?) and the TPMS after topping off and checking pressure with a high quality gage. 24-48 hours out, everything is holding. However yesterday pressures are down a couple of pounds in some of the tires. Their temps went from 68 degrees to 46.
I'm figuring the cold drop in air temp form mid / low 55 - 60 air temp to low 40s high 30s is the reason.

I'm keeping a log to chart each tire. But is it normal to experience a drop of a few pounds as temp dips?
inbd rear tires will normally run 20-30 degrees hotter than their ob mates. thus, the pressures will be higher. at 190-200 degrees, expect problems like blowouts.
10 degree temp change = aprox. 3 psi.
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Old 04-26-2015, 09:47 AM   #22
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I gotta agree with Tony Lee. There seems to be a lot of hysteria over such a simple thing.

The RV manufacturers placard gives a recommended pressure for those who haven't weighed the coach before using (the vast majority). Since the RV maker doesn't know your actual loaded weight, the recommendation has to assume max load or something very close to it. They (or their lawyers) all learned from the Ford Explorer tire fiasco that low-balling tire pressures is a no-no and borders on financial suicide.

But if you make the effort to weigh the rig, you can better tune the tire pressures for a more pleasant ride while still remaining safe. It's not something you must do, but you will likely get a benefit (softer ride) if you do. Of course, if the scale shows you are near max load anyway, nothing is going to change much.

The other advantage of a weigh-in is that it detects an overloaded tire or axle. RV Safety Foundation (rvsafety.com) has found evidence that a large percentage of RVs are overweight in at least one tire position. Yours could be one of them.

http://rvsafety.com/weighing/wheel-position-weighing
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Old 04-28-2015, 06:57 PM   #23
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Whatever happened to the good old advice to not sweat about the small stuff.

Weigh each axle with the coach in travelling trim (each wheel is even better), look up the manufacturer's load pressure tables, pump the tyres up to those pressures and hit the road.

Check them every two or three weeks and relax.

"Run the mfgrs. tire pressure listed on the placard in the coach." Nope! Tire manufacturer trumps RV manufacturer every time.

"run nitrogen, it doesn't ....." Baloney

" even one pound under is underinflated" Sheeeesh.

"oxygen molecules are small so leak through the tyres" Sigh!
I think you might find this thread interesting, especially the page 2 comments from a retired tire design engineer: Tire pressure - Page 2 - General RV Information - Escapees Discussion Forum
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Old 04-28-2015, 07:05 PM   #24
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That is common misinformation, found on passenger car tire web sites, e.g. Tire Rack. It's valid for tire using pressures in the 40-60 psi range, but rather far off for tires that run in the vicinity of 100 psi. The ideal gas laws state that the change is 1.8% of the pressure for each 10 degree change, but usually we just say 2% for a simpler rule of thumb. Mixtures of gases, moisture, etc. in the air used to fill the tire can make small differences, but it's not worth worrying your head over those.

You may also see advice that the change is 0.5 psi. That's about right for a 25-30 psi tire, fine for the smaller tires used on many cars.
If you want the full calculation in detail, here it is:

If you remember back to high school chemistry, gas pressures increase with increasing temperature. The relevant calculation is (P1)(T1) = (P2)(T2) where temperatures are expressed in Kelvins (degrees Celsius plus 273).

So if you wanted to know what the pressure of a tire would be when it is 10 degrees (Fahrenheit) hotter your equation would be:

P2 = (P1) (T2/T1)

Since 68 degrees F = 20 C and a change of 1 C =~ 2F then if your tires are pressurized at 100 psi at 68 degrees then at 78 F they will be roughly 5 degrees C hotter. Therefore,

P2 = (100 psi) (273 + 25)/(273 + 20) = (100) (1.017) = 101.7 psi

So the correct answer is that pressures will increase or decrease 1.7 percent for every 10 degree change in temperature which for a 100 psi tire translates roughly into 1.7 psi
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Old 04-28-2015, 07:13 PM   #25
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Air is about 80% nitrogen, and the pressure of nitrogen-filled tires will still change with temperature. However, since the nitrogen used for tires is dry, the pressure may not change as much as with air.
The only benefit to RV'ers is that Nitrogen is dry. The real benefit is to those dealers who charge you for it.
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Old 04-29-2015, 03:10 PM   #26
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If you want the full calculation in detail, here it is:

If you remember back to high school chemistry, gas pressures increase with increasing temperature. The relevant calculation is (P1)(T1) = (P2)(T2) where temperatures are expressed in Kelvins (degrees Celsius plus 273).

So if you wanted to know what the pressure of a tire would be when it is 10 degrees (Fahrenheit) hotter your equation would be:

P2 = (P1) (T2/T1)

Since 68 degrees F = 20 C and a change of 1 C =~ 2F then if your tires are pressurized at 100 psi at 68 degrees then at 78 F they will be roughly 5 degrees C hotter. Therefore,

P2 = (100 psi) (273 + 25)/(273 + 20) = (100) (1.017) = 101.7 psi

So the correct answer is that pressures will increase or decrease 1.7 percent for every 10 degree change in temperature which for a 100 psi tire translates roughly into 1.7 psi
The problem with trying to use the ideal gas law is that the tires are filled with air that contains varying amounts of water depending on where and when it was filled. This increases the PSI variation over temperature a lot.
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Old 04-29-2015, 04:44 PM   #27
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The problem with trying to use the ideal gas law is that the tires are filled with air that contains varying amounts of water depending on where and when it was filled. This increases the PSI variation over temperature a lot.
At 20 degrees C the vapor pressure of water is 17.5mm of Hg out of a standard pressure of 760mm. So even air that is fully saturated with water has a partial pressure of water vapor that is only 2.3% of the total. Therefore, the effects of differences in water vapor levels are small compared to the total.

I think you will find that most sources say that a 100 psi tire varies between 1 and 2 psi per 10 degree F change.
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Old 04-30-2015, 11:57 AM   #28
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At 20 degrees C the vapor pressure of water is 17.5mm of Hg out of a standard pressure of 760mm. So even air that is fully saturated with water has a partial pressure of water vapor that is only 2.3% of the total. Therefore, the effects of differences in water vapor levels are small compared to the total.

I think you will find that most sources say that a 100 psi tire varies between 1 and 2 psi per 10 degree F change.
That's not what I found when I made a several day test years ago when I thought the ideal gas law calculations would be be fairly accurate. There was a significant difference between measurements and the law.
It is easy enough to test if you have a decent difference between day and night temperatures. You might want to take a few measurements and see if your results match your calculations.
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