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Old 06-18-2019, 08:53 AM   #1
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Acceptable Tire Pressure

Recently had coach weighed by SmartWeigh and found that, according to the tire pressure/load chart for my tires I needed to take 5# out of the front tires and add 5# to the rear. I use a TPMS which I've reset for the new psi(s).

My question is, what is an acceptable tire pressure variance? That is, if a tire is suppose to have 115# and you have 114# -- that is ok. But what about 113#, 112#, 109#? What is acceptable? +/-5%? I want to have correct pressure, but I can sit in the coach for 30 minutes and the tire pressure will change.

Not sure when it's necessary put out all of the gear to add/release psi.

Thanks...
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Old 06-18-2019, 10:20 AM   #2
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tireman9, a retired tire engineer and Winnieowners member recommends adding 10% to the table pressures. Remember that the table pressures are minimums for the specified weight. The pressure on the tire's sidewall is the maximum rated pressure for the tire. As far as the tire goes, anything in between is OK, not considering ride quality and wear. The third rating is that on your vehicle's placard which is going to be something in between.

As long as you set your pressures "cold" you don't need to concern yourself with normal variations. Your TPMS is probably similar to mine with -10% to +20% settings for the low and high pressure warnings.

I know this isn't a straight answer to your question but, +5% is going to be a problem if you're already at your tire's maximum rated PSI but not if you're in the middle of the range. Similarly -5% isn't going to be a problem in the middle but could be if you're at the minimum table PSI for your MH's weight.
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Old 06-19-2019, 03:56 PM   #3
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BobC, thanks for the reply and good information. I guess what I'm asking is when you prepare to pull out of a campground and look at your TPMS and you have a tire that is -1 psi, do you haul out the air hose and fix it or you decide one pound is ok? Well, what about 2, 3 or 5? At what point do you say, I need to pull all of the stuff out and fix this?

Thanks...John
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Old 06-19-2019, 05:29 PM   #4
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Here's a link to a topic on tireman9's (aka Roger Marble) tire blog that may help:

RV Tire Safety: Cold Inflation Pressure

Read some of his other articles and I think you'll get to a better understanding of tire issues in general:

RV Tire Safety

For what it's worth, I saw one site that quoted one degree +/- results in 1 psi +/- and another that quoted 2 psi +/-.
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Old 06-19-2019, 06:27 PM   #5
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Thanks for the info and the link to Roger Marble -- Great Information. The fog is beginning to clear and a picture of 'normal' is beginning to form...
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Old 06-19-2019, 06:38 PM   #6
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Quote:
Originally Posted by BobC View Post
Here's a link to a topic on tireman9's (aka Roger Marble) tire blog that may help:

RV Tire Safety: Cold Inflation Pressure

Read some of his other articles and I think you'll get to a better understanding of tire issues in general:

RV Tire Safety

For what it's worth, I saw one site that quoted one degree +/- results in 1 psi +/- and another that quoted 2 psi +/-.
Correction: It should be ten degrees, not one degree!

Mea Culpa.
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Old 06-23-2019, 05:35 PM   #7
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Quote:
Originally Posted by jrials View Post
BobC, thanks for the reply and good information. I guess what I'm asking is when you prepare to pull out of a campground and look at your TPMS and you have a tire that is -1 psi, do you haul out the air hose and fix it or you decide one pound is ok? Well, what about 2, 3 or 5? At what point do you say, I need to pull all of the stuff out and fix this?

Thanks...John
1 psi is fine. Remember that the accuracy of the TPMS also factors in and none of them are at +/- 1% accuracy. My TST 507 is rated at +/- 3% accuracy. One side in the sun for a while and one not in the sun an cause a difference. Get yourself a good, accurate tire pressure gauge as a double check.

The true value in a TPMS is the trend you see, not the absolute pressure reading. You'll know or already know what looks normal for your coach. Deviations outside of that norm are what you need to be concerned with, especially decreases. Only worry about the initial tire pressure setting when the tires are at ambient temperature.

For example, since my car does not have a spare tire I look at the TPMS a couple of times each trip. One morning I saw that the driver side rear was about 1 psi lower than usual. I headed to work while watching it. After ten miles it should have increased a small amount as the other three tires did. Instead it decreased 1 lb. psi.

I turned around and went back home where I found a large screw in the tread. I had the tire replaced, no muss, no fuss, no flat. The TPMS never went off and it seems to be set about 15% the rated pressure.

HTH,

Ray
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Old 06-24-2019, 03:49 AM   #8
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I am able to calculate a highest pressure for the load , at which comfort and grip is still acceptable. But also a save lowest pressure for the load and speed used, at which no overheating of any part of the rubber of tire, which is the goal of all pressure - advice.


If you give those weighed loads per axle or better axle-end, and give max speed you use and wont go over for even a minute, together with tirespecifications ( maxload AT presssure and maxspeed of tire, but with sises and loadrange I can google) , I will calculate my extra safe advice , wich is higher then what you found in the table. And that you have too use for the variance I explain here.

On 110 psi for max , this gives a range of about 10 psi at which you are save, so that much it can go down .
And then even that 110 advice is calculated for 65/68 degr F ( I use 65degrF in my calculations, difference is marginal).
When colder you can do with lower for safety only, but may bring it to 110 then for fuel-saving or road-handling for instance.
When hotter outside the cold pressure ( when inside tire = outside tire temp) you may not lower to that 110 then, because the tire needs the lesser deflection it gives to produce lesser heat, because the cooling down is also less because of smaller temperature differences.


The use of cold pressure advice, is because that is the only reliable, how much has it already cooled down when you tanked, and what where the conditions to highen up the temp in tire > pressure in tire?)

And the pressure rising warm , wich you see with a TMPS system that does not calculate it back to 65 degr F, is all calculated in in the advice.
So a 120 psi max cold pressure tire, can get 40% extra pressure so 170 psi , when inside tire temp goes to boiling point of water 100degr C/212 degr F.
This can incidentially happen when descending the mountains , and using the brakes to often. The tire, rim and valve are made to stand that extra pressure.



Will try to give a table of that for the higher pressure here used.


And this all is if you are able to determine the weight , speed used, pressure measured, 100% accurate, is never possible. But it shows that 5 psi measured back to 65 degrF is not a problem.
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Old Yesterday, 10:33 AM   #9
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I find there is no way to get all my tires set for the "correct" pressure as simply setting in the yard, the pressure goes up/down by ten degrees or more just due to the sun. I have now run several long trips and do not worry the pressure varying as long as it starts low and climbs as I drive. I always watch for one being different from the others or certainly watch for tire temps to climb. But all those things have adjustable alarm levels and it took some time to sort out what level would work for the amount my tires vary, while leaving the temp set at something like 150 degrees or so that was recommended.
Perhaps a bit of info overload but still keeping me safe from when things really go bad. In other words, I get notice BEFORE the tire blows out through the side or catches fire!
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