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Old 05-27-2016, 02:47 PM   #1
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Tire pressure increasing quite a bit

We recently got a Fleetwood Storm 32 ft class A. It has a 18K GVWR, currently weighs front at 6240 lbs, rear at 10060 lbs. It has Goodyear 245/70R19.5G tires, (G670 I think).
On a recent trip the tires were aired to 85lbs, per the Yellow info label in the RV, at around 70 degrees. The weather was moderate and never exceeded 85 degrees. After several hours of driving the tire pressure increased to 99 to 104 lbs on all tires.
The TPMS manual did say some tire brands could increase 20-22 lbs, but I was wondering if other RVers had experienced that much of an increase?

The sidewall on the tires do say 110 lbs for maximum load capacity, so I assume any increase up to that pressure is safe, but how much could you exceed the 110 lbs, assuming you started at higher pressure than 85, say you started at 100 lbs, how much could the pressure increase over 110 lbs and still be ok?

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Old 05-27-2016, 02:51 PM   #2
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Tire pressure increasing quite a bit

Welcome to irv2.

The increases you're seeing are common and nothing to be concerned about. The 110 on the sidewalks is the minimum pressure needed for maximum load. It has nothing to do with the other situation you're posting about.


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Old 05-27-2016, 02:53 PM   #3
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I'm pretty sure the 110 max psi is when measured cold, leaving room for heat increasing the pressure. My tires are set at 95 front, and 100 rear according to what the manufacturer recommended based on weight.
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Old 05-28-2016, 04:09 AM   #4
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Its not about pressure rising , but the inside tire temp , that is important for savety of tires. The pressure rising can be used to calculate the inside tire temp.

AT normal driving at about 50/55m/h the tire inside temp rises to about 110/115 degr F when outside temp is about 65/68 degr F. this for a tire that is calculated in its maximum load for 99m/h so most LT tires.

But more important is the temp of the hottest part of the rubber of tire.
Wich is sertainly 115-65= 50 degr hotter then inside tire temp , if every part of rubber produces the same amount of heat, and thas not the case.
I estimate that the rubber may have a max temp of about 265 degr F.
Sertain parts of tire bend more when deflecting, so produce more heat.

So its a complex compound of data that determine the allowed deflection and inside tire temp that goes with it , and the pressure rising that goes with it.

For that normal driving the pressure rises about 10 to 11% so your pressure rising of 104/85= 22,5% and it probably means that your inside tire temp is calculated with my made spreasdsheet 165 degr F , and by that hottest part of rubber probably over 265 degr F, so tire-damage begins.

Because you gave weights and tire data , I have filled it in my made MotorhomeRV tirepresssure calculator, and gives F 95 psi and R 80 psi.
The big sise of your tires makes me suspect the maximum load to be calculated for 75m/h and so your used speed is closer tot that max then for instance a LT tire with 16inch rimms, wich is calculated for 99m/h.

So I lowered the maximum load to get the deflection and so heatproduction this tire would have when 99m/h would have been used to calculate the maxload for. This would still mean acceptable comfort and gripp, and probalby the pressure -risings of that 10 to11% when driving 50/55m/h.
Then advice F 107 psi and R 85 psi.

Probably your Front tires went hotter then rear and the inner rear hotter then outer rear tires. with that the pressure rising differences.

rising from 85 to 99 would mean an inside tire temp of 139 degr F.

Now about howmuch the pressure may rise .
The AT-pressure ( yours 110 psi) is not the maximum cold pressure of a tire.
From a document of Semperit/Continental i make that the maximum allowed cold pressure ( when inside tire temp =outside tire temp = about 65 to 68 degr F) is 1.4 times te AT-pressure ( Yes 40% higher).

Also read a while ago from Tireman9 or Capri-racer in a topic that tires are tested to can stand a pressure of 2 to 3 times the AT-presssure.

If you would fill the tire at 1.4 times the AT -pressure ( yours 110 psi x1.4= 154 psi) at freesing point of water so 32 degr F, and incidentially the temp in tire rises to boiling point of water so 212 degr F ( can be by using the brakes to often when descending from mountains) the pressure rises to about 2 times the AT pressure ( filled in my spreadsheet your data gave 216 psi) so still under the border the test describes.
So even in extreme situations the tire can handle these high pressures.
That is if its undamaged by for instance overheating by using to low pressure
And also the rimms and valves must be able to handle this pressure.
Otherwise you would still have wheelfailure.

Again became a long story , but pick from it what you can use.

End conclusion: use F107/R85 psi and see if pressure rising gets lower when driving 50/55m/h, and the temps are an indication.

And dont be afraid of the pressure rising, cold measurement is only reliable because warm , you never know for shure how warm and howmuch it has cooled down already. And the tiremakers included the pressure rising in their tire-desighn.
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Old 05-28-2016, 06:48 AM   #5
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Tire pressure increasing quite a bit

All reference pressures given for tires --all of them-- are cold inflation pressures. This DOES NOT mean cold, like below freezing or something. It simply means ambient air temperature, whatever that happens to be, before the tires have been driven on more than a few minutes or so.
So if it's 25 degrees in winter you pump your tires to the recommended 90 psi (for example). If it's 95 degrees in August, you still set your tires to 90 psi. As you drive and the tires generate heat there will be a corresponding increase in pressure (see "Boyle's Law") but the degree to which the temp and pressure increase is determined by the tire design-- it's already baked into the cake.
I usually see about 11-15 psi increase after a while, sometimes as high as 20 psi. The temp you read on your TPMS is fairly meaningless in absolute terms, since what you're reading is the temp of the air inside the valve stem. What you should look for is a change or an anomaly that can't be explained.
Example 1: a tire that's been reading 105 for the past 2 hours starts increasing about 5 degrees every 5-10 minutes, while all other tires remain constant. Strange. Something is generating that heat. 2). The right front tire on my toad runs about 20 degrees hotter than the other three tires. Why? Because it's right behind the diesel exhaust. 3) you note that all the tires on the left side-- both coach and toad-- are running about 20-25 degrees hotter than the right side. Perplexing, until you grasp that you're westbound at mid-day in winter on I-10 in Arizona. The sun is low in the sky but still bright and strong, and your left side is in full sun while your right side is shaded.
For the full skinny on tires, go to www.rvtiresafety.com. Tireman9 is a retired tire testing engineer, and knows whereof he speaks.
Good Luck!

John & Diane, fulltiming since '12
'02 DS, FL, Cat, '04 Element
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On the road since June '12 with Lincoln, the guard cat.
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Old 05-28-2016, 06:59 AM   #6
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Tom10, Yes its normal for your cold tire pressures to increase 15 - 20 psi as the ambient temp increases & as you heat them up by driving highway speeds. Their pressure also increases if you drive up to significantly higher elevations.
The 110 psi rating on the side of that tire is always the max cold inflation pressure; nothing to do with what the pressure may be after the tires heat up. If you needed to carry the max load for that tire, you'd inflate them to 110 psi when cold and after hours of driving at 65 they might be 130 psi hot, no problem.
The tire manufacturers all warn, not to adjust inflation when the tire is hot.
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Old 05-29-2016, 10:15 PM   #7
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Thanks to those who responded to my inquiry and concerns, I am now comfortable with the increase in tire pressure. I may try to increase the front pressures some and see what results...
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Old 05-29-2016, 10:31 PM   #8
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A good course of action is to get a good 4-corner weight ... then you're setting your tire pressures IAW the recommended for your actual weights across the axles. This provides a good safety margin and also gives you some indication of the need to shift your load (weight) from front to rear or left to right. There are many, many post about this topic within IRV2 ... safe travels ...
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Old 05-31-2016, 12:15 AM   #9
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Originally Posted by Mike and Cha View Post
I'm pretty sure the 110 max psi is when measured cold, leaving room for heat increasing the pressure. My tires are set at 95 front, and 100 rear according to what the manufacturer recommended based on weight.

But, I'm sure others will have a differing perspective.
There is a lot of talk about "4 corner" weights on this forum.
That is irrelevant, because the tire pressures on each axle should always be the same side to side.
"Axle weight" would be relevant, however.
Again, others will no doubt disagree. Just my 2c.
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Old 05-31-2016, 07:45 AM   #10
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Re: 4 corner weights: there are situations where the weight difference between a left and right side tire(s) can be significant. In that situation tire pressures for the axle may carry the weight - while still leaving the highly loaded tire(s) under inflated. In addition, a big weight imbalance can affect handling.

Yes - when all said and done - the tire pressures on all tires across a single axle should be the same. Safe travels ...
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Old 05-31-2016, 09:01 AM   #11
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In the last few posts I did not see it specifically mentioned, so for safety sake... If you find one side of an axle loaded higher than the other, the first thing to do is to try to balance the load. This can be impossible due to design sometimes, but do try. Next thing is to set the tire pressures of both tires according to the higher loaded tire. It's better to run over than under inflated, and you must run the same pressures on all tires on one axle. Use the "tire manufacturers load vs pressure guide". So, for Goodyear 670 RV tires 22.5" go here The Inflation Loading - Goodyear RV and download the inflation table, and scroll down to your tire size.
Again, it's better to be over inflated than under, so load your vehicle to it's normal travel load, all your stuff, people, and fuel, propane water, toads etc. then weigh it. To get 4 corner specific weights you drive onto the scale, which will then give you (in most cases) front axle load and rear axle load. Record that. If allowed, back up and run back up on the scales again. but this time only with the right side (or left if more convenient). Record both loads. Subtracting the single side load at front from the axle at front then likewise for rear, will give you the load on the other side tires. One last thing. If your tires are in the sun when you are measuring pressure, they will show higher pressures than tires in the shade, so don't make the mistake of pumping up the shaded ones or deflating the sunny ones.

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