Tire Pressure

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captainjohn0123

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I recently purchased a tire pressure monitoring system. I've been noticing a 16 to 20% increase in my tire pressure when driving, even on cool days. According to the inflation charts, having my pressure set at 100 lbs is plenty for the weight of my unit. I'm a little concerned about a tire going from 100 to 120 lbs in 50 degree weather, driving no more than 60 mph.

When manufactured, our unit had load range F tires. They have been replaced with load range G. They are 19.5" tires.

Is it normal to see this large of an increase in tire pressure while driving????
 

donn

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Since about 1920 tire pressure checks have always been set when cold.  No one worries about hot pressure.
 

lavarock1210

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Yes it is normal for the pressure increase.  What is most important is that all the tires increase by the same amount.  If you have one tire that increases much more than the others then that tire or axle needs some attention.  If it is on a trailer there may be a brake dragging, bad bearing or even a bad tire with a broken steel belt or a tread separation in progress.
 

captainjohn0123

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I have a 38 foot Winnebago Class A. The tires on each axle normally stay within 2 lbs or so of one another. Occasionally the rear may run as much as 4 lbs difference from one side to the other, but each set normally stays within that 2 lb range.

I was mainly concerned about a 20 lb increase in cool weather.
 

Larry N.

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What is most important is that all the tires increase by the same amount.

Under ideal conditions I'd see that as true. But I find large temperature variations between tires as I drive, causing somewhat uneven pressure changes (not grossly so), usually due to sun on one side of the coach as we drive.
 

captainjohn0123

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SargeW said:
John, have you weighed your rig yourself, or are you going by the weight on the sticker of the RV?
I'm going by the sticker, but have added 500 lbs per axle to the max sticker weight when doing my figuring.
 

afchap

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The sticker is not relevant as you no longer have the original tires on the rig. You need the tire mfg chart for the new tire model and size. Then you need the axle and/or wheel weights for your rig. Any other solution is just guessing.

I don't see 20% increase as extreme. Cool weather isn't much different than warm weather  on mine for psi and running temp. Extreme hot weather can be.
 

FastEagle

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captainjohn0123 said:
I'm going by the sticker, but have added 500 lbs per axle to the max sticker weight when doing my figuring.


There are industry standards written by the T&RA for ?plus sizing? your tires. Some manufacturers publish them but many more do not.

In the reference provided below, starting on page 15 (Plus Sizing), the entire procedure is explained. Just use the information as it applies to your particular situation. When all done you will have the industry accepted NEW recommended tire pressure set for your replacement tires.


http://www.tiresafety.com/images/Tire%20Replacement%20Manual.pdf


FastEagle
 

SargeW

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That's my concern John. With out a true weight, you may be running heavier than you think. And a low tire will heat faster than a properly inflated one.  An increase of 16 to 20 pounds per tire seems like a lot to me.  My rig is running down the road at about 26k while full time loaded.  I never see a rise of more than about 10# per tire, even in 100 degree temps.  The difference though is that my rig is running 22.5 tires.  How much of a difference that makes I am not qualified to say.

I would recommend weighing the rig ASAP, and a four corner weight is best.  If that isn't possible then an axle weight is the next best thing. 
 

lavarock1210

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Larry N. said:
Under ideal conditions I'd see that as true. But I find large temperature variations between tires as I drive, causing somewhat uneven pressure changes (not grossly so), usually due to sun on one side of the coach as we drive.
What  TPMS are you using?
I have been using a Pressure Pro system on my 5th wheel and truck for 5 years. Only get tire pressure.  I watch my tire pressures very close and check them often while travelling. (gives me something to do on long boring drives.)  I have found that on a given axle the pressure side to side is rarely more than 2 psi different.  Even when one side is in the sun the pressures even out after 10 miles of towing.  I also have found the ambient temperature does not change my towing pressure after 10 miles much. 
Are there any others that have noticed this?
To the OP!  What TPMS system are you using?  Also what is the maximum tire pressure for your tires?  I have read that running G and H rated tires below maximum pressure will cause heating from tire wall flex.  If your tires are rated at 120 then I can see them heating and increasing pressure until there is enough pressure to reduce the tire flex. On my trailer I run them at the max rated pressure.  Same for the truck rear axle.
 

Gary RV_Wizard

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The tire industry gurus say that a 15% rise from "cold" is within the norm, but you seem to be experiencing as much as 20%. That's still not terrible, but it does suggest that the tires may be running soft for the actual load. You should get the rig weighed, at least by axle and preferably by wheel, and use the tire inflation table for the size and brand of tires you have installed.
 

Larry N.

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What  TPMS are you using?

It's a TST  TPMS, showing both temp and pressure. My pressure differences aren't large, but they do vary from tire to tire. Of course there's usually altitude changes involved, too (I live in Colorado, and often travel through the Rockies), but I see this even on fairly flat routes over hundreds of miles (Kansas, Illinois, Michigan, for example). I see similar (but smaller pressure changes) on my Jeep toad, also.
 

Mavarick

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I think my first question would also be what TPMS are you using? First, my system is like Sarge?s, 22.5?s that under normal conditions will increase app 10% - 12%. For us that run at 100 psi normal is 10 -12 lbs. I have never seen one go 20 lbs + for any reason. Actually I think 20 lbs would ring an over pressure alarm on mine, I?d have to look. If you were to inflate at 50 deg in the am and then drive freeway speed into 80 deg weather (hotter pavement, not ambient) later in the afternoon I guess a 20% total increase is possible but I don?t think it?s likely. I have driven many miles in one day without ever raising psi that much. When just sitting, ambient will play a role but usually not as much, say 8% or so. Driving generates more heat so I have found it creates more of a psi difference and ambient usually won?t affect psi much higher/lower when driving.
I have noticed a big difference in calibration among some TPMS sensors but that would be a bad individual sensor, not all of them at once. IMO if you are see that much difference then I would look into it further, especially the true weight of your MH and proper tire inflation for that weight as others have mentioned.
In order to sort this out I would remove all the sensors and use an accurate tire psi gauge. I know gauge?s are not exactly that accurate but at least they do not have to transmit a signal to another device to just give you a readout. Too much room for error so I find it easier to just use the gauge to check things. Get your pressures set according to weight then take it for a drive. After app 10 miles check the pressures again with the gauge. Once you are satisfied with the results put the sensors back on and see how they calibrate with the gauge. They don?t have to be the exact same number as long as they react evenly. If the tire gauge sees the tire psi rise 10 lbs then the sensors should see the same 10 lb rise when doing the same test. If not I would discuss it with the mfr of the TPMS.
 

lavarock1210

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Larry N. said:
It's a TST  TPMS, showing both temp and pressure. My pressure differences aren't large, but they do vary from tire to tire. Of course there's usually altitude changes involved, too (I live in Colorado, and often travel through the Rockies), but I see this even on fairly flat routes over hundreds of miles (Kansas, Illinois, Michigan, for example). I see similar (but smaller pressure changes) on my Jeep toad, also.
Elevation will have the same effect on all the tires thus I should see the same change on all tires thus they all should be the same under all conditions where the vehicle is under way.
If I were to start at 10,000 ft at 40 degrees F and travel to sea level were it most likely will be warmer say 80 degrees F then I would see no change in tire pressure.
On very long trips I have found after towing about 10 miles my tire pressures are within 4 psi every day of the trip of what they were when I started the trip. 
 

BernieD

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Lavarock

PressurePro never felt a true need for tire temperature so resisted adding that to the system. PP gave in to marketing mania a couple of years ago and added temperature reporting also. You can give PP a call to see if your system supports it.

In general, increasing temperatures are reflected in rising pressures and PP does warn for high pressures. However, you need a large temperature increase (maybe 25%) to trigger the alarm. I keep my front tires at around 100-105psi and have seen temps as high as 120+psi when driving at the speed limit on a hot day on an expressway. No problem.

You are looking at it correctly, the spread in pressures between left and right sides.
 

lavarock1210

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captainjohn0123 said:
TPMS is Hopkins. Max cold pressure is 110.
You stated you are seeing a 20 psi pressure change.  What has been the temperature change?  For each 1 psi of pressure you should see about 4.2 degrees f temperature assuming no significant elevation change or loss of any air in tires.
 

captainjohn0123

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Myrtle Beach, SC
lavarock1210 said:
You stated you are seeing a 20 psi pressure change.  What has been the temperature change?  For each 1 psi of pressure you should see about 4.2 degrees f temperature assuming no significant elevation change or loss of any air in tires.

No significant outdoor temperature or elevation change. Outdoor temperatures were running around 50.

Outdoor temperature doesn't seem to be affecting the pressure. We were seeing the same pressure increase with temperatures in them 70's the last time we moved.

 

Foto-n-T

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Keep in mind that different manufacturers tires will vary in their temperature.  Just because tire A only increases in temp and pressure 10% doesn't mean that tire B will do the same.  As long as all of your tires are coming up in pressure approximately the same amount I don't see a problem.  Should one be higher than another I'd look for a problem.  My truck tires typically run 10%-15% higher when warmed up no matter the outside air temp although the sunny side of the truck can be slightly higher.  The trailer tires often run between 18%-22% higher but they are much closer to their operating limits on weight than the truck.
 
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