Community Forums

footdoc · 12-16-2006, 07:10 PM

Just installed PressurePro and am wondering what kind of pressure increase to expect from cold to operating temps going about 63 with temps in the 50s...so if my cold baseline is 115, how much will it typically increase?

footdoc · 12-16-2006, 07:10 PM

Just installed PressurePro and am wondering what kind of pressure increase to expect from cold to operating temps going about 63 with temps in the 50s...so if my cold baseline is 115, how much will it typically increase?

wallynm · 12-16-2006, 07:27 PM

5 to 15 pounds plus the tires will run hotter up to 125 or so. Sunndy side and inside dual will run higher and hotter

GaryKD · 12-17-2006, 12:22 PM

Hi Larry,
wallynm is about right for outside temps in the 50's. Wait until you run a day in the heat of summer in the SE or SW. Then you'll see 25+ lb increase. Now that begins to get exciting,,,,for me.

Gary RVRoamer · 12-18-2006, 04:21 AM

Pressure increases about 2% for every 10 degrees (F.) of temperature increase, whether it be ambient or from the heat of road contact (friction) while driving. 2% on a 115 psi base is 2.3 psi, so pressure climbs quickly.

You will also get some false alarms on a cold night if you set the baseline pressure on a warm day, e.g. 80 degrees. A 50 degree drop in temperature will readily trigger the 10% alarm threshold on the PP sensors.

Lug_Nut · 12-27-2006, 05:00 PM

Actually it is surprising that the sun on one side does not affect the internal temperature as much as the skin temperature would indicate. I have internal temp and pressure sensors. The variance from the sunny side to the shady side is almost nil. Yet reading the skin temp with an IR temp gun shows a considerable difference.

Cruzer · 12-28-2006, 03:30 AM

I've seen that too and I bet it's due to the RV tire's ability to shield heat. When I check the sensor readings in my Wrangler I see a temperature difference after it's been parked with one side to the sun. But not in the Bus. I'm thinking that the thinner sidewall Wrangler tires don't insulate as well as a load range H RV tire does.

That's one feature I really like about the SmarTire system. It's ability to read temps is put to good use by calculating a temperature compensated low pressure chart. If I manually cycle through and view a 60 degree tire that shows 90 PSI it'll state that it's -5 PSI. Yet, when I check a 30 degree tire that shows 90 PSI it'll show that it's right on. by the same token if the tire is at 150 degrees and reads 90 PSI it'll probably be -15 PSI. That's one of the unique advantages of the SmarTire system because it always gives you accurate warnings at any temperature.

Cruzer · 12-28-2006, 03:42 AM

Actually, to answer the original question:

Following is a pressure compensation chart that I got from my SmarTire manuals. It'll show you the temperature compensated pressure for the normal inflation pressure.

The CIP column on the left is the Cold Inflation Pressure. By taking the current temperature across the top and matching the 2 columns you'll find out what it should read at that temperature.

Lug_Nut · 12-28-2006, 03:46 AM

Interestingly I get what would be normal pressure increases with increased temperature on my MH tires. For example my front are cold at 115 PSI and after driving they climb to 128/130 PSI. My Escalade which also has real time tire pressure for each tire, gains near zero extra pressure even after 860 non stop miles. May be it is related to the size and profile. The Caddy has 20" rims but very low profile tires.

Peter

ISLAPP · 12-28-2006, 04:21 AM

Mark, if I am looking at that chart correctly, it answers something I have always wondered about. I always wondered if there was technically a certain ambient temperature that ideally should be used to set the so-called cold inflation setting. It appears that ambient temp is 65.
So do I also read the chart to say that if, for example, the ambient temp is 80 and I want to have 75PSI cold inflation setting, that I should really see 78PSI when I check or fill the tire?
Mike

RVDriver · 12-28-2006, 08:56 AM

Question

I don't have "PressurePro", BUT!

I have always had to come up with an inflation pressure at home before leaving on a trip. Now in the summer at 85 degrees outside, fill tires to XX lbs, but when I leave for Texas in January, what pressure do I put in my tires?

XX lbs minus XX lbs? I usually "Hopefully" check my tires one or two days before leaving when its above 32 degrees, and fill to the same as "Summer", is this correct? I have an IR thermometer that I check tires temps with when I stop at "Rest Areas", and do note cooler temps in winter vs summer, and the "Sunny side" is about 15 degrees warmer then the "Shady Side".

Thanks

Bruce

Cruzer · 12-28-2006, 10:31 AM

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by ISLAPP:
....So do I also read the chart to say that if, for example, the ambient temp is 80 and I want to have 75PSI cold inflation setting, that I should really see 78PSI when I check or fill the tire?
Mike </div></BLOCKQUOTE>

That's correct. Once the air temperature cools down to 65 degrees, you'll have 75 PSI in them.

Bruce,

As long as the coach has been sitting, and not driven recently, the air temperature inside the tires should be pretty close to the same as the ambient air temperature. If you are looking for a 90 PSI cold inflation pressure then I'd check them with a gauge and relate to the chart for what actual pressure to put in. If it was 32 degrees outside I'd put 84 PSI in the tires. If it was 95 degrees outside I'd put 96 PSI in the tire. Either way you'll wind up with 90 PSI cold inflation pressure once the tire is at the 65 degree baseline.

Once you drive the RV all bets are off, however. The tires will heat up substantially and the ambient air temperature won't have any bearing on how hot the air inside the tire is. Unless you have a SmarTire system or some other way of measuring the tire air pressure you'll just be taking a wild guess. Shooting the sidewalls with an infrared gun will only tell you the sidewall temps and are not a good indicator of how hot the air inside the tire is. Sidewall plies flex and rub during use and are also affected by sun and shade. However, as Lug_Nut also noticed, that temperature doesn't transfer to the tire's air temperature. That heat tends to stay in the sidewall and disipate to the outside as well as the rim itself more so than to the internal air.