Tire Pressure vs. Temperature

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W

Weewun

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Went over today to check the MH and get it ready to 'winterize.'  Winterizing is going east to I-95 and turning right.

Checked the Tire pressures (all six) and they were down about 10lbs from last month when we took our trip.  The temperature is about 40 degrees cooler and I expected some decrease.  Does 10lbs decrease seem normal.

I filled them and added 5lbs to compensate for the additional weight (105lbs front and 100lbs rear), have I made a mistake???? Do I need to lower the pressure for the reduced temperature???

When I weighed the MH and Toad the Goodyear table sid I needed 100lbs front and 95lbs rear and I don't recall there being temperature correction in the chart. 

I have rambled but please work your way thru the note and help me out, Thks.
 
IMO, you should be okay.  Tire pressure tables are for cold pressure (start of the day).  The season you're heading into is generally colder, so even when the tires heat up from running, they will have lower temperature & pressure than if they were run the same amount on a hot day in the summer.  Now if you measured them in South Dakota and you are headed for south Florida, then you may want to recheck them ("cold") after you get there and adjust them as necessary.
 
Thks, for the input. Going fron North Carolina to Key Largo for the Winter by way of Myrtle Beach, SC and Wildwood FL.  Will check the pressure before I leave Myrtle Beach and again at Wildwood.
 
Made a excel-form to see what the pressure does, when the temperature chanches.
The original idea was that for races you have to fill cold a lower pressure, to get the same pressure at 90dgr celcius, which is the working temperature then,as the normal working temperature of 45dgr Celcius.  But in your case maybe you can do something with it to .
http://members.home.nl/jadatis/pressurecalculationwithtemp.xls
 
Tire pressure will change 1 psi for every 10 degrees Fahrenheit...
Click to expand...

Not quite true - it depends on the initial pressure in the tire.

Tire pressure changes roughly 2% for every 10 degrees. Lower temps yields low pressure.  That means a 40 degree drop in temperature will result in an 4 x 2% loss or 8%. If the tire was 100 psi at 80 degrees, it would be about 92 psi at 40 degrees.

You are correct to bring the tire to its recommended pressure. The pressure tables are for a "cold" tire, menaing it has not been driven and is at ambient temperature. Make the pressure right at whatever ambient is.
 
The formula for pressure increase in a constant volume container ( tires are pretty close to a constant volume) is:
P1/T1= P2/T2

Where P1 is the initial pressure, T1 is the initial temperature.
Where P2 is the new pressure, T2 is the new temperature.
The temp must be in degrees kelvin.

Or P2 = P1 X T2/T1

For P1 = 85 psi, T1 = 294.26' kelvin, and T2 = 305.37' kelvin. (T1 = 70'F, T2 = 90'F)

P2 = 85 X 305.37/294.26 P2 = 85 X 1.038 = 88.21

So for a 20' F increase in temp the pressure should increase by about 3 psi.

The bad news is that apparently water vapor in the air in the tire can cause the numbers to be somewhat larger than those predicted by the formula.


( Kelvin:  A temperature scale whose zero point is absolute zero, the temperature of 0 entropy at which all molecular motion stops, -273.15? C. The size of a degree Kelvin is the same as the size of a degree Celsius.
K+ 'C + 273.15)
 
Clay, I love that science. There is nothing like getting the true facts. I mean it.
  Actually I would like the R?aumour Scale calculations as well. (Just kidding).

  By the way, if you want to make degree ? marks. just enter ALT+0176. Lotsa fun

Other easy symbols are: ?  ?  ? ?  ?  ?  ?  ? ?  ?  ?  ? ?  ?  ? .... etc etc, all with the ALT key plus 4 numbers. ( A digression).

carson FL



 
My 2% factor was derived using that same formula (Charles Law for Gasses). The number is actually closer to 1.8% per 10 degrees F., but 2% is close enough for most RV related purposes and few people want to deal with Charles Law or the required conversion to the Kelvin temperature scale.

 
carson said:
Clay, I love that science. There is nothing like getting the true facts. I mean it.
  Actually I would like the R?aumour Scale calculations as well. (Just kidding).

   By the way, if you want to make degree ? marks. just enter ALT+0176. Lotsa fun

Other easy symbols are: ?  ?  ? ?  ?  ?  ?  ? ?  ?  ?  ? ?  ?  ? .... etc etc, all with the ALT key plus 4 numbers. ( A digression).

carson FL
Click to expand...

That is the best way to do it for sure.
Problem is I am too lazy. Also on my laptop I have to press "fn + alt  + 0176"  The fn is a blue function key and I have to use the blue numbers on the qwerty keyboard. I usually can't remember all that.  :)




 
RV Roamer said:
Yeah, but it is nice to be able to evaluate whether the Pressure Pro Low Pressure alarm at 4 a.m. is due to a drop in ambient temperature or a leak in the tire.
Click to expand...

Yeah, but at 4AM it is much more likely to be the drop in ambient temperature than a slow leak ;)
 
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