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Tuesday, July 19, 2016

Another post on Cold Inflation Pressure and "Ambient" temperature.

Some clarification from a tire engineer. 

First off, I am not going to address what the correct Cold Inflation Pressure ("CIP") is for your RV. We will assume you have read my other posts on how to learn that number. If you haven't, simply look at the list of labels on the left side of this blog and you can review them.

There are different guidelines for Trailers vs. Motorhomes. I want to focus only on setting the CIP

Some people want to refer to a Temperature compensation chart.

Tire Rack was off a bit till they updated their tech page last year after my input. Also Wikipedia definition for Cold Inflation Pressure was also almost correct till I added a clarification aimed at RV owners. The 1 Psi for 10 F is OK if your base inflation is near 40 psi but many RVs use 80 to 110 psi.
The correct "Rule of Thumb" to use is 2% for each 10F.

Tire pressure is not based on any laboratory standard temperature (some claim 70 F) but is based on the tire not being warmed from either use, i.e. being driven in previous two hours, or from being in the Sun for previous two hours. Even partial sun can affect the reading.

Classical "Temperature in the Shade" is the "Ambient" tire engineers are talking about. Not temperature in a theoretical laboratory.

So regarding a situation of setting the pressure when the tires and air is 50 F. That would be fine and we would expect the pressure to increase by about 8% if the Ambient increased to 90 F even without driving or Sun exposure.

It is correct to say, "The ONLY time to check CIP is FIRST thing in the morning BEFORE the day's temp has had a chance to increase and BEFORE the sun has had a chance to shine on the tires and BEFORE you have used the vehicle."

However, if you are driving from the campground on top of Pike's Peak and stop for lunch for two hours in the shade in Flagstaff where it is 90 and check your air, you might find a change of a few psi. You could adjust your pressure before continuing to Phoenix, where it is 120 F, but I don't bother to adjust inflation by the 1 to 3 psi variation I observe day to day. In my mind that is too much work.

NOTE: My personal CIP is  75/80  F/R on my Class-C MH.  Both of these pressures are more than 10% above the minimum pressure needed to support the measured load on each tire so I have a "cushion".

I usually wait till I am home and am getting ready for the next trip before I adjust my inflation to my personal CIP, so I simply monitor the running inflation pressure which goes up and down as ambient temperature, driving and Sun exposure changes the inflation. My TPMS will warn me of air loss, so all is good as I motor down the highway.

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