Dick,
Thanks for the permeation information. I have posted info on this subject in the past but we have new owners on the forum and it may be good to touch upon it once again. If you look at the rubber tire structure with an electron microscope it looks porous or stranded and there would certainly be rapid air loss without the interior barrier coat on the inside of the tire. All tires have this interior barrier coating used to slow the air migration.
I live near the gulf coast and when I added air it was always humid and I paid no attention to that fact on my last coach. When I would go to check the air I would get a mist or some water droplets at the gauge and Schrader valve. I had a Michelin tire on the rear fail in my last coach and a year later I had a second one fail. I noticed where the sidewall "zipper fail" was that the steel cables looked oxidized (rusted) where they failed. I had all my tires replaced and made arrangements with Michelin to have the failed tire sent in and failure analyzed. I passed that oxidation observation onto them at the time of fail. They always assumed that the tire fail cables sat exposed and any oxidation always occurred after the fail so it was ignored in the past.
I was in the habit of checking and noting air pressure before I headed out so I had a long track record that the tires were never run under inflated. I worked with a reliability engineer at Michelin and what we surmised as a plausible fail mechanism, because in the colder months my RV sat, that under the right conditions of moisture saturated air migration through the sidewall, the moisture could condense on the steel cords. They surmised if those steel cables oxidize and the surface was roughened that they will saw themselves apart with flex and stretch cycles during rotation. They do not see this issue on truck tire use given in general they do not sit but they have noted higher failure rates in RV tire use, which they always assumed was due to improper inflation.
They have since increased the barrier coat thickness in tires in response to this possibility. I have added a water separator to a small electric compressor that I use on the road. If your coach has an air dryer then the onboard system is a good air source. So on the subject of moisture in tires, it may be one of the good reasons to consider nitrogen in RV tires if you have no other dry air source. This is a non issue in cars given there are no steel cables in the sidewalls. Something for us all to consider...especially you fogged in folks in Newport and the Florida crowd.
Later Ed
