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Friday, August 7, 2020

Another question on "Cold Inflation" vs your "Set Pressure"

Got this question from a reader of an RV Forum:

Thank you for all of your informed comments regarding proper tire care. I need one clarification. I have always considered the cold psi on the side of my 22.5 RV tires to be the minimum to carry the maximum rated load, but have assumed that psi was also the maximum COLD psi the tire should see. From your recent post, am I to understand that unless the tire states that it is the maximum cold pressure, I can exceed it by 5-10 psi?
Thank you for your time, Doug

My Answer:

The wording on tire sidewall IMO was written by lawyers, not engineers or users. Info on the sidewall is the inflation needed to support the Max load.  The difficulty is that few understand that the pressure changes with temperature and the only meaningful pressure, measurement is when the tie is "cold". This still confuses some because some want to apply Chemistry Lab practice of adjusting to theoretical 72.5°F when what "cold" really means for tires is at "Ambient Temperature" and does not include any pressure build-up"   In real life terms this means "Not warmed by being driven on or in direct sunlight for the previous 2 hours"
Now we need to address what is meant by "Cold Inflation" vs the psi to set your tires to or what I like to call your "Set pressure".
I like to suggest the "Set Pressure" for motorhomes to be the minimum needed to support the maximum load on the tires PLUS 10% inflation. 
RV trailers are different because of their Interply Shear problem.

For RV Trailers,  I would like to see a minimum of +15% load capacity over the measured heaviest loaded tire, with +20% to +25% Reserve Load capacity being better. Sadly most RV trailers come with tires that provide +0% to +10% load capacity vs GAWR
NOTE: I am not even addressing the tendency for most RV owners to overload their tires.
So for trailers I try and simplify:
- To lower, but not eliminate the Interply Shear problem I suggest the "Set Pressure" when the tires are "Cold" to be the pressure on the tire sidewall. BUT I still want trailer owners to confirm they have at least 15% "Reserve Load" over their measured scale reading.


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