Originally Posted by gljurczyk
I Think you answered your own question in your first paragraph. You have probably forgot more about tires then I know. With saying that I have not found one GoodYear dealer yet who has NOT told me to run my tires at the cold pressure rating. (they are the ones who carry my warranty) I purchased my tires at Blacks Tires in NC Lumberton one of the largest dealers out there. When they took off my Trail Express, load range C and they installed the Marathon D, I was told to run them that way. I know the weight of my trailer and axles. I showed them the #'s, and he said run them at 65lbs. I would be alot happier with the performance. I also asked and payed for balancing being the tires I was taking off were not. He said that they should be balanced, which the factory will not do. He also stated that 90% of trailers coming in for new tires there old are not balanced. That's my dilemma with just reading the inflation charts.........
Lets separate the inflation question from the other issues.
Tire companies publish Load and Inflation tables for their tires. These indicate the minimum cold inflation needed to carry the load shown in the table.
Looking at the MAXXIS table provided by WindyWest we see an ST225/75R15 is rated to carry up to 2150#@50 psi and up to 2275#@55 or up to 2540@65 etc.
Now when you get the actual load on one of your tires when the RV is fully loaded and ready to travel and you find the heavier loaded tire on your front axle is at 2205# you should go to 55psi as your minimum cold inflation. Next you need to find a tire that can carry that inflation. Again according to the table we see that a Load Range C is limited to 50psi so you need to go the next higher Load Range tire or LR-D,
Now there are two more items to consider.
1. A Safety Factor. You will find various suggestions ranging from +5psi or +10% or or +10psi. For non Passenger tires, I would suggest at least +5psi over what is required to carry the load or in this example 60psi.
2. Special consideration for multi-axle trailers. Warning, this gets technical.
When not driving in a straight line there are special side loads on multi-axle trailers because the tires are fighting each other because they are not "pointed" to the center of the radius. These loads cause interior structural tearing. Sometimes 24% higher than those seen in tires on non-trailer application. Initially at the microscopic level but with time and repeated cycles these forces can cause a belt to come off the body of a tire. You can lower these forces by either decreasing the load 24% on the tire (probably not something you want to do) or you can increase the inflation to stiffen the structure and decrease the slip-angle. In this case you could increase the tire inflation from 55psi to 65psi on the LR-D tires. BUT you need to be sure you are not exceeding the max rating of the wheel.
Video showing lateral deflection. Time from 0:46 to 1:07
Note slow speed and relatively large turning radius.
How To: Maintain Proper Lug Nut Torque on a Keystone RV - YouTube
Hope this helps.