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Friday, May 4, 2018

How much inflation in my tires? Question from new RV owner

Recently read yet another forum post on the topic of "How Much inflation should I run. This post followed the standard format of:  A. I just bought an XXX RV. How much air do I need in my tires?  B. Numerous replies ranging from "I use xx psi" to "You must always run the pressure on the tire sidewall"  to the more correct reply of "You need to know your tire loads first". Some forum threads run to dozens of back and forth exchanges Some have correct info IMO while others are still using what I consider "Old Wives Tales".  I recently replied as follows.

1. Almost all tire Inflation/Load charts have identical numbers (maybe as high as 98% )so if you can't find your brand you can reasonably use another brand till you do.

2. Yes, the charts give the MINIMUM inflation needed but to avoid the need to adjust your inflation every morning (inflation changes by about 2% for every change of 10F) I and others suggest you add a bit so you are running the minimum + 0% to + 15%.

3. You should NEVER run lower than the minimum inflation shown on the chart.

4. Since all tires on any one axle should have the same CIP (Cold Inflation Pressure) you base the minimum on the heavier end. This is why we recommend "4 corner weights" to learn what the heavier end load is. Until you learn the actual load on the heavier end you can assume one end is supporting 53 to 55% of the total on the axle (the axle weight when on the truck scale).

5. How much to add over the minimum? I think you will find various suggestions in the range of +1 psi to +10%. Since I am offering advice to a wide range of users (ST trailer tires, LT and Class-A) with minimum inflations from 50 to 120 I prefer the percentage and if people want to have their CIP ending in 0 or 5 they can add the % then round up. ( Note the +1 Psi is from Tire Rack where they are advising owners of passenger tires that normally are running 30 to 36 psi)

6 Premature wear. Most LT and 22.5 size tires can be good for 60 to 100,000 miles wear so most RV owners will have their tires "age out" before they wear out so the minor wear difference, if any due to running + 5% over the minimum if overshadowed by variations in suspension wear, alignment variations, and driving habits. The center shoulder wear info is based on car wear rates primarily with bias tires or inflation differences greater than +/- 15% from the suggested CIP.

7. "Correct" inflation is not what the charts say. They are providing the "Minimum" cold inflations is every case.

8. Adding Load % and adding inflation % is done only to compensate for not knowing the actual heavy end of an axle. If you learn the actual heavy end when the RV is fully loaded to the heaviest you ever travel at, I do not suggest you need to add any more weight when doing your calculations. How much can you be out of balance side to side? I have seen a very small % of RVs with the load at 50/50 side to side. Most appear to be in the 53/47 to 55/45 range but a few have been found with over 1,000# heavier on one end.

9. It is important to realize that when we look at actual weights of RVs in use, over half exceed one or more of the weight specifications for tires or axle loading. IMO this is a clear indication of why so many RVs have tire problems.

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