By Roger Marble
Don’t get your shorts in a bunch about tire inflation – BUT you still should consult the inflation tables.
I talked about this a number of years ago but it seems it’s time to cover this again for those new to RV living.
I have covered what I felt is the “best” inflation for tire life in my posts where we discuss “4 corner weights” – which means learning the actual load on each tire position by getting the RV on a set of individual tire scales. While large RV conventions such as FMCA or Escapees events sometimes have those scales, many times they are not convenient. But you can find truck scales at many interstate exits, where you can learn the actual load on each axle.
Since we know that almost no RV has perfect 50/50 side-to-side load balance, learning the actual load on each end of each axle is a good idea. Some RVs have been found to be 1,000# out of balance.
If you can only find truck axle scales then I suggest the following rules of thumb until you learn your “4 corner weights.”
Class A motorhomes and large (28′-plus) 5th wheel trailers with slides and especially if they have a residential refrigerator, should assume they have 53% of the axle load on one end so should use that heavier number when consulting the tire Load & Inflation charts.
Class C motorhomes and trailers shorter than 28′ with slides should assume a 52/48% side-to-side split; while class C without slide, class B, and small single-axle trailers can assume a 51/49% side-to-side load split.
Using the heavier end figure, consult the published tire Load & Inflation tables to learn the minimum inflation pressure for the tires on that axle of your RV. This “minimum” inflation is the number you would consider for the morning of every travel day. To avoid chasing inflation changes due to changes in the weather, I suggest you add 10% to the number from the tables so you can simply monitor the inflation using your TPMS and as long as you never drop below the minimum inflation needed to support your load.
I am also in favor of this plus 10% inflation margin so you don’t find yourself chasing your tail every day by adding 1 or 2 psi when it gets cooler and you find yourself 1 or 2 psi low, or bleeding off 1 or 2 psi when the weather turns warmer. You can simply monitor the morning inflation number and as long as it stays near the +10% and does not drop to +0% or go above +20% you are good to go for that travel day.
With +10% margin it would be easier to discover you are low a few psi and simply wait till the next fuel stop, where there should be high pressure air available if you need to add air.
For those that don’t know how to inflate a warm tire, here are the steps:
1. Measure the pressure when the tire is at ambient temperature (not warm from driving or being in sunlight). Many consider this their “Morning Tire Pressure.”
2. Note the number of psi you want to add to each tire to get to your goal inflation.
3. When you get to a fuel stop measure the warm pressure.
4. Add the number of psi from #2 to the warm pressure in #3 and add air till you get to at least this new warm pressure goal.
This “rule of thumb” will work for pressure changes of 5 psi or less. If you find you need to add 5 psi or more there may be something wrong, e.g., a leak unless you have seen a long-term decrease in pressure as the weather cools down.
Don’t get hyper about being 1 or 2 psi off. Remember, if you have a 10% cushion, you are good to go as long as you are within a few psi of your goal.