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Friday, September 18, 2020

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 when 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,  i.e., 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.


Friday, September 11, 2020

10% safety margin? Not Over-Inflation for more load capacity.

Originally Posted by Crasher View Post
Roger. When you refer to a 10% safety factor, is the tire any safer at 10% over it's rated load psi? If it is, why don't the manufacturers recommend a higher psi for the load? Or, is the 10% factor to cover the days when the ambient temp is lower which would lower the CIP eliminating the need to adjust the pressure? Whenever I have run tires above the load charts, the center of the tread will wear more than the outer sides. That tells me that the tire was not making optimum contact with the road for best wear and traction. Admittedly, it's a minor issue, but an inquisitive mind has to ask.

My +10% is on the set inflation and is NOT a "Safety Factor" in the normal sense. We know that tire inflation changes by about 2% for each 10°F change in temperature. The intent of this "Flex Range" of inflation is to avoid the need to mess with inflation on a daily basis.
Assume you needed 70 psi to support your heaviest ever expected load (this is why we say get on the scales when fully loaded to your heaviest). So assume you set your inflation to 70 psi and the Ambient temperature is 80F. What happens the next day if the ambient drops to 70F? Your tire pressure will have dropped by 2% to about 68psi which is below what is needed to support the measured heavy load. So you get out and increase your tire pressure back up to 70psi. A few days later it's 90F so tire pressure is now (90F - 70F = 20F so 2% per 10F = 4% increase of the 70 psi so now your tires are at about 73 psi cold so you drop your tire pressure.

See the problem? You are messing around with your tire pressure. almost every day.

However if you have a +10% "Flex Range" above your needed inflation, or in our example + 7 psi You can ignore the day to day pressure variation unless or until the temperature has dropped 50°F.

Tires can tolerate the increase in pressure with essentially no damage but low pressure can result in increased operating temperature which accelerates the "aging" of the belt rubber which can shorten tire life.

Also if you have to mess with your tires a lot, soon you will tire of the chore and stop monitoring and adjusting tire pressure which can lead to low inflation. This extra work can get old quickly and then you stop checking and setting your pressure. I Do Not Want That to happen.

RE center wear. That was an issue with bias tires but I do wonder what micrometer you are using to measure tire tread wear to 0.001" especially given that tire tread wear is normally in the .001" per 1,000 mile range and I doubt that your pressure remains constant over each thousand miles operation. Road surface (concrete vs asphalt) has a much bigger impact on tread wear.