By Roger Marble
Are “blowouts” the result of running a tire inflated to more than the tire sidewall number? I think people are over-thinking tire over-inflation.
1. Any mention of tire inflation is about “cold” inflation unless there is a specific mention of “hot inflation.”
2. “Cold” inflation does not mean you need to refrigerate your tires or get them to some artificial chemical laboratory “standard” of 68F or 70F. “Cold” simply means at the prevailing ambient temperature, i.e., the air temperature in the shade. Tires generally reach “cold” temperature after being parked for 2 hours or more in the shade.
3. All tires warm up when running. Sometimes when driving the Interstate, we even see the tires on one side are 10-20 degrees hotter than the other when constantly in direct sunlight. Think about traveling due east or west in Kansas or Iowa. I know I have seen this happen in my Class C more than once.
How to add a “margin” of inflation
4. If you have your actual tire loads (scale weights when fully loaded) and use that number to learn the minimum inflation from the tire Load & Inflation tables, you can add a margin (see below) and then be in good shape and not worry about tire temperature.
5. What “margin” of inflation? Use the tables and identify the inflation that can support no less than the “heavy end” load. (Always go up to next 5 psi step in the table.) Do not round or try to calculate an inflation that exactly matches your scale number. Just use the table numbers.
6. Now that you know the MINIMUM inflation required, it is desirable to have a margin so you do not have to get out the compressor every day because the ambient temperature has changed. Given that tire temperature will change about 2% for each change of 10F in ambient, and that a 20F to 40F change in morning ambient is in the “normal” range, that would suggest that having a margin of + 8% to 10% will make life easier.
If you are running 10% more inflation than the minimum required for the load, you will find that the “hot inflation” will not increase as much as if you are starting at the minimum + 2psi. EXAMPLE: Suppose you need to run 70 psi MINIMUM based on the measured load and the information in the tables. I think that if you start out inflating to 77 psi, you will see less of an increase in pressure than if you start out at 72 psi.
7. Maximum inflation? This causes some unwarranted concern for many. They read the tire sidewall and see something like “Max inflation 80 psi, Max Load 2780 Lbs.” IMO it would be better if the tire said “Max Load 2780 at 80 psi.” This is because there is an absolute maximum load a tire is rated for and the inflation pressure needed to support, when cold, is 80 psi. This means, in reality, that 80 psi is the MINIMUM inflation required to support the MAXIMUM load.
Testing for maximum inflation a tire can tolerate
While I cannot speak for all tire companies, I do know from my experience as a tire design engineer where testing for the maximum inflation a tire can tolerate was part of the standard process, I saw that for normal street tires, P, LT or Truck type, the new, undamaged tires could tolerate anywhere from 100% to 250% over-inflation and not fail. In a few cases some tires were able to tolerate as much as 400% for a short time before failure.
Now I need to make it clear that running tires at 100% over the number molded on the tire sidewall is not safe. The tests were conducted in an explosion chamber and not while tires were running. Tire engineers test new designs to confirm they can tolerate significant over-inflation. We run tires in overload on test wheels continuously longer than possible for anyone to run their tires on the highway so we know they can tolerate significant increase in both temperature and pressure.
8. “Blowouts” do not happen because a properly loaded, undamaged tire was run at highway speeds. “Blowout” simply is a word that is used to explain that there was a sudden loss of air that made a loud noise, and the driver was surprised. Running a tire at hot inflation of 110% to 130% of the tire sidewall pressure is not, in itself, going to cause a “blowout.”
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