By Roger Marble
Roger: “When you refer to a 10% safety factor, is the tire any safer at 10% over its 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.” —Crasher
My response:
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 80 F. What happens the next day if the ambient drops to 70 F? Your tire pressure will have dropped by 2% to about 68 psi, which is below what is needed to support the measured heavy load. So you get out and increase your tire pressure back up to 70 psi. A few days later it’s 90 F so tire pressure is now (90 F – 70 F = 20 F) so 2% per 10 F = 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.
Regarding center wear on the tire. 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 of operation. Road surface (concrete vs. asphalt) has a much bigger impact on tread wear.
Read more from Roger Marble on his blog at RVtiresafety.net or on RVtravel.com.
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Thank you Rodger. All my tire experience before our DPMH was with military & civilian aircraft tires. The tolerances were called out in a plus/minus range. I’ve adopted the same policy with my 22.5 tires, a +/- 5 psi tolerance based on your 10% model. Thank you again for your clarity.