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Friday, February 18, 2022

4 questions on inflation, a long back story and RVs are not the same as HD Trucks

 [QUOTE=yeloduster;6070640]This is a question for Tireman.

A little background first. My older brother was an independent OTR for years. The last 15 years or so of his career he had a contract that took him from his home in Roosevelt, UT to Houston, TX. His gross weight were ~80000 lbs. He typically left early Monday morning and was home by 4 PM on Friday.

In the winter he might leave Roosevelt at 5000 ft elevation and maybe -20°. He would proceed through Colorado and top out on Berthoud Pass at 10,300 feet and proceed to Houston where it might be 85° and elevation 100 ft.

His tire cold inflation pressures would experience a difference of ~5 PSI due to elevation change and ~10 PSI due to temperature change. In addition because he was hauling oil field equipment on a flat bed his load could be heavy on one side because of the odd shape of his load. I know he couldn't measure side to side weights. He relied on axle weight to set his air pressure.

He and I talked about tires a few times. I don't recall talking about specific pressures but I know he checked his pressures each morning before leaving wherever he was but didn't change the pressure unless he found a low tire.

He drove millions of miles and wore out several trucks and a truckload of tires. He says he had very few blow outs because he used good tires and monitored his pressures. The few blowouts he experienced were road hazard type events.

His experience would suggest that tire manufacturers load charts take into account a certain amount of side to side imbalance of the load and account for large changes in elevation for over the road trucks.

Actually I guess I have 4 questions:

  • If the engineers who developed the load charts took into account reasonable side to side load imbalance and large elevation changes shouldn't we just follow the chart?
  • If the engineers who developed the load charts took into account changes in ambient temperature when developing the chart would it be better to set your cold inflation pressures according to the chart and add or subtract about 1.5 PSI for each 10° the ambient temperature varies from 72°?
  • Most RV owners do not have a calibrated tire pressure gauge. I've read that moderately priced tire pressure gauges are accurate ±3%. Knowing all these uncertainties about inflation would a RV owner be wrong if he set the pressures with his gauge according to the weight on his axles and checked them regularly. As long as the pressures were his original set point ±2-3 PSI everything is OK?
  • Some of us are OCD and getting 4 corner weights, studying the charts and getting the exact pressure according to the chart and then setting pressures to ±1 PSI according to our gauge satisfies our OCD but does being that precise really matter for safety, driving comfort and tire wear?
I'm thinking if we inflate to the chart value for our axle weights taking into account the actual cold inflation temperature we should be good![/QUOTE]
First off it's important to remember that HD Trucks are weight limited not tire limited so many times they do not have to run as high a pressure as some RVs do. HD trucks also may have tandem duals which means 18 tires while Large RVs may only have 8 tires total so don't get hung up with that comparison.

1. The charts have the MINIMUM psi required to support the stated load. There is no stated or implied "safety factor" that would allow overloading.

2. Tires are to be inflated to the appropriate inflation when the tires are at ambient temperature before you start driving. 72°F is not some special temperature. The chart is based on AMBIENT air temperature wherever you are.

3. My hand gauge (Accutire brand) cost $12 from Amazon and is accurate to +/- 0.5 psi at 80.0 psi ( 0.6% accuracy) when checked against an ISO Certified Laboratory gauge. Paying more for a gauge is no guarantee of better accuracy. I prefer digital gauges over dial gauges as they are easier to read. I do not like stick gauges as they can get way off if they get worn from use or dirty. Dial gauges can sometimes be hard to read.

4 I would suggest if possible to get 4 corner weights when the RV is at its heaviest. Consult the tables for the inflation for the heavy end of each axle. That is the MINIMUM pressure to set the low pressure warning on your TPMS. I would add 10% to that MINIMUM for your "cold" tire (ambient) pressure.

5 If you are traveling back and forth between Phoenix and Pikes Peak just be sure that in the morning at Pikes Peak in the snow you are not below the MINIMUM psi shown for your scale weight and the tables advise.

6 It is the under inflation we tire engineers are concerned about. In my Class-C I tend to run +15% to +20% higher inflation than the tables suggested minimum because I have a light weight RV based on my 4 corner weights.

7. Run a TPMS that you have set properly and test at least once a year.


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