Is monitoring tires "Rocket Science?"

I get asked this question quite a bit. I think that after reading various posts on tire inflation and load, some people want to complain a bit and are hoping to use the excuse that it is too much work to monitor tire pressure every travel day.

Here is an example from a trailer owner:

"I've noticed that 50 psi cold at sea level at ambient 65F is 54-55F at 6,000 to 7,000 feet.
  While touring should I deflate to 50 psi cold at 7,000 feet if I'm hanging around there for a day or two?"


I am hoping that if I share my personal experience, it will provide a down to earth view on the topic that some want to turn into "Rocket Science"

I just finished a two-month cross-country trip.  Ohio > Oregon > Seattle  > Calgary, Canada > Yellowstone > Ohio in my Class-C MH with LT type tires that are 7 years old but have always been covered whenever parked for more than a couple of days, Always running inflation about 15 psi above what is needed for actual max load based on 4 position scale weights that are confirmed to have not changed significantly each year with a trip across CAT scales.
 Fastest I ever run is 70 mph but most of the 7,400 mile trip was with cruise set at 62mph. In other words, the tires have had a good life with good care.

Elevation ranged from about 25' at Olympic Nat Park to 8,000'+ in Rockies. Morning temperature ranged from 33 F to 94 F. I have TPMS (both internal and external) so I am able to constantly monitor both pressure and temperature.

I adjusted the tires once during the trip. I think I needed to add from 1 to 4 psi in the 6 tires. Since I run a nice inflation cushion of +25% I don't get all bent out of shape when the cold inflation is off by 3 to 5 psi from my goal.

You don't have to make the task of monitoring inflation a big deal. While I may set my pressure to +/- 0.5 psi, you certainly don't need to be that fanatical, and I would consider it completely acceptable to be +/- 2% for  the average user as long as the "goal" cold inflation has an appropriate safety margin.

With TPMS I simply hit the button a couple times a day and get a real time reading of tire temperature and pressure. Yes, the temperatures vary and so do the pressures, but unless I see a sudden drop in pressure and there have been no external changes such as a sudden rainfall or one side constantly hotter than the other after spending hours with one side in full sun, I just do not worry about it.

Relax.  Let your TPMS monitor your tires and enjoy the scenery.

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Quick Summary here

I have been following a number of RV forums and want to do a bit of a summary for those that don't have the time or interest to read a number of posts on this Blog: Remember more than half of the thousands of RVs that have been checked have been found to have one or more tire and/or axle in overload. So nect time you camp you can be pretty sure that one or both of the RVs parked next to you are overloading a component.

- You should have a Digital tire gauge. I have posted the results of my tests of dozens of gauges that RVrs were using and the bottom line is that about 15% were off by more than 5 psi which  IMO makes that tire gauge a good door stop or tent peg. I have provided information on how to do your own gauge "Calibration Check" to confirm the accuracy of the gauge you use daily and how to maintain your "Master Gauge"

- If you follow the advice from the two Tire Engineers on this thread you will be setting the pressure based on the appropriate method here:
     - On your TV inflate to pressure based on the Door sticker and Owner's manual for "Fully Loaded" operation. BUT get on a scale at least once to confirm you are not overloaded.
     - If you have a multi-Axle trailer you should be running the pressure on the tire sidewall. I have written in my Blog and provided the technical justification and need to follow this advice. You should still go across a scale and calculate the actual tire loads to confirm you are not overloading one or more tires
      - If you have Motorhome you need to confirm your unit's "corner weights". Use one of the worksheets I have provided links to. Use Load/Inflation chart from your tire manufacturer, to confirm you are not overloaded and identify the heavier loaded tire on each axle. Using the pressure for that load add 10% (without exceeding the wheel max) and that is your Cold set pressure for all tires on that axle.
      - If you follow the above there should be no reason to be chasing your tire inflation around unless you have a major change in Morning temperature ( More than say 30°F ) as you would only need to "Top Off" the air once a month at most.

- Don't be too anal about inflation. Variations of 1 to 3 psi day to day are normal. You should not need to make adjustments unless the Cold pressure is 4 or 5 psi away from your goal. (Multi-axle trailers might try and get no lower than 3 psi below tire sidewall pressure)

- TPMS are a WARNING device and like Insurance we should all be using them. Also just like Insurance they cannot "Prevent" bad stuff from happening but with luck they may give sufficient advance warning to lessen the impact of the bad stuff that does happen.

- If you are parked for more than a day you need to cover your tires that will be in direct sunlight with WHITE tire covers. This protects against both UV and higher temperature damage which can shorten tire life.

Altitude (elevation) effect on Tire Pressure

We have previously discussed tire pressure change vs temperature change. The question of tire pressure change due to altitude (elevation) has been brought up on occasion so I guess it’s time to answer that question.

Quick answer: Driving from Death Valley to Denver will result in less than a 3 psi increase in tire pressure IF there are no other changes. So it isn’t a big deal.

Now for the engineers and technically inclined the formula is p = 101325*(1 - 2.25577 * 10**-5 * h)**5.25588 according to info at Engineering Toolbox.

With p (pressure) in Pascals and h (height above sea level) in meters. After unit conversion and if we assume 14.7 psig at sea level we theoretically would have 12.2 psig at 5,000 ft. This decrease in pressure outside the tire would translate to a 2.5 psi increase in the internal tire pressure.

In reality there would be other changes that would also affect your tire pressure as I doubt that you would find the ambient temperature to be the same.

Bottom Line.
If you are checking your tire inflation the morning of every travel day as you should, and adjusting when necessary to meet your minimum inflation as published on your RV tire placard, you will not have any issues as you travel across America, no matter the altitude (elevation) or weather.


Don't forget if you have questions you can submit via email (previously published)

or

Post a comment to the June 25 topic "Do you have questions?"