Example Of Pressure & TPMS Setting Calculations

[tireman9]

I thought posting this example might help those with the recurring question of "How much air should I use?"

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I received this question.

"I am a newbie. Just 5 weeks with my 2017 Bay Star 3113. I need some help understanding just what would be the correct or appropriate tire pressure on my coach. When loaded with my wife, dog and self, partial food and clothing, tools, chairs, BBQ ladders, 2/3 water, full gas and propane and misc stuff I got the following from the CAT scales: Front axle - 7,300. Rear axle - 13,260.

Here is my dilemma: The dealer delivered the coach with about 85 lbs. per wheel cold, but when I use the Michelin Tire pressure chart for my 235/80R 22.5 XRE tires should be just 75 lbs! I am concerned that they will be "underinflated" and could build up excess heat. Should I keep the 85 lbs or lower the pressures?"

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My reply:

Welcome to RV fun.

For all things tires (except buying) I obviously suggest you check my blog. I don't expect folks to remember everything but if you spend a few minutes checking it out, you can learn how to use the "Label List" on the left to find a post of the topic of interest. There is also a search box in upper left.  Now to your specific question.

1. We want to know the heaviest load on your tires and since few RVs are perfectly balanced side to side for weight we ideally want to know the "4 corner weight" to learn the heavier end for each axle. lacking knowing that number IMO we can do a rough calculation by using 53% of the axle scale weight for the RV when it was fully loaded (the expected heaviest it will ever be).

2. 53% of your front would be 3,870# and rear would be 7,030# or 3,515 on each dual.

3. Looking at the Michelin load tables we find for your size at 85 psi can support 3,975 for single (front position) and 80 psi supports 7,050# for two tires in dual position.  Yes we always round up.

4. Based on the above your MINIMUM inflation would be 85/80  This is the number I would use for the low pressure warning numbers on your TPMS.

5. I recommend adding 10% and again rounding up that means 93/88 psi for your Cold Inflation Pressure or CIP   In your case given the close numbers for the front load I would be comfortable using 90 psi all around as a single number is easier to remember. This 10% gives you a nice cushion so you do not have to chase your tire pressure around whenever the temperature rises or falls. You could even get down to 85psi before needing to "top off" the tire pressure again.

6  All tires on an axle should be inflated to the same level for improved handling and response in an emergency situation.

7. I would set the TPMS High Pressure warning to 110 to 115 psi and your high temperature warning level to 160F.

8 Remember CIP means when tires are at ambient temperature and have not been in direct sunlight or driven on for at least two hours.

Finally, In your case you are close to some numbers when we round so if your RV is more balanced than my suggested 47/53% you may be able to lower my suggested inflations by 5 psi but only when you confirm your heavier end is less than 53% of the total.

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PS The above post will be on my blog in a few weeks. You folks just got the "Advanced notice".