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Thursday, March 12, 2015

Minimum or Maximum inflation or Load...I'm confused

Read a post of an RV forum from a motorhome owner who was a bit confused by the wording on his tires about Max Inflation and max Load when some people are telling him to learn the minimum inflation. He said
"Uhh.... So why does it say MAX PSI in front of it???  MAX means minimum???"

My answer:
Yes this is confusing.  The intent of the information is to let you know that when you inflate the tire to the pressure associated with the highest level for the Load Range it is rated to carry the stated load and no more.
Remember that "Load Range" is a replacement for the outmoded "Ply Rating"

When you look at a Load/Inflation table you will see a series of inflations in 5 psi increments. You will also see a series of load capacities.

I'm going to use a LT size but the same process would be used by other Motorhome owners.

Size LT225/75R16
Single  1790#@45  1940@50   2060@55  2190@60  2335@65 
2440@70  2560@75  2680@ 80  3000@95

Now the other bit of important information in the table is the Load Range limits for this size tire.  LR-C is 50psi  LR-D is 65  LR-E is 80 and LR-F is 95

If you had this size tire and it was a LR-E tire it would say 2680# at 80 psi Max or some such

Note some tire companies use the wording like 2680# Max at 80 psi Cold or some variation thereof.

The inflations in the table can be considered a Minimum when you start by looking at your measured load.  In the above example if you measured 2500# on the heavier front tire you would select the inflation from the table associated with at least 2500#  which means 2560# and you would then know that the Minimum cold inflation you should ever run was 75 psi.

Finally we know that pressure varies as the ambient temperature changes. This change is about 2% for every 10°F increment so to avoid having to chase around looking for a service station every time the temperature drops 10°F, I suggest you add 10% to the required inflation level which in the case of this example would be 7.5 psi

I wouldn't worry about getting the inflation to the exact 1/2 psi just know that if you add 7 or 8 psi to your minimum of 75psi you are good to go. You also don't have to worry about adding air if it gets cold and the inflation drops to 79psi as you have a nice margin built in.

Over time all tires loose air pressure so eventually your pressure will drop to the point that your day to day pressure variation will result in you having only 75 psi in the tire. I would not wait till your margin has been used up but would have added a bit of air once I got down to 77psi or so and bumped it back up to the 82/83 psi level.

Long explanation but I wanted to give you the background so you would understand why I say

1. Weigh the RV
2. Select the minimum cold inflation for your tires based on the table minimum inflation
3. Add 10% margin to that inflation to learn your "cold set pressure"
4. Inflate all tires to the inflation for the heaviest loaded tire on that axle. (all tires on each axle should have the same inflation)
5. Check your inflation in the morning before you travel more than 1/2 mile and before the tires are warmed by sunlight or monitor the inflation all the time with a TPMS.

Hope this helps.

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