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Friday, June 8, 2018

Setting Motorhome inflation for "Newbie"

Reading a forum post that said:
"We are moving across the United States for our FIRST trip in our 06 Mountain Aire. Don’t think that’s a great idea for a first trip, but that’s what’s gonna happen. LOL. I do not have access to a four corner scale. I do have access to a cat scale. I downloaded the Michelin tire pressure guide. From what I remember here, I take the axle weight and use the Michelin guide? I seem to remember someone saying to add 5 psi to compensate for the lack of a 4 corner scale?"
My reply:

You can read my RV Tire blog for more details for Motorhomes (but not towables)

1. If you have no scale reading follow the Vehicle Certification Label AKA (Tire Placard)

2. As soon as possible, get on a truck scale when you are at your heaviest (full of fuel, food, water, propane, clothes, people etc).

3. Using the axle load numbers: Confirm no axle exceeds GAWR (gross axle weight rating) (on placard) and the RV does not exceed GVWR (gross vehicle weight rating) (on placard). If you exceed either max weight rating you must move or eliminate some "stuff" in the RV.

4. If your weights are lower than GAWR then you MIGHT be able to lower your inflation a bit but there is some effort involved.

5. Since you will not have "4 corner weights" you will not know how much out of balance side to side you actually are. Some RVs have been found to be 1,000# out of balance on an axle.

6. I suggest at a minimum you assume one end of each axle is carrying 53% of the total axle load. You could use the 53% number to consult the Load & Inflation tables.  WARNING Michelin tables assume perfect 50/50 load balance which IMO is not realistic, so you need to calculate the individual axle end load ( divide Michelin number by 2)

7. Using the 53% load number and the calculated Michelin load number you now know the MINIMUM Cold Inflation aka MCI number. It is suggested you add 10% to that inflation number for inflation when setting tire inflation. This is your daily measured Cold Inflation Pressure aka CIP. This 10% covers you for day to day temperature and elevation changes.

8. If or when the measured cold inflation (or inflation reported by your TPMS) drops to your MCI number you need to add some air to get back to your 10% margin number.

9. I see no reason to ever bleed off pressure from your daily measured CIP unless your reading is higher than the inflation number molded on the tire sidewall. If you get that high you can bleed down to the tire sidewall pressure but never below the MCI.

Ya a bit of work but once you think about your goal - Stay above the MCI and below the inflation molded on the tire sidewall you are good to go. A TPMS makes the job of checking inflation on each day of travel much easier as you simply check each morning before you start out. TPMS will also give you a warning when or if you get a puncture od have a leaking valve and are losing air. A good TPMS will have some form of "early warning" when you start to lose a couple PSI while running down the road. This could prevent a serious tire failure.

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