Tire Pressure Help

[geraldlanc]

I need help determining the correct tire pressure for my mh. The tires are Michelin 12R22.5.

The front of the coach weighs 12820 and the rear weighs 23220 ( I don't have the weight for each end of the axles ). The tires say single load 7590 at 120 psi and dual load 6780 at 120 psi.

Does that mean that each front tire has approx. 6410 pounds on them and the rears have approx. 5800 pounds on them?

The maximum pressure listed on the tires is 120 psi.

Thanks for the help,

I don't know how to calculate the correct pressure.

Gerald

[desertdeals69]

Look at the chart in the Michelin manual, it will give you the exact pressure for each tire.

[wolfe10]

http://www.michelinrvtires.com/michelinrv_en_us/toolbox/reference-material.jsp

Click on "RV Tire Guide".

[tireman9]

Gerald if you don't know the actual load on each side of an axle I suggest you assume one side has 55%. That would mean your fronts need the inflation necessary to support. 7051# and the rears 6386#

Now this is conservative but I believe reasonable as we know that few RVs load their tires equally side to side.

[elkhartjim]

I have four corner weights, fully loaded with clothes, supplies, full water (I normally carry a half tank), full propane and full fuel. I added 5% to the heaviest weight on each axle when I calculated my pressures. Interestingly enough, until recently I had only axle weights to calculate from and after having the four corners weighed, I'm running the exact same pressures. I was glad to find out my axle weights are within 210# side to side so I didn't need to shift "stuff" around. Of course the "stuff" that would need to be shifted is me.

[tbutler]

From Brett's reference, download the RV Tire Guide. On Page 19 is a table for 22.5" tires. Find yours in the column on the left side of the page. Read across the table (lbs single for front tires, lbs dual for rear tires) to the weight that is greater than the weight that Tireman9 has suggested for you. Read up to the top of the table for the pressure in pounds (or other units). The table for dual tires gives the combined weight for two tires so use 2 x 6386 = 12,772.

As I read the table, it looks like you could use 115 pounds for the tires both front and rear. The rating for 115 pounds has a nice cushion (Single Tire at 7130 and Dual Tire at 13,050) over the weight you will be supporting but if you are especially cautious you could add 5% to the above which would be 6 pounds but that would take you over the 120 pound limit so I would set the pressure no more than two pounds above the 115 from the table.

Be aware also that your wheel rims have a pressure limit. On my coach that limit happens to be 120 pounds also. Check yours to be sure you aren't exceeding their limit. All limits are cold (before driving) tire pressure. The pressure will rise when driving and that is taken into account when setting the limits.

You didn't specify the condition of your coach when it was weighed. As Jim mentions, you should know the maximum weight of your coach with its normal load of clothes and equipment as well as full water, fuel and propane. For the rear axle it is wise to factor in full waste tanks. Using those weights you can be assured that your coach will be within the limits for the tires even with tanks full.

[andyshane]

From Brett's reference, download the RV Tire Guide. On Page 19 is a table for 22.5" tires. Find yours in the column on the left side of the page. Read across the table (lbs single for front tires, lbs dual for rear tires) to the weight that is greater than the weight that Tireman9 has suggested for you. Read up to the top of the table for the pressure in pounds (or other units). The table for dual tires gives the combined weight for two tires so use 2 x 6386 = 12,772.

As I read the table...

Tom and Louise are right on; plus, don't fret if your TPMS shows a higher pressure rolling down the road. The table inflation value is usually assumed to be at 70F. Unless you like to drive with Missus jogging alongside, your temps enroute will be way higher than that, around 130F. If they run higher, a few PSI added should bring them back down (don't fill to more than the cold sidewall limit, with cool tires).

[wolfe10]

Andy,

Slight correction. Cold vis a vie tire temperature for setting PSI is NOT, repeat NOT at a given temperature.

Instead, it is defined as "temperature before driving". It could be 25 degrees or 100 degrees. Going from summer temperatures to winter temperatures, you absolutely WILL need to add air to have the correct PSI.

And, as you state, the tire engineers are fully aware of temperature and PSI rise with driving and that is why all their pressure recommendations are "at current temperatures before driving".

[hsantucci]

All RV's manufacturers list the tire pressure for each RV. Do not go by Michelin spec's because the coach may have different loads.

Should be a placard on the side of the drivers area that states the proper tire pressure.

Harry Santucci

[wolfe10]

Harry,

Welcome to the FMCA Forum.

To clarify, the GVWR sticker in each motorhome by law does NOT state the correct PSI. It states the correct PSI if EACH AXLE IS LOADED TO ITS GAWR (Gross Axle Weight Rating).

Hopefully, a coach is not at the top of its weight rating. If not, THAT is when you should go to your tire manufacturer's inflation table to determine the correct minimum PSI for your coach's actual load. Best is to use the heavier wheel position on each axle to go to the table, with all tires on an axle having the same PSI based on the heavier wheel position.

[tireman9]

Tom and Louise are right on; plus, don't fret if your TPMS shows a higher pressure rolling down the road. The table inflation value is usually assumed to be at 70F. Unless you like to drive with Missus jogging alongside, your temps enroute will be way higher than that, around 130F. If they run higher, a few PSI added should bring them back down (don't fill to more than the cold sidewall limit, with cool tires).

Andy,

I need to point out that the pressure in the tables is NOT to be adjusted to a theoretical "Laboratory Standard" 70°F. The Load/Inflation charts are based on inflating a tire that is at ambient temperature. This means it has not been driven on more than a few minutes and has not been parked in the Sun for more than a minute or two.

In general it is suggested to check tires in the morning before travel. If you are parked so tires on one side are in full morning sun then maybe you need to plan on checking them the night before say 3 hours AFTER sundown.

I have many many posts on my blog on temperature & pressure. Too many to post here so it may be easiest to add a review of those posts on your "To-Do" list.