Correct PSI

[gaylemarlowe]

In prep for our upcoming trip I always check the psi in the tires. As I had it weighed again just recently (not individual corner weights) I thought I would again go to the inflation charts to make sure I was okay. But, for some reason, when looking at the inflation charts, I seem to get a bit confused about weight capacities as they seem to indicate axle "end" weights vs axle total weights and single vs dual tires.
Hopefully Tireman or another knowledgeable person can help.

Front tires = Michelin X Coach HL 295/80/R22.5Z
Front axle weight= 12,900 lbs

Drive tires= Michelin X Coach HL 295/80/R22.5Z
Drive axle weight= 13,932 lbs

Tag tires= Michelin XZA2 Energy 295/80R22.5
Tag weight= 7840 lbs

The reason I had it weighed again was I just took it to the local truck dealer to have the steering box bracket welds inspected due to the recall by Spartan. Welds okay for now. The Cat weigh station is right across the road, so I figured a quick recheck was in order. The above weights are with most all traveling goodies in the coach, full fuel, but empty water and waste tanks as it is still not warm enough to put water in till we get south. I have not included any fudge factor for the water or waste tanks.

Thanks in advance
Gayle

[tbutler]

Given the weights you report, your front tires if split evenly between the two would have a load of 6,450 pounds each. Add 500 pounds because you don't have individual corner weights. The difference probably won't be that great, others may have a more conservative number, but you really don't want to come up short here. That would give you an empty weight per tire of 6,950 pounds. On the tire chart look at the weight capacity for single tires.

For the drive tires, 13,932 divided by 2 is 6,966 pounds on each side, add 500 again since you don't have corner weights. Figure an empty weight of 7,466 pounds for each side of the rear axle. Divide this by two as you are splitting this load by 2 tires on each side. That gives you an empty weight of 3,733 per drive tire. On the tire chart look at the weight capacity for dual tires. Dual tires don't carry as much because they don't cool as well as a single tire. Being close to the other hot tire will affect the ability of the tire to support a load at any given inflation.

Your tag axle weighs 7,840, again add 500 pounds for the lack of individual corner weights, 8,340 pounds divide by two gives 4,170. Again, use the single tire column in the tire chart for these tires.

Now, before you look up weights you need to calculate the weight of a full propane tank, a full fuel tank and a full fresh water tank. If your propane tank was full and your fuel tank was also full, you don't need to calculate, it is already included in your weights above. Water weighs 8.34 pounds per gallon. You can use the water weight for the waste tanks as well as the fresh water as the wastes are mostly water. Calculate the weight of each tank and add it to the weights above. Assign the axle for each weight based on where the tanks are located in your coach. In our coach the waste tanks are right in front of the rear axle and the fresh water tank is about 4 feet behind the front axle. So most of the weight of the waste tanks on our coach goes on the rear axle and most of the fresh water tank weight goes on the front axle. Since the total capacitie of the two waste tanks almost exactly equal the fresh water tank, I simply assign their weights to the closest axle.

You will have to make a determination how best to assign the weight of the tanks in your coach depending on the locations of each tank. If midway between axles, split weights between the two axles. If they are behind the rear axle or in front of the front axle, the calculations get more complicated as part of the weight in front of the front axle will be subtracted from the rear axle. Imagine the coach like a teeter-totter. Weight on one end lifts the other end. Each axle acts like a fulcrum with the weight being levered from one end of the coach to the other end. The tag axle creates another variable, the weight it carries can be varied and as it takes more weight it reduces weight on the drive axle and adds weight to the front axle. The level of complexity of this calculation is beyond what most of us are willing to do so we'll just keep in mind the general principle. My guess is that most motor homes have the tanks located between the two main axles.

Finally, you need to compute or estimate the weight of any personal items that are not in the coach, where they are stored and assign their weights to the front or rear axles. Loading personal items into the coach is where the 500 pounds added to the end of each axle comes in as these items my be stored anywhere in the coach and can create uneven weight distribution.

Now you are ready to look up the weight supported by each tire in the appropriate column on the table. If you figure on the maximum loading of your coach you should always have sufficient tire pressure.

The above is why I do my level best to get corner weights when the fuel, propane, fresh water and waste tanks are full and we are traveling with our usual load of personal items. If you can do that, there are no or minimal calculations to do and the uncertainty factor drops almost to zero. Well worth the fee charged at FMCA rallies where they offer this service. I get the coach corner weighed every time I'm at a rally where the service is offered. That gives me a history and thus an idea of how much our load varies under different circumstances over a period of years.

[tireman9]

Gayle

Reasonable question and you get extra "points" for paying attention to your actual tire loading.

I have no idea why Michelin decided to present their information http://www.michelinrvtires.com/reference-materials/load-and-inflation-tables/#/

on for the pair of tires when in dual position, other than to set their tires as special.

The Industry tables are published on individual tire capacity

I am confident that you can do the math even is some others aren't

I am glad you check your inflation before a trip but would be more confident in your safe travels if you said you use a TPMS.

Tom Butler's approach is a reasonable one and isn't much different in the end result as my normal suggestion.

[gaylemarlowe]

Thanks for the quick responses. It looks like I was reading the charts correctly, but was not sure. Now I can feel certain the coach is riding on properly inflated rubber.

Gayle