Tire Pressure

[2012sam]

I'm very confused with all the weights and tire pressures. I'm running the Michelin XZA3 275/80R 22.5 LRH, I had it weighed yesterday for the first time and all they were able to give me was steer axle 11680 lbs, drive axle 22940 lbs (includes tag), then had me back up to weigh my tag that came in at 5800 lbs by itself. Side walls tell me to run at 120 psi and I have looked on line and found charts that tell me to run anywhere from 120 to 80 psi. What do you seasoned RVer's run at ? Sorry and confused and thanks for the help...

[RVerOnTheMove]

The weighing that you have gotten is an OK start just to ensure that your coach is not overloaded, but it's really not the information you need to know if everything is OK. You need to get each tire location weighed separately. That's the R and L front tire (axle) weight, the R and L drive axle weight and the R and L tag axle weight. With this information you can go to the Michelin website and get the proper setting for your tires based upon the heaviest weight on either side. It will also give you the information necessary to see if you need to move some of your stored equipment from front to back or side to side.

If you find a good RV weighing service such as RVSEF, they will do the weighing, explain the results and tell you exactly what the inflation should be at each axle. Despite one side being heavier than the other, you will always run the same pressure on the tires on the same axle.

[rpasetto]

If you can get your coach weighed by wheel position, like RVSEF does with their portable scales, it'll be best. Few coaches are evenly balanced, so it's not a good idea to assume that your wheel position weights are half your axle weight; some will differ by 1000# or more. The safest thing to do until that time is to inflate tires to the max spec... but ride will be harsh. An alternative to the moveable wheel-position scales, is a weigh station which will let you "straddle" the scale so you can get a weight one wheel position at a time. Not all weigh station operators will want to do that; with some, it's impossible because of the way the scale is positioned.

[tireman9]

I agree that the Michelin tables can be a bit confusing as they are different than most other tables in that they show the load for the entire axle rather than the individual tire(s). But here is how I would work through the math (until you get RVSEF or similar company to give you the individual weights):

1. You need to add 5% to the measured load as you really are not 50/50 side to side balanced. Most rigs are within 5% but some can be 1,000# or more out of balance. So you should think Front axle 12,264 Rear Duals 17,997 Tag 6,090#

2. Looking at the Michelin tables for your LR-H tire (careful as they show different for LR_G) we find:

Fronts (single) axle rating of 12,380# at 100 psi cold

Rear (dual) axle rating of 18,160# at 85 psi cold

Tag (single) axle rating of 6,090# at 75

3. The above pressures should be considered your MINIMUMS but I suggest all RV motorhome users run an extra 10% so you don't have to be chasing inflation if you run into a cold front and the pressure drops of if you have a really big Walmart shopping trip. So you should inflate your tires to at least 110, 94 round to 95, and 83 round to 85 for the tag.

4. Be sure to have a good digital gauge and stop at a Michelin tire dealer and check your gauge against their master gauge. (If they don't have a master gauge then find another dealer) Your gauge should be +/- 2% or less and most digitals are +/- 1%.

NOTE When I do gauge calibration checks at the rallies I attend I find more than 10% of the gauges are off by more than 10% so they usually end up in the trash.

If you want to learn more about tires and can't make the FMCA convention in Indy you might want to read my blog from the beginning.

[gdroke]

I'm still a bit confused about tire pressures. Once you have determined the correct pressure for the tires and you are checking them with an accurate tire gauge, when should the tire be at that pressure? I know that sounds weird, but here is why I ask.

I set the pressure when the tire is cold, before I leave in the morning, and then check them at every stop, Obviously, the pressure is higher after driving, due to heat. Am I supposed to let air out at that point to get them back to the correct pressure? If so, that could mean I am adding or removing air at every stop, due to temperature variances. Is that correct?

Additionally, in the morning, when the coach is oriented north/south, with one side perpendicular to the sun rising in the east, the outside tire of the dual tire setup will have a higher pressure than the inside tire, because the inside tire has been shaded, and the outside tire is absorbing heat from direct sunlight. On top of that discrepancy, both dual tires on the east side of the coach will be at a slightly higher pressure than the 2 dual tires on the west side of the coach. Now I've got 3 different air pressures because of how the temperature has affected them. How do you possibly compensate for that condition? If I try to, won't they be wrong again almost immediately when I drive the coach and the "tire-flexing heat" brings the temperature of all of them up?

I guess part of my problem is that my new, highly accurate tire guage is smarter than I am.

[wolfe10]

gddroke,

Set tire pressure cold (i.e. before driving). Do NOT let air out as your PSI rises with driving and higher temperatures. The tire manufacturers are very much aware of the increase due to heat and their inflation tables take that into consideration.

And, as you point out, if a tire is in the sun, PSI will and should be higher, just as if it had been driven. Cold would have the asterisk of "in the shade". So, on tires in the sun, indeed, they should be set a few PSI higher.

Easy to OVER-THINK this. It is really not that complex.

Brett

[gdroke]

Simple enough, thank you.