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Old 10-10-2012, 05:31 PM   #1
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Questions about load/inflation chart

I downloaded a Load/Inflation sheet from Goodyear to fnd out what tire pressure I am supposed to be using on the MH. Next I weighed the MH with it full of fuel, propane, and fresh water, and loaded the way we normally travel.

The front axle weighed 12,050 lbs which would be 6025 lbs load per tire. My tires are the G670 series 295/80R22.5. According to the chart for single tires I should put 95 lbs of air in the front.

The rear axle weighed 20,880 lbs and divided by 4 wheels would be a load of 5220 lbs. per wheel. Going back to the chart and looking at dual wheels I would need 90 lbs. of air in each of the rear duals.

I have been running 110 lbs. in the front and 100 lbs. in the rear. I sure don't want to run them too low and risk ruining a tire. Does anyone see a problem with my calculations? I scanned the chart but can't figure out how to show it on here! Hopefully you'll see it below.

img128.pdf
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Old 10-10-2012, 05:48 PM   #2
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I see no problem with your calculations- but do keep in mind that the loads in the chart are static, and a moving vehicle isn't.

When cornering, the load on individual wheels can shift by a fair amount- the higher pressure that you're running now provides a certain amount of "redundancy" for those situations.

There's probably a tire inflation chart somewhere that takes cornering loads into account, but if so, I haven't been able to find it...
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Old 10-10-2012, 06:14 PM   #3
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Quote:
Originally Posted by Francesca View Post
There's probably a tire inflation chart somewhere that takes cornering loads into account, but if so, I haven't been able to find it...
The tire design and existion charts already take that into consideration.

Now, if you racing circle track cars or NASCAR tracks then you'd need to take that into consideration.
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Old 10-10-2012, 08:17 PM   #4
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Your calculations are right on. Since you weighed individual axles and not wheels, you might consider adding 5 psi to each to take that into account, and any slight inaccuracies you might have in your tire gauge. I'm sure you saw it, but allowable tire loading on tires mounted as dualies is slightly different from the same tire mounted as a slngle.
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Old 10-10-2012, 08:30 PM   #5
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The only shortcoming in your calculation is that motorhome axles rarely split the load exactly 50/50, i.e. one end is typically heavier than the other. I would allow for at 10% variation, meaning one end is carrying 60% of the load rather than 50%. That gives you a higher per wheel weight and consequently higher pressures. Adding 5 psi is another way to compensate for imbalance and other factors.
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Old 10-10-2012, 08:42 PM   #6
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From personal experience
At the start of my just completed 5500 mile trip
I left Dallas and drive about 100 miles up toward witchita falls.
I finally pulled over to check the tyre pressures and found 125 all around
I dropped the fronts to 100, the drives to 110 and the tag to about 95
Remember these are hot temps
The ride change was amazing.
The storage facility do the preflight on the owners behalf - batteries tyres fridge water etc.

So don't worry about adding a bit more for this and a bit more for that - the charts are accurate and your calcs are correct.
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Old 10-10-2012, 10:52 PM   #7
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Steve: Goodyear Tire Care Guide. Has the inflation tables and under
Inflation Control and Practices

"Inflation should be checked when tires are cold"
"Do NOT bleed air from hot tires."

( I think you feel the storage facility had over -inflated the tires but tire pressures increase as tire temps increase and the inflation tables consider this. Tires may be under inflated when they cool)

Enjoy the return to downunder, looking forward to your return.....
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Old 10-10-2012, 11:00 PM   #8
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"Cold" for tires is defined as not driven on overnight nor more than one mile.
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Old 10-10-2012, 11:03 PM   #9
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Quote:
Originally Posted by Gary RVRoamer View Post
The only shortcoming in your calculation is that motorhome axles rarely split the load exactly 50/50, i.e. one end is typically heavier than the other. I would allow for at 10% variation, meaning one end is carrying 60% of the load rather than 50%. That gives you a higher per wheel weight and consequently higher pressures. Adding 5 psi is another way to compensate for imbalance and other factors.
Which is why four "corner" weights are so important when setting/discussing tire pressures. Even adding a few psi for end to end differences is merely a guess.
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Old 10-10-2012, 11:07 PM   #10
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Point taken
I considered that the 100 psi hot was close to 90 psi cold - the temp I was aiming at
However I didn't then check the cold pressures
Other than a tyre kick as a prestart check I didnt consider checking the pressures over the next 8 weeks.
Be trip I'll do the pressures cold
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Old 10-10-2012, 11:40 PM   #11
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Steve: FWIW- You may not be that far off base. This article applies to passenger tires but calls for pressure about 6 psi above cold recommended pressure for tires operated for an extended period above 45mph...
Tire Tech Information - Air Pressure: When and How to Set

Glad you enjoyed your visit.....take care...
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Old 10-11-2012, 12:09 AM   #12
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my weights put me at a 5 pound split front to rear
so after many miles i keep all 6 at 100 psi
seems to work well for us
and its easier for me to keep up with

you did good weighing and reading the chart. thats half the battle
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Old 10-11-2012, 09:50 AM   #13
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In Phx, during the middle of summer, I have measured 195 on the sunny side tire sidewall. This answers a question nobody else has been able to answer. All I got before was: "Set them cold."

"*NOTE: Tires on a parked vehicle exposed to direct sunlight will appear overinflated due to the heat absorbed from the radiant energy of the sun. Pressures cannot be accurately set on these tires until all have stabilized in the shade."

If needed, that is the way I would have done it.
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Old 10-11-2012, 10:28 AM   #14
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Thanks to everyone for your comments. I could not find a scale around here that could weigh the four corners, only the front and back.
I think I will add 5 lbs to the chart recommendations and hope for the best!
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