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Old 07-10-2014, 10:11 PM   #1
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Check my tire pressure math??

Some of you guys with later model rigs may have already done this exercise. I'm going to try to get coach weighed in a few days and get tire pressure optimum, and I'm working with a dash of "Cowboy Logic" here in the mean time:

The Stats
Tires Goodyear G670 275/70R22.5(H)
GVWR 31,000 lb
Front GAWR 12,000 lb
Rear GAWR 19,000 lb

Dry weight of my coach is 25179, does not affect calculations below:


So, assuming I load up and am near the axle weights above, and assuming a balanced left to right load, and using the Goodyear loading/inflation chart: http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

Then a minimum inflation of 90psi in the rear (each corner at 9500/2 = 4750 per tire) and 105psi in the front (6000 per tire) would cover me as per the chart, correct?

Then assuming a 5% variance from max axle weight side to side would make the front corners (instead of about 6000lb each) 5700 and 6300 and the rear corners (instead of 9500 each) 9025 and 9975 requiring a min of 110psi on the fronts (working off the heavier corner) and still require just over 90psi for the rears (9975/2 = 4987.5# vs. chart of 4980)

SO, even if I wanted to run up to 5psi over, assuming I wasn't seriously unbalanced side to side or overloaded on either axle, and if my exact rig weight was unknown, wouldn't 110 to 115 in the front and about 95 psi in the back be a good "safe zone?"

More to the point, have any of you found any situation in your setups with these tires that would make you want to put 100+psi to the rears?
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Old 07-11-2014, 07:27 AM   #2
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Timely question as I was going to be posting the same in a while.
One thing that puzzled me was the 2 lines of 275/70R22.5 on the chart.
The second one has (G159) whatever that means?
Which line are you using?
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Old 07-11-2014, 07:34 AM   #3
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I should have mentioned that initially. A quick G159 Goodyear google search indicates these were recalled production ceased in 2006. I used the other numbers.
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Old 07-11-2014, 08:27 AM   #4
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If I am reading it right, it seems with a Rear GAWR 19,000 lb. it would be mathematically impossible to 'require' 100 lb. in the rear tires?
Maybe there's another reason besides weight?
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Old 07-11-2014, 08:46 AM   #5
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Since you are going to air all tires on the axle based on the weight of the heavier corner, I would say yiu would have to be 10,780lbs on the heavier corner to require the 100psi minimum. Is that how you see it?

I wouldnt want to be 2000+ lb different side to side, so I would be trying to correct that before juicing all the tires.
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Old 07-11-2014, 10:13 AM   #6
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Gottawonder, I don't want to answer that as I'm new at this and trying to learn myself.
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Old 07-11-2014, 10:53 AM   #7
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No problem, but I think we're on the same page...more importantly I think it's the correct one
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Old 07-11-2014, 11:00 AM   #8
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Where's all the experts when you need them?
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Old 07-11-2014, 12:01 PM   #9
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I went and weight my rig loaded and the rear was 17880 lb and the front was 9620,,, just wanting to know,, what should the rear be,,, I have the same tires and do they have a updated chart that one is 2005
and I was looking and on the inside of the mh it has chart for air pressure and it has 120 psi for all the tires,,, should we use the tire chart or what fr. says
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Old 07-11-2014, 02:44 PM   #10
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po 185:
I suggest that you use the tire manufacturer's chart for your specific tires. Note that there is a separate rating for the dual tires and the front tires.

You should preferably get single wheel weighing's. It generally is difficult to find places to do that, but at major rallies, you can generally get that done for a song.

If you cannot get single wheel weighing's then go to a truck CAT scale. Weigh with your full cargo, normal water supply, full diesel and passengers.

Then use your tire manufacturer's charts and set the pressure with cool tires, like early in the morning.

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Old 07-11-2014, 03:08 PM   #11
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Also,
I heard from a tire expert who frequents some of the RV sites and he said that the tires on the same axle should have the same pressure. Even if one corner is heaver than the other, both tires should be pressurized the same.
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Old 07-11-2014, 05:38 PM   #12
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hey guys check this out on CAT scale weighing your rigs
How To Weigh | CAT Scale
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Old 07-11-2014, 06:01 PM   #13
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I have my actual weights from the Cat scales.
Here they are:
Steer Axle 10,080
Drive Axle 17,720
Gross Weight 27,800
http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

How would I calculate my tire pressure?
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Old 07-11-2014, 06:04 PM   #14
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Quote:
Originally Posted by tmmar View Post
Also,
I heard from a tire expert who frequents some of the RV sites and he said that the tires on the same axle should have the same pressure. Even if one corner is heaver than the other, both tires should be pressurized the same.
Tom
Absolutely.
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Old 07-11-2014, 08:43 PM   #15
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I have my actual weights from the Cat scales.

Here they are:
Steer Axle 10,080
Drive Axle 17,720
Gross Weight 27,800

Goodyear 275/70R22.5
http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf
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Old 07-11-2014, 09:50 PM   #16
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Quote:
Originally Posted by dan-nickie View Post
I have my actual weights from the Cat scales.

Here they are:
Steer Axle 10,080
Drive Axle 17,720
Gross Weight 27,800

Goodyear 275/70R22.5
http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf
Based on those you'd be good at a min of 85psi. Without weighing each corner you could bump the fronts 5psi to account for any left to right variance and another 5 for buffer.

Looks like the backs you could run at 90 to 95 and account for some left to right weight mismatch and still have some buffer.


I hope to get a 4 corner weight after I get the fam in this one...will definitely hit the scale to get a front & rear. Hopefully we won't have to have a flea market behind Flying J.

Per our discussion this morning, I tweaked my rears to about 101psi this morning and fronts to 109. My inner rear tires were about six pounds high; getting the 4 rears even made a noticeable difference.
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Old 07-12-2014, 09:09 AM   #17
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For these situations i made my multilingual motorhome tirepressure calculator, in wich I take a reserve to the given load or different when only GAWR's given .
Also I use a saver formula then the american tire makers use to make those pressure loadcapacity lists , and even a bit saver then the European lists.
It takes care that the deflection of the tire stays the same as when maximum load and AT-pressure on tire, wich is the goal of all calculations.

so if you weighed give the seperate wheel(pair) weights and I will calculate and give a picture of my filled in spreadsheet.

will make one in next post for your GAWR's, but first have to google your tires for the maximum load and AT-pressure.

If you want to play with the spreadsheet yourselfes,here the link to the map.
First download it to your computer by rightclicking and choosing Download .
After download and eventual virus check , open in Excel programme on your computer, but Open office Calc can handle it too.

https://onedrive.live.com/?cid=A526E...E092E6DC%21793
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Old 07-12-2014, 11:17 AM   #18
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Quote:
Originally Posted by jadatis View Post
For these situations i made my multilingual motorhome tirepressure calculator, in wich I take a reserve to the given load or different when only GAWR's given .
Also I use a saver formula then the american tire makers use to make those pressure loadcapacity lists , and even a bit saver then the European lists.
It takes care that the deflection of the tire stays the same as when maximum load and AT-pressure on tire, wich is the goal of all calculations.

so if you weighed give the seperate wheel(pair) weights and I will calculate and give a picture of my filled in spreadsheet.

will make one in next post for your GAWR's, but first have to google your tires for the maximum load and AT-pressure.

If you want to play with the spreadsheet yourselfes,here the link to the map.
First download it to your computer by rightclicking and choosing Download .
After download and eventual virus check , open in Excel programme on your computer, but Open office Calc can handle it too.

https://onedrive.live.com/?cid=A526E...E092E6DC%21793
Thanks for posting this. I saw your spreadsheet in another forum; that's a lot of labor.

Dan, I ran two with his spreadsheet. One is the "Part 1" based on the specs for our rigs, the second uses "part 2" using your axle weights. For your rig, the main difference i see in my allowances vs. his are he used 10% variance left to right using the actuals and also has an extra bump for ride control/comfort.

here are pdf's of the results:

Dan1
Dan2


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Old 07-12-2014, 12:06 PM   #19
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No math to it. Read the sidewall on the tire and air it to the posted max psi cold and you're good to go. I run these same tires on a F550 mechanics truck.
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Old 07-12-2014, 01:10 PM   #20
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Quote:
Originally Posted by Gottawonder View Post
Thanks for posting this. I saw your spreadsheet in another forum; that's a lot of labor.

Dan, I ran two with his spreadsheet. One is the "Part 1" based on the specs for our rigs, the second uses "part 2" using your axle weights. For your rig, the main difference i see in my allowances vs. his are he used 10% variance left to right using the actuals and also has an extra bump for ride control/comfort.

here are pdf's of the results:

Dan1
Dan2


Gottawonder

Pretty fancy spreadsheet.
And it's the first time I have see actual PSI instead of 5 lb increments.
Thanks.
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