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07-10-2014, 10:11 PM
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#1
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Senior Member
Join Date: Apr 2013
Posts: 240
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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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2014 Berkshire 390RB-60
2014 Jeep Wrangler Unlimited
Roadmaster Falcon A/T tow bar
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07-11-2014, 07:27 AM
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#2
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Senior Member
Join Date: Feb 2014
Location: Dallas TX
Posts: 1,675
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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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Dan
2014 Berkshire 390RB-60
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07-11-2014, 07:34 AM
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#3
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Senior Member
Join Date: Apr 2013
Posts: 240
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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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2014 Berkshire 390RB-60
2014 Jeep Wrangler Unlimited
Roadmaster Falcon A/T tow bar
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07-11-2014, 08:27 AM
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#4
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Senior Member
Join Date: Feb 2014
Location: Dallas TX
Posts: 1,675
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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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Dan
2014 Berkshire 390RB-60
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07-11-2014, 08:46 AM
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#5
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Senior Member
Join Date: Apr 2013
Posts: 240
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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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2014 Berkshire 390RB-60
2014 Jeep Wrangler Unlimited
Roadmaster Falcon A/T tow bar
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07-11-2014, 10:13 AM
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#6
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Senior Member
Join Date: Feb 2014
Location: Dallas TX
Posts: 1,675
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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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Dan
2014 Berkshire 390RB-60
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07-11-2014, 10:53 AM
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#7
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Senior Member
Join Date: Apr 2013
Posts: 240
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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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2014 Berkshire 390RB-60
2014 Jeep Wrangler Unlimited
Roadmaster Falcon A/T tow bar
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07-11-2014, 11:00 AM
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#8
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Senior Member
Join Date: Feb 2014
Location: Dallas TX
Posts: 1,675
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Where's all the experts when you need them?
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Dan
2014 Berkshire 390RB-60
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07-11-2014, 12:01 PM
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#9
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2014 360 QL
Join Date: Oct 2013
Location: gardendale texas
Posts: 153
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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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07-11-2014, 02:44 PM
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#10
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Senior Member
Join Date: Jun 2013
Location: Mount Laurel, NJ
Posts: 1,024
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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.
Hank
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07-11-2014, 03:08 PM
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#11
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Senior Member
Join Date: Apr 2008
Location: Long Island (Nassau County), NY
Posts: 4,352
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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.
Tom
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Tom and Margaret
2014 Berkshire 390bh-60
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07-11-2014, 05:38 PM
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#12
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Berkshire club
Join Date: Sep 2013
Location: Texas
Posts: 59
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hey guys check this out on CAT scale weighing your rigs
How To Weigh | CAT Scale
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2014 360QL-60 Mystic Garnet
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07-11-2014, 06:01 PM
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#13
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Senior Member
Join Date: Feb 2014
Location: Dallas TX
Posts: 1,675
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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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Dan
2014 Berkshire 390RB-60
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07-11-2014, 06:04 PM
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#14
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Senior Member
Join Date: Apr 2013
Posts: 240
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Quote:
Originally Posted by tmmar
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
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Absolutely.
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2014 Berkshire 390RB-60
2014 Jeep Wrangler Unlimited
Roadmaster Falcon A/T tow bar
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07-11-2014, 08:43 PM
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#15
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Senior Member
Join Date: Feb 2014
Location: Dallas TX
Posts: 1,675
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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
__________________
Dan
2014 Berkshire 390RB-60
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07-11-2014, 09:50 PM
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#16
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Senior Member
Join Date: Apr 2013
Posts: 240
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Quote:
Originally Posted by dan-nickie
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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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2014 Berkshire 390RB-60
2014 Jeep Wrangler Unlimited
Roadmaster Falcon A/T tow bar
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07-12-2014, 09:09 AM
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#17
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Senior Member
Join Date: Feb 2013
Posts: 418
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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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07-12-2014, 11:17 AM
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#18
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Senior Member
Join Date: Apr 2013
Posts: 240
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Quote:
Originally Posted by jadatis
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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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
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2014 Berkshire 390RB-60
2014 Jeep Wrangler Unlimited
Roadmaster Falcon A/T tow bar
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07-12-2014, 12:06 PM
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#19
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Senior Member
Join Date: Jul 2012
Location: SE Arizona's Gila Vally
Posts: 1,507
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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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2016 RAM 1500 - 2016 Keystone Laredo 265SRK
The road goes on forever and the party never ends.
Illegitimum non carborundum
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07-12-2014, 01:10 PM
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#20
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Senior Member
Join Date: Feb 2014
Location: Dallas TX
Posts: 1,675
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Quote:
Originally Posted by Gottawonder
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
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Pretty fancy spreadsheet.
And it's the first time I have see actual PSI instead of 5 lb increments.
Thanks.
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Dan
2014 Berkshire 390RB-60
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