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Old 10-14-2012, 09:29 PM   #1
Join Date: Jun 2012
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Help me understand calculating tire pressure

I weighed my rig at a truck scale today.
Front = 8900
Rear = 17380
Dingy = 3760

Does 26280 for the coach sound about right for weight?
The rig is a 39' Fleetwood Discovery. The tires are Goodyear-G670 275/70R22.5
GVWR = 31,000
Front GAWR = 12,000
Rear GAWR = 19,000
Base = 24,000

It was a very light load as we only went out for the weekend. I was down about 35-gal of diesel. Fresh tank was full. Black and grey were empty.

Do i understand this correctly?
Front weight divided by 2 equals 4450 per tire.
Rear weight divided by 4 equals 4345 per tire.

Goodyears tire chart

According to Goodyears tire chart I am well below the lowest pressure rating of 85psi which has max weight of 5170 for single and 4770 for dual tires.

Does this mean that I should use 85psi F&R? Is this the optimum psi or the minimum? What would the optimum be?

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Old 10-14-2012, 10:39 PM   #2
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The 85 psi is the minimum you should run, you can go higher as some do. Some will run 85 to 90 psi to make up for difference in axle end loading (one side heavier than the other) plus a few psi to allow for slight leakage.

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Old 10-14-2012, 10:48 PM   #3
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You are reading the chart correctly.
85 psi , is the minimum to carry, optimum gets a little complicated.
Have you driven the rig with higher pressures ?
If 85 gives a sloppy ride then you want to go up , personally I'd start at 95 frt 90 rear, and consider how it handles, with a full tank and every thing on board for a trip. then re-weigh.
Lower pressure may give a smother , less bumpy ride, but the trade off can be faster tire wear and poor fuel mileage. Like I say complicated.
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Old 10-15-2012, 07:06 AM   #4
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We are full-timers and moved from a TT, to a new-to-us MH about a year ago. In that time I learned a few things about proper tire pressure that I was not aware of. This may, or may not, help you out... and I'm sure others have differing ideas...

I've had my coach weighed three times in the last year. The first weighing was at one of the local truck scales, and the other two were at an Escapee RV Club Smart Weigh station.

The truck stop scale provided overal weight, as well as, front/rear weight split. I was under-weight overall, and under-weight on both front and rear axles. I divided the overall axle weight in half, and inflated the tires as per the inflation table. I thought that was all that was needed.

The Escapee program weighed the coach at each wheel point individually.
The first time Escapees weighed the coach, I was again underweight overall and on both axles. However, it was interesting to note how the weight was distributed left/right. I was considerably heavier on the curb side of the coach. In retrospect, that should not have been surprising, since I have more storage bay space on the curb than on the street side. Checking the inflation table, I realized I was under inflating my tires on the curbside, and adjusted accordingly.

The third weighing was again at an Escapees Smartweigh Program. I had gotten rid of some stuff, and moved heavier items to the roadside. I was now more evenly loaded left to right, and was actually able to decrease tire pressures a bit from previous pressures (explained below).

It was explained to me that under-inflating tires increases tire temperature while traveling. This increase in temperature can increases tire wear, can cause the tire to deteriorate prematurely, and can potentially cause blow-outs at speed.

Escapees instructed me to take the weight of the most loaded tire on an axle (regardless of side of axle), find the corresponding pressure on the inflation table, and inflate ALL tires on that axle to that pressure. This properly inflates the most loaded tire (on an axle), and gives you a safety margin on the other tires (on that axle).

Based on what I learned about the potential negative effects of under-inflation, I would rather err on the side of slight over inflation, as opposed to under-inflation.

As with anything, your opinion and/or results may differ from mine.
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Old 10-15-2012, 02:29 PM   #5
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Your figures are correct. At 85 psi, your side to side loading could be off by 700# front and 400# in the rear and still be safe. I have the same tires and 85 psi is also the minimum pressure for both axles. As Skip discussed, by increasing pressure to 90 psi, my ride was more stable, I increased the margin for temperature and weight fluctuations, and no adverse tire wear. I did not notice any decrease in ride comfort.
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Old 10-15-2012, 03:45 PM   #6
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Again, you are reading correctly.

At 85psi front, you have about 700lbs per tire 'safety margin'.

90psi rear would give you the same 700lbs per tire extra.

I would run 85/90 psi..

As always, when you finally load up for an extended trip (or after a few years of 'accumulation') reweigh the coach. They tend to grow heavier as the years go - just as we do
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Old 10-15-2012, 04:21 PM   #7
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Thanks for the replies. Why is it that the placard in the coach says 120-psi? I assume that is at GVWR but that does not correlate with the tire chart.

I have been at 105-psi in all 4 and the ride has been fine. I think i will drop it to 95 and see how it rides.
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Old 10-15-2012, 06:16 PM   #8
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Placards were revised at some point to show maximum allowed pressure. (CYA or Gov't regs???) The tire max is 125psi but the rims are probably 120psi max and that is the number used on the placard.

My rig has 8000# front axle and 15000# rear axle (GAWR) but only a 22,000 GVWR because the Allison tranny is rated at 22K.GVWR, but has a 26,000 GCWR
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Old 10-29-2012, 12:53 PM   #9
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I came to this topic , googling for tire-pressure.
Once got hold of the formula that the tyre-makers in Europe use to calculate the needed pressure for a sertain load, and went running with it.
Made spreadsheets for it and translated a few from Dutch to English to go worldwide with it. Came to know by collecting information trough many fora, that this formula was not that holy as I declared it in the beginning, and Especially in America a wrong system is used , wich leads to to low pressures and so to much deflection of the tire for lower pressures.

Also on this topic, there is a link to such a wrong list.
If you see different load-letters in one list, you can sertainly say this list is wrong. I did not even checked it .

Other thing is that I calculate a high pressure wich leaves reserve for over/unequall loading, pressureloss in time, misreadings of load and pressure-scales, and what I forgot.
But if you go to high , comfort and gripp gets lost.
So for that I introduced the load% , wich is, what the real weight is of the weight you calculate the pressure for.
Under 85% L% discomfort begins by bouncing, under 80% screws come loose , so this can be used for trailers/caravans.
Over 100% tire-damage begins at the maximum speed of tire, or if lower 99m/h.
Here a link to my public map of skydrive, wich belongs to my hotmail-adress with the same username as here( so if you want to mail me combine yourselfes, spamm robots cant).
See if I can in my first post
H.Huester did some good weighing, so he can calculate with my spreadsheet , first for the first weighed axle-loads with front L% of 95% and behind L% of 85%. then see if it would have lead to to low pressures for the 4 point weighing he did later. Doing this you can yudge the savety of my calculations.
for this you can use the "pressurecalculationwithtemp"spreadsheet in the head-map somewhere in the middle.
To use it , first download it by rightclicking on it then choose for Download.
After download and eventual viruscheck, open it in Excell or compatible programm to use it ( open office Calc fi) , In the browser web app wont work, it caant handle sheet-protection and data-validation I used in most spreadsheets.
In the map "All about tire pressure" several "myownformula"spreadsheets. wich produce a list the same as the tire-makers give , and the different used systems are explained.

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