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Old 08-17-2015, 06:53 PM   #1
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Totally confused about tire pressure recommendations

I have a new Winnebego 27n on a Ford F53 Chassis
The vehicle has Goodyear G670/RV 245/70 R19.5 Tires

I had the vehicle weighed with a full tank of water and 1/2 tank of gas. I could only do steer/drive weights.

The results were: 6020 Front
9900 Drive

I took the rig on a 50 mile trip and it was quite the handful. Lots of tail-wagging and I was pushed by small vehicles passing me on the left. (I have done the CHF)

Looking at the Goodyear Tire Tables their lowest recommended pressure for the single tire is 80psi for 3640 pounds and 80 psi for the duals at 3415 lbs.

Based on the weights of my rig, I am below their rated numbers on each corner. The tires were all at 85 psi

Any idea where they should be set?

Thanks for any suggestions (Except "Buy a Diesel Pusher")

Bill
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Old 08-17-2015, 07:07 PM   #2
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Never go below the minimum shown in the charts. So, you could go down to 80 at the least but then you should also run 5 psi over the pressure given just in case the pressure goes down before you check it again. So, that puts you back at 85 all the way around if you read the chart correctly which I didn't check.
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Old 08-17-2015, 08:03 PM   #3
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The 85 psi is fine.

On to the next problem!
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Old 08-17-2015, 08:27 PM   #4
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Use the TPs stated on the tire pressure placard on the driver's side door sill. That pressure is for the maximum vehicle weight. More is not better.
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Old 08-18-2015, 09:10 PM   #5
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Quote:
Originally Posted by Old Crows View Post
Use the TPs stated on the tire pressure placard on the driver's side door sill. That pressure is for the maximum vehicle weight. More is not better.
That is what the RMA =Rubber Manufucturers Association states too.
For Superlucky_ Here is the complete pdf, CH 4 pertains to MH's, pg 51 has tire inflation information.
Michelin, Goodyear, and other websites contain the same information.
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Old 08-18-2015, 09:43 PM   #6
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Its never a good idea to run air pressure as low as you can. Low pressure makes them run hotter. Heat is what causes tires to fail (blow out). Look up the term "zipper rupture" Definitely always run the max on the sidewall. Those tires should be 14 ply. Should be 110psi If I remember correctly but double check. The ONLY downfall is a stiffer ride. Small trade off if you ask me when the alternative could be a footballing RV.

Your squirmy feel could be sidewall deflection from low pressure.

I'm new to the RV owning world but Ive been mounting commercial truck and passenger car tires for over 15 years. Ive seen my share of tire blow-outs due to under-inflation.
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Old 08-18-2015, 10:07 PM   #7
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Do NOT run the max on the sidewall. That's the ultimate pressure the tire can take, with a safety factor, without a pressure vessel failure. t will cause excessive wear in the center of the tread due to deformation. Weigh the rig (front only, back only, and tag only if you have one). Then inflate to either the minimum recommended or the correct pressure for the load being carried by each tire.
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Old 08-18-2015, 10:49 PM   #8
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Originally Posted by frankdamp View Post
Do NOT run the max on the sidewall. That's the ultimate pressure the tire can take, with a safety factor, without a pressure vessel failure. t will cause excessive wear in the center of the tread due to deformation. Weigh the rig (front only, back only, and tag only if you have one). Then inflate to either the minimum recommended or the correct pressure for the load being carried by each tire.
Sorry but the pressure on the tire sidewall is NOT the ultimate pressure the tire can take but is the pressure needed to carry the load that is also molded on the tire sidewall.

Tires are designed with a significant margin such that over pressure from operation will not result in a tire failure unless the tire has been damaged.

While the cold inflation pressure should always be no less than the inflation needed to carry the measured load it is a good idea to run about 10% more than the minimum needed so you aren't chasing your tail every time the temperature drops which would result in an under-inflated tire.

Center wear can occur if you run significant over inflation (maybe 25% high or greater) but in reality center wear is almost never a concern for RV application as the tires will most likely age out before they wear out.
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Old 08-19-2015, 12:22 AM   #9
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Originally Posted by frankdamp View Post
Do NOT run the max on the sidewall. That's the ultimate pressure the tire can take, with a safety factor, without a pressure vessel failure. t will cause excessive wear in the center of the tread due to deformation. Weigh the rig (front only, back only, and tag only if you have one). Then inflate to either the minimum recommended or the correct pressure for the load being carried by each tire.
This is precisely why I tell people to take internet forum advice with a grain of salt. Too many misinformed people dishing out "facts"
The sidewall pressure is meant to be set at cold. Allowing for pressure raise and drop with temp raise and drop. The tire is manufactured to be able to withstand the heat and pressure variation without failure.
Setting at minimum possible pressure is askingbegging for a blowout. Its betting your tires that you can run them at their stress limit and they'll be ok.

14 ply truck tires don't bulge in the center like an over inflated 4 ply car tire. So premature, unusual wear is not a concern. And on that subject, Most passenger car tires sidewall at about 35psi. They would need a constant life of over-inflation by at least 25% before you would see any center tread wear.
But nobody ever sees that other than the chart hanging on the wall at the tire shop. Because nobody ever runs over-inflated. Most people used to run under-inflated. That's what prompted the federal mandate of TPMS in all passenger cars starting in 2007. The amount of blowouts and catastrophic failures due to under-inflation has dropped significantly since then.
OP: Call any truck shop you like. Call multiple. They will all tell you inflate to max sidewall pressure. Because its right.
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Old 08-19-2015, 12:33 AM   #10
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Quote:
Originally Posted by Bill n Karen View Post
This is precisely why I tell people to take internet forum advice with a grain of salt. Too many misinformed people dishing out "facts"

OP: Call any truck shop you like. Call multiple. They will all tell you inflate to max sidewall pressure. Because its right.
You are correct about too many misinformed people dishing out "facts". Please read what Tireman said above and what I quote below

The pressure on the sidewall of a Michelin RV tire and many others is not the "Maximum" the tire should ever have (unlike car tires) it is the minimum to support the maximum rated carrying capacity of the tire.

From the Michelin RV Tire Guide:
Quote:
"If you look at the tire's sidewall, you'll see the maximum load capacity allowed for the size tire and load rating, and the minimum cold air inflation needed to carry the maximum load."
From page 6 of the GoodYear RV Tire and Care Guide:
Quote:
"How much air is enough?
The proper air inflation for your tires depends on how much your fully loaded RV or trailer weighs. Look at the sidewall of your RV tire and you’ll see the maximum load capacity for the tire size and load rating, as well as the minimum cold air inflation, needed to carry that maximum load."
From TOYO:
Quote:
Q: What are the consequences of inflating the tires to accommodate the actual loads?
A: If the inflation pressure corresponds to the actual tire load according to the tire manufacturer’s load and pressure table, the tire will be running at 100% of its rated load at that pressure. This practice may not provide sufficient safety margin. Any air pressure loss below the minimum required to carry the load can result in eventual tire failure.
But then they go ahead and publish a weight/pressure chart allowing lower pressure for RV's!!

From the August 2010 Motorhome Magazine "Tread Carefully" tire article:
Quote:
The maximum load capacity allowed for the size tire and load rating and the minimum cold air inflation needed to carry that maximum load are located on the tire’s sidewall.
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Old 08-19-2015, 12:38 AM   #11
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Totally confused about tire pressure recommendations

Well I don't agree with max pressure and I certainly don't agree with min pressure but I do agree with using your weight to determine the appropriate pressure. With that +10% as already stated is advised.

Forgot to add;

Here is Goodyears site regarding your tires. Do as THEY say when come to inflation.

http://www.goodyearrvtires.com/tire-...n-loading.aspx


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Old 08-19-2015, 07:54 AM   #12
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Originally Posted by Bill n Karen View Post
This is precisely why I tell people to take internet forum advice with a grain of salt. Too many misinformed people dishing out "facts"
The sidewall pressure is meant to be set at cold. Allowing for pressure raise and drop with temp raise and drop. The tire is manufactured to be able to withstand the heat and pressure variation without failure.
Setting at minimum possible pressure is askingbegging for a blowout. Its betting your tires that you can run them at their stress limit and they'll be ok.

14 ply truck tires don't bulge in the center like an over inflated 4 ply car tire. So premature, unusual wear is not a concern. And on that subject, Most passenger car tires sidewall at about 35psi. They would need a constant life of over-inflation by at least 25% before you would see any center tread wear.
But nobody ever sees that other than the chart hanging on the wall at the tire shop. Because nobody ever runs over-inflated. Most people used to run under-inflated. That's what prompted the federal mandate of TPMS in all passenger cars starting in 2007. The amount of blowouts and catastrophic failures due to under-inflation has dropped significantly since then.
OP: Call any truck shop you like. Call multiple. They will all tell you inflate to max sidewall pressure. Because its right.
You equate running what the tire inflation charts say to "under inflated". That is wrong on so many counts.

Yes, running under inflated is inviting tire trouble.

No, running what the charts say for your weight is NOT under inflated!
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Old 08-19-2015, 08:14 AM   #13
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As I had mentioned in my original post, I did use the Goodyear table to determine correct pressure. My sticking point was that both axles did not weigh the "minimum" weight used in the chart.

The minimum PSI listed was for a weight that was much higher than my weights.

I will start with 80 (lowest PSI listed) and start playing around with it.

Thanks for all of the support.

Bill
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Old 08-19-2015, 10:46 AM   #14
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Well, opinion corrected. What I said was what a tire shop told me a few years ago.
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