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Old 10-17-2014, 11:57 PM   #15
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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.
Quote:
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."
Inflation Pressure Safety Margin
Toyo Tire does not recommend an “inflate-to-the-load” policy for RV tires. Tires that are inflated to accommodate the vehicle’s actual loads do not have any inflation safety margin. Consequently, even a minor loss of air pressure will cause the tires to be under-inflated and overloaded. Toyo Tire’s policy is to observe (as a minimum) the tire pressure established by the vehicle manufacturer as indicated on the tire information placard. There are multiple reasons why a safety margin
(by inflation) makes sense:
• All tires lose about 1-1.5 PSI per month due to natural permeation of the tire’s internal air pressure through the tire’s rubber membrane.
• In the event of slow air leaks from punctures, an inflation “reserve” may allow detection and repair of the leak prior to reaching a dangerously low inflation level.
• A safety margin is prudent for users who are apathetic regarding tire inflation maintenance.
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.
[/QUOTE]
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Old 10-18-2014, 11:54 AM   #16
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Originally Posted by 2-lane View Post
Tireman9...Recently purchased Samson GL283A 275/70R-22.5 H rated 16 ply. Load index is 144/141, or 6175 single 5675 dual @ 120psi. Every chart I could find all listed a H rated tire at 146/144 or 6610/6175. It seems the load index for the Samsons are more of a G rated, or 14 ply, that have max @ 110psi. My RV is in the neighborhood of 26,000 pounds. These tires @ 120psi are rated at 35,000 pounds. Is there some kind of formula? I've looked everywhere I can think of on the web, but nothing.
OK looks like you are getting info from a variety of sources and possibly mixing part info from one with part from another.

Lets get the info organized and confirmed and I think you will arrive at the correct solution.

1. Read the tire sidewall again. Confirm the words "Load Range H" or "LR-H"
2. Read the info on your tire Placard.
3. Stop using the number of ply. Radial tires have never to my knowledge had the actual number of body ply represented by the number associated with "Ply Rating". In the 1930 -50's tires may have had the actual number but after WW2 with the introduction of stronger materials it was soon discovered that fewer layers of material was needed to achieve the same level of strength. Simply put which is stronger a single strand of steel rated at 1000# or ten strands of cotton rated at 100# each?
4. Get your FULLY LOADED RV on a scale. Ideally you follow the worksheet on THIS page but as a minimum get the total for each axle. If you do not get the individual ends of each axle measured you should assume a side to side un-balance of 45/55 untill you learn the real un-balance. Side to side differences of 1000# are not unusual.
5. Using the 55% figure and the Load Inflation charts (Ideally from your tire MFG but if you look at GY, Bridgestone, Firestone and Michelin you will find all 4 the same or at least 3 of the 4 are identical.
6. Be sure to use the "Single" load for front tires and "Dual" for your rear paired tires. The "Formula" is very complex and that is why there are published tables that are identical across tire companies for 97+% of tires.
7. Once you have used the tables to learn the MINIMUM cold inflation needed to carry the load you simply increase that inflation by 10% to give yourself a "Safety Margin".
8 Get and use a TPMS. Too many times I read of people ending up loosing one or two tires at costs that are two to four times higher than the cost of the TPMS system.
9 Get and use good digital pressure gauges at least once a month. This will confirm the TPMS is still working properly. Use the hand gauge as the primary tool in setting pressure as TPMS can be off by a few psi. Once you get use to using a TPMS you will quickly learn the range of pressures to expect from the individual tires. The readings will NOT be identical and this is normal.
10 Read the posts in the links above
11. Put on your "to-do" list reading the other related posts on my Blog over the next few weeks. With 160+ posts I sometimes feel I must have covered every topic twice but as long as I continue to get questions I guess I need to keep posting.

Hope this information helps clear up your confusion
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Old 10-18-2014, 12:10 PM   #17
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In the absence of an inflation chart, use the max load value on the sidewall. You can probably safely go under that by maybe 10-15%, i.e. assume that 10% less psi reduces the load capacity by 10%, but that's just guesswork. Educated guesswork, but still a guess. At 20% you are starting to get risky because all tires have a minimum safe inflation, regardless of loading, and you don't want to get anywhere near that.

If you can find another tire with the same max load and psi rating, you could probably use that inflation table as well, but again, that would be taking a risk. Probably a small one, but no way to know for sure.

Tire dealers have shown to be a poor source of tire pressure info - they tend to use a one psi fits all approach, either a psi they got out of the air ("we always use xxx") or the max load value shown on the sidewall.
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Old 10-18-2014, 01:45 PM   #18
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This is a worst case suggestion:

Everyone says "The tires say xx PSI max" this is simply not true

They say Max Load #### at max pressure of XX psi

Cruise around to other tire maners and see if someone else makes a tire of the same Max load at the same max pressure.

Hopefully you will find 2 or 3 that do and which have charts

hopefully the charts will all agree at the pressure for lighter loads.

And if they do.. USE IT. plus 5PSI.

Best suggestion I can make, IS to contact Cuss-some-More support at the tire maker's web site and ask them however.. .If they can not answer your question ... Next time you buy tires.. BUY some brand that can.
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Old 10-18-2014, 02:14 PM   #19
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Originally Posted by Gary RVRoamer View Post
In the absence of an inflation chart, use the max load value on the sidewall. You can probably safely go under that by maybe 10-15%, i.e. assume that 10% less psi reduces the load capacity by 10%, but that's just guesswork. Educated guesswork, but still a guess. At 20% you are starting to get risky because all tires have a minimum safe inflation, regardless of loading, and you don't want to get anywhere near that.

If you can find another tire with the same max load and psi rating, you could probably use that inflation table as well, but again, that would be taking a risk. Probably a small one, but no way to know for sure.

Tire dealers have shown to be a poor source of tire pressure info - they tend to use a one psi fits all approach, either a psi they got out of the air ("we always use xxx") or the max load value shown on the sidewall.
Not sure what you are saying here.
You do not need to take an "educated" guess. Simply look at published Load inflation charts from the major tire companies. Since it is basically the air pressure & air volume (tire size) that establishes the load capacity of a tire you use a chart. Do not guess.

"use the max load on the tire and go under than by 10%" What is the goal here. Are you saying there is a minimum load that must be on a tire?

"assume that 10% less psi reduces the load capacity by 10%, but that's just guesswork" Why guess? BTW the relationship is not linear so 10% air is not = 10% load capacity.

The minimum inflation published in the tables is the minimum recommended inflation unless the tire MFG provides specific information otherwise.

Bottom line is we have learned a bit of a lesson here. The decision on what brand to buy was probably price driven and now we are seeing that low cost tire dealers & companies do not provide the same service as found with larger companies. Here are some tips I suggest when shopping for your next tire brand.
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Old 10-18-2014, 03:18 PM   #20
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Thanks for replies...My RV placard calls for 95psi front and rear with 255/80R-22.5 tires @GVWR. PO replaced w/ 275/70R-22.5. Two newer on front, so replaced rears with same tires, Samson GL283A. I felt very secure on the Samsons, thats why I purchased four more. They don't match any of the charts I have looked at, so no chart to follow makes guess work. I did find this..dawsengineering.com. It is a pdf on tire pressures at reduced loads. Doing the math with this (just assuming weights for now, haven't weighed in yet) would show 9500lbs@front=92psi..1550lbs@rear=82psi, based on the 120psi 6175 single, 5675 dual. With the absence of an accurate inflation chart for Samsons, this might a good bet.
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Old 10-19-2014, 05:18 PM   #21
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Also note that the rim has limits!

Both for load and pressure.

They may have a stamp someplace.

Ours are 22.5 alcoa rims rated for 95 psi max.

So limited to what tire can carry weight at that psi.

Not all are the same but most are close enough to same.

Many folks fail to check rims...former employer had to change tires and rims on emergency equipment trailer after they discovered they overbuilt the capacity of the rims.

Axles were fine.
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Old 10-20-2014, 10:53 AM   #22
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Thanks for replies...My RV placard calls for 95psi front and rear with 255/80R-22.5 tires @GVWR. PO replaced w/ 275/70R-22.5. Two newer on front, so replaced rears with same tires, Samson GL283A. I felt very secure on the Samsons, thats why I purchased four more. They don't match any of the charts I have looked at, so no chart to follow makes guess work. I did find this..dawsengineering.com. It is a pdf on tire pressures at reduced loads. Doing the math with this (just assuming weights for now, haven't weighed in yet) would show 9500lbs@front=92psi..1550lbs@rear=82psi, based on the 120psi 6175 single, 5675 dual. With the absence of an accurate inflation chart for Samsons, this might a good bet.
Published tables are always better than someones personal formula.
I simply Googled 275/70R-22.5 load and found this info from Continental. You have to go down a couple pages to your size but the info is there.

I have to wonder about a tire company selling truck tires but not publishing a Load/Infl chart.
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Old 10-20-2014, 01:24 PM   #23
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The posted chart you suggested is the same as all that I have already seen. Not one has a load index 144/141 on a 275/70R-22.5 tire. They all show 148/144. The load index is the weight they carry at max psi 148=6610lbs, 144=6175. These inflation charts don't reflect the tires I have. As far as Daws Engineering goes, Dr John Daws PHD in engineering, has more than 20 years in the tire industry. His tire equation is more than "some guys personnel formula" Anyone can say look at the charts, but if its not there, then what? Daws engineering has an answer, not as easy as a chart, but maybe even more accurate.
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Old 10-21-2014, 12:12 PM   #24
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Tireman9
The posted chart you suggested is the same as all that I have already seen. Not one has a load index 144/141 on a 275/70R-22.5 tire. They all show 148/144. The load index is the weight they carry at max psi 148=6610lbs, 144=6175. These inflation charts don't reflect the tires I have. As far as Daws Engineering goes, Dr John Daws PHD in engineering, has more than 20 years in the tire industry. His tire equation is more than "some guys personnel formula" Anyone can say look at the charts, but if its not there, then what? Daws engineering has an answer, not as easy as a chart, but maybe even more accurate.
Interesting info. Dr.Daws may have developed his personal theories on load & inflation and I am sure that helps his clients when he is working for the lawyers. The only problem is that I see is that his formulas are not used by any of the major tire companies or US DOT. Wonder why.

Another observation. Wonder why Sampson rates their tire lower than the competition? Have you compared the dimensions of the tires as published by the mfg? It is possible that the Sampsons are actually smaller than the tires covered by the charts. Sometimes a tire company will take a mold that is close but of a different size and re-machine it in the bead area to make it for a different rim diameter. Like taking a 12.00-20 tube tipe and turn it into a 275/70R22.5. They then do the calculations for their end product.

I go back to my observation of wondering why a tire company who admittedly is making a non-standard tire doesn't make the Load Infl table available for their product.

Have you considered asking Dr. Daws why the Load Index would be different for Sampson than for other tires?
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Old 10-21-2014, 12:28 PM   #25
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I've looked everywhere I can think of on the web, but nothing.
2-lane
Ask the dealer that made his $$ by selling you tires for a "Samson tire pressure chart"....(or contact the tire manufacturer directly).

If there is "no such thing", IMO it's time to "bite the bullet" and buy some good tires.

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Old 10-23-2014, 08:16 PM   #26
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First off, my question was how to overcome the lack of an inflation chart. To respond "go look at an inflation chart" is no answer. Or to 'bite the bullet" and go buy good tires, that's a real doozy of a reply. Samsons are widely used around the world, and priced right. I have found a solution, just put 400 miles on them, so on to the next challenge.
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Old 10-23-2014, 08:45 PM   #27
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I checked the pressure today with a digital tire gauge at roughly 70-72.5 PSI (45 on passenger inside rear wheel and driver front at 82.5).
What am I reading wrong here?
One tire was 82.5 another was 45, and the rest were between 70-72.5 psi?

How often do you check your tires?????
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Old 10-24-2014, 07:58 AM   #28
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First off, my question was how to overcome the lack of an inflation chart. To respond "go look at an inflation chart" is no answer. Or to 'bite the bullet" and go buy good tires, that's a real doozy of a reply.
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You're welcome!
Happy to help.
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