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Old 06-05-2015, 04:20 PM   #29
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Originally Posted by Tireman9 View Post

Load Capacity = Air Pressure to a power x tire air volume x K
If I understand it you are saying Load capacity, generally quoted in pounds, equals Air pressure, generally quoted in pounds per square in. to a power, of unknown units of measure, times air volume, generally quoted in cubic ft., time a factor of unknown units of measurement.

Would you walk us through factoring out the different units to bring us to Load capacity POUNDS.

It used to be that both sides of an equation had to equal out in units of measurement used.
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Old 06-06-2015, 06:06 AM   #30
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Originally Posted by sheriff1 View Post
Now I am confused... The attached Maxxis table shows anything but max pressure according to the weight.
Max load on my axles is 3800 per axle (per Airstream) , I figure 2000 per tire (rounding up) , + 10% for uneven load , + 10% to go over 65mph puts me at about 2400 lbs. puts me at 60-65 lbs pressure. All good with a ST 225/75/15 LRD tire.
I have the tire shop recommended LRE tires with max pressure of 80 psi which I now know will beat the rivets out of our Airstream.
What pressure should I run?

http://ecx.images-amazon.com/images/I/71x0iLT9jAS.pdf


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Sherriff1,

You're going in the right direction. The whiole idea is to find the WORST condition and inflate for that!

Allow me to parse your post so you can see the steps:

Quote:
Originally Posted by sheriff1 View Post
.....Max load on my axles is 3800 per axle (per Airstream) , I figure 2000 per tire (rounding up) , + 10% for uneven load , + 10% to go over 65mph puts me at about 2400 lbs......
Since I have the floor, I will go ahead and quibble about a few things.

1) First, we don't know how well AS did on selecting the axles. It's quite possible they did a good job, but we know that some trailer manufacturers don't. So weighing the trailer - tire by tire, if possible - is the only way we can be sure.

2) I would use a 15% value for the uneven load, but that's just me.

3) 10% for over 65 mph, only puts you up to 75 mph. I think you should go the whole route up to 85 mph by adding another 10 psi.

4) I am of the opinion that tires shouldn't be selected to do more than 85% of their rated load.

So doing all of that, I come up with a bit more than 80 psi.


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Originally Posted by sheriff1 View Post
....I have the tire shop recommended LRE tires with max pressure of 80 psi which I now know will beat the rivets out of our Airstream.......
I think this is a great exaggeration. The spring rate of a tire is proportional to its inflation pressure. So 80 / 65 is 123% - less than 25% greater.
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Old 06-06-2015, 06:13 AM   #31
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Quote:
Originally Posted by HowieE View Post
If I understand it you are saying Load capacity, generally quoted in pounds, equals air pressure, generally quoted in pounds per square in. to a power, of unknown units of measure, times air volume, generally quoted in cubic ft., times a factor of unknown units of measurement.

Would you walk us through factoring out the different units to bring us to Load capacity POUNDS.

It used to be that both sides of an equation had to equal out in units of measurement used.
I am sure Roger could do that, but suffice it to say the units work out. I am sure that you could too if you thought about it for a bit. Think in terms of the K factor having the proper units to balance everything.
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Old 06-06-2015, 10:05 AM   #32
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I am sure Roger could do that, but suffice it to say the units work out. I am sure that you could too if you thought about it for a bit. Think in terms of the K factor having the proper units to balance everything.
That is an interesting approach. Just use what ever units fit the bill. There was a time when science followed some rules but in today's taught approximation math you can dial in what ever you need.

No wonder they need pictures on the cash registers at McDonald's for the kids to figure out the bill.

While I don't like that method used in our discussions I love it while shopping because I can pay what ever I want and the cashier usually has NO Idea.

You have been given 2 reasonable methods here of determining tire pressure for a give load. The manufactures inflation chart and the road temperature test for final adjustments. But I am sure there are many that will read a number on the tire sidewall or let a kid at the tire store do what ever he thinks is right.
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Old 06-06-2015, 02:11 PM   #33
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Gee guys I try and keep it reasonable. OK since you asked I can give an example Note. You should NOT do this calculation for your personal tires. The company that made your tires may have deviated from this formula for a number of reasons. If they did, the numbers in their published tables take precedence.

The formula is in metric units. There is of course rounding and then conversion to pounds and psi that make things more difficult.

For example for Metric Truck tires with nominal section less than 315 which were standardized 4/20/1989

For 60 series and larger
L=K x 6.075 x 10^-5 x p ^0.7 x Sd^1.1 x (Dr + Sd)

P is infl in Kpa
L is load in Kg
Sd = H(2-.78A-(.05/A)-.7(A-.55)^2
Dr is nominal rim diameter converted to mm using 25.4 rounded to nearest mm
A is Nominal Aspect ratio H/S.75
H = Section Height
K ranges from 0.64 to 1.088 depending on rim angle ( 5° or 15°) and A

Calculate load for each 5psi using exact conversion to Kpa

Remember don't use this formula as the tables have priority

++++++++++++++++=

Enough of this
Please use the frekin tables published by the manufacturer of your tires.

The suggestion that you can increase the max speed rating of st type tires above 65 only applies to Goodyear as so far they are the only company to my knowledge that has published a tech bulletin allowing an increase to 75 ONLY if you increase the inflations in the table by 10%. The Gy bulletin did not advise any higher than 75.

Using the Gy bulletin for a different brand would be like using a GM service guide for the oil filter to use in a Ford engine.
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Old 06-06-2015, 02:17 PM   #34
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Originally Posted by HowieE View Post
That is an interesting approach. Just use what ever units fit the bill. There was a time when science followed some rules but in today's taught approximation math you can dial in what ever you need.

No wonder they need pictures on the cash registers at McDonald's for the kids to figure out the bill.

While I don't like that method used in our discussions I love it while shopping because I can pay what ever I want and the cashier usually has NO Idea.

You have been given 2 reasonable methods here of determining tire pressure for a give load. The manufactures inflation chart and the road temperature test for final adjustments. But I am sure there are many that will read a number on the tire sidewall or let a kid at the tire store do what ever he thinks is right.

Road Temperature test is NOT an accepted method for establishing the correct inflation. There are just too many variables that affect the temperature of the tire.
I have done tests with a tire running at 50 mph and can give you temperatures that have a 50°F range depending on where and how I take the measurement. If I were to put that same tire on a vehicle I could probably extend the temperature range by another 20° or more due to additional vehicle induced variations.

Please use the tables after you know the actual load on the individual tires and an accurate ( +/- 1psi) gauge. All tires on an axle should use same inflation.

and none of this addresses the INterply Shear induced problems.
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Old 06-06-2015, 04:01 PM   #35
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The correct inflation pressure for your trailer tires has been recommended by the trailer’s manufacturer. (In that context, recommended means correct).

It you have replaced your original tires, the correct inflation pressure for the replacements will be the amount necessary to provide the load capacity equal to or greater than the original tires provided.

Those are the facts. When an owner deviates from them the only valid finger pointing will have to be at their own actions.

It is assumed that everyone knows what their recommended - cold - tire pressures should be. It’s in every vehicle’s owner’s manual and on every vehicle’s tire placard. Deviations are few and they are also listed in the vehicle owner’s manual.

An over inflated tire is one that has exceeded the value displayed on it’s sidewall. An under inflated tire is one that is inflated below it’s recommended value. A “run flat” tire is one that is found to be inflated 20% below it’s recommended value.

Here is a Q&A from a well known tire manufacturer.

“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.”

Here is a reference from the UK. They kind of keep it simple but the information is interesting.

http://www.national.co.uk/information/caravan-tyres.aspx

Also interesting is the recommendation to change tires sooner when they are using higher inflation pressures.

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Old 06-07-2015, 07:31 AM   #36
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That is an interesting approach. Just use what ever units fit the bill. There was a time when science followed some rules but in today's taught approximation math you can dial in what ever you need. .........
HowieE,

First, most formulae that describe relationships between things have constants. Those constants get the dimensions correct.

For example. the ideal gas law: PV = nRT

P = pressure (pounds per square inch)
V = Volume (cubic inches)
n = number of moles of the gas
R = a constant
T = Temperature (in degrees Kelvin)

So on the left side of the equation is pound-inches and on the right side of the equation is moles-Kelvin.

Note that the 2 sides aren't the same - EXCEPT for the constant. From that equation, you could derive the dimensions of the constant R. The same thing applies to the load formula. Those constants have dimensions such that the dimensions work out.

Quote:
Originally Posted by HowieE View Post
........ No wonder they need pictures on the cash registers at McDonald's for the kids to figure out the bill.

While I don't like that method used in our discussions I love it while shopping because I can pay what ever I want and the cashier usually has NO Idea.

You have been given 2 reasonable methods here of determining tire pressure for a give load. The manufactures inflation chart and the road temperature test for final adjustments. But I am sure there are many that will read a number on the tire sidewall or let a kid at the tire store do what ever he thinks is right.
Have you ever wondered why vehicle manufacturers are REQUIRED BY LAW to have a vehicle tire placard that lists the original tire size and the proper pressure for that size? It's because the method to get the tire size and pressure is way, WAY more complicated than what we have been discussing. There's the tables, but there is also testing to see if the pressure causes any vehicle handling issues. This is NOT a simple process.

Using your hamburger analogy, it's like asking what the price of the burger is and demanding it be tied to the price of cattle at the stockyard. The process way, WAY more complicated than that.
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Old 06-07-2015, 07:34 AM   #37
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Road Temperature test is NOT an accepted method for establishing the correct inflation. There are just too many variables that affect the temperature of the tire. .......
I agree with this completely. The idea behind the inflation pressure build up is to get a handle on IF you are over stressing the tires, by using the heat buildup as a guide. It is NOT a way of determining the correct pressure. It is a way of determining if it is WRONG! Even at that, it is but one of many ways it could be wrong.
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Old 06-07-2015, 08:11 AM   #38
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Whoa! Interesting posts but there are quite a few assumptions. First, the tire companies and vehicle manufacturers test their products and provide consumers with charts. We do not have to calculate anything other than PSI and load. While that may sound like an assumption in itself, it is also the legal route of responsibility. As far as temperature test goes, it is very accurate and a good check for correct pressure when monitored and compared. While temperature is not exact (sun on one side, even bad bearing, etc) it does provide data that relates to fine-tune the PSI EITHER DIRECTION. I could not find the chart for Michelin MS/2 but it is identical to the information for the Rib of the same size. Once the load is known (CAT scale- we do not know its calibration but nonetheless we use it) the Single tire on each side of axle rating is used. Two axles and, well, the rest is known. It is the same idea with cars/vehicles. Note how manufacturers do not say "look at the tire sidewall and inflate as indicated" but they provide an optimum control/ride PSI based on OEM tire PSI.

This is an important issue for many reasons- safety, ride, reliability, control. Another interesting comment here on the forums is how people are upgrading to E rated from D. I have but E rated tires are harder per se. Add to that people using maximum air pressure and read about screws falling out, etc on their campers. They have not posted a relation that I have witnessed just my own thought. Inflate your vehicles tires to maximum and drive it around for a bit. "feel the road" then take it down to manufacturer spec. Big difference but still specified to carry a "regular load". Capri Racer, I agree with you that there is a lot to it but also that most has been done for us. As far as Airstream and tire pressures go, I find it interesting that ALL their trailers have the same rating - MAX on D rated tires. I believe that is just more lack of load rating on both their part and the part of ST trailer tires, which constantly fail BTW.
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Old 06-07-2015, 09:12 AM   #39
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Maybe all the new trailers have the same max rating. My 1988m 25' Excella, specifies load range C at 45 lbs.
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Old 06-07-2015, 08:34 PM   #40
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Whoa! Interesting posts but there are quite a few assumptions. First, the tire companies and vehicle manufacturers test their products and provide consumers with charts. We do not have to calculate anything other than PSI and load. While that may sound like an assumption in itself, it is also the legal route of responsibility. As far as temperature test goes, it is very accurate and a good check for correct pressure when monitored and compared. While temperature is not exact (sun on one side, even bad bearing, etc) it does provide data that relates to fine-tune the PSI EITHER DIRECTION. I could not find the chart for Michelin MS/2 but it is identical to the information for the Rib of the same size. Once the load is known (CAT scale- we do not know its calibration but nonetheless we use it) the Single tire on each side of axle rating is used. Two axles and, well, the rest is known. It is the same idea with cars/vehicles. Note how manufacturers do not say "look at the tire sidewall and inflate as indicated" but they provide an optimum control/ride PSI based on OEM tire PSI.

This is an important issue for many reasons- safety, ride, reliability, control. Another interesting comment here on the forums is how people are upgrading to E rated from D. I have but E rated tires are harder per se. Add to that people using maximum air pressure and read about screws falling out, etc on their campers. They have not posted a relation that I have witnessed just my own thought. Inflate your vehicles tires to maximum and drive it around for a bit. "feel the road" then take it down to manufacturer spec. Big difference but still specified to carry a "regular load". Capri Racer, I agree with you that there is a lot to it but also that most has been done for us. As far as Airstream and tire pressures go, I find it interesting that ALL their trailers have the same rating - MAX on D rated tires. I believe that is just more lack of load rating on both their part and the part of ST trailer tires, which constantly fail BTW.

RE temperature - Please re-read my post #34

RE tire evaluation by MFG. While this is done by car manufacturers I have never heard of a RV company doing an on-vehicle evaluation. Tires are selected that will meet the goal load capacity.

Per regulations tires on highway vehicles must be capable of supporting the published GAWR for the vehicle. For cars you will see that the inflation is specified higher than than minimum needed for a variety of reasons such as ride & handling & fuel economy etc. For RVs however the industry seems more interested in shooting for low price so they select tires that meet the bare minimum so this means the tire is the "weakest link" The inflation is selected at the tire max while the lowest Load Rating (cost) is also selected. This sometimes results in an axle being de-rated so the RV company doesn't have to spend a couple bucks on a higher capacity tire.

There are technical reasons to run the tire max (lower Interply Shear)but given the apparent lack of design engineers working in the RV industry I am not holding my breath waiting for an RV company to step up and provide higher load capacity than the bare minimum needed to meet regulations.
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Old 06-07-2015, 09:10 PM   #41
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Ok so I have been following this thread from the beginning and to be honest, the more posts there is the more confused I'm becoming.

Here is my situation:
I have a 28' Trailer with Michelin 225/75/R16 E/10P MIC LTX M/S2 Tires. The trailer loaded and ready for camping comes in with a weight of 6084 LBS.

Right now I'm running the tires at 60PSI (when cold) but I'm concurred that this is not correct and would like input as to the proper preasure I should be running.

Thanks

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Old 06-08-2015, 05:46 AM   #42
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Ok so I have been following this thread from the beginning and to be honest, the more posts there is the more confused I'm becoming.

Here is my situation:
I have a 28' Trailer with Michelin 225/75/R16 E/10P MIC LTX M/S2 Tires. The trailer loaded and ready for camping comes in with a weight of 6084 LBS.

Right now I'm running the tires at 60PSI (when cold) but I'm concurred that this is not correct and would like input as to the proper pressure I should be running.

Thanks

Doug
OK, 6084# / 4 = 1521# (4 tires holding up the weight)

1521# * 1.15 = 1750# (accounts for side to side and front to rear load variation)

1750# * 1.15 = 2011# (Load the tire to no more than 85% of its rated load)

So I get 55 psi.

So what makes you think 60 psi is not correct?
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