Tire Pressure vs. Carrying Capacity

[BlackAces]

Tire load inflation charts/tables should have TRA approval.

Most of the current GY charts used on the WWW have a 2005 date. It's on the bottom of the PDF pages. That date is before the last tire rule change approval of 2007.

When tires of the same size are manufactured with differing materials in their construction the inflation charts will not even come close to pressures needed at each increment to achieve the desired load capacity.

The ST235/85R16E has a maximum load capacity of 3640# at 80 psi.
The LT235/85R16E has a maximum load capacity of 3042# at 80 psi.

Airing your trailer tires to coincide with the load being carried requires you to go to the scales every time you make a significant change in your cargo load. Airing your tires to the trailer manufacturers recommendations will normally provide a reserve load carrying capacity providing your load is balanced.

In any event, overloading and under inflation MUST be avoided.

BA

[CapriRacer]

Originally Posted by jadatis

I would strongly advice, not to use the ST part of that document of Goodyear...........

I strongly disagree with that. The rationale that follows is faulty in that EVERY load table derives from the same set of formulae.

While we are aware of the issues with ST tires, we can NOT completely assign those issues to strictly the load table. There are many other possible causes.

[tjdonahoe]

With a little over 16000 miles on original gym's ,I marked them and took them to tire shop and rebalanced them, one wanted 4 oz, rotated the tire on the rim and it took 2 oz, when replacing they got Xed, all have even wear at 65-70 lbs, they are being watched very closely .

[SteveH]

Originally Posted by tjdonahoe

they are being watched very closely .

If they are over three years old, I suggest you watch very closely in your rear view mirrors because that's where you are most likely to see them when they come apart and leave the trailer.

[Doug C]

Originally Posted by CapriRacer

No.

Long Version: Barry's Tire Tech

Short Version: The load curve is a curve. (I think I just made a funny.) While it is close to linear, it is indeed a curve.

And in the region we use, inflation pressure is not linearly proportional to the load carrying capacity. Put another way, a 10% reduction in inflation pressure does not result in a 10% reduction in load carrying capacity - it is less.

That's why you need the load tables.

BTW, this applies to ALL tires - passenger car, truck, trailer, aircraft, lawn and garden, bicycle, motorcycle, etc.

Thanks for the link to Barry's site. Very informative.

[jadatis]

Originally Posted by CapriRacer

I strongly disagree with that. The rationale that follows is faulty in that EVERY load table derives from the same set of formulae.

While we are aware of the issues with ST tires, we can NOT completely assign those issues to strictly the load table. There are many other possible causes.

But fact is that ST tires are calculated in their maximum load for more deflection allowed. This brings the loadcapacity to the edges of what is wise to laws of Nature.
If you then calculate it for lower pressure with a formula that gives even more deflection in the lower pressures , the tire deflects to much and produces more heat , more then it can cool down , so spots of tire get to hot and harden and tire damage begins.

Fact also is that these pressure/loadcapacity lists are made using a formula.
I will give picture I used in my warning mail to Goodyear.



compare calculation 1 and their list copied at the bottom.
In exeption to 35 psi and 60 and 65 psi all the given loadcapacity's are rounded from the calculation for Diagonal tires in list 1 I gave with power 0.585.
Explanations for the exptions is 35 and 65 are AT-pressures for B-load and D-load. What they did is calculating with power 0.585 for C-load and replaced 35 and 65 psi by the maximum load belonging to B-load and D-load. The 60 psi I think is because of roundings and conversion from KG/kPa to lbs/psi.
For me this prooves that they used this formula to determine this P/LC-list.

I see no yustification to calculate P/LC lists for Radial ST tires with the old formula for diagonal tires.

I will try to explain the effect of this by an example.
If you would asume the american LT calculation with power 0.7 to be sufficiënt, and probably it even is not, and calculate for 30 psi , it gives in picture calculation 2 =1504 lbs loadcapacity, and for calculation 1 =1595 lbs loadcapacity ( for diagonal tires).
This will give aproximately 1595/1504=1.0605 times more surface on the ground so service length ( because surface with stays the same).
I determined that this gives about squaire 1.0605=1.1246 times as much deflection , so as much heatproduction . That to the already to the edges calculated deflection for the speed of 65m/h of ST gives significant more heatproduction and more then it can cool down , so tire-damage in the long or shorter run.

[Tireman9]

To the OP and others
Simple question but complex answer. There are two actual tire engineers on this thread plus some who just play tire engineer on TV

There is a formula used to generate most of the numbers in the published tables. In general the formula gives
Load Capacity = Air Pressure to a power x tire air volume x K

Now the K factor is generally based on expected usage based on the type of tire covered. I think we can all understand that passenger tire usage (laod, speed, type of surface etc) is clearly different than say a mining tire of identical dimensions.

The power valus on the inflation pressure also varies based on general shape of a tire. An ultra low aspect tire like a 235/35R18 has a different factor than a 235/75R15 as the deflection characteristics are different. (this comment will make our tire engineer hobbyist happy to know)

With regard to ST type and trailer application. I strongly recommend you use the inflation associated with the max load capability molded on the sidewall of the tire as this will lower the "Interply Shear' that is unique to multu axle trailers. This is because the trailer tires, unlike the tires on your TV are not rotating around the center of the turning radius but are being dragged around the curve which increases the shear force that is trying to detach the belts from the rest of the tire.
If you want to know more you can simply Google "Interply Shear" to see both tire blog posts and technical papers on the topic.

Finally there is the reality that some tire sizes needed to be "grandfathered" as their load capacity was developed many years ago and the load/infl table was based on actual service results not some formula developed later to fit the curve of the individual points in the table. There is simply no single formula that will fit all sizes and type of tires.

The above is why you need to use the table your your specific type & size tire as published by the manufacturer of your tire. While the vast majority of tires of the same type and size have the same capacity there are some that vary a bit so you should not use a Michelin table for a Goodyear tire even if both were the same size and type.

Now as to the suggestion from our tire engineer wanna-be. I would warn that the suggestions he offers have not been adopted by any tire company or regulatory agency in the world. But you are of course free to decide if you want to go with his idea or follow what all the rest of the tire engineers in the world do.

Tires are designed to pass certain well defined durability and strength tests that specifically point to the published load & inflation numbers for that specific tire. If you decide to ignore the industry tables and invent your own personal guide please don't come back here and complain about hving a tire failures "for no apparent reason"

PS I do like the suggestion to have at least 15% "head room" between the tire capacity at the inflation you are running and your measured individual tire load.

[sheriff1]

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


2015 F350 CC 4X4 6.7 Diesel
2010 27FB Silver Cloud "The Silver Spoon"

[HowieE]

Originally Posted by Tireman9

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.

[CapriRacer]

Originally Posted by sheriff1

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


2015 F350 CC 4X4 6.7 Diesel
2010 27FB Silver Cloud "The Silver Spoon"

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:

Originally Posted by sheriff1

.....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.

Originally Posted by sheriff1

....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.

[CapriRacer]

Originally Posted by HowieE

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.

[HowieE]

Originally Posted by CapriRacer

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.

[Tireman9]

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.

[Tireman9]

Originally Posted by HowieE

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.

[BlackAces]

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.

CW

[CapriRacer]

Originally Posted by HowieE

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.

Originally Posted by HowieE

........ 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.

[CapriRacer]

Originally Posted by Tireman9

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.

[rodsterinfl]

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.

[Bill M.]

Maybe all the new trailers have the same max rating. My 1988m 25' Excella, specifies load range C at 45 lbs.

[Tireman9]

Originally Posted by rodsterinfl

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.