Tire Pressure vs. Carrying Capacity

[Doug C]

Tire pressure question: Is tire pressure and tire carrying capacity directly proportional all the way through the range of possible air pressures? In other words, is the following hypothetical analysis correct?

For the sake of simplicity with the math, I’ve used round numbers:

Let’s assume that a particular tire will safely carry 4000 lbs. at 100 psi.

Does this mean that the same tire will safely carry 3000 lbs at 75 psi, or 2000 lbs at 50 psi, or 1000 lbs at 25 psi???

Is this how it really works?


Thank you.

[BoldAdventure]

No.

[Tony S]

No is a good answer to an interesting question but could someone chime in with a little more info. I have had the same question myself.

Many Thanks

Tony

[HowieE]

You will find the answer here. The chart is good for any manufacture.

http://www.goodyearrvtires.com/pdfs/rv_inflation.pdf

[jadatis]

I also thought it was that easy when I asked an European tyre-manufacturer it.
What you use is the logical math and is not even that bad to use.
The Tyre-maker answered me also back with NO its more complicated.
But then I mailed back if they use some kind of formula for it, and they sent back a copy of formula to calculate pressure for a sertain load, and the extra's for higher speed and camber angle ( alighnment wheels like this /-\ on the axle).
Went running with that and learned myself excell to make spreadsheets for it.
The formula al over the world has a power in it , Europe uses 0.8 to calculate loadcapacity for a sertain pressure ( so the other way around then the formula I got) and this formula is more often used to make pressure loadcapacity lists.

When I declared this formula holy , I made some spreadsheets and bothered also American fora with it, and came to a document of american IR J.C.Daws who introduced another formula wich is lineair but with still a loadcapacity at zero pressure. I babtised that the construction load. He compares it with the different calculations in Europe and America ( USA uses different powers as 0.5( root) 0.65 , 0.7 but stepped over for the P- tires and XL/reinforced/Extraload to the EUR calculation with power 0.8 as late as 2006.

To begin with I will give a picture of several calculations for , then you see the logical calculation you do , is not that bad, but also give an "on the road methode wich is also simple to use, and comes pretty close to the ever to be constructed ideal formula.

Also my determined formula and see how small the differences are between that and logical and on the road methode.



Also I will give link to the public map on my One-drive that belongs to my hotmail.com adress with same username as in this forum .
3/4th about tire-pressure so sniff around.

https://onedrive.live.com/?cid=A526E...e092e6dc%21128

Sertainly other questions will pup up after you read this all.
then in this topic ask this Dutch pigheaded self-declared tire-pressure-specialist.

[Arcticfox]

interesting..... thanks

[HowieE]

That formula may hold for P tires but P tires are not recommended for trailer use.

[tjdonahoe]

My 1973 23' avion came with car tires and it was all it ever seen, never any problems, it's weight was around 6000 lbs.

[SteveH]

Just my opinion, but I would try to avoid using "P" metric radials on a travel trailer. I would, however, use an "XL" rated SUV tire before I would use an "ST" tire. They're usually rated for more weight than a "C" weight rated tire.

[jadatis]

Nothing wrong with P-tires for a TT as long as the maximum load is enaugh.
an ST ( special trailer) tire is calculated in its maximum load for lower speed ( 65m/h) so more deflection allowed.

Will give link to topic I started on RV forum, about maximum load in relation to maximum speed of tire.
http://www.rv.net/forum/index.cfm/fu...d/27477930.cfm

A stiffer tire needs more pressure for the same load then a less stif tire.
Compare LT C-load to P-tire of same sises if you can find it.

HowieE writes that the formula would only hold for P-tires, bus do you mean the official one or the logical one.
For P-tire and XL// the offical one with Eur 0.8 power comes close to the to natures law ideal one, but the higher the AT-pressure , so LT tires E-load for instance the logical formula is better then official EUR one.

[larryglarson]

As an old truck driver, a man who has driven over 3 million miles and is now a retired camper man you have the right approach. More weight more air up to the tire rating. Too much air you wear out the center of the tire, too little air you wear out the edges of you tire. More air it rolls easier so less fuel. Too much air you give up ride. I have car tires on my pickup and run 35 with out trailer (number on the door) and 40 pulling an 30 ft. airstream. I have regular tailer tires on trailer and run 60. This has been working good. Tires wearing well and good ride.

[r carl]

Originally Posted by SteveH

Just my opinion, but I would try to avoid using "P" metric radials on a travel trailer. I would, however, use an "XL" rated SUV tire before I would use an "ST" tire. They're usually rated for more weight than a "C" weight rated tire.

P metric tires do have a flimsy sidewall don't they, I like LT tires.

[CapriRacer]

Originally Posted by Doug C

Tire pressure question: Is tire pressure and tire carrying capacity directly proportional all the way through the range of possible air pressures? In other words, is the following hypothetical analysis correct?

For the sake of simplicity with the math, I’ve used round numbers:

Let’s assume that a particular tire will safely carry 4000 lbs. at 100 psi.

Does this mean that the same tire will safely carry 3000 lbs at 75 psi, or 2000 lbs at 50 psi, or 1000 lbs at 25 psi???

Is this how it really works?


Thank you.

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.

[Hophead]

PSI is really dictated by the load the vehicle will have to carry. There should be no issues with having the psi lower than the maximum rating as long as it is sufficient to carry the load of the vehicle, and you would want to consider the full load with water, luggage etc.. when factoring total weight of loaded vehicle and appropriate PSI required for the load.

Load and PSI works as you assumed. Attached is an industry standard for trailer tire loads and comparable PSI.

Any reputable and knowledgeable tire dealer (like Les Schwab) can explain how it works and what tires are right for your load and trailer/driving conditions.

Monitor tire wear and adjust pressure accordingly. Over inflated tire on an Airstream rides rougher and may shake out the rivets (per my dealer). So I called Airstream (Ohio) and they have no real justification on what tires are installed, other than that is how we have always done it.

Goodyear inflation .pdf

[larryglarson]

The Good Year Tire web site does a good job. I know I am "out voted" on truck v/s trailer tires for an airstream, but at least read what Good Year has to say. I would never put truck tires on a trailer any more than I would put airplane tires on. Good Year know tires after over a 100 years of making them.

[SteveH]

[QUOTE=larryglarson;1625597]The Good Year Tire web site does a good job. I know I am "out voted" on truck v/s trailer tires for an airstream, but at least read what Good Year has to say. I would never put truck tires on a trailer any more than I would put airplane tires on. Good Year know tires after over a 100 years of making them.[/QUOTE]

If this were true, a person would think the GYM's would have a much lower failure rate.

[larryglarson]

I have Good Year Marathon Tires on my 30' classic and I do not have any trouble. They wear well, the air does not leak out of them, just no trouble. I hear all kinds of stories about trouble so I guess there must be some. I am 72 and have driven over 3 million miles. I have had bad tires and rigs that "ate" tires, but in the last 30 years I have not had any trouble with Good Year tires. I know there are other good tires out there, but for cost and quality, I think Good Year's are OK.

[Action]

Originally Posted by CapriRacer

That's why you need the load tables.

I would suggest the load table of the tire size you are using from the manufacturer of the tire you have.


Using a ST tire table on your LT tire may get you into some shortened tire life.

>>>>>>>>>Action

[jcanavera]

Originally Posted by Hophead

Any reputable and knowledgeable tire dealer (like Les Schwab) can explain how it works and what tires are right for your load and trailer/driving conditions.

Monitor tire wear and adjust pressure accordingly. Over inflated tire on an Airstream rides rougher and may shake out the rivets (per my dealer). So I called Airstream (Ohio) and they have no real justification on what tires are installed, other than that is how we have always done it.

Attachment 238946

Many reputable tire dealers I have talked to in my locale will only recommend and sell ST tires for trailer use. (That liability issue pops back into the picture). As a matter of fact, some even refuse to mount anything other than ST's on a trailer. To keep things clean I ordered from the Internet and took the tires and wheels to a dealer to get my LT's mounted. No questions asked and no issues.

As far as matching pressure to load, make sure to add enough pressure to give you at least 15% reserve load capacity over what your maximum weight of the trailer would be.

Jack

[jadatis]

Originally Posted by Hophead

PSI is really dictated by the load the vehicle will have to carry. There should be no issues with having the psi lower than the maximum rating as long as it is sufficient to carry the load of the vehicle, and you would want to consider the full load with water, luggage etc.. when factoring total weight of loaded vehicle and appropriate PSI required for the load.

Load and PSI works as you assumed. Attached is an industry standard for trailer tire loads and comparable PSI.

Any reputable and knowledgeable tire dealer (like Les Schwab) can explain how it works and what tires are right for your load and trailer/driving conditions.

Monitor tire wear and adjust pressure accordingly. Over inflated tire on an Airstream rides rougher and may shake out the rivets (per my dealer). So I called Airstream (Ohio) and they have no real justification on what tires are installed, other than that is how we have always done it.

Attachment 238946

I would strongly advice, not to use the ST part of that document of Goodyear.
The loadcapacity's are calculated with the formula for diagonal tires.
this is the formula that was first introduced in 1928 , and later adjusted ( about 1970 ) in its power for radial tires.
EUR all kind of tires to 0.8 power.
USA P-tires H/W div down to 50 power 0.5
P-tires lower H/W div then 50% power 0.65
LT tires up to truck tires power 0.7.
I already warned Goodyear a year ago about it but they wont see my point.
But who would believe this gus from Holland Europe , where they put Gremlins in all the stuff the sent to America .
As late as 2006 USA TRA swiched over for P-tires and XL to the power of EUR
of 0.8 but left LT at 0.7 power.
The higher the power the lower loadcapacity it calculates for a pressure.
Power 1 is the same as no power and is the logical calculation.

I made my own pressure/loadcapacity lists and you can find them here.
https://onedrive.live.com/?cid=A526E...e092e6dc%21904
Set the formula that tight that sooner to low loadcapacity then to high, wich is always saver, but think its verry close to the ever to be ideal formula to laws of nature.

In this first go to the USA map and then choose if you want it given per tire, axle single or axle dual. Ten you dont have to devide by 2 or 4 anymore.
Open the PDF for your AT-pressure and search the loadindex/maximum load of the tire, then find the loadcapacity yust above the tire or axle load you want to know it for, and read below or above the needed pressure.
Best is to add 10% to the weighed load you use before looking back the pressure.