Recommended Minimum Tire Pressure

[bachler]

The origenal post asks what tire pressure.. You will find the Max. Pressure for that tire. On the coach specs. you will find a listed/reccomended pressure.. Now you have 3 choices. Tire/coach/yours. Take/use the one that make you happy. The rest really don't matter. Life is good.

[rvtime2]

My tires have 80 psi cold stamped on all six tires on our Type C MH. Based on weighing our MH and comparing to load vs psi chart for our typical tire size, my tires could be set at 60-65 psi front and 75-80 psi rear cold. My label in driver door panel recommends, 65 psi front and 80 rear, my load tests and data seem to support recommendation by WB suggestions on door label. Therefore, I have decided to use the 65 front and 75-80 psi rear, but I am glad to confirm by scales and review of tire data. What I did find, is that we should probably add more weight to load our front axle. This is our first MH, so forgive my rambling as to what we have done, but thought it may be pertinent.

[jadatis]

The minimum pressure at wich TMPS gives warning of to low , should be the calculated minimum pressure for the real loads, with the ever to be constructed ideal formula, wich mine comes close to. This pressure it must have in the coldest outside temperature you ever ride into, so probably below freesing point.

Then converse this pressure from that cold outside to an average outside temperature of 18dgrC/65dgrF and use that as the preset pressure of TMPS.
Then the high alarm is not that important, because highest inside temperature and pressure belonging to that can get incidentially get as high as boiling point of water 100dgrC/200dgr F, by the heat of the brakes transported trough the rimms. I dont think a large tire as here will ever get to that.
So if in those conditions high alarm goes off, you know that you dont have to worry that much about it.

The calculation of the lowest needed pressure though is a bit dangerous.
You can misyudge the loads and mosty to low then, but even if weighed things can be a bit different because of shifted loads and extra load.
Also pressure and load-scales , even if calibrated , can give misreadings.
A digital pressure or weight scale suggests a high acuracy because of the 2 digits behind the point, but still can have some inacuracy.

But still its best to weigh, preferably per wheel(pair), other per axle.
But as long as you have not weighed, you still need an advice to use .
So in lack of weighing, you have to use the GAWR's ( gross axle weight ratings) wich mostly are on the same plate as the pressure advices.
Then front use 5% reserve and behind even 18% to make the % to use of load the pressure is calculated for of 85% under wich discomfort begins by bouncing ( so I concluded by reactions, so discussable).
This makes the reserves as large as possible for , little overload or unequall load R/L per axle.

[Tireman9]

Originally Posted by bachler

I do not want to get into a He said she said.. When the tire manufacturer Was having wa hard time Making rubber Hard/elastic enough to be road worthey. he had a piece of rubber in his hand , when he got called outside of his lab/shop he accidently throw the piece on the stove that had gone out but it was still warm. when he came back in he found it put it asside when he picked it up later he found it had Cured to the appx. consisity he wanted.. Hens heat is not good. and is a result of underinflation. Now for side Zipper Blow outs. That is/was a common Blowout for the ZRV tire. I inflated to Factory LBS. and Blow 2 of them Out Less then 2 years old. 2.75 X 22.5 Michilen. The dealers said this was a common thing to happen... I know some will dissagree. That is good.. This happened to me... I now use the Green line, Michalin. Life is good

Interesting interpretation on the work of Charles Goodyear and his first patent for vulcanization from 1844. "Zipper" sidewall failures are not the result of rubber reversion (de-vulcanization) but of the fatigue failure of the steel cord due to bending past the yield point. Yes Sidewall failures are relatively common but given that a majority of RV owners have failed to properly inflate their tires such that they have one or more tires underinflated sidewall flex failures are not surprising. This is based on actual measurements of thousands of RVs.
The fact that someone checks the inflation of a tire in no way guarantees they will not have a puncture or valve leak develop as they drive down the road. In all the thousands of tire autopsies I conducted in my career I have never seen a tire sidewall "blowout" that was not the direct result of either a run low flex failure or being operated at excessive speed (+50 mph or more above design limit).

[NeilV]

Tireman - My Dad was an engineer with Tyre Rubber and Roll and developed formulatons for all types of rubber products including tires used in automotive, trucking, construction, farm, aviation and military applications for 47 years and would agree with you 100%.

Yes an RV tire that has gone down to 30 psi should be considered functionally flat and requires dismounting from the rim and inspection to certify that it is undamaged from the event and still safe for use otherwise you are taking a roll of the dice with the safety of everyone inside your RV and those near by who could be harmed by it if you should have a tire failure.

I saw one guy whose coach was riding rough after refilling a low tire that was functionally flat so he called for a mobile tire tech to put on the spare. The tech turned white as a sheet when he got his light out and inspected the inner tire on that dual. He had everyone stand back at least 50 feet while he used a 15 foot air hose with a bleed valve to deflate the tire from the far end of the hose and when he pulled the wheel from the coach even deflated you could see how the tread grooves had spread and the steel belting had started to errupt from between them. It could have been quite devastating if the tech had just swapped it when it still had 110 psi in it. Would it have blown from the vibration from the impact wrench or possibly when he started to walk it out off the hub or maybe when he lay it flat on the ground? Who really knows but that is was too close for comfort for all concerned. I'll have to see if I still have the pictures of that one.

[Tireman9]

Yes I have written of a fatality and a serious injury. There is also a Youtube of a fatality. People have no idea of the potential energy of a fully inflated truck tire.

A quick search of Youtube on tire explosion is a real eye opener.

[Mr_D]

The pressure shown on the side of a truck size tire is the minimum for the maximum weight the tire will support, it is NOT the maximum the tire should have. Lots of posting about this as well as quotes from the manufacturers RV tire manuals as well as magazine articles.
The pressure shown on the label on the RV wall is for the original tires at the maximum rated load of the axle. Use this if you've never weighed the rig or if it's loaded to it's maximum.

[Tireman9]

Originally Posted by Mr_D

The pressure shown on the side of a truck size tire is the minimum for the maximum weight the tire will support, it is NOT the maximum the tire should have. Lots of posting about this as well as quotes from the manufacturers RV tire manuals as well as magazine articles.
The pressure shown on the label on the RV wall is for the original tires at the maximum rated load of the axle. Use this if you've never weighed the rig or if it's loaded to it's maximum.

Mr_D You are absolutely correct on the Min-Max pressure issue.
While passenger tires may have a "Safety Warning" on the tire bead seating or "Mounting" pressure, that is completely different issue than operating pressure.

5 degree taper wheels (Passenger, LT and TT wheels in full inch diameter) usually have a "safety hump" which provides an additional support to prevent a tire de-seating and loosing air when the vehicle is turning a tight corner and is not at full pressure. There is even a regulatory test for this. When mounting and inflating tires the bead needs to "button-hook" over the hump. If there is insufficient lube or the tire or wheel have been damaged it is possible for the tire to center on the hump and not fully seat at normal pressures. Tire techs are supposed to be trained to limit the inflation to minimize the chance of explosion so a "max seating pressure" is specified. Seating pressure is when you hear the "pop" during the inflation process. After seating higher pressure is OK.

19.5, 16.5, 22.5 and 24.5 inch rims with 15 degree taper do not have these humps so the process does not have a "pop" to indicate full seating pressure.

Tires are designed to tolerate two or more times the inflation molded on the side of the tire. If you read the words on your RV tires you will see they do not say Maximum Pressure but "Maximum load at xxx psi"


The above is one reason inflating tires from 0 psi should be left to professionals. Inflating truck size tires from 0 psi should be done in safety cages if at all possible and never with someone close to the tire being inflated.
Adding 5 to 10 psi to a tire to achieve the proper cold inflation is different. You can do that as long as the tire has not been run more than 20% low on pressure or if it was a dual when its mate failed.

[bachler]

Bewear Of the false Information that is posted BY so called Professionals/that know It. I took Machanical engineering Then Electronic Engineering. Our instructor in the last block. Had a saying. He had written on the Chalk board.. It was . A true engineer is a person that Has all common sence Removed from his head and it has been filled with a bunch of usless Numbers. And cannot convay with normal minds.. In retrospect I think he may have been onto something....

[edgray]

aBsoltle. Y. Amasing. pOt caling the keTTle Black. (on something)

[UFO Pilot]

Beware of false information from people who know nothing about the subject at hand. I will always trust the professional's opinion before I trust the layman's.

[jadatis]

Originally Posted by UFO Pilot

Beware of false information from people who know nothing about the subject at hand. I will always trust the professional's opinion before I trust the layman's.

But also keep in mind what the goal is of the tire-makers, and that is to make money.
That is why they dont give you information that can be used against them.
Yust gather information from profesionals and those who think to know about it and can support it with logical arguments ( Like I do). Then draw your own conclusions.

What I discovered in the years that I have been busy with tire-pressure calculation is next.
Before 2005 the calculation in America came to to low pressure , or better to much deflection of the tire in the lower pressure/load range.
As late as 2006 American TRA swiched over to the calculation of European ETRTO wich used this since about 1970 for all kind of tires and prooved to be save for standardload/P-tires and XL/extraload/Reinforced, but for C-load and up still is not ideal.
For C-load and up American TRA still uses the old formula so for those tires still to much deflection in the lower load/pressure range.

In the Ford Firestone affaire around 2000 to low tire-pressure was an issue.
Because a bandaid was used for P-tires and XL// on SUV of substracting 10% of the maximum load of the tire before doing the old calculation , the calculated pressure was repaired adequately. So the wrong calculation was not the cource of the blewing tires wich courced lots of roll over accidents with death people also, so serious thing.

The real cource is to my opinion the larger profile blocks of the treath , that cover a large part of the sidewall , wich made that sidewall lesser part to flex without getting damaged.
Also for low Aspect Ratio tires ( hight/width division) lesser deflection is allowed then the tire-makers use to calculate the maximum load.
Because I dont have a laboratory in my back yard, I am not working in the tire-business, I cant say howmuch should be substracted from the maximum load of both kind of tires , but calculated it rougly that it can rise to substracting 20% of the maximum load that is written on sidewall.
Mayby its not that bad and only 10 % has to be substracted, but even then it can cource to low pressure when calculated with the ever to be constructed ideal formula.

[Dr4Film]

Jadatis,

Could you please summarize what you stated in your post because after reading through it twice I still didn't have the faintest idea what point you were attempting to convey to the group.

Sometimes my head is TOO thick.

Thanks.

Dr4Film ----- Richard

[Tireman9]

I find it interesting how some want to use logic based on personal theories rather than data to solve complex mathematical problems. It is even more interesting when they make statements which are demonstrably not accurate.
In the US there is a standards group called the Tire & Rim Association or TRA that publishes both formula and tables. The US Dept of Transportation follows this published information. In Europe there is a similar organization called ETRTO and in Japan there is JATMA and even in Australia there is a similar organization. Many countries choose to follow DOT regulations when it comes to tire performance and safety issues.
All of these organizations follow almost identical formula with the exception of "English" vs metric dimensions and of course the associated rounding differences. Tire manufacturers almost universally follow the published information found in the tables. They do not follow the information offered by people who do not work for or have experience working in the industry as to do so would place themselves in an untenable position of having to defend their decision of going against many decades of history and the rest of the tire manufacturers in the world.

A review of the historical published load/inflation tables will show that the loads established in 1995 and earlier for popular tire sizes was the same in 2000 and in 2010 so to claim that there was a change in calculations for these tires is not a claim based in fact.

Vehicle tire inflation specs as published on the vehicle placard are governed by DOT regulations and are always the responsibility of the vehicle manufacturer not the tire manufacturer, so if a lower than desirable inflation results in tire failures, the formula is not the cause.

With the industry move to ever lower aspect ratio passenger tires the tire industry has adjusted the formula for low aspect tires, based of actual test data rather than a guess.

Of course people can choose to follow the opinions of self appointed "experts" rather than those with a background of actual tire design responsibility but if they do so they need to remember who they will want to complain to about tire failure issues.

[Mr_D]

Originally Posted by bachler

Bewear Of the false Information that is posted BY so called Professionals/that know It. I took Machanical engineering Then Electronic Engineering. Our instructor in the last block. Had a saying. He had written on the Chalk board.. It was . A true engineer is a person that Has all common sence Removed from his head and it has been filled with a bunch of usless Numbers. And cannot convay with normal minds.. In retrospect I think he may have been onto something....

Looks like they left out English composition and writing too.

When the newly minted EIT's and PE's showed up on the job it took about two years of experience before they were ready to actually work as an engineer.

[UFO Pilot]

I run 150 pounds in all my tires and you should too.

I don't trust the experts, it just seems right to me.

[edgray]

Happy New Year, Wayne!

[UFO Pilot]

Ed

Happy New Year to you too! Leaving for 3 weeks in Palm Springs tomorrow. Roberta has knee replacement scheduled for Feb 20 so we will be staying close to home after that.

[edgray]

Originally Posted by UFO Pilot

Ed

Happy New Year to you too! Leaving for 3 weeks in Palm Springs tomorrow. Roberta has knee replacement scheduled for Feb 20 so we will be staying close to home after that.

Enjoy. Hope Roberta will be well enough for Palm Desert rally in April.

[UFO Pilot]

Originally Posted by edgray

Enjoy. Hope Roberta will be well enough for Palm Desert rally in April.

We have 2 Rallies scheduled for April up North. We are hosting one and I am being installed as President at the other. Hope the dates won't conflict with Palm Desert.