Correct Air Pressure

[dominicoliveira]

Just installed  Toyos 295/80r/22.5   

Front axle=15,000lbs==115 lbs

Tandem=16,000lbs=  105 lbs

Tag axle=9200 lbs= 105 lbs  are these a/pressures correct?

When I tow  add another 4000 lbs  please advise     dabigtigerexpress@sbcglobal.net

[manholt]

dom.  Welcome to the Forum!

When your on a trip.  Do you know how much your coach weights?  Each axel and corner to corner is best, but at least each axel.  In your coach, mine is behind driver seat on wall, there is a plaque or paper that shows recommended tire inflation for steer axel (front). drive axel (dual wheels) and tag axel.  That does not include what you carry and tow!  I do 115, 95, 90 that's cold tire pressure and there is only me in coach, with empty holding tanks, 50% or less fresh water and full fuel...150 gallons!

Any truck stop with a CAT scale will weight you and your toad since that is part of your total weight.  Escapees also have some portable scales...Google them.  FMCA does it also at our Rally's! 

What your currently carrying is fine, until you get weight...might not be a smooth ride or not enough air. 

Carl

[hermanmullins]

We will have vendor in Texarkana in October at the Six-State Rally that will be weighting all corners.

Date is October 25th to 28th, 2017.

For information give me PM and I will get it to you.

Herman 

[rhinderber]

What is the reason for the different pressure in different positions  

[desertdeals69]

The weight of each corner determines the correct cold pressure.  The chart that each brand of tire has will list the correct pressure. The heaviest position on each axle is the determining pressure for the tires on that axle.

[wolfe10]

Correct.

All tires on an axle require PSI based on the heavier wheel position.

And front/rear/tag can all have markedly different PSI.

[wildebill308]

I recommend you add 10# to whatever the chart says as a safety margin. Some say 10% but it is easier to just add 10 psi. The tables are the minimum you can run at a given weight.

Bill

[rayin]

On 6/20/2017 at 8:12 AM, WILDEBILL308 said:

I recommend you add 10# to whatever the chart says as a safety margin. Some say 10% but it is easier to just add 10 psi. The tables are the minimum you can run at a given weight.

Bill

(thumbs up!) Running the minimum is risky; if the owner checks pressure on a warm day, then begins driving on a cold morning they can be running an underinflated tire.

No tire mfgr. I'm aware of recommends running less than the federal tire placard in/on the vehicle. Michelin has detailed minimum inflation RV charts that have, in small print, an admonition to never run less than the federal tire placard.

[wolfe10]

10 hours ago, RAYIN said:

(thumbs up!) Running the minimum is risky; if the owner checks pressure on a warm day, then begins driving on a cold morning they can be running an underinflated tire.

No tire mfgr. I'm aware of recommends running less than the federal tire placard in/on the vehicle. Michelin has detailed minimum inflation RV charts that have, in small print, an admonition to never run less than the federal tire placard.

While I absolutely agree with adding a cushion to the "minimum PSI for a given load", I do take exception to the statement: "No tire mfgr. I'm aware of recommends running less than the federal tire placard in/on the vehicle." You might provide a link to such a statement by a tire manufacturer. Were there such a statement, it would be based on their corporate ATTORNEY, not their engineer!

The inflation PSI on the GVWR plaque is the correct PSI IF, repeat IF you are running axles at their capacity (GAWR).  Your axles may be well under or over that.  Inflate based on the heavier wheel position on each axle, all tires on an axle to have the same PSI based on that heavier wheel position.

[tireman9]

Rayin,  I would be comfortable with suggesting lower than placard inflation IF I am talking with an RV owner that is making an effort to properly care for their tires. Info on placards is based on an assumption that the vehicle owner is not aware or interested in getting a better balance of performance and durability. Those inflations are assuming two things. First that the RV owner will not ever get on a scale and Second will never overload either axle of the RV.

The problem is that RVSEF, who does 4 corner weights and provides inflation info at FMCA events, that has the data that shows that over half of the RVs on the road probably have at least one tire or axle in overload so the Second assumption about placard numbers is not supported by reality.

RE tire company suggestions. While there are probably documents suggesting to inflate to placard pressure, I think Brett is correct in the idea that such info is written by the lawyers to protect the RV company when the owner fails to do proper maintenance of the RV tires.

 

Lets look at what a couple tire companies offer for advice and information.

Goodyear has an RV specific web site with information on how to weigh an RV and adjust tire pressure

Michelin provides similar information

Even Bridgestone tire Co that does not actively market to the RV community offers info on how to weigh and then set inflation

 

Now there may be some small importer that has no engineering staff that wants to avoid having to answer technical questions that decided it is easiest to tell people to just follow the placard. This is not wrong but just no optimal advice.

 

Then there are couple actual tire engineers, like myself, who monitor RV forums and attempt to provide even more detailed info to the small percentage of owners that want the details. But as I said to my knowledge there are only two of us on line and I may be the only one monitoring FMCA forum.

 

[rayin]

On 6/22/2017 at 0:43 PM, Tireman9 said:

Rayin,  I would be comfortable with suggesting lower than placard inflation IF I am talking with an RV owner that is making an effort to properly care for their tires. Info on placards is based on an assumption that the vehicle owner is not aware or interested in getting a better balance of performance and durability. Those inflations are assuming two things. First that the RV owner will not ever get on a scale and Second will never overload either axle of the RV.

The problem is that RVSEF, who does 4 corner weights and provides inflation info at FMCA events, that has the data that shows that over half of the RVs on the road probably have at least one tire or axle in overload so the Second assumption about placard numbers is not supported by reality.

RE tire company suggestions. While there are probably documents suggesting to inflate to placard pressure, I think Brett is correct in the idea that such info is written by the lawyers to protect the RV company when the owner fails to do proper maintenance of the RV tires.

 

Lets look at what a couple tire companies offer for advice and information.

Goodyear has an RV specific web site with information on how to weigh an RV and adjust tire pressure

Michelin provides similar information

Even Bridgestone tire Co that does not actively market to the RV community offers info on how to weigh and then set inflation

I read Bridgestones pdf you  provided a link to; under the section "find the RECOMMENDED inflation pressure", the last sentence is: " NEVER use inflation pressures lower than those printed on the vehicle placard."

 

Now there may be some small importer that has no engineering staff that wants to avoid having to answer technical questions that decided it is easiest to tell people to just follow the placard. This is not wrong but just no optimal advice.

 

Then there are couple actual tire engineers, like myself, who monitor RV forums and attempt to provide even more detailed info to the small percentage of owners that want the details. But as I said to my knowledge there are only two of us on line and I may be the only one monitoring FMCA forum.

 

I realize we've discussed this before on irv2.com, but;

Are you saying we should ignore those admonitions as stated by Bridgestone, Goodyear and Michelin as  "just corporate attorney talk"

[wolfe10]

Ray,

I don't follow discussions on irv2, so here goes:

Best way to determine correct tire pressure is to weight each wheel position when loaded as you go down the road. Take the heavier wheel position on each axle to go to your tire manufacturer's inflation table.  That will tell you the MINIMUM PSI for that load. Most of us add 5-10% to that as a fudge factor/safety factor. All tires on an axle get the same PSI based on the heavier wheel position.

Next best-- get axle weights (not as good as it ASSUMES perfect left/right weight distribution which is very unlikely on a motorhome). You will want to add at least 10% as a fudge factor/safety factor.

These last to could be very close or WAY off in PSI.  And, you hope they are WAY OFF, because if they are close, you have tires loaded to their max carrying capacity. And, if that is the case, were it my coach, I would look at upgrading to a tire with more carrying capacity.  Said another way, I don't want tires to be run at 100% of their capacity:

1. Go by the PSI on the GVWR sticker.  Yes, it is the correct PSI IF, repeat IF the axle is, indeed loaded to its GAWR. Again, I would not want this to be the case!

2. Go by the PSI on the sidewall of the tire.  Again, this ASSUMES that the tire is loaded to its max carrying capacity.  As in 1, I would not want this to be the case!

[tireman9]

Ray

Neither Brett nor I  nor others are saying ignore the sentence you seem to focus on. What we are suggesting is to consider the entire set of instructions from the tire companies.

If you know for a fact you have no tire in overload you are more than OK to run the inflation shown on the tire placard. That inflation is based on the assumption you are running your axles at GAWR.

If you would like a bit better ride then you might consider following the section in the tire companies current detailed instructions and consult the charts. and ensure you always have no less than the minimum inflation shown.

 

I suggest you do an even better job and set your TPMS low pressure warning level at the minimum pressure needed to support the heaviest end of that axle. You set your CIP to the minimum +10%  and set the high pressure warning on the TPMS to the CIP + 25%.  I would consider the best all around solution to the question of what pressure to run.

 

or if you prefer just follow the Placard. Your choice.

 

Any chance you will be at Indy. If so I suggest you attend one or both of my tire seminars.

[rayin]

I had to cancel our Indy reservation. DW is not healing as Dr.forecast. That cost me over $300 in lost fees, but DW's health is foremost.

I considered what someone said about "corporate attorney"- load/inflation charts and their statement about inflating to federal tire placard pressure. Appears to me they covered their 6 for both positions. I remember the Ford Explorer low tire pressure/roll-over issue. 

I will continue to follow the federal tire pressure placard, just as I do for my pickup and DW's Jeep(wonder how many use a load/inflation chart for pickups or autos?)

Following a load/inflation chart figure means you're running tires at their maximum weight limit, airing to +10% means you're running tires 10% over their maximum weight limit. We both know as sidewall flex increases so does flex generated heat.

Goodyear says " IMPORTANT: It's a common practice for RV owners to lower tire pressure in their search for a smoother ride. This is not only dangerous, it's relatively ineffective, as the difference in ride quality is not significant. When minimum inflation pressure requirements are not met, tire durability and optimum operating conditions are compromised. Tire inflation pressure should always meet at least the minimum guidelines for vehicle weight." To my knowledge, Goodyear is the only mfgr. that makes an RV specific tire and so states on the tire. Michelin makes truck tires and publishes a RV load/inflation chart. Is my thinking correct on that?

 

 

[desertdeals69]

Michelin makes the XRV series which is on more motorhomes probably than all the other mfgs combined.

[wayne77590]

I don't think that is correct. Michelin lists the XRV tire in the truck section but it is a tire that is made for rv's.  From their information: "All-position radial designed specifically for exceptional performance on recreational vehicles and motor homes in coach applications..."

I don't think anyone has said to inflate 10% over max tire rating capacity.  What is listed on the tire is the maximum inflation pressure of the tire to handle the indicated load on the tire.

The following from information I researched on the marking on the sidewall are:

Max Load Single 1380 kg (3042 lbs) at 550 kPa (80 psi) Max Pressure Cold
Max Load Dual 1260 kg (2778 lbs) at 550 kPa (80 psi) Max Pressure Cold –

So for a single tire on a vehicle with an 80 psi max pressure cold could support a load of 3042 pounds.  No one is indicating to go over 80 pounds.

The 10 percent addition would be if the load were 2500 pounds and the tire chart said 70 psi, then 77 pounds could be used as a minimum pressure to support 2500+ a few pounds.

Make sense?

[wayne77590]

I was typing as you were posting.