Tire Pressue Puzzle

[rpbuttery]

I had my 2011 Allegro Open Road RED (purchased "previously enjoyed" a year ago) four corner weighed at the Freightliner Service Center in Gaffney, SC recently.  Both the front and rear axle weights were very close to the manufacturer's maximum axle weight ratings.

Based on these weights, the Freightliner techs said the indicated tire pressures for my Michelin XZA3 tires was 90 PSI, and set all six tires there.

The manufacture's weight label (the label next to the driver's seat in the cockpit) indicates recommended tire pressures of 110 PSI for all six tires.

Why would there be that much difference between the two when the actual axle weights--both front and rear--were very close to the manufacturer's maximum axle weights?

[dickandlois]

rpbuttery, You might look at the load range chart for the new tires and at what pressure the tires match the load requirements. The science of tires keeps changing.

Rich. 

[jleamont]

http://www.michelinrvtires.com/reference-materials/load-and-inflation-tables/#/

 

above is is the link from Michelin 

We have a tire expert on here if you have any other questions he will most likely chime in.

Good luck

 

 

 

[elkhartjim]

"The manufacture's weight label (the label next to the driver's seat in the cockpit) indicates recommended tire pressures of 110 PSI for all six tires."

Is this the recommended pressure or maximum allowable pressure of wheel/tire combination?

[dickandlois]

Hi Jim, Regarding the tire pressure. The OEM label is the maximum pressure level. One needs to air up the tires relative to axle weight. 

Different tire sizes have different load capacities, but you still air them up to the pressure level needed for weight. Not exceeding the maximum pressure of the tires or rims.

Rich. 

[wildebill308]

To add to what Rich was saying. The sticker in the coach probably relates to the gross (maximum) weight set by the manufacture. If you run that pressure, it will cover the maximum weight you should have in your coach. The tire manufactures tables are based on the actual weights recorded for a given tire. I would always run a 10lb cushion to compensate for weight creep or changes in elevation or the normal air loss. Remember if you are running at the exact pressure recommended by the manufacture, if you lose one pound you are running under inflated.  

Now the tier man will correct all I have said.

Bill

[wolfe10]

Actually, the Federal GVWR sticker indicates the correct tire pressure FOR THE OE SIZE/LOAD RANGE TIRE WHEN EACH AXLE IS LOADED TO IT'S GAWR (Gross Axle Weight Rating).

That is the recommended PSI ONLY, repeat ONLY if the axle IS loaded to it's GAWR!!!

[tireman9]

WildBill and Brett have it right.

While I have posted the process a few times on my blog I am willing to do a personal calculation and analysis if rpButtery could provide the tire size including the Load Range, brand and the 4-corner scale weights I would be happy to look up the numbers and post the process so all feel a bit more comfortable.

[elkhartjim]

11 hours ago, Elkhartjim said:

"The manufacture's weight label (the label next to the driver's seat in the cockpit) indicates recommended tire pressures of 110 PSI for all six tires."

Is this the recommended pressure or maximum allowable pressure of wheel/tire combination?

It was a rhetorical question.

[wildebill308]

12 hours ago, Elkhartjim said:

It was a rhetorical question.

Well to me "Is this the recommended pressure or maximum allowable pressure of wheel/tire combination? " Does not read as a retorical question. So I thought I might be wrong. After all I can't spell and have ben wrong before.

Simple Definition of question: a sentence, phrase, or word that asks for information or is used to test someone's knowledge

• : a matter or problem that is being discussed: a subject or topic
• : doubt or uncertainty about something

A rhetorical question is a question that you ask without expecting an answer. The question might be one that does not have an answer. It might also be one that has an obvious answer but you have asked the question to make a point, to persuade or for literary effect.

Bill

[manholt]

Quote

 

Good night, Bill!

[elkhartjim]

It might also be one that has an obvious answer but you have asked the question to make a point, to persuade or for literary effect.

There's your answer, Bill.

[rpbuttery]

Tireman9,

Actual corner weights were:

LF--4,500

RF--4,860

LR--8,980

RR--9,080

I have new Michelin XZA3 tires--275/80R22.5, load range G.  They replaced Michelin XZE2 tires (the MH manufacturer's original equipment).

Thanks.

(May have answered my own question.  In looking at the Michelin RV Tire Reference Chart, I see that the maximum pressure--for both the single and dual positions--is 110 psi.)

[wayne77590]

My onion: First I'd redistribute some of the front load to reduce the difference - or have the passenger loose some........never mind. A large distribution difference can have it's toll on components.

Looking at the chart for the 275/80R22.5 for Michelin and using the present front value I would set the pressure at 90 psi and that would give you a 570 pound leeway for anything you would add to the front. (Trinkets the passenger may purchase)  The minimum for your RF is 80 psi @ 4970 psi but this only gives you a leeway of 80 pounds of trinkets to add to the front load. You have to inflate all tires to the same PSI across the axle so even your LF would have to be at 90 PSI based on your current values.

Let's assume you load to 90 psi. That gives you a maximum load capacity for that tire of 5370 pounds and is well above your 4860 pounds.  The "Fudge" factor is 10 PSI, so if you drop pressure by 5-10 pounds you don't have to worry about putting air in.  As you go up in elevation you will have a change of negative .47 PSI for each 1000 feet of elevation change so as you go up your PSI will go down and as you go down 1000 feet you will have a positive .47 increase in PSI. Going from sea level to 10, 000 feet would drop the pressure 4.7 psi. That would still be within your minimum PSI rating. If you start at 10,000 feet and go to sea level you would still be within the recommended PSI settings   For temperature you can figure about 2% of pressure differential for every 10 degrees of temperature change.  Going from 70 degrees to 100 degrees would be about 6 PSI and would still be under your 110 PSI maximum.  Going from a start of 100 degrees to 0 degrees would decrease the pressure by 6 PSI and still be withing the chart recommendations. 

I like the fudge factor way of airing up my tires.  I set the pressure so that it can drop several PSI and still be within load range, or increase by several PSI and still be under the max PSI recommended by the tire manufacturer.

If you have not done so, go to Tireman9's web site and read up on some of his tips an tricks. Roger is very well studied on the subject of tires.

[tireman9]

10 hours ago, rpbuttery said:

Tireman9,

Actual corner weights were:

LF--4,500

RF--4,860

LR--8,980

RR--9,080

I have new Michelin XZA3 tires--275/80R22.5, load range G.  They replaced Michelin XZE2 tires (the MH manufacturer's original equipment).

Thanks.

(May have answered my own question.  In looking at the Michelin RV Tire Reference Chart, I see that the maximum pressure--for both the single and dual positions--is 110 psi.)

Thanks for the numbers.

For the fronts we look for 4860 and find 80 psi is the minimum so I would suggest +10% or 88 rounded to 90psi for your CIP set point.

Rears (duals) we see the capacity for two tires in dual to be right at 9080 with 80 psi so I would again suggest +10% or 88 for your CIP set point Since you were right on the nose I would actually consider the minimum to be 85 so the CIP would be 94  rounded to 95psi.

 

I used the Michelin RV tire guide pub 04/15 for the above numbers.

Have you confirmed your digital pressure gauge is accurate to +/- 2 psi or better from a certified gauge?  When I check gauges at events I normally find 10 to 15% are off by 5 psi or more and fewer than 10% are accurate to +/- 1 psi.

 

The above variations are one reason i suggest a +10% over your minimum inflation. That way when cold weather or altitude or just air permeation add up and you start to see - 3 or -6 psi you still have a margin above your minimum needed and can top up the air at the next fuel stop and never run at or below your minimum inflation.

 

[rpbuttery]

Thanks all for the great information.  (The specifics to my data from Wayne77590 and Tireman9 were especially helpful--non-technical info that even I can understand.)

If you could backstop me on one other thing. 

I use the TireMinder Model A1A tire pressure monitoring system.  The "normal pressure loss" alert (as contrasted to "rapid" or "semi rapid" air loss) in this system is issued when the pressure in a tire has dropped 15% below the baseline pressure.  (TireMinder uses baseline pressure to be the cold inflated pressure the tire is to contain.)   

Based on a synthesis of the comments above, if I set the MCIP for the front tires at 90 psi, and the rear at 95 psi, my TireMinder normal pressure loss alert would happen at approximately 77 psi and 81 psi, respectively.

Does that sound reasonable and acceptable?

P.S. to Tireman9--I use my coach air compressor system with NAPA 90-475 air hardware with bleed valve.  I haven't confirmed its accuracy to a certified gauge, but I will. 

 

[wayne77590]

A loss of 15% is going to put your tire weight PSI below the 80 psi needed for your weight and the same for the rear tires.  You may wish to use Roger's 10% value as that would keep you just above your minimum recommended value. same for the rear(s). A 10% drop from 90 psi is 81 psi and anything lower than that you want to know immediately. A 15% drop would put the front at 5 psi below the recommended minimum for the tire(s). (I think - Roger can explain it better)

[manholt]

Wayne.  Don't sell yourself short.  You and Tman said the same thing in about the same manner....10% good rule....KISS works!

[tireman9]

Yes there are a couple steps I would suggest for everyone once you get the RV weighed and consult the table to learn in MINIMUM inflation you should ever see in your tires.

 

- The +10% is to avoid chasing your tail with adding 1 or 2 psi when temperature, elevation or even barometric pressure and normal air permeation result is a loss of a pound or 2. Some think they must always run exactly the pressure in the tables. This could mean they are adjusting inflation daily which I am definitely opposed to unless you are running a race car    which is a different story.

 

With a 10% "window of opportunity" you can tolerate day to day variation without any worry. Too much worry or attention to tire pressure will drive you crazy and can lead some to just give up and to stop doing due diligence for tire maintenance.

 

- With a good TPMS you need to figure out how to adjust your settings (different for different systems) such that the -15% will be no lower than the minimum inflation needed to carry the load ALL THE TIME. If you can't figure this out you should contact your TPM dealer and ask how to set the TPM such that it will warn when you hit xx psi.

 

.

[rpbuttery]

Let me try this again.  I think I confused by mixing definition and terms.

Using the terms from a Tireman9 blog post, I am going to use 80 psi for the front tires minimum cold inflation pressure (MCIP), and 85 psi for the rear tires MCIP, as suggested by Tireman9 above.  Then, adding 10 psi to each, I will use 90 and 95 psi as my goal cold inflation pressure (GCIP), or set point (I.e., the pressures at which I will maintain the tires).

However, because my TireMinder system uses a 15% (which I cannot change) below set baseline for normal air loss alerts (rapid air loss alerts are,of course, immediate), I will set the TireMinder baselines at 95 psi for the front, and 100 psi for the rear.  Thus, normal air loss alerts (which in theory should never happen) would happen at 80.75 psi for the front, and 85 psi for the rear. 

Is this sufficiently muddy? ?

[rpbuttery]

Thanks, Tireman9.  (I think our posts crossed in the air.)

I think I've got it now.

[obedb]

Wilde Bill 308/ you been hanging around with RodgerS.

[wildebill308]

4 hours ago, ObedB said:

Wilde Bill 308/ you been hanging around with RodgerS.

No He must be off learning how to cook snails. I just dislike someone who ask a question then says it is a “rhetorical question” and didn’t need an answer. Then argues about what he said (that is in print) You should have seen the response that was deleted probably because of my spelling.

Bill

[manholt]

Bill.  Hang in there!  We have all had some deletion of late!

[wildebill308]

Well to keep this on track I checked my tire pressure and it was within a pound of when I last checked it. I saw a devise that ties the rear tires together (kind of a Y valve stem extension) so they run at exactly the same pressure. I was wondering if anyone had tried this. My concern would be if one gets a leak they both go down. This was a real and actual question.

Bill