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Old 05-16-2015, 12:13 AM   #15
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Quote:
Originally Posted by Tireman9 View Post
Dd you miss this on pg 51? "Determining Proper Inflation Pressure - With actual weights of the loaded RV acquired by weighing, it is possible to compare them against the GAWR, GVWR, and tire capacities posted on the vehicle tire placard or certification label. These actual weights are also what should be used to determine any increase in inflation pressure for the tires, if required. (See “How to Determine an RV's Actual Weight” on p. 53.)
Inflation pressure recommendations may also be determined based on the tire manufacturer's specifications, which define the amount of inflation
pressure necessary to carry a given load. These inflation pressures may differ from those found on the vehicle tire placard or certification label.
However, never use inflation pressure lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall
."

I also have different interpritation of the info on pg 55 that advises to use the Load Inflation tables in "Note #1"

Could you explain what is wrong with using the tables to establish the inflation needed to carry the actual load on a tire in RV application?
Highlighted in red is my reasoning. We all interpret what we read or hear differently.
There is nothing wrong with using the tables to determine the absolute minimum air pressure to carry the actual load. Adhering to Michelin and other load/inflation charts-up to a point; because there is no margin for pressure fluctuations, if a large drop in ambient air temperature causes your tires to lose pressure, such as driving from S. Florida in March to your destination in MI a week later; you have under-inflated/over-loaded tires, which is the #1 cause (approx. 90%) of all tire failures; again, according to the RMA.
From page 50, here is another reason to never run load/inflation chart pressure:
Crowned road surfaces (which can cause inside dual tires to support more of the load than the
outside dual tires)
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Old 05-16-2015, 05:59 AM   #16
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Quote:
Originally Posted by Tireman9 View Post
Dd you miss this on pg 51? "Determining Proper Inflation Pressure - With actual weights of the loaded RV acquired by weighing, it is possible to compare them against the GAWR, GVWR, and tire capacities posted on the vehicle tire placard or certification label. These actual weights are also what should be used to determine any increase in inflation pressure for the tires, if required. (See “How to Determine an RV's Actual Weight” on p. 53.)
Inflation pressure recommendations may also be determined based on the tire manufacturer's specifications, which define the amount of inflation
pressure necessary to carry a given load. These inflation pressures may differ from those found on the vehicle tire placard or certification label.
However, never use inflation pressure lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall
."

I also have different interpritation of the info on pg 55 that advises to use the Load Inflation tables in "Note #1"

Could you explain what is wrong with using the tables to establish the inflation needed to carry the actual load on a tire in RV application?
So in my case the placard shows the same pressure for all tires which match the inflation pressure molded on the tire sidewall. After weighing, the weights from the chart say I should reduce the pressure by about 20PSI. So as a consumer should I use the pressures from the charts and go against the warning in the Rubber Manufactures Guide which states, "However, never use inflation pressure lower than specified by the vehicle tire placard, certification label or owner’s manual. Nor should inflation pressure exceed the maximum pressure molded on the tire sidewall[/I]." This is where I am confused. My interpretation is if the placard has a lower psi than what is molded on the tire, then it is ok to raise the psi to an amount which is less than the psi on the tire, but you should never run the tire at a psi lower than the placard(like in my case).
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Old 05-16-2015, 06:31 AM   #17
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It sounds like, from all of this information, that, because Ford recommends 65 psi in my front tires and the tires max psi is 80, it would only benefit me to weight the front axle, and adjust the front pressure, somewhere between 65 psi and 80.

I only wonder if a weight, suggesting a higher psi, would also be a weight, over my front axle, weight rating.
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Old 05-17-2015, 10:50 AM   #18
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Keep in mind, all the load/inflation charts show the absolute minimum air pressure to carry the corresponding load, not the optimum.
Somewhat like flying on an airplane with the absolute minimum fuel to reach the destination, if anything goes wrong, something bad happens.

twinboater, Ford (now) and all other vehicle mfgrs. recommend an air pressure to safely support that vehicles maximum GVWR. That's what got Ford in trouble with the roll-overs, their recommended air pressure was established for passenger comfort instead of vehicle control; to which, they finally admitted. And, you are right according to that RMA paper.
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Old 09-16-2015, 07:04 AM   #19
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Tire Pressure

Quote:
Originally Posted by twinboat View Post
I read from Ray,IN's post;

" never use inflation pressure lower then specified by the vehicle tire placard, certification label or owners manual. Nor should inflation pressure exceed the maximum pressure molded on the sidewall"

In my, case both of those numbers are 80 psi, for the rear tires, on my MH.

Why would I need to get weights and tire pressure charts, f or a recommended pressure and then add 10% to it.
YES!!
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Old 09-16-2015, 11:52 AM   #20
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The inflation tables provide the opportunity to tune (optimize) the pressure for the actual load. Optimizing the pressure may yield an improved ride and/or handling and maybe even a bit more tire life, but we are talking very small differences in most cases.

The placard values are intended to make it possible to drive safely without any inflation tuning, but even that assumes the coach is not overloaded (GVWR & axle GAWRs not exceeded). These days, coach builders err on the side of caution when establishing the placard psi values. Nobody wants a repeat of the Ford Explorer rollover kind of problem (caused by inflation that was inadequate for some operating conditions).
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Old 09-17-2015, 07:07 AM   #21
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Originally Posted by Ray,IN View Post
Keep in mind, all the load/inflation charts show the absolute minimum air pressure to carry the corresponding load, not the optimum.
Somewhat like flying on an airplane with the absolute minimum fuel to reach the destination, if anything goes wrong, something bad happens.

twinboater, Ford (now) and all other vehicle mfgrs. recommend an air pressure to safely support that vehicles maximum GVWR. That's what got Ford in trouble with the roll-overs, their recommended air pressure was established for passenger comfort instead of vehicle control; to which, they finally admitted. And, you are right according to that RMA paper.
I respectfully disagree. The load inflation charts ARE the optimum pressure to run. If you know exactly your weights, then you can use the OPTIMUM pressure for that weight.

You may want to allow more air pressure in case you are going to a colder climate, if you think your weight may change, etc. But, at the weight given, those pressures ARE optimum.

I run semi back and forth from ND to FL. I run 100# rear, and 120# front. I go from -30 in ND, to 80 in FL in the winter. So, I need to set pressures for ND, and they are higher in FL! You may need to adjust those pressures according to your usage.
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Old 09-17-2015, 09:06 AM   #22
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Originally Posted by mpierce View Post
I respectfully disagree. The load inflation charts ARE the optimum pressure to run. If you know exactly your weights, then you can use the OPTIMUM pressure for that weight.

You may want to allow more air pressure in case you are going to a colder climate, if you think your weight may change, etc. But, at the weight given, those pressures ARE optimum.

I run semi back and forth from ND to FL. I run 100# rear, and 120# front. I go from -30 in ND, to 80 in FL in the winter. So, I need to set pressures for ND, and they are higher in FL! You may need to adjust those pressures according to your usage.

Ray is correct in his wording. The pressures shown in the chart is the minimum pressure required to support the indicated weight. The pressure indicated on the sidewall is the minimum pressure required to support the maximum weight indicated on the sidewall. Optimum pressure is a relative term that the tire companies don't address. You may find that 5-10 psi more pressure improves the handling feel of the coach. The tire companies are primarily concerned with safe pressures. Under inflated tires lead to build up of heat. Heat kills tires.

Make double sure, if you use all corners weight and the chart, to never run with less pressure in each tire on an axle than in indicated for the tire onthe heavy end of that axle.


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Old 09-17-2015, 10:03 AM   #23
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The pressure indicated on the sidewall is the minimum pressure required to support the maximum weight indicated on the sidewall.


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I read this over and over, from some on this site, but when I read my tire sidewall, I read this;

LOAD RANGE E
MAX LOAD SINGLE 1215 kg ( 2680 lbs ) AT 550 kpa ( 80 psi ) MAX PRESS
MAX LOAD DUAL 1120 kg ( 2470 lbs ) AT 550 kpa ( 80 psi ) MAX PRESS

To me, this means no more then 2680 lbs of weight on the tire.

It also means that I should NOT put more then 80 lbs of air pressure in that tire.

I can't find anything about Minimum pressure.

They are 16" light truck tires on my Class C, MH, maybe larger tires have something else on the sidewalls, but I have never seen it.
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Old 09-17-2015, 10:16 AM   #24
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Originally Posted by twinboat View Post
I read this over and over, from some on this site, but when I read my tire sidewall, I read this;

LOAD RANGE E
MAX LOAD SINGLE 1215 kg ( 2680 lbs ) AT 550 kpa ( 80 psi ) MAX PRESS
MAX LOAD DUAL 1120 kg ( 2470 lbs ) AT 550 kpa ( 80 psi ) MAX PRESS

To me, this means no more then 2680 lbs of weight on the tire.

It also means that I should NOT put more then 80 lbs of air pressure in that tire.

I can't find anything about Minimum pressure.

They are 16" light truck tires on my Class C, MH, maybe larger tires have something else on the sidewalls, but I have never seen it.

I suspect this is a difference between light truck tires and the 19.5 and 22.5 commercial tires. The following is copied directly from Michelin's RV data book:

The amount of pressure required in each tire depends on the weight of the fully loaded vehicle. So the RV owners cannot determine the tire’s correct pressure unless they know their vehicle’s actual weights. The maximum load capacity allowed for the size tire and load rating and the minimum cold inflation pressure needed to carry that maximum load are located on the tire’s sidewall. The lower the pressure, the lower the load that the tire can carry. A complete load and inflation table is availableatwww.michelinrvtires.com;MICHELIN® RV Tires: Guide For Proper Use and Maintenance and RV Tire Information – MWL43146; and the MICHELIN® Truck Tire Data Book – MWL40731.


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Old 09-17-2015, 12:49 PM   #25
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Tire Pressure Confusion

[QUOTE=Steve Ownby;2748078]I suspect this is a difference between light truck tires and the 19.5 and 22.5 commercial tires. The following is copied directly from Michelin's RV data book:

The amount of pressure required in each tire depends on the weight of the fully loaded vehicle. So the RV owners cannot determine the tire’s correct pressure unless they know their vehicle’s actual weights. The maximum load capacity allowed for the size tire and load rating and the minimum cold inflation pressure needed to carry that maximum load are located on the tire’s sidewall. The lower the pressure, the lower the load that the tire can carry. A complete load and inflation table is availableatwww.michelinrvtires.com;MICHELIN® RV Tires: Guide For Proper Use and Maintenance and RV Tire Information – MWL43146; and the MICHELIN® Truck Tire Data Book – MWL40731.

I agree with Steve completely. The ONLY Way to inflate your tires on an RV accurately is to get the weight of the axles and refer to the manufacturer tire inflation chart. Adding 10% as a safety factor seems wise (overinflation is safer than under inflation as long as you do not exceed the maximum pressure). Not only is the ride smoother when properly inflated, but I am convinced that properly inflated tires are the safest. We have a 35,000 pound class A diesel pusher. I was using the maximum inflation and the vehicle bounced around dangerously, at times. At proper inflation the maximum tire tread is gripping the pavement. Our Maximum inflation pressure on our Michelin tires was 120lbs. After weighing, the manufacturer recommendation was 95 pounds. Huge difference!

By the way , I resisted getting the vehicle weighed because it seemed complicated. It was REALLY easy. Just find a local CAT weigh station at a gas station and it takes 5 minutes, tops. Use the axle weights and look up the tire inflation in the manufacturer's chart. Very simple.
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Old 09-18-2015, 01:13 PM   #26
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I think the problem is that many are operating to slightly different understandings of the words being used.

Load Inflation tables identify the minimum inflation for a number of specific load placed on the tire.

Tire sidewalls tell you the maximum load capacity for a given tire when inflated to the maximum pressure for that Load Range in that size tire

Optimum implies we have common agreement on which performance requirements we feel are most important.
Optimum also implies that there is agreement in the inherent trade-offs of the numerous performance characteristics

If the only performance we are concerned about is load capacity and if we want the maximum capacity possible for the size and load range of a specific tire then we have a situation where the Optimum inflation is the Maximum for the Load range which is also the Minimum inflation specified in the tables.

If however we do not need to support a load that corresponds to the tire's maximum load then it is possible that other performance criteria may be considered and as a result there will be trade-offs to arrive at a new Optimum.

Since we are talking about Motorhomes there are normally performance characteristics other than just load capacity. Some might be Fuel Economy, Noise, Ride Comfort, Tread Wear, Steering Response etc. Inflation pressure will affect each of these characteristics. Some positively and some negatively so clearly the "Optimum" depends on the clearly understood and agreed on priority of these and other characteristics.

Bottom line
As a tire engineer I suggest that people select an inflation pressure that will provide at least 15% extra load capacity over the heaviest loaded tire on an axle. All tires on an axle should run the same cold inflation
The above will still provide acceptable ride and provide improved durability and fuel economy.
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Old 09-18-2015, 01:40 PM   #27
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Tireman9-

A question came to me as I looked at the manufacturer's tire loading charts for a Class A I'm considering.

Suppose the measured (i.e., scale-weighed) weight on an installed tire was lower than the lowest charted pressure for the tire?

I know this is an uncommon problem. I can see four ideas right off:

1) Run the installed tires at lowest charted pressure
2) Run the installed tires below charted pressure
3) Install tires that can run within their pressure chart at those weights
4) Increase the coach weight (e.g., carry liquids)

Each of these has drawbacks. If smooth ride was the goal and money not a problem, I'd pick 3), but I'm interested in your take- and have probably overlooked some other options.

Thanks!

Mark

P.S.- Here are the actual values that have me asking this question:

Monaco Monarch 30PDD SVE (Ford F53 chassis)
Gross vehicle: 20,500 lbs
Gross combined: 25,500 lbs
Front gross axle: 7,000 lbs
Rear gross axle: 13,500 lbs

Michelin tires
Size: 245/70R 19.5
Max. speed: 75 MPH
Minimum charted pressure: 80 psi
Single tire, 80 psi: 3640 lbs
Dual tire, 80 psi: 3415 lbs

These values are from the Owner's Manual. But, they also match the Michelin Truck Tires Data Book, for this size tire, models XRV (LRF) and XVE (LRG).

2 x 3640 lbs = 7280 lbs (exceeds front gross axle)
4 x 3415 lbs = 13,660 lbs (exceeds rear gross axle)

Maximum tire load at minimum pressure exceeds axle load maxima in both cases.
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Old 09-18-2015, 05:26 PM   #28
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Tire Pressure Nirvana

I have been reading and studying about this for months. I finally did what the Tire Manufacturer and most experienced RV Owners have been saying and had our RV weighed. Then, I adjusted our tire pressure using the data in Michelin's tire pressure chart for our tires and weight. Increased the pressure by 10% for safety(clearly, a bit too high is much safer than any amount too low). Up until now, I have been using the maximum load pressure on the door of our RV and the sidewall of the tire.

OH MY GOSH! The proof is in the pudding!

We are on our way to the Grand Canyon and Albuquerque Balloon Festival fron Michign. I cannot believe how much BETTER and SAFER the ride is!!!

The tires are gripping the road better on both dry and wet pavement. How can I tell? The wind and air pressure from passing trucks are having much less impact on moving the RV around and the need for steering adjustments are dramatically less. Virtually negligible.

The vehicle is so much easier to control on the road. It is particularly noticeable in construction zones with narrow lanes and numerous bumps and pavement height changes. It is SO much easier to steer and control the vehicle. Not jumping all over the place.

And the ride is dramatically smoother. My wife has noted it and commented on multiple occasions.

I almost didn't make this change because my wife and I are very conservative when it comes to safety issues. We didn't work all these years to retire and end up in a crash heap. We figured maximum pressure is best/safest. I no longer believe that.

I am sold on weighing the RV and setting the tires to manufacturer's recommendations plus 10%. Now I see why so many posts recommend it. Sorry I didn't do this two RVs and a couple of years ago when we started this post retirement RV journey.

PS. Any post about tire inflation safety would not be complete without stressing the critical importance of having a Tire Pressure Monitoring System. We bought one the first day we RV'd. $400 seemed like a lot of money at the time. But, no matter what you set your tire pressure at, having a device that let's you know if you have a pressure or temperature problem with one of your tires can save your life. I have read many times that THE biggest risk for an RVer is a tire problem. A TPMS is worth every penny!

Happy Travels,

Bob & Nancy M, and Lexi our Maltese
2014 Tiffin Phaeton 40QBH, Cummins Diesel, Blue Ox with Patriot Brake towing 2014 GMC Acadia.
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