Reader's questions on inflation and type tire load capacity.

 Questions from Readers

"Is there a way to verify the tires on my trailer have been inflated with nitrogen? My tire dealer said they did but I am not sure if I can trust them. The pressures stay even tire to tire and not a lot of change while driving which I think tells me that the tires are inflated with nitrogen."

My answer: As a Tire Design Engineer, I see no reason to worry about the claimed use of Nitrogen, The air we breathe is 78% Nitrogen. While using N2 to inflate tires is not bad it just isn't worth spending any money on it.

About your question on inflation:    Are you using your TPMS to monitor tire pressure or are you using a hand gauge? TPMS are generally accurate to +/- 2% and they can warn of puncture while driving down the road. I have seen a tire fail because the owner used his hand gauge but the valve core stuck slightly open and the tire failed 50 miles down the road.

Here is a post of my inspection of that tire. I believe that if he had been running a TPMS he would have avoided that failure.
https://www.rvtiresafety.net/2020/11/claims-of-defective-tires-causing.html

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A question on "LT 225/75/16E. vs. 225/75/16 Commercial tires"

"I’m looking to replace tires on E-450 chassis motorhome. The OEM tire is a LT 225/75/16E. Reading the forums, a number of people are replacing the OEM tires with 225/75/16 Commercial rated tires. They are doing this because the OEM Load Capacity is very close to the sum of the load of the four E rate tires. Four Commercial adds appx. 500 lbs. per tire load capacity or 2000 lbs. for four tires."

My research has turned up seven tires that have both an E rated and Commercial in the same model tire. What I found and surprised me was the as the weight difference between the two tires. With the exception of one brand, all show that the Commercial rated tire weighed less than the LT E rated tire. As a lay person, it seems counter intuitive to me. While there are other tires, here are two tires I am considering:

Michelin Ailis CrossClimate
Commercial: 37 lbs.
LT E rated: 39 lbs.

Toyo Celsius Cargo
Commercial: 36 lbs.
LT E rated: 38 lbs.

I would have thought the commercial tire would have been heavier given that the commercial tire has appx. 18% more load capacity. Can you explain and shed some light on this?

Thanks

My answer:

ED We need to be careful when comparing tires as the actual "TYPE" and Load Range can result in our selection.The Michelin Agilis is a good example. This is a tire developed in Europe and does not carry the "LT" designation as in Europe they call some lines "Commercial" and other tires of the same size are not considered "Commercial".  The Load Index number is the tip off.   You can consider this a bit like the "Load Range" AKA "Ply rating" in the US market.
The additional load capacity differences come about because of the conversion of Metric to "English" dimensions. I think that if you look at the actual specifications for the tires you are looking at, you will find the stated inflation numbers are slightly different. Close but different.  The expected usage can also result in some tires being considered "Commercial"  vs "RV".
I do see many people asking about or posting information on tires but they do not include the "Complete" designation in their Internet posts. Some include the ST or the LT on US sizes. European tire sizes do not have "ST" type tires or "LT" as leading letters but have a "C" after the rim diameter standing for "Commercial". (Not to be confused with Load Range C) To further confuse the issue some tires are available with two different Maximum Inflations and different Maximum load capacities.
For your question I would suggest you look at the Maximum Load Capacity and the inflation necessary to support that load. If the inflation is higher than that shown on your Certification Label, which I think might show an LT type tire in Load Range E and 80 psi. Your "Commercial grade" European tire may specify inflation a bit higher than 80 psi. If so you need to confirm the max inflation rating for the new tires.

I would not be concerned with the weight of the tires you are considering as design and wear goals can add or subtract a couple of pounds per tire
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I got this request:

How about a recommendation for a quality tire pressure gauge? I no longer trust the typical stick gauge.

I have found the Accutire MS-4021R Digital Tire Pressure Gauge from Amazon to be very accurate.

I have 3 and they always read +/- 0.5 psi from the ISO Laboratory Master I used to check my gauges.

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This question from a Class-A owner:

We have a Class A diesel MH. Our maximum cold pressure on the front tires is 120 psi. After weighing our coach we need to run 113 psi. We add an extra 5 psi for safety, so our cold pressure is 118 psi.

We live along the Texas coast, so our summer morning temps can be in the high 70's or low 80's. Before leaving for a trip I set my cold pressure to 118 psi first thing in the mornings.

During the summer we often travel to Colorado or the mountains of New Mexico, chasing the cooler temperatures. A few weeks ago while in New Mexico the morning temperature was in the high 40's and my tire pressure had dropped to 106. We were leaving that morning for Amarillo TX. The high that day in Amarillo was to be 101. My dilemma: add air to increase my pressure to 118 and chance once getting to Amarillo of having my pressures into the high 130's or low 140's; or drive several miles on the lower pressure tires knowing that they would increase in pressure in several miles to a safe traveling pressure.

What I did was to add about 5 psi to each of my front tires. Upon reaching Amarillo my pressures were in the mid to high 130's. The next morning before starting our day I checked the pressures and released about 5 psi in each front tire to get me to my normal pressures of 118 psi.

My answer:

Yes, you set the COLD Inflation Pressure in the morning before you start to travel and before the tires have warmed up from travel or being in the direct Sun. You do not have to worry about the increase as you travel into warmer parts of the country or as the tire warms up from travel.
You never want to travel on underinflated or low-pressure tires. Your 130 or 140 psi is not a problem.
You can adjust to the cooler temperature and lower PSI the next morning after the tires have cooled down.

Your +5 psi is a good idea. That is about what I use on my RV.

Side question: Where did you get the suggestion to inflate to 113 psi cold? The Load Inflation tables are usually arranged in increments of 5 or 10 psi. You should not interpret the inflation between the individual boxes found in the tables. If you need more then jump up to the next level.

 

Roger Marble

*****

 ##RVT1173

Proper care of tires is NOT rocket science

 Looking at some posts and questions on various RV forums, it seems that some folks think that proper care of tires is as complex as rocket science. While I will admit I can get very wrapped up when talking about tires, I do try to be reasonable, as I believe that if the instructions are too complex, detailed, or long-winded, some folks will throw up their hands and give up. I definitely do not want you to give up.

Four basic steps for proper care of tires

Here are four basic things I think every RV owner should do. These are probably the biggest “bang for the buck” actions you can take to avoid having a tire “blowout.”

1. Find and record the information on your certification label that looks like this:It contains the VIN, tire type and size, load range, GAWR (Gross Axle Weight Rating), and inflation. This information provides the foundation of the knowledge you need, whether your RV is a 45′ Class A Diesel Pusher with three axles…… or a 10′ bumper-pull teardrop trailer.

2. Learn the actual weight of your RV. This means that you need to get on a truck scale such as this.

Don’t worry, getting your RV weighed is easy (check out YouTube videos) and only costs about $15. By “actual weight,” I am saying you need to have the RV loaded to the heaviest you ever expect it to be. This means all the clothes, water, fuel, tools, and even your bowling ball collection on board.

3. Then compare the scale weight and confirm the number is BELOW the GAWR for your RV. If not, that means you need to carry less stuff when you travel, so it’s time to put your RV on a diet.

4. Once you have confirmed your actual weight is lower than your GAWR, you can simply inflate your tires to the PSI on your certification label.

Extra information

The PSI number on the tire sidewall is just identifying the inflation the tire needs to support max load number on the tire sidewall. That is NOT the highest inflation the tire can tolerate.

So that’s it. Four steps and you are good to go. Yes, there is more you can do such as installing a TPMS, which I highly recommend, or even learning your “4-corner-weights.” But the above four steps are what I would consider the absolute minimum steps needed.

How much air pressure is too much in RV tires?

 With the introduction of Tire Pressure Monitoring Systems (TPMS), many RV owners are presented with what to them is new information on the status of their tire pressure and temperature.

We can start off with a question:

“I have a serious concern with the G-rated tires on my 5th wheel trailer. When I run the cold max pressure at 110 PSI, I get TPMS readings up to 134 PSI rolling down the road. Lately, I have been running 95 PSI cold and am getting 115-119 PSI rolling. The tires and rims are rated for 110 PSI. I can live with 120 PSI, but the 134 PSI concerns me.”

The inflation ratings for wheels are based on a “cold” pressure. An increase in pressure due to operation is considered by wheel manufacturers and I would not be concerned as long as the proper cold inflation is used along with appropriate limits on load and speed.

Tire wording “Max Pressure” can be confusing. In reality, the pressure stated on the tire sidewall is the cold pressure needed to support the stated load. That load is the maximum load the tire should be subjected to. I advised the owner that he should NOT underinflate his tires and plan on seeing the operation temperature to result in an increase in the pressure sufficient to support the load. The “cold” pressure is the only pressure you should be concerned with.

Tire pressure increases with increase in temperature

Tire pressure increases by about 2% for each increase in temperature of 10° F. If you don’t remember the science from high school, you can read THIS post from my blog on Tire Safety. If you are seeing a 21% increase in pressure (110 > 134 psi), that means you are seeing about a 100° F increase in internal tire temperature. I would consider that increase a bit much for normal tire operation. In my 50 years of designing, testing, and working with tires, I only remember measuring a temperature rise of 100° F in Indy race car applications.

If you are seeing a 25% increase in pressure (95 > 119), then you are getting a 125° F increase in temperature. This indicates you are working the tires very hard. This extra “work” that is generating a greater temperature increase is not good for long-term tire life. You are “aging” the tire rubber faster. Some might want to review the “Key Point” of tire life and how increased temperature can shorten life as covered in THIS post.

Possibly overloading tires and/or driving too fast

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading. It is recommended in tire company data books that your operating speed for any tire in RV application be no greater than 75 mph. The “Speed Rating” is just a short-term rating and should never be considered acceptable in day-to-day RV applications.

You need to confirm your pressure is 110 psi AND that your gauge is giving an accurate reading at that level.

This was the reply:

“Today drove 250 miles and my 95 PSI tires were running 115-119 PSI and the tire temps were at 20F above outside ambient temp of 50F and 71F tire readings. I still contend that 134 PSI is way too dangerous for tires to run on 110-rated tires.”

So I responded:

Few people realize that the pressure increase as a function of temperature is based on well-established and confirm able physics. A TPMS is not reading the actual tire temperature but is actually reading the temperature of the brass valve stem and the metal base of the TPMS itself, which is being cooled by outside air.

Air is a good insulator

Air is a very good insulator. If you think about it, you have a small column of air running up the inside of the valve stem which makes it difficult for the heat to travel up the center of the stem and past the valve core itself. This is all while the valve is moving rapidly around being cooled by the outside air.

I am aware of laboratory tests that go against what “common sense” might indicate. That being that the air inside a tire is not uniform in temperature but it is always cooler than the hot spots of a tire. It is the hot spots that can result in a tire failing if hot enough for long enough.

I have no doubt that the TPMS was indicating only 20° F above the cool 50° F outside air temperature. You should not be concerned about the hot pressure of 134 psi on tires that have a cold pressure rating of 110 psi for its max load capacity rating, that is, as long as you have confirmed the actual load you are placing on your tires is no greater than 90% of the load capacity shown in the Load & Inflation tables for your tire. By “confirmed” I mean with actual scale readings for each axle when the RV is fully loaded

As a tire design engineer with 50 years’ experience, I trust the science of the “Gas Law” and the knowledge that air is an insulator and metal conducts heat from a hot source to a cooler one.

I don’t know what to advise other than to decrease the operating load and speed and to confirm your hand gauge is accurate. Also, always inflate the tire when cold to 110 psi, as continued operation at current load and speeds will certainly result in pressure readings that are well above the cold pressure of 110 psi.

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Another question on "Cold Inflation" vs your "Set Pressure"

Got this question from a reader of an RV Forum:

Thank you for all of your informed comments regarding proper tire care. I need one clarification. I have always considered the cold psi on the side of my 22.5 RV tires to be the minimum to carry the maximum rated load, but have assumed that psi was also the maximum COLD psi the tire should see. From your recent post, am I to understand that unless the tire states that it is the maximum cold pressure, I can exceed it by 5-10 psi?
Thank you for your time, Doug

My Answer:

The wording on tire sidewall IMO was written by lawyers, not engineers or users. Info on the sidewall is the inflation needed to support the Max load.  The difficulty is that few understand that the pressure changes with temperature and the only meaningful pressure, measurement is when the tie is "cold". This still confuses some because some want to apply Chemistry Lab practice of adjusting to theoretical 72.5°F when what "cold" really means for tires is at "Ambient Temperature" and does not include any pressure build-up"   In real life terms this means "Not warmed by being driven on or in direct sunlight for the previous 2 hours"

Now we need to address what is meant by "Cold Inflation" vs the psi to set your tires to or what I like to call your "Set pressure".

I like to suggest the "Set Pressure" for motorhomes to be the minimum needed to support the maximum load on the tires PLUS 10% inflation. 

RV trailers are different because of their Interply Shear problem.

For RV Trailers,  I would like to see a minimum of +15% load capacity over the measured heaviest loaded tire, with +20% to +25% Reserve Load capacity being better. Sadly most RV trailers come with tires that provide +0% to +10% load capacity vs GAWR
NOTE: I am not even addressing the tendency for most RV owners to overload their tires.
So for trailers I try and simplify:
- To lower, but not eliminate the Interply Shear problem I suggest the "Set Pressure" when the tires are "Cold" to be the pressure on the tire sidewall. BUT I still want trailer owners to confirm they have at least 15% "Reserve Load" over their measured scale reading.

##RVT962

Max Load - Max Inflation & Cold Inflation confusion continues

From an RV Forum "My understanding is that the number on the side of the tire is not a recommendation. Rather it is the maximum cold temperature that the tire is rated for. You should never run above that number. Without actual weights, I would run the recommendation from the motorhome manufacturer on the placard near the driver's seat, or a little higher, but never over the sidewall number."

The information on the tire sidewall is a statement of the maximum load capacity and the cold inflation required to provide that load capacity.

Different tires have the word "max" placed in different parts of the phrase which leads to the confusion.

Here is a post on passenger tires.

Here is a post on when the Minimum inflation is the Maximum.

Note: A a statement of Max Load "At" stated inflation such as "Max load 3,000 lbs at 80 psi" is leaving off the implied "minimum" before the 80 psi.

Also introducing the word "cold" seems to confuse some.

Maybe this will help:

Tire inflation should only be set when the tire is at ambient temperature and has not been warmed by being in sunlight or driven on for the prior two hours.

A tire may be loaded to the number of pounds stated on the sidewall, only when the tire has been inflated, when cold, to the stated pressure on the tire sidewall.

The problem is that these phrases don't fit in the same place as current "Max Load 3,000 lbs at Cold inflation 80 psi"

Setting tire pressures on cold days?

Here was his post...
"I am looking for a formula for adjusting "cold inflation" pressures for large tires. I have read that for smaller (car) tires the recommendation is 1 psi per 10 degrees F.

What is also not clear is what the assumed "cold" temperature is. Is assumed to be 65 F, for example. So if we are sitting at 25 F, what is the adjustment?"

My reply

Here is my blog post that explains "Cold Inflation"

We are not in High school chem class so there is no adjustment need to get to "standard " temperature.

If you are running a TPMS, which you should be, you will soon recognize the normal range of temperatures and pressure variation in your tires.

I would not worry about adjusting inflation pressure is today's expected temperature is +/- 10F from yesterday. After all, you should be running +10% over the inflation you need to support the measured load.

Let's say your minimum needed is 90 psi. Adding 10% that means your cold inflation (cold meaning the tire was not driven on or in direct sunlight for the previous two hours)  should be 99 psi so you round to 100 psi. All is good.

The next day's weather is 20F colder. That would mean the tire inflation will drop 4%  ( 2% per 10F change)

So now your tires would be 96 psi but since you have to stay above 90 I would say there is no reason to worry about adding air. Just enjoy life and head out.

If the next day your weather went up 10F instead of down that means tour "cold" inflation would go up 2% from the 96 or to about 98psi. Again you don't need to do anything.

I would not worry about lowering the pressure until I saw pressures 10 psi above the pressure molded on the tire sidewall. When you get to that point you could bleed off a few psi but remember if the weather gets cold over the next couple of days you will be needing to add air again.

Don't get your shorts in a bunch about inflation. Just watch your TPMS and you will learn what is normal for your coach.

Do inner duals fail more often? If so, why?

Read this on an RV Forum thread:

"Isn't it weird how it always seems to be the inner tire that goes bad or blows? Seemed to always be my experience (bad luck) when I was driving semi's for a living."

While I have never seen a documented study, I can certainly believe this observation can be made by many. There is Science behind the "Why" this may be true.

For those that have reviewed the post on Tire Covers that explains the effect of temperature on tires, you have learned that it is the higher temperature that accelerates the tire "aging" process. If you look at the temperature readings on sets of dual tires you will see that when the tires are inflated to equivalent pressures the temperature of the inner dual on Motorhomes will usually show as a bit hotter. The difference isn't a lot but the effects of that difference I believe are cumulative.

It is also true that older tires are more likely to fail due to the degradation of the rubber flexibility and strength.

Please do not take this observation and assume you need to start adjusting the inflation in your duals to run more air in the inner tire. Doing this could end up resulting in a shift in loading between the pair of tires to place more load on the inner tire, and we know that increased load results in increased operating temperature.

Tire operating temperature develops from complex actions of flexing of the belts and of the lower sidewall which are the two hottest locations on a tire. You might be able to lower the temperature in one location while increasing the temperature in a different part of the tire.

The best practice I can suggest is that you:
1. Confirm the tires in each pair of duals are a "Matched Set" (See THIS older post).
2. Ensure you know the actual load on each set of duals not just the total axle load.
3. Use the Load and Inflation tables to learn the MINIMUM Cold Inflation Pressure for the heavier loaded axle end.
4. Add a 10% inflation to that minimum number to establish your CIP.
5. Inflate all tires on the axle to the same CIP. (matching the inflation within +/- a couple psi is good enough.
6. Run a TPMS to monitor your pressure whenever driving.

How much inflation in my tires? Question from new RV owner

Recently read yet another forum post on the topic of "How Much inflation should I run. This post followed the standard format of:  A. I just bought an XXX RV. How much air do I need in my tires?  B. Numerous replies ranging from "I use xx psi" to "You must always run the pressure on the tire sidewall"  to the more correct reply of "You need to know your tire loads first". Some forum threads run to dozens of back and forth exchanges Some have correct info IMO while others are still using what I consider "Old Wives Tales".  I recently replied as follows.



1. Almost all tire Inflation/Load charts have identical numbers (maybe as high as 98% )so if you can't find your brand you can reasonably use another brand till you do.

2. Yes, the charts give the MINIMUM inflation needed but to avoid the need to adjust your inflation every morning (inflation changes by about 2% for every change of 10F) I and others suggest you add a bit so you are running the minimum + 0% to + 15%.

3. You should NEVER run lower than the minimum inflation shown on the chart.

4. Since all tires on any one axle should have the same CIP (Cold Inflation Pressure) you base the minimum on the heavier end. This is why we recommend "4 corner weights" to learn what the heavier end load is. Until you learn the actual load on the heavier end you can assume one end is supporting 53 to 55% of the total on the axle (the axle weight when on the truck scale).

5. How much to add over the minimum? I think you will find various suggestions in the range of +1 psi to +10%. Since I am offering advice to a wide range of users (ST trailer tires, LT and Class-A) with minimum inflations from 50 to 120 I prefer the percentage and if people want to have their CIP ending in 0 or 5 they can add the % then round up. ( Note the +1 Psi is from Tire Rack where they are advising owners of passenger tires that normally are running 30 to 36 psi)

6 Premature wear. Most LT and 22.5 size tires can be good for 60 to 100,000 miles wear so most RV owners will have their tires "age out" before they wear out so the minor wear difference, if any due to running + 5% over the minimum if overshadowed by variations in suspension wear, alignment variations, and driving habits. The center shoulder wear info is based on car wear rates primarily with bias tires or inflation differences greater than +/- 15% from the suggested CIP.

7. "Correct" inflation is not what the charts say. They are providing the "Minimum" cold inflations is every case.

8. Adding Load % and adding inflation % is done only to compensate for not knowing the actual heavy end of an axle. If you learn the actual heavy end when the RV is fully loaded to the heaviest you ever travel at, I do not suggest you need to add any more weight when doing your calculations. How much can you be out of balance side to side? I have seen a very small % of RVs with the load at 50/50 side to side. Most appear to be in the 53/47 to 55/45 range but a few have been found with over 1,000# heavier on one end.

9. It is important to realize that when we look at actual weights of RVs in use, over half exceed one or more of the weight specifications for tires or axle loading. IMO this is a clear indication of why so many RVs have tire problems.

How I program my TPMS

As you know I am a strong supporter of having all RVs being equipped with Tire Pressure Monitor Systems. Since 2004 cars and many pickups with ratings less than 10,000# GVWR have come with TPMS installed by the factory.

RV companies have not been providing TPMS even as optional equipment from the factory, so we owners have had to make the purchase and install the warning systems ourselves. This means we need to ensure the warning levels are appropriate for our RV.

While I can't address how to program every different type or brand system, I can tell you how I have the system I purchased set up. I hope that after reading this post you can sit down with the information that is specific to your RV,  i.e., load on each tire position, tire size, Load Range and inflation info, and ensure your settings are appropriate for your personal situation. So I suggest you dig out the instructions that came with your TPMS and review your settings.

First off, some terms:

MOP - Minimum Operating Pressure - This is the minimum pressure that I think you should ever operate your tires. This pressure is found in the Load/Pressure tables and is the minimum pressure needed to support the actual measured tire load.

CIP - Cold Inflation Pressure. This is the pressure you would set your tires to before driving on them. The tire should not be warmed by either driving on them or having the tire in direct sunlight for the previous two hours. Some folks call this their "Set Pressure" or "Baseline Pressure". I suggest the CIP be set to MOP + 10%, or +10 psi, but not to exceed the pressure molded on the tire sidewall associated with its max load, as this tire pressure may be the intended max CIP for the wheel.

Tire Load - This is the actual measured load on the tires when the RV is at the heaviest you would ever expect to be. Ideally this has been learned by having individual scales under each tire position on your RV. If you can't get this number then at least get the RV on a truck scale such as a CAT scale at a truck stop and learn the actual load on each axle. We want to know each tire position or each axle, as loads are almost never completely evenly distributed. It is not unusual to have tire on an axle or on one end of an axle to have 500# to 1,000# more than another position. There are a number of posts in this blog on load and how to get the loads on each tire position. Note duals are considered one position. If you can't get individual axle ends measured then I suggest you use the measured axle load and assume one end has 53% of the axle load. This would provide at least a partial margin for error.

Hot Running Inflation - This is the pressure we see on a tire after it has been running down the road. I have previously covered the relation between Temperature and pressure in THIS post. Normally for properly inflated tires this will be 10% to 20% above your CIP.


When I bought my Class-C unit in 2008 the first "add-on" was a TPMS. At the time I was not really aware of the selection of TPMS available for the RV aftermarket but I found a system made by German electronics company Hella. It was an internal system but it was designed for passenger cars so I had some problems as my hot tire pressures occasionally exceeded the high pressure warning level programed by Hella. I learned to ignore the occasional high pressure warning levels. In 2009 I started to go to FMCA RV conventions and rallies where I discovered the aftermarket systems designed for the RV use. Based on features I felt important, I purchased a TireTraker TPMS.
When we bought our new 2016 Class-C coach, I needed a new TPMS and selected the TireTraker TT500 as I felt the features plus the lifetime guarantee made it the best option available for me.

Programing the TPMS:
This is usually a two-step process. First, you need to have the monitor/display "learn" which sensor is on which tire. For the TT500 it involves stepping through the monitor settings, identifying a position and installing a sensor at the appropriate position. Other brand TPMSs may have different setup steps or even may come pre-programed with each sensor marked by position. It's the next step that I think is also important to get right for your specific needs.

Setting the warning levels:
Before we start the actual process of setting the levels, it is important to know what the various levels should be. You also need to know which pressure warning levels you can set and which may need extra work if you can't set the levels yourself. Again I can't address every brand TPMS so you may need to re-read your manual or even contact the support people at your brand TPMS.

1. Low Pressure warning level. Some systems have a fixed % below your baseline pressure; others may allow you to set this level independent of the CIP. You need to know how to set this level or what the % below the baseline your system works on. My TT500 is pre-programed to warn at -15% from the "Baseline" or CIP.

2. High Pressure warning level. The TireTraker is pre-programed at "Baseline" + 25%.

3. High Temperature level. The TireTraker, as are many TPMS, is set to warn of high temperature at the 70 C to 75 C (158 F to 167 F) range. IMO it should be remembered that it is low pressure that generated high tire temperature. If you get a High Temp warning but the pressure is OK, then your sensor has detected high temp from metal components that transfer heat faster than rubber. This might indicate a bearing or brake problem which should be checked as soon as possible.

Here things get a bit complex:
We do not want to operate tires when under-inflated but if we set the CIP or Baseline pressure to just what is needed to support the load the TPMS will not warn till we are 15% underinflated. If our CIP is MOP + 10% (See definition of MOP above) we still could end up with an underinflated tire. So to meet my goal of never operating in an underinflated condition, I will need to set my TPMS "Baseline" pressure to MOP + 15%.

My situation and solution:
As measured by RVSEF (RV Safety & Education Foundation):
My front tires are loaded LF 1,900#   RF 2,100#   and
Rear Duals  LR 3,550#   RR  3,850#
Tires are LT225/75R16 LR-E

Based on Load & Inflation tables for my size tires, my MOP for my front tires on my Class-C is 60 psi based on the 2,100 load and also happens to be 60 psi based on my read dual load of 3,850.

I add 10 psi to 60psi and get 70 psi for my CIP.

If I want my TPMS to warn me before I get to my MOP, I need to set the "Baseline" pressure on my TPMS to MOP + 15%, which for 60 psi is 9 psi. So in my case my "Baseline" for the TPMS and my CIP are within 1 psi of each other. I don't worry about trying to measure or set my tire pressures closer than +/- 1 psi. I would say that if you are within +/- 2 psi when setting your CIP you are in good shape.

We need to be careful when discussing safety margins or pressure changes as people with Load Range C, D and E tires will have different calculations than the folks with LR-G & H tires where 15% might be 18 psi.

The High Pressure warning level for TireTraker is Baseline +25%, so with a Baseline of 69 to 70 psi, that equates to 87 psi. On a recent trip to Indianapolis in the heat of the day I noted my tire pressure had gone up about 15%, so my settings worked out for me.

##RVT805

Another example of learning correct tire inflation

Quote:

Originally Posted by Ron Tireman9 and others....I just ran my coach over to the local Cat Scales at a truck stop nearby. As I stated earlier they do not make things easy and a 6" curb on each side of the scales prevented me from putting each side on separately so that I could get a close four corner figure. I settled on the two axle approach. All tanks were full, (water tank was actually just over 2/3, it fills very slowly and I got bored). Front axle came in at 11,400 lbs, rear axle was 16,560 lbs. My front axle is rated at 13,300 lbs and the rear is 21,000 lbs with a GVW of 34,300 lbs. 295-75R-22.5 tires. I have looked at the tables but would like to see what you folks come up with for approx. front/rear inflation? Thanks, Ron

Ron, you didn't say the brand tires. To my knowledge all major brands except Michelin have the same info for load & inflation and Michelin is same on maybe 95% and those that differ may only differ by 5 psi or so.

Since you only have total for an axle and do not know the heavy side we estimate one side at 53% -- So Front would be 6,042 and rear 8777 (4,389 per tire).

Most 295/75R22.5 LR-G need 110 to support 6,175 F (single) and 80 psi to support 4,540 in Dual.

Since I recommend a +10% of air for a good margin, that would suggest 121 psi front and 88 for the rear. BUT a LR-G is only rated for 110 psi so you need LR-H on the front. Also need to confirm the front wheels are rated for 120 psi and since even H are only rated for 120 that makes the recommended Front inflation 120 psi.

Clearly the estimate of 53% is causing the problem so it increases the need for you to learn the actual side to side load on the tires by going somewhere other than CAT truck stop scale.

It is CAT policy to not do one side weights and that is why they are making it harder to only get one side on the scale.

I suggest RVSEF

EDIT AND UPDATE   8/16/20

Locations with scales
https://www.dieselboss.com/service.htm
https://www.allstays.com/c/weigh-scales-locations.htm
https://www.publicscaleslocator.com/
SmartWeigh is another option

I checked around home and found a local building supply that had a scale that would give me 4 weights for $10. Only about 3 mi off I76 just East of Akron OH.

##RVT799

Another example of pressure and TPMS setting calculations

David said:

"I am a newbie. Just 5 weeks with my 2017 Newmar Bay Star 3113. I need some help understanding just what would be the correct or appropriate tire pressure on my coach. When loaded with my wife, dog and self, partial food and clothing, tools, chairs, BBQ ladders, 2/3 water, full gas and propane and misc stuff I got the following from the CAT scales: Front axle - 7,300. Rear axle - 13,260.

Here is my dilemma: The dealer delivered the coach with about 85 lbs. per wheel cold, but when I use the Michelin tire pressure chart for my 235/80R 22.5 XRE tires they should be just 75 lbs! I am concerned that they will be "underinflated" and could build up excess heat. Should I keep the 85 lbs. or lower the pressures?"


My reply:
David, Welcome to RV fun.

For all things tires (except buying) I obviously suggest you check my blog. I don't expect folks to remember everything but if you spend a few minutes checking it out you will learn how to use the "Label List" on the left to find a post of the topic of interest. There is also a search box in upper left.  Now to your specific question.

1. We want to know the heaviest load on your tires and since few RVs are perfectly balanced side to side for weight we ideally want to know the "4 corner weight" to learn the heavier end for each axle. Lacking knowing that number, IMO, we can do a rough calculation by using 53% of the axle scale weight for the RV when it was fully loaded (the expected heaviest it will ever be).

2. 53% of your front would be 3,870# and rear would be 7,030# or 3,515 on each dual.

3. Looking at the Michelin load tables we find for your size at 85 psi can support 3,975 for single (front position) and 80 psi supports 7,050# for two tires in dual position.  Yes, we always round up.

4. Based on the above, your MINIMUM inflation would be 85/80. This is the number I would use for the low pressure warning numbers on your TPMS.

5. I recommend adding 10% and again, rounding up, that means 93/88 psi for your Cold Inflation Pressure or CIP. In your case, given the close numbers for the front load, I would be comfortable using 90 psi all around, as a single number is easier to remember. This 10% gives you a nice cushion so you do not have to chase your tire pressure around whenever the temperature rises or falls. You could even get down to 85 psi before needing to "top off" the tire pressure again.

6  All tires on an axle should be inflated to the same level for improved handling and response in an emergency situation.

7. I would set the TPMS High Pressure warning to 110 to 115 psi and your high temperature warning level to 160 F.

8 Remember CIP means when tires are at ambient temperature and have not been in direct sunlight or driven on for at least two hours.


Finally, in your case, you are close to some numbers when we round so if your RV is more balanced than my suggested 47/53% you may be able to lower my suggested inflations by 5 psi, but only when you confirm your heavier end is less than 53% of the total.

##RVT794

Why isn't my TPMS accurate?

Comment on an RV forum
"I just bought a new TST 507  TPMS (tire pressure monitoring system) and  I gotta say they could improve the documentation.
Definitely confusing as setting up is a bit complex. At one point you are told to press a button and wait for the second beep (not the first!) before setting the pressures or whatever.
But once that's over, it works as advertised. It does seem to take a while, once turned on, for all the sensor readings to be displayed. After that it just cycles through them every 5 s, and beeps when one is over or under.
Not sure how accurate the psi readings are. Will have to get out my pressure gauge and compare."
+++++++++++++++=
My reply

TPMS are primarily designed to WARN of a loss in pressure and are not intended to be your primary source of accurate pressure measurement. In my experience ALL TPMS will have one or more sensors reading a psi or two different than a hand-held digital gauge. For that matter even hand-held gauges are seldom accurate to +/- 1.0 PSI based on the calibration checks I offer at my tire seminars.

IMO best practice is to set your tire pressure as close as possible to the desired CIP (cold inflation pressure) using your personal, master digital hand gauge and then install the TPMS sensors. Wait about 5 min for pressures and readings to stabilize, then do a pressure check on the monitor. You will note slight variation, but all should be within the specified range (some spec +/- 2%  some spec +/- 2 psi) You will find that after a few days use you will get comfortable with the pressure readings showing some variation from each other. With a little experience you will get comfortable and know the range of normal variation.

What I do...
Each morning when I get up, I first turn on the TPMS monitor and go get my coffee. By the time I am done with the 1st cup - about 10 min - all the sensors have sent in a reading. I step through each tire on the system and based on experience, I know if the tires are ready to go as far as being properly inflated. Yes, the numbers vary, but as long as they all are in a range of a couple psi from what I got when I last set the tire pressure all is good.

With the above practice I am getting the "cool tire" pressure reading which is the only reading of importance. With my +10% over the minimum inflation needed for my Motorhome, I don't have to worry about a daily +/- 2 to 4 psi variation. However, if I have a few days in a row with the display pressures consistently showing a few psi lower than my CIP goal I know it's time to plan on a stop at the truck stop for a shot of high pressure air.

##RVT789

Question on TPMS settings

Got this question:  "Tireman, I would like your opinion as to what would be good alert values to set in the TPMS on my trailer. I need to set high and low pressures and temperature alerts."

First lets be clear that Trailer Cold Inflation Pressures and Motorhome and Tow vehicle CIP are established differently.
While I have covered the specifics and the Why for each application in other posts, I will summarize here.

1. Both trailers and motorhomes need to confirm the actual load on their tires by getting on a truck stop scale or similar when the RV is fully loaded.

2.Using the Load & Inflation tables published by tire companies we should all learn the MINIMUM inflation needed to support the measured load for each tire.

3. Motorhomes should set their CIP at the MINIMUM Inflation in #2 + 10%

4. CIP for Trailers is the the pressure on the tire sidewall.

5. The Low pressure alert for Motorhomes and Trailers should be the MINIMUM inflation needed to support the measured load.

6. Trailer tires should not be loaded morer than 85% of the "Max Load" as molded on the tire sidewall.

7. Most TPMS come set with the max temperature warning at 70°C ( 158°F)  which is just fine.

8. Based on input from many RV owners with TPM I would suggest the High Pressure warning be at least 25% higher than the CIP.

Send your questions to me, Roger, at Tireman9 (at) gmail.com


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Another post on Cold Inflation Pressure and "Ambient" temperature.

Some clarification from a tire engineer. 

First off, I am not going to address what the correct Cold Inflation Pressure ("CIP") is for your RV. We will assume you have read my other posts on how to learn that number. If you haven't, simply look at the list of labels on the left side of this blog and you can review them.

There are different guidelines for Trailers vs. Motorhomes. I want to focus only on setting the CIP

Some people want to refer to a Temperature compensation chart.

Tire Rack was off a bit till they updated their tech page last year after my input. Also Wikipedia definition for Cold Inflation Pressure was also almost correct till I added a clarification aimed at RV owners. The 1 Psi for 10 F is OK if your base inflation is near 40 psi but many RVs use 80 to 110 psi.
The correct "Rule of Thumb" to use is 2% for each 10F.

Tire pressure is not based on any laboratory standard temperature (some claim 70 F) but is based on the tire not being warmed from either use, i.e. being driven in previous two hours, or from being in the Sun for previous two hours. Even partial sun can affect the reading.

Classical "Temperature in the Shade" is the "Ambient" tire engineers are talking about. Not temperature in a theoretical laboratory.

So regarding a situation of setting the pressure when the tires and air is 50 F. That would be fine and we would expect the pressure to increase by about 8% if the Ambient increased to 90 F even without driving or Sun exposure.

It is correct to say, "The ONLY time to check CIP is FIRST thing in the morning BEFORE the day's temp has had a chance to increase and BEFORE the sun has had a chance to shine on the tires and BEFORE you have used the vehicle."

However, if you are driving from the campground on top of Pike's Peak and stop for lunch for two hours in the shade in Flagstaff where it is 90 and check your air, you might find a change of a few psi. You could adjust your pressure before continuing to Phoenix, where it is 120 F, but I don't bother to adjust inflation by the 1 to 3 psi variation I observe day to day. In my mind that is too much work.

NOTE: My personal CIP is  75/80  F/R on my Class-C MH.  Both of these pressures are more than 10% above the minimum pressure needed to support the measured load on each tire so I have a "cushion".

I usually wait till I am home and am getting ready for the next trip before I adjust my inflation to my personal CIP, so I simply monitor the running inflation pressure which goes up and down as ambient temperature, driving and Sun exposure changes the inflation. My TPMS will warn me of air loss, so all is good as I motor down the highway.


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