Tire Inflation. We all need to pay attention to it

 

It doesn't matter if we drive a Diesel Pusher that is 48' long or pull a 6' "Pop-Up" trailer with our car. It doesn't matter if our vehicle runs on Gas or Hydrogen or Diesel or is an EV, our vehicles currently run on tires that are inflated with "air". Even if that inflation "air" is 78% Nitrogen or 98% Argon we must use some gas in our tires. What is constant for all vehicles on the roads today is that we have tires that require inflation with some sort of "gas".

It is basic Physics that It's the "air" pressure that supports the tire load, not the tire structure, and while we all know that there are folks on the Internet, who will claim to know more about any topic than the people with years of training, I can only hope that the majority of readers of this blog will trust the actual Science of tire design and engineering rather than some random guy spouting his opinion on what level of inflation you should run.

I follow a number of RV forums and find a number of folks offering their interpretation on what the proper inflation is for tires in RV application. Some insisted the only correct inflation pressure was what was shown on the "Tire Sidewall" while others insist the "Vehicle Certification label" is the only correct level and of course there are more than a couple that have developed their own inflation tables and will be happy to tell you what their personal calculations indicate.

Here is my take on the subject:

We have two separate issues to consider when selecting the minimum cold tire inflation pressure in RV application.

A vehicle MFG has DOT requirements they must follow. They are to specify the minimum inflation needed in the tires they provide on the new RV. That inflation is required to be sufficient to support the stated GAWR divided by the number of tires on that axle. Basically this means the axle rating is used to establish tire inflation based on the published load & inflation tables from the tire industry.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or GAWR or the individual tire load capability.

You will immediately note that the RV company is making an assumption that is just not a reflection of reality, that being that axle loads are always split exactly 50/50 side to side. While many RVs may have the load split only a couple percent off of 50/50 some have discovered one end of an axle may carry more than 1,000# more than the other. Others have learned their unbalance is closer to 40/60 and some have discovered that their tires are overloaded as soon as they fill their water and fuel tank before loading anything else into the RV. Obviously a responsible owner needs to do a better job than some RV companies are doing.

While some feel it is sufficient to just read the regulations about the vehicle certification label, I and other tire engineers know that "policy" is not always the best way to get the best combination of ride, handling and tire durability.

Lets see what Goodyear says in their RV tire application web page.
"Correct tire inflation is a key component in tire care. The recommended inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the measured load on each individual tire. Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard."

Maybe you don't think Goodyear knows more than some RV owners. What does Michelin say in their RV documents Here on pg 4:
"The amount of inflation pressure required in each tire depends on the weight of the fully loaded vehicle, to include passengers, cargo load, fuel, and water."
" To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear indication of how the weight of the recreational vehicle is distributed across the axle. "

Also in their tire maintenance document on tire inflation:
"Unlike commercial light truck and truck applications, we recommend weighing motorhomes at each wheel position of the vehicle (right front corner, left front corner, etc.). This is the only way to get a clear indication of exactly how the weight of the motorhome is distributed. Otherwise, one wheel position may be significantly overloaded even though the GAWR has not been exceeded. Tire inflation for a given axle should be set according to the pressure corresponding to the higher of the two end loads."

Others are allowed to have and express their opinions and their interpretation of how the regulations are to be applied, but it was my job as a tire engineer to understand and work within FMVSS regulations for some 40 years.

I will leave it up to you to decide if the only acceptable inflation is what is on the vehicle certification label or that inflation is learned by reviewing what major tire companies say in their  published guides or if some guy pontificating at the campfire is a more reliable source.

Tire Loads & Inflation - A refresher

 Just read another post on a FB page where more incorrect info was published....Soooo lets go over this topic again

 Today’s key points: Know the minimum tire inflation based on manufacturer estimates. At a MINIMUM Check your inflation with a good gauge at least monthly and every morning before travel. Better yet get, program and use a Tire Pressure Monitor system  TPMS to the knowledgeable.

Tire inflation seems to be a topic that confuses some and has others believing in misleading or just plain incorrect information. Tire inflation is one item that directly affects the safety of your RV, truck or car as you travel down the highway. Many of my posts have been background information that you really don’t have to know or fully understand. The intent of these topics is to give you a better foundation of understanding more about tires but, if you only pay attention to one series of posts this is it.

Tires do not carry the load. They are just a container of air. It is the inflation air that does the work. Think for a moment of an impact wrench. It can’t do the work of loosening or tightening nuts on your wheels without the air, and a tire can’t do the work of carrying the load or provide the traction needed to turn, start or stop if it doesn’t have air. The load a tire can carry is basically based on the air volume of the tire and pressure of that air inside the tire.

If the load was carried by the tire construction we would have Tire Load and Construction tables. But we don't. We have Load and Inflation tables because, for a given type and size tire the load is just a function of air pressure. More pressure gives more load carrying capacity.

If you want to get a feel for how little load your tire can carry without air in it, you can test this yourself. Simply take an unmounted tire and stand on the beads of the tire. I think you will find that the tire can support less than 5% of the rated load before it deflects more than an inch or two. Some people believe that tires with higher Load Range can carry more load at the same inflation. This is just not correct. You would be hard pressed to measure the uninflated load capacity difference between a Load Range D and E tire or between a G or H Load Range tire because almost any significant load would compress the tire to be flat.

The term Load Range replaced to old "Ply Rating back in the early 70's when tire construction materials became stronger and with the introduction of Radial construction.

If you are not sure, the "Load Range" is marked on the sidewall of your tires. If you don't see the words "Load Range" look for "LR" followed by a letter usually between C and G. "Load Range" may not be in large letters but you should be able to find them on your tires.

TireRack has a good information page on "Load Range" HERE.

The simple answer to how much air you need is on your Certification Label AKA Tire Placard that I wrote about last week. Depending on the year your RV was made, this information is on a sticker on the side of your trailer or near the driver’s seat in your Class-A or on older RVs it might be glued to the wall inside a cabinet or on the inside of your entry door. Yoyr car or Pick=up has the sticker on the driver door jam. Where ever it is, you should also have the information in the stack of owners manuals you received when you bought the new RV. Find this data sheet and write down the information for loads, minimum inflation, tire size and load range and place this info where you can easily find it. This inflation is the minimum based on what the RV manufacturer thought you would load into your vehicle. They may have guessed correctly or you may have gone way over that estimate depending on how much “stuff” you carry.

##RVT1038

What inflation to run when changing Ply Rating or size?

Following a few different RV Forums, I see a number of people, that clearly do not follow this blog,  have questions on what inflation to run. While many of these folks are discovering that the new Goodyear Endurance line seems to come with higher Load Range or only in larger sizes they end up not knowing what to do about inflation.
In one case the person didn't even know they were buying a larger size so I do wonder sometimes how people managed to ever buy tires for their car if all they seem to know is that tires are round and black and seem to cost too much.

When you replace tires on any vehicle, you should always be sure the load capacity of the new tires is equal to or greater than the capacity of the original tires on the vehicle.

Regular readers of this blog know that load capacity depends on BOTH tire size and INFLATION level. You also know that there is a "Tire Placard" or "vehicle certification label" on or in all your vehicles that identifies the original size tire and the inflation recommended by the manufacturer. In the case of RVs the label also should tell you the GAWR which is the maximum load you should ever have on each axle confirmed by scale readings.

So back to the original question of what inflation to run ?

 .

Example 1:  There was a change in tire size but the Load Range stayed the same.
Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-C
 A quick check of a Load & Inflation table for ST type tires we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can support 2,150#@50 psi. This is a nice 18% increase in load capacity.  I would suggest that in this case the owner continue to run 50 psi and enjoy better tire life.

Example 2:  There was a change in tire size AND in Load Range as can happen with the new Goodyear Endurance.
 Original tires were ST205/75R15 LR-C and you mistakenly bought ST225/75R15 LR-E
Back to the Load & Inflation table for ST type tires we see that the ST205  is rated to support 1,820# @ 50 psi and the ST225 can now support 2,830# @ 80 psi. This is a 55% increase in load capacity.  If the owner were to run the 80 psi as indicated by the tire sidewall the "ride" would probably be hard on the TT. In this case I would not follow the inflation on the tire sidewall. I would suggest that in this case the owner continue to run 50 to 55 psi and enjoy better tire life. In this example the owner expressed concern about running lower inflation than marked on the tire. Some people even incorrectly said that running less than 80 psi would somehow overheat the tire. I pointed out that I saw no problem with running less than the sidewall inflation as long as the actual load had been confirmed with actual scale readings and that there was a good margin of capacity over the actual loading.

Example 3. A Class-A owner wanted to change from a Michelin 275/80r/22.5 to a Firestone FS 591 295/75r/22.5. His reasoning was two-fold. First he was concerned about the historical reports of  delays in getting the sometimes hard to find the Michelin size and second was the significant for him, cost difference.
Both tires were LR-H . The owner did a good job of consulting the tire manufacturer's data sheets to learn the actual tire dimensions. One poster had offered a generic tire size comparison web page but the numbers the page generated were clearly different and could lead to problems.
The load capacity for the Michelin and Firestone tires is 7,160 @ 120 The owner had also confirmed the dimensions were acceptable for the application.

Summary for Newbies

This is a summary of a discussion I had on RV Forum on tire pressure that started with a question and some comments.

"I'll give my take on maybe why, we have ----- Travel Trailer owners ----- Motor Home owners that never had a TT ---- TT owners that moved up to a MH ----- MH owners that still act like they still own a TT -- maybe there is any other I can not think of right now.

THE POINT IS TIRE PRESSURES ARE FIGURED DIFFERENTLY between the RV's owner's involved.

I have friends that own MH's and suffer blow outs all the time and their tires look good at the pressure they are using and not a Run Low Flex Failure that will most likely cause a blowout.

Maybe Tireman9 will answer some of the differences between TP's of TT's and MH's to clear up some of the misconceptions people have."

Here is my answer to the broad question

Yes, the proper inflation for MH and "tow-able" RV are different. There is actually strong science behind why there is a difference. THIS blog post is a short explanation. You can read the technical info HERE.

The other issue is that some folks just do not want to hassle with vehicle maintenance required with RV ownership. They are used to cars that have thousands of man-hours of engineering design, testing and development to make those vehicles extremely reliable.

Some of us are old enough to remember that when we learned to drive we were taught how to change a tire, as 10,000 miles was good tire life. Many probably learned how to change oil or adjust a carb. Some even knew how to set the engine timing and change a set of spark plugs. Nowadays it's hard to even see the plugs but with their life now at 50,000+ and the ignition computer controlled and many tires good for 40 to 60k miles who needs to know how to maintain a car?

RVs have a number of systems that do need maintenance. The tires on RVs are an outlier. While top tier tire companies use essentially the same rubber compounds and tire building equipment. This blog post explains the concept of "Tiers" in the tire industry. If you have a MH you will probably recognize the brands and names of the tires on your RV. 

However if you have a "towable" and by this, I mean both 5th wheel and more basic "Travel Trailers" many of you probably have tire brands that don't even fall into the 5th tier level. Some of these off-brand tires may not have a lot of up to date engineering built into them but IMO the main problem is the selection of tire size and load/speed capacity made by the RV company.

Again IMO many RVs are sold based on low cost and the unit has lots of "bling". I don't think I have ever heard of a salesman touting the benefits of the tires that come on an RV. I doubt that many even know the size or maybe even the brand provided on the various units they sell.

So the bottom line is that tires on RVs need more maintenance than the tires that came on their cars. Partially because of their usage but also because there is such a small margin of extra capacity provided based on the tires being undersized to keep costs down. Maybe if RV companies spent more, or even some time evaluating better options, i.e. had better quality for the tire sizes and type they offered, the incidences of tire failure on RVs would be less frequent just as they are less frequent on today's cars. 

##RVT818

Will confusion on how much inflation be resolved?

Hi Roger. Do you think the tire pressure issue will ever be resolved to people accepting the values recommended by the tire manufacturers based on actual loading, or will some always be confused between the tables, the placard and quotes of Federal FMVSS Regulations from some on the forum? It seems so simple and logical, but some just don't get it. I still think a sticky written by you could settle the issue, but the moderators disagree. I suspect a concern for liability for the forum to advise something that contradicts the MH placard. I have a question for you about my RV. I carry 5 psi above the minimum for the load on my steer tires on a 60* morning @ 1100' altitude. When traveling, I had a morning @ 5000' and temp in the upper 30's. This produced a pressure of 1-2 psi below the minimum. I was going to be traveling into a warmer climate (95*), so did not adjust pressures. This type of condition can happen when we are traveling. I don't want to chase the pressures, so if traveling into warmer weather, I just go and watch the TPMS. I don't feel this is a problem but wanted your thoughts. Thanks for your thoughts and time.

Not sure if there ever will be a resolution to the Inflate to the placard vs Inflate to the actual tire load. I would liken this to the change oil every 3,000 miles vs change when the car's computer advises.

DOT has a goal of trying to make things simple and keep people safe. DOT also knows about the data that indicates that over half of the RVs on the road today have one or more tire overloaded (either too much load or too little air for the actual load) I really can't fault their approach as IMO many RV owners aren't willing to make the effort needed to learn the proper inflation and to then maintain it.

We have had tire inflation stickers in cars for many decades and there have been massive vehicle recalls because their tires were underinflated, I have read that many as much as 30% low. We don't hear anyone making a case that the accidents and fatalities were the driver's fault. That would be blaming the victim which isn't acceptable, even if true.

OK to your question. I am guessing your CIP is in the 90 - 110 psi range so IMO +5psi isn't enough to avoid the pressure fluctuation that results in your overload. This is why I suggest a +10% value for inflation margin over the table minimum number..
In my Class-C RV, with LT225/75R16 LR-E tires, I am lucky as my RV is rather light. I really only need 60 psi F & R based on my "4 corner scale readings. My certification sticker says 65 / 80 and my dealer delivered it at 64 psi all around. 

For my application +10% would indicate 66 psi and a  +15% margin > 69 psi. I run 70-75 
Even though I can be a bit "anal" about inflation I also wrote THIS post and I really do not mess around with my tire inflation.

My TPMS is set to warn before I would be in overload and I run across a scale once a year just to confirm I am not too far from my original "4 corner weights".

In 2014 I drove Ohio > OR > Calgary > Yellowstone >OH over a seven-week period with temperatures ranging from 90's to snow and elevation of 20' to 8,000' and never had to adjust inflation.

Who to believe on tire inflation?

I was reading an RV forum and found a number of folks offering their interpretation on what the proper inflation is for tires in RV application. Some insisted the only correct inflation pressure was what was shown on the "Tire Placard" or "Vehicle Certification label"
.
Here is my reply:

We have two separate issues to consider when selecting the minimum cold tire inflation pressure in RV application.

A vehicle MFG has DOT requirements they need to follow. They are to specify the minimum inflation needed in the tires they provide on the new RV. That inflation is required to be sufficient to support the stated GAWR divided by the number of tires on that axle. Basically this means the axle rating is used to establish tire inflation based on the published load & inflation tables from the tire company.

The owner of the RV needs to confirm the actual loading for their RV and should confirm they are not exceeding the GVWR or either GAWR or the individual tire load capability.

You will immediately note that the RV company is making an assumption that is just not a reflection of reality, that being that axle loads are always split exactly 50/50 side to side. While many RVs may have the load split only a couple percent off of 50/50 some have discovered one end of an axle may carry more than 1,000# more than the other. Others have learned their unbalance is closer to 40/60 and some have discovered that their tires are overloaded as soon as they fill their fuel tank before loading anything else into the RV. Obviously a responsible owner needs to do a better job than some RV companies are doing.

While some feel it is sufficient to just read the regulations about the vehicle certification label I and other tire engineers know that policy is not always the best way to get the best combination of ride, handling and tire durability.

Lets see what Goodyear says in their RV tire application web page.
"Correct tire inflation is a key component in tire care. The recommended maximum inflation pressures for your tires are indicated on the certification label or in your owner's manual. Since RVs can be loaded with many different configurations, the load on each tire will vary. For this reason, actual air pressure required should be determined based on the load on each individual tire.
Inflation pressure should be adjusted to handle the tire carrying the heaviest load, and all tires on the axle should be adjusted to this standard."

Maybe you don't think Goodyear knows more than some RV owners. What does Michelin say in their RV documents Here on pg 4:
"The amount of inflation pressure required in each tire depends on the weight of the fully loaded vehicle, to include passengers, cargo load, fuel, and water."
" To determine proper inflation pressure, Michelin recommends weighing each wheel position of the recreational vehicle individually. Weighing each axle end individually will give a clear
indication of how the weight of the recreational vehicle is distributed across the axle. "

Also in their tire maintenance document on tire inflation:
"Unlike commercial light truck and truck applications, we recommend weighing motorhomes at each wheel position of the vehicle (right front corner, left front corner, etc.). This is the only way to get a clear indication of exactly how the weight of the motorhome is distributed. Otherwise, one wheel position may be significantly overloaded even though the G.A.W.R. has not been exceeded. Tire inflation for a given axle should be set according to the pressure corresponding to the higher of the two end loads."

Others are allowed to have and express their opinions and their interpretation of how the regulations are to be applied, but it was my job as a tire engineer to understand and work within FMVSS regulations for some 40 years.

I will leave it up to you to decide if the only acceptable inflation is what is on the vehicle certification label or that inflation is learned by reviewing what major tire companies say in their  published guides.

__________________
Retired Desi

What is a good tire gauge?

You need a system, as any gauge can be damaged or fail.

Pressure gauge system
Over the past six years I have offered pressure gauge calibration check services. With well over 70 gauges of almost every description checked we have learned that many are not much better than door stops or for use as a tent peg.

There are some broad generalities we have learned.

Digital gauges

are usually the most repeatable and accurate with dial gauges.

Next are "stick"

or "pencil" gauges lagging far behind.

While it is also true that I have found some gauges of each type to be accurate, those tend to be those that are brand new. The stick gauges with the sliding bar tend to get worse with age. This is probably due to dirt and nicks in the slide stick causing inconsistent and inaccurate readings.

Before I get into my suggested "system" some are probably wondering how I can be confident that my master gauge is itself accurate.

Having worked as an ISO 9001 certification auditor, it was part of my job to understand how to establish systems and procedures that when followed could be relied upon to deliver accurate information. Even though I am retired, I still can gain access to tire laboratory test equipment and occasionally compare my gauges to current ISO certified laboratory gauges.

System:  You need at least two gauges. One is your own personal "Master Gauge". This gauge is not used to check the pressure in your tires. It is only used as a reference for your regular use hand gauge. The Master gauge should never be stored in your tool box but should be carefully packed away where it will not get dirty, wet, dropped or knocked around.

Calibration check procedure: Using one of your front tires as a source of high pressure air you would take a reading with your Master gauge and a reading with your hand gauge. Hopefully when you first set up your system they will both read within 1.0 psi of each other or better. If not you may want to check your gauges against other digital gauges to try and confirm if it is your master or hand gauge that is most accurate. Majority wins if you have two that match to within 0.5 psi. For your two gauges you should record the two pressure readings and the date. You can then take readings of all your tires using your hand gauge, recording the pressures as you go. You could also record the readings of pressure from your TPMS (tire pressure monitoring system). Now I would expect the TPMS readings to be within +/- 3 psi of your hand gauge.

Remember, TPMS are primarily a tool to warn when there is a change not to be used as an absolute accurate gauge. Again record the measured pressures. Once you have finished this initial check you can set your tires to their appropriate Cold Inflation Pressure (CIP) level using your hand gauge.

Now when you look at your morning TPMS CIP readings, you should see similar movement up or down of the pressure of all tires as daily morning ambient temperature changes. Remember the tire pressure will change by about 2% for each change of 10F ambient. I covered this change is THESE posts.

As time goes on you might see a slow decrease in pressure. This is due to air permeation through the tire carcass and O rings around the valve and TPMS sensor. Eventually after losing about 5 psi you should "top off" the tire pressure. At that time you would use your hand gauge to set the tire pressures. I would suggest that before setting the tire pressures you again do a gauge comparison to learn if there has been any change in the accuracy of your hand gauge. If the difference between the two gauges has changed then you need to learn why.

Reliability: It is very unlikely that both your master gauge and your hand gauge would both go out of calibration by the same amount and in the same direction at the same time.

My System:  I use an Accutire MS-4021B

Digital Tire Pressure Gauge that I bought from Amazon for $12 as my master gauge. (Note I confirmed this gauge is off by 0.5 psi.)  I also use one as my hand gauge (seen above). Since this gauge does not have a truck "dual foot"

head I also run hose extenders on my rear duals. 

I also bought a Tire Traker TPMS. 

I have not had any leaking problems as I check the valve stems, TPMS sensor and hose fittings with a spray of soapy water to ensure there are no leaks.

Some may want to save $12 and not bother with having a master they can trust. But I don't think it's too hard to imagine a gauge reading a bit higher with time as the internal springs lose strength, so you would get misleading high readings which means your pressure is actually lower. This can lead to a failure that will cost a lot more than $12.

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Am I reading Load & Inflation tables correctly?

Wayne M. asked

Tireman, Here is some information on my Toyo 265/75R22.5 LR-G tires. Can you confirm I am reading the tables correctly?
Toyo 265/75R22.5
PSI     70     75     80     85     90     95    100    105   110
Dual 3625 3705 3860 4040 4205 4410 4525 4685 4805(G)
Sing 3875 4070 4300 4440 4620 4805 4865 5150 5205(G)
Measured scale weights on each corner
RF: 4650        LF 4750
RR: 9000       LR 8250
Right side has Refer. There isn’t anything in the bays (not all the way through bays) that I could possibly switch around for even distribution so I have to go with the heavier weight.
I’m thinking 100 psi for the front.
What would my rears be with a fudge factor?
++++++++++++++++++++

First off , I have to thank you for providing the information on your specific tires and your actual tire loads for each end of your axles. I prefer data to guessing.

With 4,750# on the heavy end of the front axle we see that 95 psi can support 4,805 so that would be the MINIMUM Cold Inflation Pressure for your fronts (remember all tires on an axle should have the same CIP) and I suggest you add 10% (10 psi) cushion to get 105 psi CIP on the fronts.

The rears are dual, so looking at those inflations.  9,000 divided by 2 gives 4,500# per tire, with the 90 psi being the minimum inflation needed to support the measured load. Adding 10% or 9 psi gives us a suggested 99 psi. 100 is easier to remember and measure so lets go with 100 CIP on each of the 4 rear tires.

Comment: Please always adjust inflation up and avoid going down no matter how close the weights are.

##RVT786

Tow vehicle tire question

Dear Roger,

"Last week I purchased new E rated tires for my F150. Went with the BF Goodrich KO2 (275/65/18) and so far I'm very happy with them. These are larger than the tires that came from the factory (265/60/18). I have a quick question.

"The tire shop originally put the PSI at 35 based on the door sticker, but after reaching out to BF Goodrich they suggested 45 PSI for everyday driving. They didn't give any recommendations for tire pressure while pulling a trailer since there are so many variables like trailer size, weight and cargo. The tire shop suggested going to 50 pounds and then deflating back down after the trip. I'm just curious if others actually bump up their PSI while towing, and have you noticed a difference in doing this? Just trying to figure out if there is a benefit to doing this.

Some important information is missing, so let's identify our assumptions:

1. We are talking about the tow vehicle tires - the F150.

2. The original poster didn't identify the OE (original equipment) tires as being P type or LT type. This makes a major difference and providing that information up front will eliminate guesswork for those trying to help with the answer. With a 35 psi door jam sticker number, I suspect the OE tires are Passenger type and the replacement tires are LT type.

3. Since it is the air pressure and not the tire construction that supports the load on a tire, buying LR-E (80 psi) tires when you never run higher than 65 (LR-D) would be a waste of money. Also if you are changing from P type tires with a max inflation of 36 to 41 you need to confirm the wheels are rated for higher load and inflation of LT tires.

4. The door jam pressures are based on the car company making some estimates on how much and how often you have the vehicle empty or fully loaded. You might check the owners manual and see if they give an empty and loaded tire inflation suggestion. The inflation in the OE tires is what is needed to support the GAWR but few people run that heavy all the time with their pickup.

5. I suggest you get the F150 on some truck scales when empty and again when fully loaded with the trailer also fully loaded, and learn the real facts of the various axle loads under both conditions.

6. Knowing the real loads, you can use the tire Load & Inflation tables to learn the MINIMUM cold inflation pressure for the F150 for the two situations. I suggest you use a margin of +10% of the table inflation to establish your CIP (cold inflation pressure). See other posts on weight and inflation if you need more details.

7. Trailer tires are a completely different situation and should always be inflated to the tire sidewall inflation. You should also run no more than 85% of the trailer tire max load rating when on a scale. Cornering, sway and side wind loads have been shown to shift loads side to side by 10% or more. Also the Interply Shear on tires in trailer application needs to be considered as this is a major contributor to trailer tire belt separations.

Hope this info helps clarify.

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##RVT781

How to Maximize tire life with minimum effort

If you are new to the RV lifestyle or an old hand with many thousand campfires under your belt, there is one thing that is true: What you knew or learned about tires from years of car ownership probably did not prepare you for the task of maintaining the tires on your RV. This is true if your RV is a 18’ Travel Trailer or a 45’ Diesel Pusher.

I think we all understand the concept that a system is no stronger than its weakest link. For many RVs, the weakest link that can interrupt long term, problem free travel are the tires. Car tires have evolved with improved durability and reliability to the point that many people may never need to touch their tires or even know how to change a tire. Today, some cars don’t even have a spare tire and if they do the driver may not have even bothered to learn where it is as the expectation of needing to use the spare is so low it has completely dropped off the radar screen.

As a retired Tire Engineer with 40 years experience in design, evaluation and quality assurance of tires I would like to offer some suggestions that I believe if followed, will lead to many years of problem free travel. I also believe that once you take the initial steps I outline, you will also find that you will only need to spend about 15 minutes a month, or less on average with tire maintenance.

The basics that are needed to have a tire problem free experience with your RV tires involve two simple steps:

1. Knowing the proper level of tire inflation on your personal RV and

2. Ensuring your tires are always inflated to that level.

There it is, just two simple and rather basic steps and, you don’t need to be experienced in vehicle mechanics or tire engineering to accomplish these two steps as I am going to share the steps that have worked for me and thousands of others for decades.

Step 1: Knowing Your Proper Inflation

So the first step is to know the proper level of tire inflation on your personal RV. The important concept here is “Your RV”. Not the RV your brother-in law uses, or the inflation the guy in the campsite next to you uses, or even the level provided by the RV manufacturer on the tire certification label, otherwise know as the “Tire Placard”. Now some will point out that the RV manufacturer has the responsibility to inform you of the minimum tire inflation you should use, and this is correct, but this is based on an assumption which may mean the numbers are too high or too low for your RV when you have it packed with the stuff you want to take along on your next trip.

This step takes the most effort, but you really only need to do it once over the years you own your RV – unless you make a major change such as add a generator or refinish the interior with granite counter tops; you know, do something that significantly changes the weight of the RV.

To learn your real weight you need to pack your RV with all the clothes, food, fuel, water and other “stuff” you expect to carry, including the proverbial "bowling ball collection." Once it is fully loaded, you and the family need to take a short trip to a local feed store or gravel pit or truck stop that has a large truck scale with enough side clearance that will allow you to get the tires from just one side of the RV on the scale. Obviously you will need to do a bit of preparation to find such a scale, so some time on the Internet or with a phone book is in order. It is suggested that you review the worksheet before heading off to the scale so you get all the facts you need in one trip.

Here is a company that travels the US and specializes in properly weighing RVs and giving the owner the load numbers for their personal unit.  " RV Safety & Education Foundation (RVSEF) This leader in safety education in the RV community offers wheel by wheel RV weighing services for all types of RVs.

Resources

If you will not be in one of the locations where RVSEF is offering their services then you will need to do a little research, as not all truck scales have the side clearance needed. You should know that some truck scale operators discourage side to side weights and have installed guard rail to prevent this. When you check out the work sheet you will have a better understanding of what is needed.

Our friends at Fifthwheel St have a web site has information including a video that focuses on trailers but has video showing the process which is similar for motorhomes.

Bridgestone has a page just on RV tires with a worksheet for calculating tire loads

Goodyear has a nice site too

Here is the Michelin RV tire page

If you look at more than one of the above links I think you will see that everyone offers the same basic information on the process. You do not need to worry about which worksheet you use as the math is the same for everyone.

After you complete your worksheet you should know the actual load on each tire position. The next step is much easier and also only needs to be done once. You need to confirm the minimum cold inflation your tires need to carry the load. It is always a good idea to use the tables published by your brand tire. Here is my blog page with links to many different brand tires. Simply go to your brand, find the table that has your size tire and look for the inflation that supports your load or more. Don't worry if your brand doesn't offer tables as almost all companies follow the same published industry standards.

Also don’t be tempted to go to a lower level of inflation if your numbers are close. Always go up.

Now you know the minimum Cold Inflation Pressure or CIP.  Wait! What do I mean by CIP? Whenever tire inflation pressure is measured or set the tire needs to be cool and at the same temperature of the air and in the shade. This means not having been driven on or in direct sunlight for at least 2 to 3 hours. Tires get warm when moving and the Sun heats them up which results in a higher temperature which results in artificial increase in pressure. Many people simply do their tire pressure check in the morning before starting out or late evening after a day of travel. Either way the tire needs to be cool and in the shade when you check your pressure.

I feel the CIP needs to above the minimum needed to carry the load as if there is a drop in temperature you may end up below the minimum if you cut things too close. I suggest you run +10% above the Minimum for your personal CIP.

A couple of details and a suggestion:

All tires on each axle should have the same inflation. This means that whatever inflation numbers you have for your tires select the highest from the tires on any one axle and use that number for all tires on that axle. This will give you more uniform stopping and steering response.

Over the past few years technology has evolved that allows you to monitor your tire inflation as you drive down the road and can warn you if you pick up a nail or other damage. You have this in your car and it is that little orange symbol with the exclamation point in it.

and now you can add this safety feature to your RV trailer or motorhome. It is called a Tire Pressure Monitor System or TPMS for short. A quick internet search will identify a number of units specifically designed for RV application or you can review THIS web site for a discussion of features to consider when shopping for a TPMS.
A bit of Truth in Advertising here. This blog is currently sponsored by Tire Traker brand TPMS. As you might expect that is the brand I have run for years. What you may not know is that I bought my TPMS from Traker before I started writing this blog.
One nice feature of having such a system is you do not need to get out with your digital pressure gauge and check your tires every day of travel as the TPMS will let you know your inflation in the morning while you stay inside with your coffee and more importantly while going down the road. You will quickly learn the details of your pressure reading in just a few days of observation.

So there you have it. Get the RV weighed when fully loaded and do the calculations using the worksheet to learn the actual tire loading and the inflation your tires need.

Step 2: Ensuring That Inflation Is Properly Maintained

This step is much more straightforward. Just check your tire pressure every travel day, or better yet get and use a TPMS (Tire Pressure Monitoring System) to constantly monitor your tire pressure. I even set my low pressure alarm level so I would get a warning before the pressure drops below the minimum inflation I need to support the measured load on my RV.

I believe that if you follow this advice you should reduce what otherwise might be a less than 5% chance of having a tire failure to less than ½% chance of tire failure.


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What temperature for CIP "cold inflation pressure?

I occasionally see posts on the need to do an adjustment of your Cold Inflation Pressure ( CIP )when the temperature is not at some "standard".

Many times the "standard" is stated to be 70F or 68F. Neither of these are correct.

From a tire design standpoint CIP means when a tire has not been warmed up either by being in direct Sunlight or from having been run.
We are not talking about some chemistry lab experiment but real life. This is defined by the Tire & Rim Association, the organization that published the standards book for tire dimensions and recommended Load & Inflation for all kinds of tires, wheels and valves.
 These standards are primarily intended to provide "interchangeability" as we want to be sure that every 15" tire fits properly on a 15" wheel of the appropriate type. Or that the valve will properly seal against air leaks by having the hole in the wheel of the correct diameter.

Now you don't have to get all wound up with temperature probes or IR guns to confirm a tire has not been warmed up. Just follow the guideline that when checking or adjusting tire pressure to your CIP, the tire should not have been driven more than 2 miles in the previous 2 hours, AND that the tire has not been in direct sunlight or otherwise artificially warmed up in the previous 2 hours.
"Cold" really means when the tire is at the temperature of the surrounding air or what is called "Ambient" temperature.

So unless you are taking your RV to the Antarctic or to the Sahara desert you can use the above as a simple guideline. Even if I were planning a trip from the top of Pikes Peak to Phoenix in a single one day drive I would not worry about making adjustments based on expected temperature. Tires are designed to have a large tolerance for pressure increase due to variations in the surrounding temperature.

Remember the rule of thumb is that tire pressure will only change about 2% for every change of 10F so even going from 30F to 100F might only result in a pressure change of 14%. Now you would probably need to adjust the CIP the next morning before setting out but again you would set the CIP when the tire is in the shade and not driven on for a couple of hours.



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