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Tuesday, December 13, 2016

Top 10 reasons to not put nitrogen in your tires

For those that missed this info in RV Travel newsletter:

The language in this video from Australia is a little off-color at times, but the information is solid. The message: Putting nitrogen in your tires is a waste of money. Watch the 17-minute video.

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Tuesday, December 6, 2016

Important consideration if changing tire size: "Minimum Dual Spacing"

Knowing about the extra details of dual tire application is not anything car drivers were ever called upon to know. When you joined the RV world your first RVs were probably trailers and you don't have to know about special considerations with duals. If you now own a motorhome that has two tires on each end of the rear axle, then you have "duals" and need to know and consider some new information. I have written a number of posts on this topic and I suggest you take a look at each post by clicking on the word "dual" on the list to the left.

If you are considering changing tire size on your motorhome, there are a number of items you need to consider. I have previously covered the importance of matching tire OD if only changing one tire of a pair of duals. While you may remember reading that POST, you may be telling yourself that you don't need to worry about the matching dimension if you are changing all your tires. While you may understand the need to pay attention to load capacity and the physical dimensions of the tires themselves, there is also a CRITICAL dimension that you can't measure with a tape measure and that is called "Minimum Dual Spacing". You may or may not have noticed this dimension when looking at a table with tire dimensions. Not all tables have this dimension but it is very critical.

I have highlighted the Min Dual Spacing information and the rim width dimension which is also critical. It is important to remember that not all tires are approved for a specific rim width when looking at the upper range of load capacity.

Tires in a side-by-side, i.e., dual position MUST have a certain amount of clearance between them or the tire-to-tire contact of the sidewall may lead to failure of BOTH tires. Going from a 255 to 275 wide tire will probably mean you will not have the specified clearance unless you also change wheels. Only way to know is to check the wheel specifications from the manufacturer.
Here is what that dimension means.
It is the distance from the center of one tire to the center of its mate, when mounted on the actual wheels you will be using. Now there is no single place you can make this measurement on a wheel as it requires calculation, as the mounting surface of the inner dual wheel is not the same surface as the mounting surface of the outer wheel.

Tire charts and dealers can tell you the minimum dimension needed as seen above but you need information from the wheel manufacturer to learn the dimension from the mounting surfaces to the wheel center-line.

So, along with load capacity, OD and tire width you also need to confirm your wheel width meets the tire specification as well as calculate the dual spacing from the dimensions provided for your wheels.

Special warning. I know that some motorhomes have steel inner wheels and aluminum outers for the dual position. You will quickly learn that the offset spacing is different for steel vs aluminum.

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Friday, November 11, 2016

Surviving a tire "blowout"

I have many posts on the technology and terminology of tire failure but no matter what the cause or what you call it, a tire failure is not fun. Here is some information on what to do.

Over the past few months there have been a couple items in the news and on the Internet about tire failures on RVs and buses. The videos are pretty dramatic:

Many on this forum are in motorhomes. Many also pull a toad, but others own or have friends with trailers or may even find themselves pulling a trailer. I will also include some information for those times too.

First, for tire failure on a toad or trailer or the rear duals or tag of a motorhome it is critical that the driver is notified as soon as possible that there has been a failure or that one is about to occur. The only way I know of gaining this knowledge is with a tire pressure monitoring system (TPMS) that can alert the driver of air loss. Some TPMSs can even alert the driver in the first few seconds when the inflation has dropped just a few psi from the hot running pressure.

If you do not run a TPMS then you will not learn of the pressure loss before damage has been done, as you will be depending on passing motorists to get your attention. By this time, damage has been done, but hopefully the toad or trailer hasn't rolled over or separated from the motorhome, which could raise the level of severity of consequences dramatically.

For motorhome or bus drivers the failure of a front tire can mean a significantly different outcome, as there is the real potential of a complete loss of control if the wrong response is taken. Here we know that a warning of initial air loss may provide enough time for a thoughtful response from the driver, but even having a TPMS is not a 100% guarantee as there are failures that do not involve air loss. So the question then is what actions need to be taken in the first fraction of a second after a front tire comes apart?

Thankfully there is a good instructional video from Michelin of what a driver needs to do. Here is another instructional video.

Yes, the advice is not intuitive to the average driver but it can work. It has been demonstrated numerous times that there is both proper and improper driver response to a tire failure. Sadly, many drivers have ended up turning an inconvenience into a tragedy.

A driver needs to stop and think about what to do and to take a moment — frequently — to help implant the correct response so it can become an automatic response. You do have plenty of time to think about this as you drive down the highway. I would suggest that if you spent as little as 10 seconds once an hour every hour when driving, thinking about the correct response of maintaining control first then slowing down second — rather than just stomping on the brakes — the action may become automatic. We all know that practice and repetition can make athletes better at their "game." Well, in this case, practice, at least in thought, can make you a safer driver in the "job" of getting yourself and family safely to your destination a reality.

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Wednesday, October 26, 2016

"I inspected my tires but still had a failure"

Read this on an RV trailer forum:

"Checked the air in all 4 tires and inspected them before we pulled out. We made it 20 miles from the house when the first tire let go. The tire service center and I both checked the remaining 3 and the spare. They had the correct air pressure and no signs of any issues. We made about 15 miles down the road and the second 1 blew out -- 2 roadside assistance calls and 2 insurance claims. I am not happy with dynamic tire or (RV trailer company name) at the moment."

I offered the following comment:

Sounds like the "inspections" being done are not sufficiently detailed. A "proper and complete" inspection doesn't consist of checking the outside sidewall and most of the tread for % tread wear. A LOT more is required if you want to improve your chances of finding the early signs and indications of impending tire failure.

In this blog, I have previously covered what I would consider a thorough inspection with example of what a failing tire looks like externally, and then I did an autopsy to show the actual condition of the subject tire. Note that the person making the video felt the tire had failed and cut the tire looking for the belt separation but even though he has an engineering and mechanical background, he failed to properly identify the location of the failure. IMO this was because he simply had not had enough experience in tire forensics, as you probably have to do a minimum of a few dozen autopsies before you can easily and quickly know where to do the cutting -- and that would be what is needed to find a large separation as seen in the subject tire. Smaller issues are harder to find and take a more experienced eye.

The free spin can be sufficient to establish that the subject tire is in the process of failing and should not be driven on.

Today it is popular in politics to complain about "the elite" and to disdain experience, but I believe that there are many fields where actual experience is necessary if you want competent results.

Very few tire service people have been given the opportunity to do investigative-level tire inspection, as their job normally doesn't require issuing a detailed report that identified the root cause of a tire failure. This not the tire tech's fault nor is it the fault of the tire store owner. You might liken this to the store clerk at a CVS or Walgreen pharmacy. They are simply not fully knowledgeable in the interaction of medications, which is why a Pharmacist is the person responsible to fill prescriptions, but I would not trust a Pharmacist to do surgery.

Sometimes competence only comes with experience.

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Thursday, October 6, 2016

How old is too old part 2

Last week I addressed the topic of tire age. Some are still asking why there isn't a better and clearer age limit on tires in RV use. It may help if there is a better understanding of the complexities of what is involved in making the prediction.

I think that most can understand that the numerous variables involved such as ambient temperature, operating temperature, load, and operating speed all can affect the "life" of a tire. Also there is the obvious variation in the numerous components and assembly practice of each tire itself. All of this adds up to an impossibly complex equation if trying to predict the life of a tire.

While many may understand that if a tire spends its life in Arizona, Texas, Alabama, Louisiana or Florida, it will have a definitely different life span than one that spends its life in New England, even if the load, inflation and speed were magically identical. I do wonder how many have even a passing understanding of the complex nature of manufacturing a tire as seen in THIS animated video. Actual shots of the process are here.  Here is an alternate view of the truck tire process, which would be essentially identical to what is seen in both LT and 22.5" size tires. If you watch only one video, IMO this 3rd one is the best and most informative.

Unlike materials like steel or aluminum, rubber is not a homogeneous material so even minor variations in the raw materials can affect raw material properties and ultimately tire life.

Here is a video showing the basic process of preparing rubber before it is applied to steel or polyester cords. Note these are videos of very low tech methods. Modern equipment is much larger and the process is harder to see as there is much more automation behind closed chamber shields.

The estimates of maximum tire life are based on assumptions of the variability of the tire and the variability of the use of the tire. The estimate must also consider the probability of the variables stacking up and the potential for the severity of the tire failure.

Some companies may feel that no greater than 0.5% probability of failure at 7 years is acceptable IF the tire is always operated in North America, never exceeds its max speed rating or is overloaded for the inflation in the tire. Another company may feel that less than 5% at 12 years is the goal.

It is also important to remember that only after thousands of tires have completed years of service can a company know if it made the correct calculations. I do not know of any company that is not constantly working to lower the failure rate or extend the usage time while at the same time trying to meet customer demands for better wear, fuel economy, traction, ride and lower cost, and the order of priority for these and other variables the customer wants in their tires.

While compromising safety is never a consideration, the rank order of the dozens of performance characteristics can have an effect on tire life.

Here is a challenge with hypothetical information. Would all RV owners be happier if they were told that the maximum age of tires was 8 years   BUT it would be illegal to operate on a tire older than 8 years or operate without a load, temperature and pressure monitor and recording system "black box" system and tire warranty would only be good 4 of the 8 year life?  Oh ya, the "black box" monitor system would add $500 to the initial cost of the RV and there would be a required annual fee of $75?

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Monday, September 26, 2016

How old is too old?

Been following an RV forum thread on tire age and have been trying to explain why there is no strict guideline for exactly when a tire should be replaced. I summed up my position...

The general guidelines for motorhomes is to have the tire inspected starting at about 5 years and to replace at 10 years no matter what the inspection indicates. Due to the Interply Shear effects on belt durability, trailer tires need to be closely inspected after a couple of years and it appears that 5 years may be the max life for most applications.

I do understand that people would like a nice clear precise answer but the problem is that with load, speed and temperatures all having an impact on the life of a tire it is impossible to give a precise time of when to replace a tire.

In today's society no company can give a specific answer to the question as they would be sued if a tire failed before the "end of life" time and they would be sued if the tire lasted past the "end of life" time. We are talking about probability.

You have a sticker on your RV telling you the inflation to use based on an estimate of how much "stuff" you will be carrying. Federal Regulations say the tire load capacity at the specified inflation must be able to support the load rating of the axle. This assumes an exact 50/50 side-to-side load split all the time. It also assumes you do not put more or less load in your RV than what would result in each axle being exactly at GAWR. Many have learned of the importance of getting the actual load on each tire.

Maybe it would help if we thought about tread depth instead of age.

Exactly how much tread can be worn off before a tire becomes "unsafe'. Most states say 2/32" for passenger car tires but does that mean the tire with 3/32" tread will always perform equally to a tire with 10/32"?  Of course not. As the tread wears the wet traction capability goes down. Do you always wait till each tire gets to 2/32" before replacing it? If not, why not?

Conversely the dry traction can go up as tread depth goes down, so there is a trade-off. I dare say that if you live is an area with lots of rain or even snow, you probably change tires before they are that worn. However, if you live in the dry Southwest you may be tempted to run less than 2/32" tread as you have seldom if ever have wet traction problems.

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Thursday, August 25, 2016

Why aren't there more recalls of bad RV tires?

In addition to writing this blog, I monitor a number of different RV forums for discussions on tires and related issues. I find that many times I am suggesting to people that they should file a complaint with National Highway Traffic Safety Administration about their tire problems. Many times I get replies asking "Why Bother". While this post focuses on trailer tires the information also applies to other RV applications.

"I get tired of hearing that most ST tire problems are caused by the RV owner. I simply don't believe it based on my own experience. NHTSA; never heard of them, much less knew I could file a complaint. So I asked my tire dealer. She said that they do turn in reports on tire problems themselves and/or turn the tires over to their tire reps for analysis. That went for tires they sold or other brands they replaced. So I don't believe the argument that NHTSA doesn't have enough data to see a trend for ST China bombs. Doesn't make sense if tire dealers are turning in info to them to protect their sales and customers."
Scott I have a number of posts on NHTSA. Here is my take on recalls and filing complaints.

They have a limited budget so must focus on most important first. That means things that can result in death or personal injury. TT only see financial loss so obviously would come last.

While your dealer may have sent information to the tire importer I have noted that many times the volume of tires in a "family" may be low enough to avoid the requirement for data be sent to NHTSA.

in 2014 207 Million passenger tires were sold, 29 Mil LT 18 Mil Truck or 254 Mil tires sold each year where a failure might result in personal injury

According to RVIA data there are 9 Mil RVs on the road but this includes approx 1.1 mil motorhomes so if we have 7.9 mil trailers with say 90% on ST type tires buying a set of 4 tires once every 4 years we are looking at maybe 7 mil St type tires sold each year

So if you were NHTSA where would you spend your research tax dollars? On the quality of 254 Mil tires where there might be injury or on the quality of 7 mil tires with almost no chance of injury?

There have been a few recalls on ST type tires but I am also aware of times when ST tires are not recalled simply because the records are poor as most are imports. In some cases the importer is a small company so does not have the money to do a recall that can cost many million dollars even for a small recall. IMO sometimes a small company would simply go out of business rather than try and handle a large recall.

The RV owners seldom file complaints and many times when they do they do not include the DOT serial so that complaint can not be used in the tally of number of complaints.

Here is a reply to another reader:

Tireman, this seems to me to be a big picture economics issue. Is it true that the auto and truck industry does not experience the failure rate or short life that the ST tires experience? If true then the RV tire industry and rv chassis mfgs apparently are not willing to manufacture to the same expectations of the auto industry leading to all the woes that we experience. Look in any parking lot and many of the cars are running on poorly inflated tires. My guess is that these do not fail at the same rate as ST tires. I also have an engineering/QA background and have abused tires for 50 years in racing, towing and with heavy Ag equipment and never seen the failure issues we see with ST tires. I do properly maintain and store my tires, even so I have had 3 failures out of 8 tires in the last three years. Anecdotally I also travel enough to see too many RV trailers on the side of the road with failures. If not for economics, why would ST tires only be made to be rated at 65 mph when we live in a faster world? I try to stay under this for fuel economy but find myself at higher speeds frequently. Impossible to practically operate within rated limits. Is this whole issue because RV mfg do not want to pass on the cost of safe tires? I understand Motorhome tire life is 5 to 7 years and tractor trailer operators get hundreds of thousands of miles on tires. ST tires 2 years? Please explain.

IMO the main reason RV TT mfg do not provide better tire fitment (load capacity) is they sell based on "Bling at low cost". When shopping the RV market you will almost never meet a salesperson who tries to sell based on specifications. Yes tires applied to TT are designed to meet 1970 driving speeds (55 mph speed limit) IMO RV Industry lobby actively opposes any change in requirements that might increase costs by even a few dollars.
Yes ST tire life is MUCH shorter than Passenger, LT or Truck tires.

Universal use of TPMS starting in 2005 on passenger cars has significantly lowered tire failure rates on cars but RVIA (mfg association) does not support TPMS as OE on RVs is one example. They also do not support applying 2002 DOT test requirements on ST type tires so you are stuck with 1970 performance in the 2016 world.

You could write to RVIA at but I have been told that as an industry representative organization they may not be very interested in individual complaints.

If you want to learn more, read some of my posts about NHTSA.

PS While I have done training at NHTSA in Washington DC I do not work for DOT or any Gov agency.

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Tuesday, August 2, 2016

Can you "contain" a tire failure ?

I read a story about someone that had a couple of tire failures on their trailer. He was trying to build a solid steel shield over the tires in the wheel well in an attempt to contain the "Exploding " tire.. I offered a reply...

Sorry, but IMO trying to "build a shield" just is not a reasonable approach. I doubt that you can spare the 500 or more pounds of steel it would take for the shield and structure to support the shield. Since a failure of a tire can be explosive event a solid wall will not let the force dissipate. You need an open grate. Check out this compilation of exploding tires to see the forces involved. Note that most of these did not involve a tire spinning at 50 to 65 mph which would add significant force to any explosion.

You need to remember that tires simply do not fail catastrophically without some reason. There is no magic involved.

Starting with a new tire...
1. It can fail in the sidewall if it is run at highway speeds (30+) while significantly under-inflated ( 40 to 80% low). Polyester melts. Steel tires do not need the speed but steel fatigues so after maybe a couple thousand cycles you get a "zipper" failure. Properly functioning TPMS can warn of the air loss in the first couple seconds of a loss ( of just a few psi for some brands). In most cases this early warning will come way before the tire has lost enough air to result in steel fatigue or body cord melting. If you have a TPMS have you tested it? Can you hear the buzzer over the loud radio?

2. Radials can have a belt/tread separation. This takes many hundreds or even thousands of miles to grow large enough for the tire to come apart. This is where the close inspection in my blog post comes in. As I showed a tire with even significant separation does not have to come apart at once but it does leave visible clues.

The reason for belt separation is a combination of initial tire design and material selection and the long term use. Initial design can not prevent all damage done through excess heat and age but current technology in first class radials should deliver 5-6 years or 30 to 60,000 miles at specified inflation and a max of 80% load, except for multi axle trailers.

Due to trailer suspension design there are unique forces "Interply Shear" placed on TT tires that result is about 24% higher shear forces than seen in motorized vehicles. This means you would need to run very much decreased load ( maybe -25% to -50%) to get the same life on a TT application than the same tire on a TV application.

I do not know of any direct comparison real life testing so can only guess at the above figures other than the 24% that comes from Finite Element computer simulation that is a well developed tool in automotive circles  other than the RV industry.

Rubber strength degrades with time and heat with HEAT being an over-riding contributor. Do you cover your tires with white covers? This can result is a very significant lowering of tire temperature. Every hour of full sun exposure can be equivalent to two to 3 hours of use running down the highway at top speed.

Quick example: 8 hours a day 7 days a week for two months each summer can be the equivalent of 10,000 miles use as far as rubber degradation is concerned. So if we assume a tire is good for 40,000 miles  and you park it as in the above example after 3 years you may have "consumed the equivalent of 30.000 mile tire life, just while parked.

IMO making some effort to prevent a failure in the first place (TPMS & frequent inspection) would be a better use of time and money than trying to prevent damage to the RV with some sort of shield. 

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Tuesday, July 26, 2016

Unhappy with the quality of the tires on your RV?

Post on an RV forum where there was a thread about "poor tire quality". There was this interchange:

"ST tires may be approved by government regulatory agencies. They still have a 3 year life. When I had tread separations, documenting the event for a government agency was not my focus. Getting new tires and going camping was my focus."

I responded...

First off, there is no government agency that is "approving" tires. The DOT does publish performance regulations that the tire company is "certifying" all the tires they make are capable of passing the requirements. If a tire company wants to game the system and make tires that do not meet the regulations there is no way for the DOT to learn this unless they conduct an investigation. The DOT will not start an investigation without justification or they could be accused of wasting taxpayer money. A major part of that justification is the number of complaints on file.

Sending a complaint to NHTSA not being your focus after a tire failure is clearly understandable. But if you never bothered to record the DOT serial number (S/N) of your tires and simply disposed of them, then you will never be able to file an actionable complaint. [Editor: The link to NHTSA is for filing a complaint regarding a tire.]

No actionable complaint =>  No Investigation =>  No findings of low quality =>  No recall => No improvement in quality of ST tires.

IMO there are some in the business of low cost tire production and sales who know the average RV owner will never complain. They are playing the odds that there will never be a recall, so with no future penalty there is no incentive to improve quality.

Even if you can't focus on filing a complaint at the time of the failure you certainly could file it a few days later, but that would mean you had made the initial effort to record the S/N for your tires.

If RV owners can't make that minimal effort of recording the S/N and spend the few minutes it takes to file a complaint,  I simply do not understand why they feel they can take the moral high ground and complain about poor tire quality.

Filing a complaint only takes a couple of minutes. Maybe less time than what many are willing to spend complaining on an RV forum.

Have you recorded the DOT S/N for your tires?

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Tuesday, July 19, 2016

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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Tuesday, July 12, 2016

Change Tire size

When considering a change in tire size there are many different areas you should consider.

Tire load capacity. You need to be sure you go with tires that have equal or greater load capacity than original.
It is possible that the original tires delivered a very small load capacity such that the coach is easily overloaded. Recalls may have been ordered or initiated but sometimes a manufacturer went bankrupt so no recall action took place or no new tire certification label was provided.
So it is up to you, the current owner to do your homework and get the facts.

- You need to learn the real load capability of the axle by contacting the manufacturer if there is no plate on the axle with GAWR info.
- You also need to learn the real loads you are placing on your tires with your coach fully loaded. Here is a worksheet with instructions on the steps and calculations you need to do.

Now load capacity is not the only information you need. You also have to be sure the tire will never rub and contact any portion of the coach or its suspension. part of this is obvious and can be discovered by rolling on your back with a flashlight to confirm sufficient clearance around all your tires. Don't forget to check both front tires with wheels turned completely both directions. I have read that 3" on all sides is a reasonable clearance.

Rim width - There is a list of approved widths for each tire size. You must stay with the dimensions listed by the tire company

One final clearance check that is sometimes overlooked is the clearance between dual tires. This requires more than just looking at the rear tires. In tire company specification charts there will be a dimension called "Minimum Dual Spacing" for the tires you may be considering. This dimension is controlled by the wheels. Sometimes this information may be marked on the wheel but most likely you will need to contact the wheel manufacturer to learn the specification. This is CRITICAL, as too small a clearance can result in tire damage or even failure.

I can't address if a specific size will fit but simply going up one size would be reasonably close to the 1/2" width increase.

For example if your current size was an ST215/75R15 going to run ST225/75R15 would give you a tire that was approx 10mm ( about 3/8" wider over all)
Now it is important to remember that tire dimensions are approximate and if you change brands the actual with may be a little different.

Also don't forget that the OD would also increase which means the clearance between the tread and wheel well or the companion tire would change also.

You should be able to find published OD and width dimensions from your tire companies web page.

One other item is that a change in wheel width (as published in the literature) will affect the tire width by about 40%. So a tire with a published width of 10" on a 6" wide rim would be about 10.2" on a 6-1/2" wide rim (40% of the 1/2" rim change = about 0.2")

 But in some cases on older coaches tires may have been changed in size or even the size designation such as an old 7.50-15.  If you find yourself in that situation you might consider dropping me an email so I can help walk you through the things you need to consider.

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Sunday, July 3, 2016

How you might run ST type tires at 75 to 80

RE Speed Ratings and operation speed.

My comparison to engine redline seems to me to be the easiest for many to understand. It is possible to run an engine right at redline or even above for a short time but I can't find anyone that advocates operating an engine at the rated max speed for any significant time or distance.

High Speed test is not a DOT regulatory test but is a test based on Society of Automotive Engineers testing. To be marked with a speed symbol a new tire needs to run for 30 minutes on a smooth drum at the stated speed.

There is no requirement for a tire to be "conditioned" with a few thousand miles at lower speed over potholes and up on curbs or with 110% of the rated load or for the tire to be able to pass DOT regulatory durability tests after running the SAE High Speed test.

Yes, an increasing number of ST type tires now come with a speed symbol molded on the sidewall. The primary reason for this seems to be to avoid import duties. What many want to ignore is the fundamental truth that the load capacity of ST tires is much higher than an LT type tire based on the premise from 1970 that the ST tire would be on a single axle trailer that was limited to 50 mph operation speed.

Molding the letters "ST" on a tire is not magic. Physics still applies. If people want to drive at 75 or 80 while towing as they would with their LT type tires and not have failures, then I suggest they pay attention to the Physics and limit the actual tire load as if it were an LT type. This is easy to do.

First simply look for an LT type tire with the same dimensions i.e. ST235/75R15 > LT235/75R15 in the Load tables and limit their measured load to the number found in the LT tables.

If you do that you will probably see a significant reduction in tire failures. Of course this also means you are not in the 50+% of RV owners that operate your tires under-inflated.

If you ignore the Facts and Physics of reality you will have to live with the consequences no matter how much you want to believe otherwise.

Bottom Line. If you want durability and life more like LY type tires then treat them as if they were LT type tires.

As with the engine in your RV or tow vehicle it may be possible to run faster but it does not mean you will avoid all problems. This post is just about tires and not about safe and reasonable operation of your RV. Personally I think 70 is too fast to drive a "big rig" and 75 is certainly too fast for towing. I have heard comments about an increase in truck tire failures due to increased speed limits in many states. A MAX of 75 is stated in some tire information guides published by major tire companies and as with any maximum, the closer you are to it the more likely you will have some negative consequences.

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Monday, June 27, 2016

Quick TPMS test and a couple of tips.

As the use of TPMS in the RV world continues to spread, I am noticing a number of comments on usage, pressure and temperature accuracy and general reliability of different systems.

I am in the process of making a test fixture to allow calibration check of pressure and possibly of temperature reporting also. I hope to have the first round of testing in a few weeks. I have a target of offering a "TPMS Pressure Test" in August in West Springfield at the FMCA Reunion for folks to learn the actual accuracy of their individual system. I will collect and report the findings in Sept.

As I do some initial runs, I am noting some interesting results and readings that need additional investigation as I am not always seeing numbers that I am expecting. This is especially true when it comes to temperature readings.

One thing I have learned is that it might be a good idea for everyone to do one test on their own unit.

With all tires properly inflated you can confirm that your system can provide a quick response and that your monitor is receiving the radio signal properly. With someone watching the monitor, go and quickly unscrew the sensor and have the person watching the monitor beep the horn as soon as the sensor issues its warning. Hopefully you will see that your system consistently reports the loss of pressure the sensor is seeing in one or two seconds.

Be sure to securely re-attach the sensor and maybe even give it a little squirt of soapy water or even Windex to check for leaks. I certainly do not want anyone reporting back that they had a tire failure because they lost air due to a loose sensor.

A side note. My sensors, and I believe many others do, have an "O" ring seal around the battery chamber. As with any device containing a battery that is exposed to water, I give the battery chamber a little squirt of WD-40 to help prevent corrosion of electrical components. You might take this opportunity to also inspect the "O" ring for tears or cracks. Don't forget this little rubber piece ages just as rubber tires do. I keep a couple spare "O" rings on hand just in case. Your TPMS supplier should have a package of replacements for a nominal fee or you can pick up some at your next RV convention or show.

 If you have a monitor that is small and portable, I suggest you not carry it around with you as one of the features we want to test is the ability of the monitor to receive the signal over the distance from the tire to the location you have mounted the monitor. This would especially be important for those monitoring sensors on a toad. You don't have to actually hook up the toad but at least park it in an appropriate location and distance behind the RV.

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Monday, June 20, 2016

"Why do I need a TPMS?"

I occasionally run into folks who say "I check the air in my tires every morning and at every fuel stop so I don't need a TPMS." I might then ask if they have an oil pressure gauge or warning light on their dash. After all, they can check the engine oil every morning and at each fuel stop. How about an engine temperature gauge?  Same follow-up about checking the water in the radiator.

TPMS were mandated in all cars and light trucks back in 2002 with a 3-year roll-in. The primary reason for this was the abysmal record of people simply ignoring tire inflation. This despite the news at the time of hundreds of fatalities being traced to tire failure due to low air pressure.

While the regulations exclude vehicles with gross weight over 10,000 pounds or trailers, where most of the RV market resides, it didn't take long for aftermarket units to hit the market and for smart RV owners to start adding this important safety device to their unit. Yet almost every day I read a post on one of the various RV forums I monitor from owners complaining that they had a tire failure.

I have written more than 30 times about tire failure and the real causes. IMO almost all tire failures in RV application can be traced to one of three causes:

1. Tread Separation -- Which is a condition where the belts and tread separate from the body of the tire. This takes months and thousands of miles to develop and grow to the point of coming apart. This condition can many times be discovered with good tire inspection effort and practice by the RV owner.

2. Run Low Flex Failure or more commonly called a sidewall blowout. This failure can develop in only a few miles and much less than an hour, so the loss of air is seldom discovered when the RV owner does a walk-around visual inspection or even daily check of inflation of the tires. This condition can be prevented with the use and proper set-up of a TPMS.

3. Impact Break can occur when a tire is run over a curb, across a large pothole or even just hitting some road trash. While a TPMS cannot prevent the break, there is a good chance that as soon as air loss occurs, the TPMS would inform the RV owner so he can pull over sooner rather than later and not have to depend on some passing motorist to flag him down by pointing to the disintegrating tire. This may reduce the damage to the RV and save some money.

I believe that some RV owners may have failed to look at the cost of a TPMS, which can be less than  $99 for a 4-tire system as seen on eBay with only the most basic of features. A much better system for 6-tire RV application is the brand I bought from TireTraker. The cost needs to be balanced against the cost of replacing a tire or tires plus the potential of hundreds to thousands of dollars by avoiding  RV damage. A TPMS can also help avoid finding yourself along the side of an Interstate trying to change a tire. In some circumstances and with a better system, the TPMS may provide sufficient advance warning to allow the driver to continue, at reduced speed, to a location where there is more room and it is safer to pull over.

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Monday, June 13, 2016

Post a complaint with NHTSA - Part 3

 Lets finish up this thread.

How to write a useful Description of tire failure for NHTSA

Lets take a short break from all those screen shots and review how to write a meaningful "Description" of the failure as seen in Step 1 of my post of Jan 8.

You need to remember that NHTSA needs facts if they are to take any further steps as they consider if an investigation is warranted.

Just ranting "My tire exploded, tore up my RV and I want new tires and compensation for the repairs and my expenses." is not useful to NHTSA. If you can't be bothered to provide critical information such as tire DOT serial why should NHTSA make an effort to investigate? Other information such as location of the incident, your cold inflation and info on what warning you received from your TPMS, if you still have the tire and can provide pictures or contact information on who has the tire, would be bonus information and potentially very helpful to the engineers at NHTSA. I previously outlined the MINIMUM information needed in my post on Dec 31 2012.


Looking at the Power King catalog on pg 10, I do find a Towmax STR in ST235/80R16 LRE size but ST type tires are not approved for use on passenger carrying vehicles such as a GMC 3500 pickup. Moving on to LT tires on pg 14 & 15 I find no metric sizes such as LT235/80R16.

So we are confronted with a few questions.   Did the owner provide the correct tire name but wrong size?
Did the owner provide the correct size but wrong tire name?    Did the owner mean the tire on the passenger side or that the tire was actually a passenger type tire?   Was the tire that failed on the GMC?
No mention of an RV or trailer brand is made by the owner. I also note that the tire manufacturer is entered as Cooper Tire and Rubber Co but according to my contacts at Cooper, they do not have a Power King or Towmax tire line and have not made ST type tires for many years.

If you were NHTSA would you consider this person a reliable witness to what actually happened  with a tire?

Now you are wondering what a good report looks like. Lets look at #10471651

See the difference?

You do not need to write as detailed a complaint as in the last example but IMO the MINIMUM information you need to provide would be:

1. A brief description of the tire condition, i.e., "Tread and belts came off" or "sidewall disintegrated"  along with a list of damage done to the RV with the associated costs would be desirable.

2. Vehicle VIN - This is on the vehicle label and probably on your registration slip.

3. Tire DOT serial - The complete serial number including the date code at the end.

4. Brief statement letting NHTSA know if you have additional information such as: "I have photographs of the tire" or where the tire is: "Tire is in my possession" or "Tire delivered to Fred's tire in Anytown, USA.

5. If you were using a TPMS that would definitely be information I would include.

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Wednesday, May 25, 2016

Question on Interply Shear on trailer tires vs tow vehicle tires

A question from a reader of a post on a trailer forum

"Tireman, your concerns over shear puzzle me. G614s are LT tires as are the tires on my 2500 pickup. Only difference is G rated vs E rated. The truck mfg recommends 60 front 75 rear with 80 psi max on sidewall. Why is there no concern about shear on the front tires of my truck. It seems to me the frequency of shear forces is much greater on the truck than the trailer."

Ya, I understand your confusion
 But the reality is that when computer analysis is used to look at the internal structural loading, the fact that tow vehicle tires are all operating at very low "slip angle" (difference between travel direction and angle the tires are pointed to) is significantly lower than for tires on a trailer.

The reason for this is that the center line of of tire rotation for the tow vehicle tires points to the center of the radius while the trailer tires, especially on tandem axle trailers, does not.

This translates into a higher slip angle which means higher internal structural twisting forces on the belts. The computer model suggests 24% higher on the TT tires than TV tires even if all tires were the same with identical vertical load and inflation.

TV front tires have "Ackermann" alignment designed into the suspension but TT have no allowance other than bending of tires, springs, spring mounts and bushings but the forces to bend the springs etc have to go through the belts of the tires.

This is a MAJOR reason for travel trailer tire life to be much shorter than motorhome or tow vehicle tire life.

Hope this helps folks understand a bit of what makes tire engineering a challenge.

Editor: Here is an earlier article Roger wrote about "interply shear," if you want more information. 

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Tuesday, May 17, 2016

Tire Dry Rot is a misnomer

I see the term "Dry Rot" used by many in the RV community when describing old tires that have visible external cracks.

Some have attributed this to the dry air in the Southwest part of the US. Some with specific references to Arizona.

Sidewall cracking occurs for a number of different reasons but these reasons all end up at the same place. The elastic properties of the rubber have been degraded over time and when the rubber is flexed it cracks rather than stretches.

Some reasons for the loss of flexibility or "stretchyness" can include exposure to UV or Ozone or simply old age. Each of these items is different but they each attack the bonds that exist between the various  materials such as rubber, sulfur, carbon black, oils waxes and numerous trace materials used in the process of manufacturing rubber. When the chemical bonds break or "crack" the loads in the rubber get transferred to the nearby material which then has to resist the forces trying to stretch it.

These cracks may start out at the molecular level but they do not repair themselves so they can only  continue to grow. Eventually they are large enough to be seen on the outside of the tire and if the tire is exposed to the damaging elements long enough the cracks can grow large enough to allow air to escape or for tire components separate.

I have previously discussed the way increased heat can actually accelerate the aging process of rubber in posts about direct exposure to sunlight and that the use of white tire covers can reduce the accelerated aging process.

For those interested HERE is a report issued by NHTSA on tire aging. You will note that on page 3 of the report titled "Background" they identify that "degradation is accelerated with higher temperatures",  You may also note that there is no mention of UV as a significant contributor to the aging process.

IMO Sidewall cracking is in itself seldom more than a cosmetic issue. However it can be an indicator of possible "old age" and degradation of the internal structure of a tire. Maybe a good analogy is when you run a temperature.

I do not recall ever hearing of someone having a temperature for no reason. Your elevated temperature is almost always an indication or symptom of some other medical problem that needs attention.

Since the consumer has no good, low cost way to learn the condition of the tire structure you are confined to looking at various symptoms.  Spotty tread wear is one symptom. Tread and/or sidewall snaking is another and of course sidewall cracking can be another.

Bottom Line
Tires do not actually suffer from "rot" as one might see in a piece of wood or some old food. They can have signs of surface cracking but as long as the cracks are shallow and a tire dealer has completed a full inspection of a tire and said it was okay to run I would go with the dealer finding.

You might want to review my post on How do I inspect my tires and note that signs other than just cracking can be much more telling than just sidewall cracking.

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What do do with your old tires?

Most informed folks in the RV community know that our tires will normally "age-out" before they "wear-out" so they are confronted with spending a bunch of money on new tires and sometimes even being charged by the dealer to take the old tires.

The other day we had some service done at home and I noticed the truck had 14 tires on it and they were all 22.5 size but they were not all the same size ( same on each axle but different between axles)

Now obviously this truck is used for local short haul service with probably a lot of relatively sharp turns in and out of driveways that result in rapid tread wear and a good portion of the time at less than max load.

Just something to consider when confronted with the question of what to do with your old tires. IMO there is a good chance companies like this might be interested for the right price.

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Tuesday, May 10, 2016

Which brand tire is better ? I can setup a test if you really want the answer.

Many people ask the question in various RV forums "I need new tires. What is the best brand tire I can get?" or "Why do tires made by company Q seem to have a high a failure rate?"

Invariably the thread quickly devolves into people providing their opinion but I do not recall ever seen the suggestion that one brand is better than another supported with any data based on direct apples to apples comparison of two brands of tires.

I believe there are two reasons for not getting a clear picture of which tires provide better service in RV application: One reason is personal pride and the other is a lack of direct comparison based on equivalent exposure to service problems.

Personal Pride
I think you will find that many posts IMO are based on the concept that people do not want to consider the possibility that the product they bought isn't the best. To think so could be an indication that someone might not be making the best purchasing decisions. For example, if someone asks about Acme brand anvils, and someone just happened to have purchased an Acme anvil they are more than willing to "badmouth" anvils made by almost any competitor even when they lack either meaningful data or solid experience to justify their position. After all why would they admit to just having purchased a good quality anvil?

Direct comparison of tire performance.
You may not be interested in ST type tire but I would hope that if you review this information it may help you make a more informed decision when asking what brand to buy.

If you consider that the OE market in ST type tires in RV trailer application is dominated by Goodyear and many dozen 3rd or 4th tier tire companies who make tires in small lots and don't even apply the name of the tire company to the tires they are making. With GY having a majority of the market and no other brand having much more than 5% I think you can see how it easy to end up with skewed impression that GY tires are bad, based simply on the observation that the greatest number of complaints are about Goodyear ST type tires. This is much like the concept that tires made in China must be poor quality because so many trailer owners that have problems with their tires discover they are made in China. I have to wonder how people expect to be able to make a reasonable comparison of tire quality based on country of origin when probably 90+% of RV trailers with ST type tires come OE with tires made in China. Wouldn't it seem reasonable then that 90+% of problems experienced on RV trailers with ST type tires would have tires that were made in China?

Now in 22.5 rim diameter tires found on Class-A RVs,  I think we see Michelins with sidewall cracking complaints and Goodyears with irregular wear complaints and since those two companies probably are on 70+% of coaches of course those are the ones that would have the most and therefore most noticeable complaints. Even if another company with maybe 5% of the market had tire failures  at twice the rate of GY or MI cosmetic issues, it would still appear that GY and MI are the only tires with problems.
Check out my post on Causation vs Correlation for more on the topic.

Thorough tire testing costs in the neighborhood of $30k to over $100k per brand for just basic outdoor wear and general durability is it any wonder that no independent agency has stepped up and run such a comparison?

Now I have the contacts in the tire industry to an independent testing company that could run such a comparison. All I need is for people to send me contributions - maybe in $500 increments and I will develop the specifications for a direct comparison between Goodyear ST type tires and Maxxis brand ST type tires plus a set of TowMax or similar ST type tires. I would use a popular size and load range made by all three companies. To keep costs down we would limit the test duration to about 10,000 miles or till there are enough failures to discontinue running that brand. I figure two failures would be enough to stop running a given brand.

So do you really want to see actual data? All I need is $35,000 in the bank to start the evaluation. Test results would be sent only to those making the minimum $350 contribution or more.
If interested send me an email and I can send you specific details of this offer. We only need 100 individual contributions to get started.

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Tuesday, May 3, 2016

Tire Temperature always seems to be a hot topic

From a couple of RV forum threads.

Hot tire temperature is important but the problem is in learning what the actual temperature is.
The average temperature is not a good indicator and that is about all you can get by reading the Contained Air Temperature. While many seem to believe that their external TPM is giving them a reading of the CAT, I can tell you that the TPM will always read significantly cooler than the CAT and that the CAT is always cooler than the hottest and most critical location in the tire.
Rubber is a good insulator and does not transfer heat from the hot spots to the cooler locations very well. As a result you can have a tire failure from extreme heat in one location while another location only inches away can be perfectly OK.

I read that many TPMS have set the high temp warning point at about 158F. That is all well and good if you are measuring the actual CAT but I do not know how an external TPM will sense that temperature especially if it is out in the cooling air stream on a metal stem or at the end of an extension hose

I would always be more concerned about the pressure measurement as that is not going to be affected by cooling of the valve stem or extension hose. We know that for our purposes pressure does follow the "Ideal Gas Law" and it doesn't matter if you use air or Nitrogen the results are essentially the same unless you are measuring your pressure to the 0.1 psi.

So knowing that tire pressure will change by about 2% for each change of 10F in temperature of CAT we can get a reasonable estimate of the CAT and will note that out TPM is showing a lower temperature change than indicated by the pressure change.
I would not use the temperature reading from a TPM to set tire pressure. If I got a high temperature reading but the pressure reading was reasonable I would suspect a mechanical problem such as bearing or brake issue.

A motorhome properly loaded with the inflation set based on actual measured load on the tire and consulting the load Inflation tables will not result in an over-heated tire. NOTE Trailers should use different method to set pressure due to the Interply Shear issue of trailer axle alignment and should set the cold inflation to the tire sidewall pressure.

If the above isn't enough detail for you you can review the other posts that have Temperature as part of the topic.

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Tuesday, April 26, 2016

How to get better MPG in your Class-A

While responding to an RV forum question on tire performance for Class-A size tires, I found a potentially very useful web site for those in the market for new tires for Class-A RVs i.e. 22.5" sizes.

The EPA has a program called SmartWay that many may not be aware of. This program identifies tires that can provide better fuel economy than tires not on the list.

Here is the web site with the tire brand and design name of tires that are certified as meeting the minimum requirements of this regulation. Being on this list vs not being on the list is about the only tool available to tire owners to compare tires where claims have been made about improved fuel economy.

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Sunday, April 17, 2016

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.
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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