Can Black tire "Covers" be used without causing damage?

Back in June 2011 I did a post that asked the question  "Tire Covers - Do they do any good?"  In that post I showed the numbers from a test I had run where I collected tire temperatures showing the effects of shielding my tires with white vinyl covers vs the tire temperature when exposed to direct sunlight for just a couple hours.

 I also covered the science of the damage that excess heat can do to our tires. The bottom line of that post was that white vinyl can help extend the life of tires by protecting them from both the affects of UV and the IMO more serious damage done to the internal structure of tires from excess heat.

I also did a rough check using dark trash bag covering a tire and in that case I found that the black cover actually resulted in a tire being hotter than when it was just in the Sun with no cover. Based on that limited data I have recommended against the use of Black or dark color vinyl tire covers.

On more than one occasion I have observed some Class-A RVs with what appears to be a mesh shield than hangs down off the side of the RV. This is different than the vinyl "bag" that hangs directly over the outside of my tires. I was able to collect a few data points while in Redmond , OR in 2014 at a large RV Convention, and that data suggested it might be possible to use this mesh material and not increase the temperature of the tires. Finally this Summer while at another RV Convention I struck up a conversation with a representative of ShadePro Inc who offered to send me a Tire Shade to test. In Aug & Sept i had some health issues and then I ran into difficulty with clouds here in NE Ohio but I was finally able to collect the data I felt comfortable with that would allow me to reach a conclusion of if this black mesh material could be used.
 

Here is a shot of my test set-up with a white vinyl on front, control sidewall in center and the black mesh shielding the rear. After 2 hours in the full sun 

In the shade a tire gave 92°F

In the sun the white cover was 126°F

The reference tire sidewall registered at 147°F

and the black mesh shade showed 136°F

Under the cover the front tire was at 114°

While behind the mesh shade the rear tire was only 101°F

Conclusion:
The data shows that in this test the black mesh did a better job of keeping the tire cool than the white vinyl.

I can think of a couple of reasons for this.
1. The vinyl cover was in direct contact with the front tire so heat was being directly transferred to the tire.
2. The mesh allowed better air circulation around the rear tire.
3. The fact that the black plastic was also in direct contact with the tire probably contributed to the poor results.

Observation:
I was wrong to suggest that all black shields were worse than white covers, as this test shows that data is better than opinion when it comes to facts. This is one of the wonderful things about Science. 

Does tire pressure follow the "Gas Law" ?

OK kids, today's science lesson is about the stuff we inflate our tires with. If you don't want to take the lesson simply skip to the "Bottom Line."

If you took engineering, chemistry or perhaps some other science class in school, you might remember something called the "Ideal Gas Law"  PV=NRT.  I covered the formula in THIS post, and the resultant "Rule of Thumb" is that for every 10°F increase or decrease in temperature the pressure in your tires will increase or decrease by 2% . Now we need to be careful and remember that some may be discussing passenger tires where 2% of 36 psi is rounded to mean 1 psi change for 10°F change but I am discussing tire pressures that may range from 45 psi to 130 psi so percentage is more accurate.

But that formula is based on basic scientific theory of a dry gas used in a non-expandable container.

The first "fly in the ointment" is that most people are inflating their tires with compressed air and air is a combination of different gases such as 78.084% Nitrogen, 20.946% Oxygen, 0.934% Argon 0.03769% other gases such as  CO2 , Neon, Helium, Methane, Krypton, and Hydrogen. Note the concentrations of these trace gases along with various "pollution" changes over time so the specific percent varies slightly. Wikipedia has the details if you are interested.

The other major variable is the percent moisture. Water vapor varies between 1% and 5% depending on temperature and geographic location. It also varies based on the process of compressing air and the quality of maintenance the air compressor receives. In general the percent of water vapor will be raised in an air compressor, so unless special procedures are taken you are going to end up with a few percentage points of water vapor. Making this even more complex is that the effect of moisture on your tire pressure varies with temperature. This can range from less than 1 psi to almost 5 psi. I have a post on how you can make the air you use to inflate your tires "dryer" if you feel that is important.

In the equation the V stands for volume and since tires do expand a little bit as the pressure goes up and inflation gas isn't 100% dry or 100% of any one gas but a mixture of Oxygen, Nitrogen and other trace gases, these facts have an effect on how close to PV=NRT you will get with your tires.

Remember that even if you pay to "fill" your tires with Nitrogen you will never reach 100% nitrogen as there are always some other gases mixed in. In some experiments I have seen that the best you can reasonably expect to get would be about 95 to 98% N2.

Another problem is to know the actual temperature of the gas inside your tire. Most people have either an external TPM sensor or use an IR gun to measure the temperature of the tire and mistakenly assume that is the temperature of the gas inside the tire. About the only time you might obtain an accurate reading of tire inflation air temperature is many hours after last moving the tire and hours of storage in a constant temperature room so both in internal and external temperature is the same.

In an effort to get closer to the actual numbers I decided to try and do a quick experiment using an internal TPMS and an External TPMS on my Class-C motorhome. I ran into a number of problems.

Some of these problems include the fact that external sensors did not completely "wake up" in the first minute of operation so I was not able to easily get a zero point temperature reading.

Also we need to accept that despite claims to the contrary, TPM sensors are not calibrated laboratory grade instruments so they introduce some error into the experiment. Next we need to consider that external sensors are only providing the temperature of the metal in the sensor body which is affected by the outside air temperature. This will usually result in a cooler temperature reading than the actual temperature of the gas inside the tire.

I did collect some data but with all the variation I was able to identify I felt the experiment did not deliver any meaningful data. I did see variations in pressure readings between the internal and external sensors over time but the numbers were inconsistent.

What does all this mean?

BOTTOM LINE
1. Reality is much more complex than nice controlled laboratory experiments
2. Tire inflation pressure does vary with tire temperature and the pressure does increase as the temperature increases.
3. Dryer inflation gas will have less variation of pressure than "wet" inflation gas.
4. In my opinion none of the above observations is significant enough to change the "Rule of thumb" that we can expect our tire pressure to vary by about 2% for every change of 10°F.
5. We all need to remember that not every change or difference that is measurable is meaningful
6. Best advice I can give is not to get your shorts in a bunch about inflation.


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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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Help It's hot and I need tires

That's the message I read on an RV forum.
Here is the pertinent background info.

   "I am having a real issue with Michelin tires.  I live in Las Vegas and there are 3 tire companies that carry Michelin tires.  The problem is they all tell me they are back ordered and have been for awhile.  They have no idea when they will get any.  I have a 40' 2008 RV that came with Goodyear tires and have serviced us well.  I just don't know what do do since I can't get Michelin tires which seems to be the tire to get. Should I settle for Goodyears that are in stock?  My other issue is we are going on a trip next week that we can't get out of and because of the extreme heat here in Vegas we are afraid that our old tires might just fly apart. What should I do?"

IMO

While I can't address tire availability from Michelin, you need to remember that there are generally considered to be 3 tiers of tire companies. Michelin, Bridgestone & Goodyear as #1 with the tires made & sold by these companies as being essentially the same  ie Firestone & Dayton ~ Bridgestone, Kelly & Dunlop~ Goodyear and Goodrich & Uniroyal ~ Michelin.

Now there are some minor differences within a corporation but many times the rubber compounds and reinforcement materials are identical or very similar. Tread designs may be a bit more advances in the Corporate brand and the top of the line tires may not have an equivalent in the 2nd brand  but for most people you will not see a difference in performance.

#1 tier have their own tire stores with many hundred nationwide.

Now Tier #2 would be tires made by corporations that have their own tire plants. These would be companies like Toyo, Pirelli, and others seen on this list. They may not have any company stores and jsut depend on large tire distributors to market their tires.

3rd tire would be companies that are not on the list above. These may actually be just sales companies the have other companies make lower cost product for them as "private brand" , Sometimes the same tire is made for a number of importers with only a name-plate change in the mold to differentiate the "brand" These companies seldom have their own stores but sell their products through wholesalers.

Check the tire warranty and I think you can get a good feeling for the real quality of the tires you might be considering. Are there lots of exceptions or if the warranty short could be a tip off that the seller isn't real willing to stand behind the products they are selling



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Now to your concern about your current tires and the hot temperature.  It's kind of hard offering an opinion or suggestion on if you need new tires right away or not, but tires simply do not fail simply because it is hot outside. Excess heat is not good for long term durability but if the tires have always been properly inflated, never been punctures or run even a few psi low you should be OK for a while.

You didn't offer your actual age of your tires but they are probably 7 to 8 years old. You also failed to provide info on how you store your tires. Inside or out in the Sun? Do you have a lot or a little extra load capacity based on actual individual tire position weights? Do you run a TPMS and have never run lower than what is needed to carry your actual measured load. All of these items can contribute to making a suggestion to change your tires right away or suggesting that you can wait a few weeks or months.

If concerned about road or ambient temperature just drop the speed down. Don't run 70 when 60 will still get you there. Last Aug I drove Ohio to Oregon at 60 - 62 with no problem.

Can you increase your tire pressure without exceeding the tire or wheel max?

Where are you headed?  Have you checked to see if there is a large tire dealer at a future location that has tires for your RV in stock?

UPDATE:
In the meantime I would suggest you go to a Goodyear truck tire dealer and have the tires inspected. Let them know your concern about the tire age and current hot temp in Vegas. Let them know your actual scale weights (Go to a local truck scale location first. There are a number around Vegas I'm sure)  and your normal cold inflation. WHile there ask if they would confirm the accuracy of your pressure gauge. If a stick type +/- 5 is about what I would expect. If digital (better) +/- 2 would be good. If outside these numbers you need a new gauge. Check my blog on tire gauges and how to have a reliable "Master Gauge" of your own for less than $15.
The tech should be familiar with the Goodyear tires you have so can offer first hand information on the condition of your tires.
Remember you can always get just the two fronts replaced as a failure there would be the greatest concern. With a TPM on the rear duals a failure would be less dramatic and you should get plenty of warning so you can safely pull over quickly before serious damage is done.

===========================
It would be nice to be able to offer more specific answer to his question but all to often people fail to provide important information that would allow more than a wild guess.


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Is monitoring tires "Rocket Science?"

I get asked this question quite a bit. I think that after reading various posts on tire inflation and load, some people want to complain a bit and are hoping to use the excuse that it is too much work to monitor tire pressure every travel day.

Here is an example from a trailer owner:

"I've noticed that 50 psi cold at sea level at ambient 65F is 54-55F at 6,000 to 7,000 feet.
  While touring should I deflate to 50 psi cold at 7,000 feet if I'm hanging around there for a day or two?"


I am hoping that if I share my personal experience, it will provide a down to earth view on the topic that some want to turn into "Rocket Science"

I just finished a two-month cross-country trip.  Ohio > Oregon > Seattle  > Calgary, Canada > Yellowstone > Ohio in my Class-C MH with LT type tires that are 7 years old but have always been covered whenever parked for more than a couple of days, Always running inflation about 15 psi above what is needed for actual max load based on 4 position scale weights that are confirmed to have not changed significantly each year with a trip across CAT scales.
 Fastest I ever run is 70 mph but most of the 7,400 mile trip was with cruise set at 62mph. In other words, the tires have had a good life with good care.

Elevation ranged from about 25' at Olympic Nat Park to 8,000'+ in Rockies. Morning temperature ranged from 33 F to 94 F. I have TPMS (both internal and external) so I am able to constantly monitor both pressure and temperature.

I adjusted the tires once during the trip. I think I needed to add from 1 to 4 psi in the 6 tires. Since I run a nice inflation cushion of +25% I don't get all bent out of shape when the cold inflation is off by 3 to 5 psi from my goal.

You don't have to make the task of monitoring inflation a big deal. While I may set my pressure to +/- 0.5 psi, you certainly don't need to be that fanatical, and I would consider it completely acceptable to be +/- 2% for  the average user as long as the "goal" cold inflation has an appropriate safety margin.

With TPMS I simply hit the button a couple times a day and get a real time reading of tire temperature and pressure. Yes, the temperatures vary and so do the pressures, but unless I see a sudden drop in pressure and there have been no external changes such as a sudden rainfall or one side constantly hotter than the other after spending hours with one side in full sun, I just do not worry about it.

Relax.  Let your TPMS monitor your tires and enjoy the scenery.

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Question on UV & Ozone tire damage.

Got this question and thought I would share my answer
"Tireman,
First of all, thanks for all your informative comments here, and also for the mounds of useful information in your blogs.
Have a question though... I've always thought that damage to sidewalls from ozone (which is always in the atmosphere) is going to cause sidewall damage more quickly than UV rays. Therefore, unless your motorhome spends a significant amount of its life in direct sunlight (as it might if you were full-timing or even half-timing), it really doesn't matter much if you cover the tires, since you are going to be replacing them in 7 or 8 years anyway. Is this faulty thinking?"

Yes Ozone can do real damage to tires. I have seen sets destroy because they were parked in a garage that had a leaking Ozonator.  BUT Heat is the primary killer of tires.
Ozone and UV can only attack the rubber surface but heat damages the internal components and structure in addition to the surface rubber.
The UV protection test in the link above shows how almost anything can stop the UV damage to tires.

The stuff sold that promises UV protection is like suntan lotion in that it may extend the time you can be in the sun without getting burned but I know of no spray on protection that cuts UV to zero.

 Heat accelerates the degradation of the molecular bonds which can lead to belt and tread separations.

I did a test on covers

that shows the significant increase in tire temperature. Since the affect of heat DOUBLES with each increase in temperature of 18F this translates to an effective doubling of the "aging rate" of tires (or reducing the tire life).

I cover my tires whenever parked for more than an overnight stay where the tires can be exposed to direct sunlight. The WHITE covers block all the UV and keep the tires at about ambient temperature rather than baking the life out of them.

With proper care:
Washing with same cleaners and cloths I would use on the RV body.
Having the tires "under-loaded" by about 20% (a 20% Safety Factor if you like) when setting cold inflation.
Never getting lower that 5% above the inflation needed to carry the load.
Multi-axle trailers should run the tire "max" inflation unless they have significantly upgraded their tires with higher Load Range and larger size, but they still need to run a much larger "Safety Factor" due to the unique loading caused my their tandem axles.
Always running TPMS and checking pressure every AM and after each stop.
Using digital gauges that have been shown accurate to +/- 1psi.

I am hoping for 9-10 year life on my Motorhome. If I had a multi axle trailer and took the same precautions I would hope for 5 to 7 year life due to unique loading from suspension design.

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Fire Safety is not a focus of my blog

RV Fire Safety is not a focus of my blog.

However, I was sent a link to a news story about an RV fire that occurred not too long ago involving three fatalities and according to the report there was also a tire failure. Here is the link to the story. It was suggested by a reader that I use this as a topic for my blog.

The main reason I am writing about this tragedy, is to point out the importance of having plans in case of fire. You need a plan on what to do if there is a fire when you are parked and you need a different plan on what to do if the slides are in and you are traveling down the road.

Here is a new video from a friend Mac McCoy, that shows one possible plan that involves using the emergency escape window. Some of you may know him as "Mac the Fire Guy". Check his web site for where he is giving his seminar and fire demo and go see it.

One thing Mac mentions in his seminars on RV Fire Safety, when he covers the use of the escape window, is that some windows like the one in my Coachmen cannot be opened without breaking the window so you need to contact your RV manufacturer to confirm that a “Fire Drill” will not result in a broken, expensive window.  

Now I want to make it clear that it is difficult for a tire failure by itself to start a fire. Once the tire looses enough air to “fail” you can no longer drive on that tire. The temperatures involved do not approach the self ignition point of rubber. According to this web site

the ignition temperature of rubber is 500°F to 600°F. The fact is that once rubber gets to above the 340°F softening or “melting” point, it loses its all its strength, falls apart under load and the tire would “blow out” or shred and scatter the pieces along the highway which would result in the vehicle stopping and the heat would no longer be generated as the failed tire is no longer carrying any load. Every week hundreds of tires fail somewhere yet there is no fire so the numbers support the degree of difficulty in having a tire self ignite.

It is possible that there are other ignition sources such as spark from damaged electrical wiring, grease, oil, fuel or even brake fluid and some of these might ignite if the RV is damaged and then there would be flame which could then ignite the tire. To put the temperatures in perspective, diesel has a flash point of 156°F, brake fluid 374°F, engine oil flash point is 300° to 440°F, wood catches fire at 374°F and fiberglass and many plastics seen in RV construction have ignition point temperatures from 190°F to 450°F. You will note that these temperatures are all lower than the ignition point of rubber.

Some other web sites with information on RV Fire safety; HERE and HERE is one on how to create a plan. RVBookstore has a DVD on RV Safety which is also available in electronic version 

Bottom Line: RVs should not catch fire if there is a tire failure. Maybe the best advice I can give is that you can go a long way by simply preventing tire problems in the first place by being sure your tires are properly inflated all the times. That means knowing the inflation needed, checking the inflation at the start of every travel day and now you can have tools that can warn you if a tire starts loosing air while driving, so I strongly recommend having a TPMS. I have TPMS on my car and on my RV.

What do you think of IR guns? – Part 2

In Part 1, I covered the problems of obtaining a sufficiently accurate temperature reading due to the high level of temperature variation across a tire tread. In this part I will cover a comparison of the temperature readings of a TPMS sensor hat mounts inside the tire air chamber with an IR gun and a tire temperature probe as used by a tire manufacturer in professional level tire racing.

Again this will be more technical than some may want to read so the summary is that.  

TPMS sensor

 1. The TPMS provides a lower average temperature than seen in the tire hot spot. 2. The IR gun and race tire probe give similar readings but the IR gun gives lower temperature readings than the probe. 3. None of the gauges provided a reading that identified the tire that was 10 psi (15%) low. This confirms that you cannot use a temperature gauge to set the proper inflation for your street tires.

The set-up:  My normal cold inflation is 70.0 psi in my LT235/85R16 LR-E tires on my Class-C RV. I have had my unit weighed and know the actual corner weights. According to my loads and my placard, my minimum cold inflation should be 55 psi. I set one tire 10.0 psi lower than the other six tires.

The test  -  It was cold (49°F) the morning I drove 13.5 miles (8 on Interstate at 60 mph) to meet with the manager of race tire development for one of the major tire companies. We then drove 1.3 mi on city streets to the interstate followed by 5.9 miles at 65 mph. We stopped and recorded the temperatures of all six tires in less than 30 seconds of stopping. 

Notes
It was the RF that was 10psi low.
The Race probe reads to 0.1°F

Conclusions: For normal tread depth street tires you cannot use tire temperature to set your tire pressure for if you try to set pressure so all tires read the same temperature you may be off by 20% or more from what is needed based on your load. The internal air temperature as read by a TPMS is cooler than the hot spot of a tire. The surface temperature as read by the IR gun will be lower than the probe which is closer to the hottest spot on a tire.

Bottom line: IR guns are OK for conductive metal parts such as hubs and brakes but are not sufficiently accurate for items with large temperature variation over small distances. Probes will give a more accurate reading than a surface reading. To get the actual hottest temperature a probe needs to be able to reach the edge of a tire belt.

How do I know what my hot pressure and temperature should be?

Bill P. asked

“I just read your article on tires and speed. Seems that most RVers would stay around 60 mph here in the USA since it is the best speed for gas economics along with tire wear. That being said I would love to hear your thoughts on tire pressure/temperature monitoring systems. I have a class C 28' Four Winds and as a new person to the RV world purchased a monitoring system.

In my travels I get a variety of readings from tires and sometimes am concerned as to the pressure max and temp max. All six tires are exactly the same but in all my reading I find it impossible to get opinions or facts on what to expect in pressure and temperature from cold temps to normal driving pressures and temps.

I understand there are a lot of factors such as weight of vehicle to the temperature in the area and highway...but there should be something on what the absolute limits above the max cold pressure and possible the same comparison with temperature. I find nothing. Example: I run the duals at between 75 psi to 80 psi and 80 is the max at cold. Does this tire allow the pressure to rise up to 90 or even 100? If you need the tire size and weight of the RV let me know. If there is some sort of industrial chart concerning these issues that would also be great.”

Great question Bill and welcome to the world of RV ownership. I know you would like an answer like “the Max temperature is 149.6°F and pressure increase of 12.8 psi” but as you are learning with tires nothing is that simple.

First off I strongly support having a TPMS. Even if you check your pressure with a good digital gauge every morning, how will you know if you drive over a nail as you pull out of the campground at the start of your day’s travels?

Next you need to get your real individual tire loads with the RV fully loaded to be sure you are not overloading any individual tire. You have a Tire Placard that indicates the recommended cold tire inflation you should follow until you know your real loads and calculate your personal minimum inflation levels.

 As you work through my earlier posts you will see the general guideline for pressure increase of about 2 percent for every 10°F, so if you set your cold pressure to 80 psi in Oklahoma City in November and it's 45°F you may see your cold pressure has increased to 86 psi in Phoenix the next morning when it is 85°F. If you watched your tire pressure during your trip you might see the tires run from five psi to 20 psi above your cold pressure as the tire temperature warmed up to 20° to 50°F above ambient. This increase in tire temperature is affected by many variables such as load, speed, air movement around the tires (inner duals will usually run hotter) and moisture content of the inflation air and actual tire inflation. Also if it starts to rain you will see a very quick drop in tire temperature and an attendant drop in pressure as the water draws away the heat being generated in the tire.

Be sure to check out “Quick post on Max tire temperatures” and Tire Temperature & Pressure - A Hot Topic and Are tire pressure monitoring systems “TPMS” worthwhile?

Why do tires fail?

Simple question but as with almost every question about tires, the answer is not a one liner. Stick with this post. It's not short but it really isn't technical and I'm sure every reader will understand the concepts.

When looking at why tires fail we need to first exclude the obvious damage caused by external objects i.e. pot holes, curbs, chunks of metal, glass or rock or other trash and junk on the road. Second we'll exclude broken valve bodies, leaky valve cores, valve gaskets, extension hoses, and cracked rims.

What's left will be a structural failure which would almost 100 percent of the time will be a detachment of different parts or components of a tire one from the other.

Most tires are made up of 20 to 30 different components such as different steels (belts and bead are not the same steel) textiles such as Nylon, Polyester, Rayon, Aramid. Then there are the various different rubber components such as Tread, Sidewall, Innerliner, Steel skim, Wedge, Flippers, Chaffer, base tread, Inserts, and other bits and pieces.


Each component is selected for different reasons and contributes both advantages and disadvantages to the total. Each component must "stick" to its neighbors both in the un-cured and cured state. The interface between two different components is weaker than the individual components but it must hold together through millions of cycles over a 200 degree temperature range.

All would be just fine if a tire were made completely of inorganic materials such as metals, stone or even ceramics. There are numerous formulas for the strength and fatigue limits for these inorganic materials.

The issue with organics (wood, rubber or other materials made from oil such as plastics) is that their strength has a "T" or time function. If you build a bridge of stone, you can calculate the maximum load it can sustain and as long as the structure isn’t changed due to external damage its strength will be the same the day it’s built and 50 years later. If I build a pressure vessel of steel and put 150 psi in it and it doesn’t fail I would have every expectation for it to continue to hold that pressure for decades, again excluding external damage.

However, if I build a pressure vessel of organic materials (a tire) it might hold 150 psi the first day or maybe the first few days but at some point it can fail. This principle is not something many engineers think of today because they don’t design bridges of wood but 100 or 150 years ago when wood was common they learned that a bridge that was strong enough to hold a railroad train to drive across, it could fail if the train parked on the bridge.

I educated some fellow tire engineers about this “T” function when I proved, through lab experiments that it was possible to fail a tire with high pressure above its stated max 18 days after it was initially inflated.

The other condition that affects and changes the “T” in the equation is temperature. Organics experience constant change (loss) in properties as the temperature increases. The rate of change (aging) doubles about every 20 degrees of increase in temperature, so it's not easy to calculate or predict the time it will take for an organic structure to fail unless you can control the temperature over time.

Bottom line. It’s a combination of temperature (heat) and time that causes tires to fail. A tire that spends its life in Flagstaff, Ariz., could last twice as long as a tire that spent its life in Phoenix if all other operating conditions were identical.

As the owner of an RV you can significantly affect one factor and that is the heat generated internal to the tire. When you run fast or overloaded or under-inflated you are running hotter. This means you are speeding up the rate of "aging" of your tires and can expect to see a tire fail before it wears out.

Quick post on Max tire temperatures

I was recently asked "What do you recommend for high pressure setting with a TPMS?"

The person said after driving about 50 miles in 92°F weather he was seeing 148°F on his right front and the left front went to 136°F on his Class A RV. His tire pressure monitoring system (TPMS) has a setting of 158°F for high temp. He wants to know; At what tire temperature should he be concerned when driving?

A couple of quick points.
If the TPMS is one of those that screws onto the end of the valve, then it is reading lower than the hottest part of the tire. If he has extension hose on the dual then the temp reading is even lower. I have tried to find a TPMS manufacturer that has data on the temperature difference but so far either they do not answer my email or they haven't bothered to do a test.

My information indicates that a properly loaded and inflated Class-A tire will probably see 140°F to 170°F operating temperature and these temperatures are OK as the tire is designed for this temperature range.

If you have actual side to side weight and know you are not overloaded, you may see temperatures in the 140 - 170°F range. If you see higher then I would be a bit concerned and want to know why.

Setting your TPMS Max warning temp to 158°F sounds reasonable to me.