Why do tires fail?

There are basically two reasons that explain the vast majority of tire failures.

1. Over-load / under-inflation which are almost the same thing since it is the inflation air that carries the load not the tire.
Under-inflation: A tire operating at less than 80% of the inflation needed to carry the load, is considered to have been run flat and there is a good chance that there has been permanent internal structural done to the tire if driven on in that condition. Related to this is having the tire inflated to a level that just barely is rated for the actual load on the tire. Obviously you can be under-inflated because of cut or puncture or a valve leak or if you use an inaccurate gauge. If you run sufficiently low in pressure at highway speeds for a just a few miles you can have a Run Low Flex Failure or more commonly a "Blowout". Steel body tires can have "Zipper" failures due to fatigue of the steel body ply.

Over-loaded: Few people realize that by design, most passenger vehicles have 13% to 20% or more "Reserve Load". That means that they are actually under-loaded by that much for a vast majority of the time. Most RV trailers on the other hand have tires selected that are at the tire max load and can just barely carry the actual load while motorhomes seem to have some reserve load but not as much as passenger cars.
Data on actual loads measured shows that over half of RVs measured (out of many thousands) have one or more tires overloaded based on actual inspection.
While at a campground last week I got into a discussion with the guy parked next to me with a 5 week old RV.  I bet him a beer that at least one of his tires was at least 10% low. We found all his tires were between 18% and 21% below spec. I did enjoy the beer. :-)

2. Heat. Heat damage occurs at the molecular level and degrade the ability of rubber to flex and stretch and not break the chemical bonds. Once cracks form the rubber does not repair itself, the cracks just continue to grow. If they grow enough eventually you may have components come apart. Heat comes from a few different sources. This heat is generated by the flexing of the tire with the hottest region being at the belt edges (edge of the tread) in radials, not the sidewall if the tire is properly loaded and inflated. Increased speed generates more heat. Sometimes the heat is generated faster than it can be transferred to the surrounding air. Over-loading generates more heat. Under-inflation generates more heat. Having 0% reserve load generates more heat than having 10% reserve load which generates more heat than being 15%  under-loaded etc.  This heat can soak into the structure of the tire and actually accelerate the aging of the rubber in the tire. As rubber ages, it looses it's flexibility so this contributes to the breakdown of the rubber at the molecular level mentioned above.



Part of Organic Chemistry is chemical reaction rate.

For every 18F increase in temperature the rate of aging doubles. Heat also comes from being in the sun when parked. So if the RV is parked with tires in direct sunlight you can see the tire achieve 36F increase or more which means it is aging at more than four times the rate it would have if in full shade.
If you want to understand the technology behind this accelerated aging due to heat I suggest you can read some of these sources if you have a few hours.
http://en.wikipedia.org/wiki/Reaction_rate

http://www.chm.davidson.edu/vce/kine...tionRates.html

http://www.chm.davidson.edu/vce/kine...fReaction.html

http://chemistry.about.com/od/stoich...actionrate.htm

Here are some specific references on tires
http://www.rubberchemtechnol.org/doi...5254/1.3547913

I provided temperature data in my post on  white tire covers which protect tires almost completely from this heat damage. Many times the cumulative damage from excess heat can result in a separation of the belts and tread from the rest of the tire. Remember aging of rubber makes it less able to tolerate flexing.

Hope this helps others understand the causes of the vast majority of tire failures.

Tire Life. Why can't I get a straight and consistent answer?

 Have seen a number of posts on RV forums and Facebook asking why there are different answers to the question of when to replace tires in RV application. Some are told "Replace at 5 years" and hear " you will die if you drive on a 6year old tire.

Discount tire has presented this diagram.

You will note that they do not show some cliff that you fall off.  I responded to a question on tire life where the person thought that the tire companies were simply pushing tire sales.

Tire "Life" is not an on-off switch. Rubber begins to lose its strength and flexibility the day it is put in the tire warehouse. Temperature and time are the primary drivers of the loss of strength and flexibility. Tires can fail for a variety of reasons. Hitting potholes creates cracks in the internal tire structure. Most are microscopic but all cracks grow and none repair themselves so the number and size of cracks simply grow till one day the rubber will not be strong enough to tolerate hitting a pothole or piece of road debris and the heat generated with a long run at high speed on a hot day simply lowers the remaining strength of a tire.

The more a tire is driven the more flexing it experiences. Older tires, having lost some of their flexibility, will experience more actual tearing rather than stretching. Driving faster increases the rubber temperature. The higher the rubber temperature the faster the rubber loses its ability to stretch and recover. 

My post on tire covers pointed out the "aging rate' of tires doubles with each increase in operating temperature of 18 degrees F.

Part of Organic Chemistry is chemical reaction rate.

For every 18F increase in temperature the rate of aging doubles. Heat also comes from being in the sun when parked. So if the RV is parked with tires in direct sunlight you can see the tire achieve 36F increase or more which means it is aging at more than four times the rate it would have if in full shade.
If you want to understand the technology behind this accelerated aging due to heat I suggest you can read some of these sources if you have a few hours.
http://en.wikipedia.org/wiki/Reaction_rate

http://chemistry.about.com/od/stoich...actionrate.htm

Here are some specific references on tires
http://www.rubberchemtechnol.org/doi...5254/1.3547913

 

The idea that tires be replaced after 5 years of use is based on probability. Some tires fail at 3 years of use and some are still running after 9 but it's the odds that can get you. If you have an RV trailer I can assure you that the science shows that backing into an RV site is much harder on the tires than pulling through. This is because the Interply Shear is much higher backing in because the side forces are much higher. I wrote about that force in this blog post. https://www.rvtiresafety.net/2018/09/warning-super-technical-post-tire.

 

##RVT1007

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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#rvt743 

How to avoid tire failure and RV damage. And I had a tire failure!

A general comment on tire inspection and how it may be able to prevent RV damage due to tire failure if done properly.

There are three basic types of tire failures.

- The least likely is a sudden large impact with some object in the road. This is the least likely and is actually difficult to do. By this I mean to have a tire that is OK with no damage having been done due to excess heat, high load, low inflation or improper repair. It could even be a new tire. You drive over something like a Railroad spike or into a foot deep pot hole or over a 10" chunk of scrap steel that fell off a truck. It has to be large and you have to hit is just right for the tire to suffer immediate failure. I know from personal experience (doing a special test project of tire "Rapid Air Loss") that even driving over a piece of 2" pipe sharpened at one end and standing 2" up in the road with sharp end up doesn't always cut through the steel belts of a tire.  Yes you might run over something that cuts the tire but you would see it in the road and hear it hit but most of the time the air loss is not immediate. The good news about this type of failure is that it has a very low probability of happening. I would guess that fewer the 5% and maybe less than 2% of RV tire failures are of this type.

- Next is the belt separation. This is when the tire tread and belts come off the body of the tire. This is usually the result of tire aging and long term cumulative heat related damage that reduces the flexibility of the rubber to the point that rather than bending the rubber develops microscopic crack which do not heal themselves but will grow. Excess heat and tire aging can come from many sources. Even parking in direct sunlight with "tire protectant" spray does not lower the temperature of the tire. Excess heat can accelerate the aging or the tire rubber properties and drastically reduce the tire life. I would expect that if properly diagnosed this type of failure occurs 25 to 40% of the time. The good news is that with proper and frequent tire inspection this can be discovered and the tire replaced before it comes apart enough to cause damage to the RV. I did a blog post just on this topic "How do I inspect my tires" back in Aug 12 2014. You can Google the phrase and find a number of web pages on the topic but many simply are telling you to look at tread depth but this is not sufficient if you want to do a complete and competent inspection. My link included a YouTube video showing the inspection of a tire with belt detachment that has not come apart and the result of the "tire autopsy" I was able to perform. You can even see the separation between the belts in the above post.

- Finally there is what is commonly, and incorrectly, called a "Blowout". This is really a failure of the tire sidewall due to excessive flexing from running with significantly under-inflated ( probably below 50% of the inflation needed to carry the load. For Polyester tires (mainly ST and LT type) This heat due to flexing can be enough to reduce the strength by half and in extreme cases even melt the cord. For Steel body tires the bending of the steel can result in a fatigue failure similar to bending a steel paper clip till it breaks. This type of failure may be 60 to 80% of the failures on RVs. The good news is that if you run a TPMS you will get a warning of the air leak and hopefully you will not ignore the warning as too many do with other warning indicators on their dash, and take appropriate action which is to stop and pull over as soon as safely possible. Amazingly some people, even when verbally warned that they have a tire that is significantly under-inflated simply choose to continue to drive off. This has happened to me a number of times. As the saying goes "You can lead a horse to water but you can't make them drink".

So how doe this information help you "avoid" failure and RV damage. While nothing is 100% I bet you would like to be able to prevent 90% to maybe 98% of tire failures you might experience.

To do this I suggest that you run a TPMS and to get plenty of advance warning I suggest you set the warning pressure to be the minimum needed to support the load. Then your cold inflation pressure would be about 15% higher.

Next do a "free spin" inspection with lots of light. This is relatively easy with a trailer but harder to do as your axle load increases. You may even need the HD jack from a truck tire center and the spin balance machine to get that big 295/75R22.5 tire spinning at at least 20 to 30 RPM to allow you to see if there is "wobble" similar to the video in my post on How to I Inspect my tires" above. While the tire is in the air also do a slow rotation to inspect 360° of the tire tread as well as 360° of each sidewall looking for cuts and bulges. If any are found you should get the tire to a service center for a more thorough inspection that may include de-mounting the tire.

I believe that if you don't see any bulges or wobble and don't have any localized irregular wear spots in the tread you can be reasonable confident there is no large separation.

Remember nothing is 100% but if you make an effort you can significantly improve your odds of avoiding a tire failure.

Information to show you how this can work. Three weeks ago I was doing my annual 360° inspection. My tires are 7 years old so even though I know they have always been properly inflated (4 corner weights + TPMS from 2nd week of operation), I knew I needed to be sure all was OK. I discovered one of my duals had developed stress cracking from long term parking. This was a real surprise for me but it shows that even with the best of care rubber can get old and tires do need to be replaced.

  This tire was scrapped (I cut two slices from bead to tread to prevent its re-use by a "dumpster diver") as it still had lots of tread but I felt it should not be on the road.

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Black tire covers may do damage.

Last June I published the results of a test I ran on tire covers. I used my white vinyl covers as samples in the test. I mentioned that I did see some companies advertising black tire covers. I do not think are a good idea but need data to offer an informed opinion, so here is my test simulating black tire covers.

Here is a shot of my test set-up.

I used a trash bag that was actually dark green to simulate a black tire cover. The tire is just covered by the bag not in it.

Here we see the reference white cover at 93.5°F which is 5 degrees cooler than in the original test but close enough for this test.

The old passenger tire I had registered 113.3°F in the part that was in direct sunlight. This tire was dirty and gray so it did not register as hot as a clean black tire would.

Here we can see that the black (dark green) plastic cover was at 136.4°F which is almost identical to the uncovered tire in the original test at 136.1°F so this dark cover is actual heating the tire in this test.

Here we see that black tires and black covers can run almost 40° hotter than when under a white cover. This means the rate of aging of this tire under the dark cover is 12 to 15 times as fast as when a tire kept is cooler under a white cover.

Some will ask if lowering the temperature for a few hours each day is worth the effort, so lets set up a hypothetical situation.

Assume you spend 6 months out of the year in sunny Florida or Arizona or similar location with lots of sunshine. You are parked so one side of the RV is in the full sunshine for 8 hours a day and the sun shines 25 out of each 30 days. Now to make the math easier let’s assume it is always 85°F ambient. Now based on the original test our tires in full sun would be at 132° but if shielded by white covers would only be 96°.

So the question is: How much “older” would the tires without covers or with black covers be than tires that were covered with white covers?

Tire A has a cover. So lets figure that as our control so there are 25 days times 6 months times 8 hours per day of solar heat exposure for a total of 1200 hours.

Tire B has no cover, so it’s temperature is two times 18° hotter. Remember the “aging” rate doubles (times 2) with each 18° increase so this tire spends 25 days times 6 months times 8 hours times 2 times 2 or 4,800 hours.

The difference is 3,600 hours which equals 150 days or almost a half year. In this example each year your uncovered tire “ages” almost one and a half years vs one year for the covered tires. This means that after 6 years of staying only 6 months in a sunny location your uncovered tires have rubber properties that are more like nine year old tires. If we consider tha additional damage done by UV light I think you can see that the uncovered tires will probably have serious sidewall cracking after only 6 years of real time aging.

Covering tires with dark or black covers may shield the UV but they do nothing for the heat and could make the heat worse through radiation.

Bottom Line. If you want to get the most out of your tires you should cover them with a white or light colored cover when the tire will be exposed to extended time in direct sunlight.

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.

Do you have to replace valve stems when installing TPM sensors?

Got this question on an RV forum:
"I understand from the thread that you should use metal tire stems with TPMS caps. The ends of the tire stems on our trailer are metal with the only visible rubber part right where they go through the wheel. I am guessing these are still considered rubber stems and should be replaced prior to installing TPMS?"

"Metal" stems can be a bit misleading as some newer stems may have some brass showing. I posted pictures on THIS post showing standard passenger rubber valve, a "bolt-in" metal valve, and a new style rubber/brass valve.

When it is recommended that "metal" stems be used, what is actually meant is "bolt-in". You will see the nut that retains the bolt-in stem on the outside of the wheel for most of these stems.

The concern is that over time (months or maybe years) the extra weight of TPM external sensors may cause some vibration and movement of the rubber stem body which might result is a failure of the rubber portion of the stem.

I am not aware of any testing done on the new style rubber/brass stems with a TPM sensor screwed on, so I and others are erring on the side of caution when we recommend "bolt-in" metal stems.

Cracking and failure of the rubber part of valve stems is another time & temperature aging thing, just as it is with your tires. You visually inspect your tires monthly, as outlined in your RV  owner's manual, so just include the rubber part of your valves and just as you would replace your tires when signs of significant signs or aging (cracks) are found do the same with your valves. 

You could consider running your current valves until you get new tires and at that point have the bolt in valves installed. In the meantime, I would take a close look at your stems when doing your monthly visual inspection. You can push sideways a bit on the rubber/brass stem and look for any cracks. If you find any, that would mean you need to replace them sooner rather than later.

Tire Covers - Do they do any good?

I have seen some posts and comments about tire covers. One post caught my eye.

The owner was complaining about the cover degrading in the sun so he was of the opinion the cover wasn't worth the cost. I have to wonder how he failed to properly interpret the proof that the cover was protecting his tires. How would he answer the question: Which would you rather have degrade due to being in the sun? Your tires or the low cost cover?

As an Engineer I always prefer to have data rather than just someone's opinion. So I set up a quick test on my own RV.

As you can see I have covers for my tires. They were the second purchase after buying my rig. The first was a Tire Pressure Monitor System.





The Test
With one side of the RV in full shade I checked the temperature of the side of the unit.
You can see I recorded 85.1°F.




Then I took the temperature of the side of the RV in the full sun.

Here we see 107.9°F








Next the temperature of the white tire cover in full sun.

We get 98.6°F
I believe the cover is cooler than the side of the RV because air can circulate behind the cover.


I then removed the cover to see what the tire temperature was under it.


We see 99.5°F
Only 1 degree hotter than the cover.







I then waited 30 minutes to see how hot the black tire got while in full sun.

We see the black tire was at 136.1°F.








KEY POINT
There are three things that can "kill" your tires. Ozone, UV and High Temperature. The Ozone & UV directly attack the surface of the tire making it crack when flexed. Temperature works no only on the surface but deep down inside the tire structure.
Increased temperature causes continued and accelerated chemical reaction which "ages" a tire faster than when the tire is cool. A rule of thumb would be that the rate a tire ages doubles with every 18°F increase in temperature. We can see the result of old rubber on the surface. What we don't see is the more brittle rubber of the internal tire structure. As rubber gets more brittle with age it also looses strength.

NOTE Overload and Underinflation can overheat a portion of a tire to the point the rubber and reinforcement materials loose all their strength.

Based on my simple test it would appear that by covering my tires I am significantly reducing the artificial aging for all the daylight hours my RV is parked and the sun is out. If I didn't have covers my tires would be "aging" FOUR times faster than with the covers in place.

I don't have black tire covers so can only guess at their temperature. While they may provide protection from UV I would be surprised if they can offer much temperature protection.

So I will let you answer the question of Tire Covers being a good investment or not.

Tire "Dressing" and "do not use covers"! Where's the test data?

Been following a long thread on an RV Forum on the topic of "Tire Dressing" aka tire treatment or "Tire Shine".

The good news is that most posters knew to not use any product that contains Petroleum Distillate. Too bad some RV dealers don't follow the guidelines on this. I have seen a number of display coaches almost dripping with some slippery fluid. Might be Brake Fluid or even motor oil as the tires were shiny and I could scrape the coating off, leaving a "slimy" substance on my finger.

I saw a number of different products mentioned and many suggestions that a product called 303 was a good UV protectant.

I saw no one provide any actual direct comparison test data for any product that would support the claim of protection against UV damage. With a little work getting through the maze of retailers selling "303" I finally made contact with their customer service. I asked "Can you provide any test data on 303 vs other tire protection products. Also info on if 303 application removes any of the wax or oil or anti-ozone chemicals built into tires." The answer was that they would ask their chemists. This seems strange that a company making claims on the performance of their product would not have comparison data available that would back up and support the claim. Please note, I am not saying that 303 does not offer some "protection" against UV damage to tires. I am concerned that the actual application as seen on YouTube may be removing the special waxes and anti-ozinate chemicals tire companies put in our tires.

If I receive actual test data that compares 303 against other products that make similar claims I will post on this blog.

What was most concerning were a couple of posts where the writers claimed

"I got tired of reading all the opinions on tire dressing so went to the source, Michelin. They stated that washing tires with mild soap is the only thing they support on their tires, no dressing no covers, no nothing. You don't see shiny tires at a truck stop and these are the people who put on the miles."

Another said "Goodyear RV tire guide says. Just keep them clean, no dressings, no covers, zip. You can use the same stuff you wash your RV with perhaps use a medium brush on stubborn spots."

I responded:   Interesting comment on "no covers". Wonder what Goodyear RV guide you are looking at? 
I found in THIS Goodyear RV Tire Guide and under storage the advice to store tires "in sunless area" and "Don't store tires where they are subjected to direct sunlight or extreme temperatures".
Based on the above, how does telling us to not use tire covers make sense?

I have confirmed with actual test data that covering tires can reduce the tire temperature by about 40° F which could extent tire life by many months, depending on how long you protect them from artificial heat aging.

I checked and found just the opposite on the shielding from the Sun. In fact I even contacted an Engineer at Michelin and he said "Our position regarding tire dressing and protection has not changed. It is still recommended that tires are cleaned with a mild detergent and water, and they are protected from direct sunlight when the vehicle is parked for extended periods of time. This is usually accomplished with the use of some type of cover. "

So I don't know where the "no cover" info from either Goodyear or Michelin comes from and if you read my blog you know that this tire design engineer is a strong proponent of using White vinyl on Class-B & C tires and the flat mesh of any color on Class-A.

I have nothing against 303. I am wondering why, for the price they don't have any actual direct comparison performance data on UV protection.

PS. If you don't want to protect your tires from the heat aging due to sun exposure please don't ask me to explain why you don't get longer life from your tires. Also next time you run into someone selling an brand  "Tire Dressing" ask to see the test data and see what happens.

Too much pressure increase

Reading an RV forum thread on TPMS usage. One comment jumped out at me.

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


I replied that inflation ratings for wheels are based on a "cold" pressure. Increase in pressure due to operation is considered by manufacturers and I would not be concerned as long as the proper cold inflation is used along with appropriate limits on load and speed.
Tire wording "Max Pressure" can be confusing but in reality the pressure stated on the tire sidewall is the cold pressure needed to support the stated load and that load is the maximum load the tire should be subjected to. I advised the owner that he should NOT underinflate his tires and plan on operation temperature to increase the pressure. "Cold" pressure is the only pressure you should be concerned with.

Tire pressure increases by about 2% for each increase in temperature of 10F. If you don't remember the Science from High School you can read THIS post.. If you are seeing a 21% increase in pressure (110 > 134) that means you are seeing about a 100F increase in internal tire temperature, which I would consider a bit excessive for normal tire operation.

If you are seeing a 25% increase in pressure (95 > 119) then you are getting a 125F increase in temperature which indicates you are working the tires even harder . This extra "work" that is generating a greater temperature increase is not good for long term tire life. You are "aging" the tire rubber faster. Some might want to review this "Key Point" of tire life as covered in THIS post.

I cover these points on Temperature, Inflation and Aging in various posts on my RV tire blog.

Your temperature increase indicates you are possibly overloading your tires and also possibly driving faster than desirable for your tire loading.

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

The poster then responded:

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

So I responded back:
 Few people realize that the pressure increase as a function of temperature is based on well established and confirmable Physics and that a TPMS is not reading the actual tire temperature  but is actually reading the temperature of the brass valve stem and the metal base of the TPMS itself, which is being cooled by outside air.

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

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

I have no doubt that the TPMS was indicating only 20F above the cool 50F outside air temperature. If you are still concerned about the hot pressure of 134 on tires that have a cold pressure rating of 110 for its max load capacity rating.

As a tire design engineer with 40 years experience, I trust the science of the "Gas Law" and knowledge that air is an insulator and metal conducts heat from a hot source to a cooler one.
I don't know what to advise other than to decrease the operating load and speed and to confirm your hand gauge is accurate and to always inflate the tire when cold to 110psi, as continued operation at current load and speeds will certainly result in pressure reading that are above the cold pressure of 110 psi.

I just had a tire failure, for no reason

If you follow any RV forums you probably see a statement like this every few weeks. I can tell you that tires do not fail because of some RV-Magic. It comes down to actual Science.

Why do tires fail?

Simple question but as with almost every question about tires, the answer is not a one liner. 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 either a detachment of different parts or components of a tire one from the other, OR a heat/fatigue failure in the mid sidewall due to driving with extreme low inflation.

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 200F 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 two 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 18 degrees F 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 probably last twice as long as a tire that spent its life in Phoenix if all other operating conditions were identical so even knowing the state where the RV was used isn't sufficient.

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. You can also protect and shield your tires from the heat generated when in direct sunlight while your RV is parked. HERE you can read about an experiment I ran on the benefits of shielding your tires from Sun exposure.

##RVT1048

Was your "Blowout" caused by parking at Quartzsite two years ago?

This is a bit of a continuation on the topic of "tire Dressing" from last week's post.

I do wonder how many that complain of sidewall cracking or less than desirable tire durability makes the effort to protect their tires from the heat and UV degradation from direct sun exposure.

While there might be some benefit and shielding from applying such as "Dressing" or "Shine" or other "stuff" to the tire sidewall for UV shielding. No dressing will protect a tire from the accelerated aging process due to being "baked" by the sun's heat.

There is no standard SPF for tire dressings as there is for sun-tan lotion, It would not be that difficult to measure the level of UV shielding by measuring the UV going through some clear plastic sheeting and then applying the tire dressing to the sheeting and measuring the level of reduction if any in UV rays.

In many cases, the rubbing of the tire sidewall when applying a dressing can remove the special anti-oxidants and UV protection provided by the chemicals the tire companies use in all tires.

All rubber, natural or synthetic, loses strength and gets stiff over time. The rate of degradation is also temperature-dependent with the rate effectively doubling with each increase of 18° F. As I have shown in my blog on the use of tire covers sunlight can easily accelerate the rubber degradation by 4 times with 8 times not unreasonable in Southern states. This loss of strength and flexibility can eventually lead to a belt separation, which many sometimes call a "blowout". Maybe that unexplained "blowout" in June was brought about by the eight months of direct sun exposure in Quartzsite, AZ, over the last two winters.

I have confirmed, with temperature measurements, the significant temperature drop with the use of white vinyl covers on Class-B, Class-C and RV Trailer sizes, and the use of a flat mesh "Tire Shades" on Class-A motorhomes. I would recommend against the use of black or dark color solid vinyl covers as they would act more like an oven by both transferring the sun's heat to the tire and preventing cooling air circulation around the tire.

##RVT948

Don't damage your tires when you clean them

Recently there was a Tech Article in an RV publication about cleaning the tires on an RV. The reply reminded me of some investigations I did a number of years ago on some strange looking sidewall damage. After doing a confirming test, where I was able to duplicate the damage, I concluded that the damage was done by a High Pressure Power Washer.

Here are some examples of the damage. You can click on the image to see an enlarged view.








Another example is seen here:











In sample three you can see the cut that was made to confirm the depth of the damage.



I have never seen a advisory posted on one of these High Pressure Power Washers warning of the damage that can be done if you get too close. There are many different pressure ratings so I cannot give an exact limit on how close you can get without doing damage but I would certainly stay more than a foot away if I felt I needed to use one.

This damage has been documented in Tire Industry publications so if you do cause this type of damage, there is a good chance you may not be compensated under the tire warranty, so be careful. The best way to clean your tires is to use the same soap you use to clean your car or RV and a sponge or cloth just as you would on the paint on your vehicle. I would not use a stiff brush or anything like steel wool on the tire as you will scrub off the protestant waxes built into the tire which will accelerate the tire sidewall aging and could promote sidewall cracks. I would limit the pressure I use to what I get out of a garden hose.

Shield your tires from exhaust heat

Some folks are noticing the inner dual on one side seems to always run hotter than the outer dual or the inner on the other side of their coach.
IMO it is the radiant heat from your exhaust system that is causing your tire to always run a bit hotter.

Remember that the "aging rate" of tires doubles with each increase of 18°. This means you are potentially cutting the life in half.

I suggest you take a look underneath your coach. You do not need to wrap your exhaust. This can cause problems and may shorten the life of your exhaust system. A simple metal shield can shield the tires from the IR heat.

Here is what GM installed on my C4500 for my Class-C. Simple sheet metal with a bottom lip to provide some stiffness and to prevent cuts I guess. Just some galvanized 16 to 20 Ga steel can do the job. I bet if you check with a Home heating shop that does ductwork they can cut and bend a piece for not much $.  My shields were welded (1" every foot or so) and show no signs of failing after 17,000 miles. Could also drill and use some heavy metal screws too.

What effect does speed have on tire failure?

Question:
"What effect does speed have on tire failure?

I have an 8000 lb trailer with 4 Goodyear Marathons. The truck is a Dodge 2500 diesel so I can cruise at 75 mph subject to wind and hills.

How much if any does speed factor into tire failure? I replace tires every 4-6 years regardless of what the look like, and keep them inflated at the PSI marked on the tires."

Basically, increased speed means increased temperature in the critical areas of a tire. Here is a graphic showing the relative temperature in different areas of a tire.

with Red being the hottest and dark blue being the coolest.

When you increase the temperature of rubber you are effectively increasing the rate the rubber is aging. I believe we all understand that old rubber is not as flexible as new rubber. If it isn't as flexible you end up with cracks or cracks that grow every time the rubber is flexed. More cracks and larger cracks are what result in tire failure.

For the above reasons plus others, the major tire companies suggest that in RV operation tires never be driven any faster than 75 and you can see this max speed stated in their literature for many of their RV tires.

But wait, you say your tires have a "Speed Symbol" that relates to 87 or 99 or maybe even 130. HERE is some information on speed ratings. Just because you have a tire with a fast symbol doesn't mean it can be driven at those speeds for 20 or 40,000 miles. About the only thing you can count on is that tires with a "higher" symbol will provide better steering response than a tire with a lower rating.

Now back to the question of how fast can you drive or tow your RV. I previously mentioned 75 as the upper limit but for some tires, there are other things to consider. With ST type tires as found on many trailers, we need to remember that the load capacity is based on a formula that originally specified a max speed of 65 mph. In the past, there were Technical Bulletins that advised that inflation needed to be increased (but not above the sidewall max) by 10 psi to go 70 mph. In addition, load needed to be decreased by 10% if you want to go 75. Still, there was that 75 max.

Many trailer owners know that they need to replace their tires before they wear out. If they don't they are probably going to have some type of failure. But those same people will not replace the tires on their pick-up till they are significantly worn. Why?

The answer can be seen if you simply compare the load and speed ratings of an LT tire and an identical size ST type tire.

LT235/75R15 101Q LRC1985# 50 psi 99mph

ST235/75R15 110R LRC2340# 50 psi 106mph

So exactly what type of magical engineering has tire company X used to achieve both increased load capacity plus increased speed capacity when it is the air pressure that supports the load? If they have the ability to make that ST tire really perform at those speed and load conditions what is wrong with their LT type tires?

IMO what we are looking at is a marketing plan taking over the engineering reality.

Bottom line

You may be able to pull the trailer at speeds above 65 or drive the Class-A diesel pusher faster than 75 but you will be consuming the finite life of the tire and can expect a failure before you wear it out no matter how you maintain your tires.

##RVT817

Why do tires fail?

Some comments on the general question of tire failures and why they seem to be so prevalent on RVs.

Previously I posted on how tires may be selected by the RV manufacturer. Lowest cost with the least margin of safety allowed by law seems to be the general trend for many manufacturers. We also know that well over half of all RVs that even bother to get weighed have one or more tires overloaded. These facts seem to be something that many want to ignore. Many blame the geographic location i.e. China where the tires were manufactured as the cause of the failures. Some think that the tire mfg intentionally makes poor quality tires. The fact is that tires do not fail for no reason at all. They do not fail because the factory was painted green instead of blue. They fail because their finite life is used up or they suffer some external damage or puncture which lets the air out. Punctures are not always obvious as this one with a 6" piece of aluminum from a broken truck rim poking through the tread.

Some punctures are quite small. So small that you need magnification to see them.

 This puncture resulted a 2 psi per week leak and I only found it after noticing the very slow air loss and the need to add air every week to 10 days.

Impacts do not always result in immediate loss of air. Some tires can run for many miles, days or even weeks and still might not “Blowout”.

 Can you honestly remember every pot hole or curb or piece of road trash you ran over in the last 10 or 20 miles or yesterday or 5 days ago as you pulled into the campground?

 

Valves can fail because the rubber components age-out just like tires can but how often have you replaced the rubber gasket in your metal valves? Do you even have bolt in valves? Standard “snap-in” valves are rated 60 psi Max but since they don’t come out of the wheel immediately after you inflate your tire to 65 psi how many owners assume all is OK for the next 5 years and 40,000 miles?

Valve cores can develop slow leaks if a single grain of sand gets lodged in the valve core seal. How many make sure they have a good o-ring in their metal valve caps, or do they simply use the cheap plastic caps as seen on most cars?

Tires age out. This does not mean that they will fail after 2782 days. It means that after sufficient time has elapsed at elevated operating temperature (which accelerates the aging process in a non-linear and ever increasing rate as the temperature goes up) the strength and elastic properties of the rubber degrade to the point that microscopic cracks that start out tearing molecular bonds apart, grow. The cracks never get smaller. They only grow. The rate at which they grow also depends on the physical properties of the rubber and the properties change with the temperature. Once the cracks grow sufficiently large, the structure starts to lose its ability to retain its integrity and things start to come apart in an ever accelerating manner.

I personally have run tests where I exceeded design range for load and inflation. The tires did not fail for 18 days when the load, inflation, speed and temperature were controlled in a laboratory. This test is repeatable so this is not magic. It is sound science. How many people would relate a sudden tire “Blow-Out” with conditions established 18 days previous?

 I do not intend my comments to mean that there has never been a tire failure that was caused by a manufacturing defect. What I want to show is that simply because you do not find a railroad spike lodged in your tire does not mean there was no reason for the failure due to external causes. It takes years of experience and sometimes hours of detailed examination sometimes including chemical analysis, X-Ray and even Electron Microscope examination to find the real reason for the failure. I know of no tire dealership that has any such equipment or that has provided man-weeks of training in forensic analysis to any of their employees that would allow them to arrive at the real answer of whay a given tire failed.

10% safety margin? Not Over-Inflation for more load capacity.

Originally Posted by Crasher

Roger. When you refer to a 10% safety factor, is the tire any safer at 10% over it's rated load psi? If it is, why don't the manufacturers recommend a higher psi for the load? Or, is the 10% factor to cover the days when the ambient temp is lower which would lower the CIP eliminating the need to adjust the pressure? Whenever I have run tires above the load charts, the center of the tread will wear more than the outer sides. That tells me that the tire was not making optimum contact with the road for best wear and traction. Admittedly, it's a minor issue, but an inquisitive mind has to ask.

My +10% is on the set inflation and is NOT a "Safety Factor" in the normal sense. We know that tire inflation changes by about 2% for each 10°F change in temperature. The intent of this "Flex Range" of inflation is to avoid the need to mess with inflation on a daily basis.
Assume you needed 70 psi to support your heaviest ever expected load (this is why we say get on the scales when fully loaded to your heaviest). So assume you set your inflation to 70 psi and the Ambient temperature is 80F. What happens the next day if the ambient drops to 70F? Your tire pressure will have dropped by 2% to about 68psi which is below what is needed to support the measured heavy load. So you get out and increase your tire pressure back up to 70psi. A few days later it's 90F so tire pressure is now (90F - 70F = 20F so 2% per 10F = 4% increase of the 70 psi so now your tires are at about 73 psi cold so you drop your tire pressure.

See the problem? You are messing around with your tire pressure. almost every day.

However if you have a +10% "Flex Range" above your needed inflation, or in our example + 7 psi You can ignore the day to day pressure variation unless or until the temperature has dropped 50°F.

Tires can tolerate the increase in pressure with essentially no damage but low pressure can result in increased operating temperature which accelerates the "aging" of the belt rubber which can shorten tire life.

Also if you have to mess with your tires a lot, soon you will tire of the chore and stop monitoring and adjusting tire pressure which can lead to low inflation. This extra work can get old quickly and then you stop checking and setting your pressure. I Do Not Want That to happen.

RE center wear. That was an issue with bias tires but I do wonder what micrometer you are using to measure tire tread wear to 0.001" especially given that tire tread wear is normally in the .001" per 1,000 mile range and I doubt that your pressure remains constant over each thousand miles operation. Road surface (concrete vs asphalt) has a much bigger impact on tread wear. 

##RVT965

Do you need an external spare tire cover?

Reading a forum where the owners of one specific brand of trailers were complaining about the $21 the RV company wanted to charge for a spare tire cover.

I have previously covered, in detail the advantages and the "why" we should be storing our tires inside or under our RV or under light colored covers. It is the HEAT that kills tires and the aging effect on tires is exponential with an increase in temperature (doubles each 18°F) you could end up with a tire failure with ZERO miles as seen in this example.

 I pointed out, with temperature data, in my blog post on why white tire covers are good and much better than black covers.
This shot shows what can happen with a black cover that effectively places the spare in an "oven" where cooling air can't offset any of the Sun's heat.

You will note that in both the above examples the top side of the tire (location with most direct Sun heating) is the location of the failure.

These folks do it right.

I know of no tire shine or protectant spray that provided protection from heat.

Why do tires fail? Tire expert has the answers

 As an actual Tire Design and Quality Engineer, I know from my 50 years of tire experience, including 25 with a primary focus on failed tire forensic inspection, that tires fail from two basic causes:

• Low air pressure, and/or
• Long-term degradation of the rubber, usually from excess heat.

Low air pressure

An active leak from a puncture or loose valve stem or valve core is the most common reason. Also, poor or no inflation maintenance contributes to low air pressure as all tires lose pressure at about 1% to 2% a month, excluding the pressure change due to changes in ambient temperature, and can lead to a sidewall flex failure, more commonly called a “blowout”.

The sidewall cord can melt (polyester) or fatigue (steel). Many travel trailer and fifth wheel owners fail to realize that they will never “feel” the results of a tire losing air until it is too late. Then they are surprised when the sidewall “lets go.” The rapid air loss “bang,” even when the tire only has about 10 to 20 psi in it, is a big surprise IF they even hear it. A tire pressure monitoring system (TPMS) can provide warning of air loss, so it is good insurance. A TPMS can easily pay for itself many times over.

Long-term degradation of rubber

The long-term degradation of the rubber at the edges of the belts in radial tires can lead to a belt and/or tread separation. Even if the tire keeps its air, you can have this type of failure, so a TPMS will not provide a warning in this case.

This degradation comes with age, as the rubber is always losing flexibility. Just think of those rubber bands you found in the back of the old desk drawer. Even in cool and dark they got brittle. However, running at, near or above the load capacity of a tire will result in increased heat generation. Increased heat actually can accelerate the aging process with a doubling of the rate with each increase of 18 F. Running a margin of at least 15% between tire capacity and measured load is a good first step. Running at higher speed will also generate excess heat. For ST-type tires, this means faster than 65 mph, no matter the Max speed rating.

Realizing that more than half of the RVs on the road have one or more tires in overload is one main contributor to the high tire failure rate. Simply thinking that a tire will fail because the tire plant building is painted blue rather than green is not logical. But many still have feelings along those lines when it comes to where a tire is made.

Buying the lowest cost “no-name” tires is, IMO, a major contributor to poor results. If the main objective is the lowest cost tire, why would anyone be surprised with short tire life?
Just paying more, however, is no guarantee of better quality. I believe the best tire shopping tool available is comparing warranty and service support.

Can you get a multi-year warranty on the tires? Is it possible to get road hazard coverage? Is there a nationwide network of dealers who stock the brand you are considering? The answer to these questions may provide a better indication of tire quality than the opinion of other RV owners published on the internet.

There are some self-appointed “experts” out there who think that selling tires for a numbers of years qualifies them to make technical pronouncements on tire technology. I would suggest that people review "The Pneumatic Tire"   published by the US DOT before making a claim of being a “Tire Expert”.

 

 

 

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