How old is too old

 I read a comment on an RV motorhome forum:

"I continue to read posts indicating that tires will calendar out, in 5 or 7 or even 10 years. After some searching I have yet to find a technical document regarding tire life expectancy. I'm not interested in sales brochures or salesmen guide lines. I'm looking for a document written by a subject matter expert in the tire manufacturing industry. IMO tire life is more a function of proper storage, inflation, maintenance, environment, and care. I believe a physical inspection by a tire professional is more reliable than an arbitrary date."

So I offered the following:

As an actual Tire design Engineer with 40+ years experience, I believe I can provide some information BUT if you are looking for an answer such as 62 Months 2 weeks and 5 days you are out of luck. It's just not that simple.

Maybe you can tell me, in hours, how long a gal of milk will stay "good". Since you know you can't answer that simple question why do you think the answer for a complex structure such as a tire age should be that simple. Do you think that all 27 basic components age at identical rates?

Most tires fail for one of a couple reasons. You should have read THIS post if you searched on "why tires fail?" I have also covered this in detail in a number of posts on my blog "RV Tire Safety"  But we can cover the topic again.

Polymer Cross Link Density is the property that determines how flexible a piece of rubber is. If it is too flexible or elastic it will not hold its shape. If it is not flexible or elastic enough it will develop microscopic cracks. These cracks will grow with every revolution of a tire and also some will grow simply given enough time.

Heat and time will change the cross link density. The rate of change is not linear but doubles with each increase in temperature of 18°F. This means 4 times if it is 36°F hotter or 8 times if the rubber is 54°F hotter. This is why tires that are on RVs that spend most of their life in Southern Tier states like FL, GA, TX, AZ will fail earlier than tires that spend most of their time in ND, MI, NH, ID, OR. But a tire that spends it life in Phoenix will "die" in maybe 4 years an identical tire that spends it's entire life in Flagstaff may live to 8 years. But there are other factors that can have significant effect on tire life.

 

If we were to put a set of tires on a LT and another identical set on a 4 tire trailer, then load all 8 tires to identical load and inflate to identical level the tires on the trailer may have a life that is 25% to 50% shorter than the tires on the truck. This is due to Interply Shear which is the force in all radial tires at the belt edges that is trying to tear the tire apart from the inside (see "Interply Shear" if you want to see my posts on that topic.

This post is on a Class-C thread but I can guarantee that information published here will be incorrectly applied to information on a TT thread because people do not understand the significant different forces internal to a tire.

Michelin has published a guide on tire inspection which basically suggests at 5 years the tire be inspected inside and out annually and re-applied if no problems are discovered. BUT they still put a MAXIMUM life on the tire as unless you have "X-Ray" vision the structure can have cracks that are not visible on the tire surface, even not visible on the internal air chamber surface.

##RVT1035

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

Expected tire life

I Saw a question on tire life:

"I was wondering could some one cover a topic travel trailers ,5th wheel, bus , semi's and semi trailers, and anything recreational  W H A T   IS   T I R E   LIFE  FOR  THEM?  I have never seen this covered   HUM !"

I have covered this in a few different posts on this blog and some RV forums but this post may put it all in one place:

Generally, tire life for Bus and HD truck is based on wear, not time, as these vehicles may drive 50 to 100,000 miles a year with the tires wearing out at 50 to 80,000 miles.
Daily drivers (cars  & P/U) drive about 12,000 a year and may get 3 to 5 years life again most based on wearing out.

With RVs (Trailer & Motorhome) mileage might be as low as a few hundred miles a year to a few driving up to 20,000.  BUT if you consult your owner's manuals you will probably see them point out expected life to be 3 to 5 on trailers. Motorhomes probably hit 7 to the suggested max of 10 years, again before wear-out due to low miles driven in most cases.

 The primary reason for the earlier "end of life" on trailers is the result of the unique radial belt shear forces identified in this blog and in the industry technical papers as "Interply Shear" that comes from a combination of tires being dragged rather than steered around corners and tires "fighting" each other when going around corners as the tires on different axles are not all rotating around the same center of the turn. It is the Interply Shear that initiates the cracks between the belts and accelerated the crack growth that can end up as a belt separation before the tires wear out. The Interply Shear damage is augmented by damage from improper (low) inflation, improper (high) load and in some cases, excess heat due to speeds higher than the basic design called for.

 I hope this helps others why we have different tire life experiences on our RVs than with our cars.

##RVT895

How much Reserve Load or "Headroom" is enough?

Following an RV forum discussion on How much extra load capacity or "Reserve Load" or "Head Room" is enough. Some folks had moved from LR-C to LR-D  others to LR-G but were concerned about running the tire sidewall pressure as it would be too hard on the trailer.

There was also some confusion about the "Maximum" allowable inflation.  I said:
"The inflation on a tire sidewall is the MINIMUM inflation pressure needed to support the MAXIMUM load (also molded o the tire sidewall).
There is a sound scientific reason to run higher inflation when running tires on multi-axle trailers. It is called "Interply Shear". This force is what can result in belt/tread separations. I have covered this in detail along with references to technical papers on the topic.
IMO you need a minimum of 15% "head-room" of load capacity over the measured load of the heavier end of an axle. (CAT scale only gives the total on an axle and very few RVs have 50/50 load split. Some folks have discovered unbalance of 500 to 1,000#)
Most cars come with 20% to 40% tire load capacity headroom. I run 25% on my Class-C."
Which resulted in this reply:
"So if a given pressure (below the tires maximum) still provides the necessary headroom over the measured load, it's okay to run it, correct? Or am I not understanding something? "

This leads me to respond:
"I wish it were that simple but all radial tires have the Interply Shear force. It's a question of how much "tire life" is being "used up". The force results in microscopic cracks forming between the two steel belts at the outer edge. Once cracks form they do not repair themselves but can only grow.
No good tool or math model can turn Shear Force into Miles of tire life is "consumed" as there are just too many variables.
Finite Element models simply indicate that higher inflation can result in lower shear force every time you turn a corner or every time a tire completes a revolution. 
About the best you can do is to do a close visual inspection (Free Spin) to look for signs or out of round or side to side runout which are strong indicators of internal structural tears. I have a video and pictures of the autopsy I did on a tire where you can see the separations running across 50% to 70% of the tread width. This large separation might run a thousand miles or under certain circumstances might fail tomorrow.
Think of it like your Blood Pressure. Higher is worse. So the question is at what number can you expect to have a fatal attack?
If you consult your owner's manual you will see a suggestion that 3 to 5 years is the max expected life but obviously someone that runs at or above Max Load and above 65 mph and drives 4,000 mi a year is more likely to suffer a failure than someone that only drives 2,000 mi, never faster than 62 and at only 80% of tire load capacity.
Moving up one Load Range C>D or D>E can probably expect longer tire life if they also go from 50 to 65 or 65 to 80 psi. Going to LR-G in a Commercial grade all-steel tire should result in longer life even if you don't run 110psi.
Since load capacity is shown in the Load/Infl tables then I would suggest trying to get to 20 to 25% Reserve Load would be a good move. You still need to watch max speed and minimize sharp turns (as seen when backing) and of course, do a good free spin inspection once a year or once every 2,000 mi whichever comes first. Plus run a TPMS to confirm you never run lower pressure than your goal."

I realize this is a long winded post but hopefully, you can think about the ideas and concepts we are discussing.  Easy questions do not always get simple answers, Especially when talking about a complex topic.

##RVT888

 

How old is too old part 2

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


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

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

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

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

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

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

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

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

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


Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 16th year. Learn more or subscribe. 

How old is too old?

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


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

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

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

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

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

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

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

Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 16th year. Learn more or subscribe. 

Can you "contain" a tire failure ?

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

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

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

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

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

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

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

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

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

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

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

Subscribe to the weekly RVtravel.com newsletter or one of our other newsletters about RVing. Great information and advice. Now in our 15th year. Learn more or subscribe.